Symposium Organizers
Thomas Ambrose Seagate Research
William Bailey Columbia University
David Keavney Argonne National Laboratory
Y. Daniel Park Seoul National University
Q1: Magnetic Tunnel Junctions
Session Chairs
Tuesday PM, April 18, 2006
Room 3020 (Moscone West)
9:30 AM - **Q1.1
Giant Tunneling Magnetoresistance and Tunneling Spin Polarization in Magnetic Tunnel Junctions Using MgO Tunnel Barriers.
Stuart Parkin 1 , See-Hun Yang 1 , Hyunsoo Yang 1 , Christian Kaiser 1 , Brian Hughes 1 , Phil Rice 1
1 , IBM Almaden Research Center, San Jose, California, United States
Show AbstractRecent advances in generating, manipulating and detecting spin-polarized electrons and electrical current make possible new classes of spin based sensor, memory and logic devices [1]. One key component of many such devices is the magnetic tunneling junction (MTJ) - a sandwich of thin layers of metallic ferromagnetic electrodes separated by a tunneling barrier, typically an oxide material only a few atoms thick. The magnitude of the tunneling current passing through the barrier can be adjusted by varying the relative magnetic orientation of the adjacent ferromagnetic layers. As a result, MTJs can be used to sense the magnitude of magnetic fields or to store information. The electronic structure of the ferromagnet together with that of the insulator determines the spin polarization of the current through an MTJ -- the ratio of 'up' to 'down' spin electrons. Using conventional amorphous alumina tunnel barriers tunneling spin polarization (TSP) values of up to ~55% are found for conventional 3d ferromagnets, such as CoFe, but using highly textured crystalline MgO tunnel barriers TSP values of more than 92% can be achieved for otherwise the same ferromagnet [2]. Such TSP values rival those previously observed only with half-metallic ferromagnets. Corresponding giant values of tunneling magnetoresistance are found, exceeding 350% at room temperature and nearly 600% at 3K. Perhaps surprisingly the MgO tunnel barrier can be quite rough: its thickness depends on the local crystalline texture of the barrier, which itself is influenced by structural defects in the underlayer. The tunneling probability in an MTJ may depend on the symmetry of the electronic wave-functions which can lead to spin filtering, as theoretically predicted for Fe/MgO/Fe [3], when the symmetry of the majority and minority bands are distinct. The tunneling probability is also strongly influenced by chemical bonding at the ferromagnet/ barrier interface so accounting, for example, for little change of TSP and TMR when Co is diluted with significant amounts of Pt [4] and for high tunneling spin polarization in rare-earth transition-metal alloys with nearly zero magnetization [5]. 1S. Parkin, X. Jiang, C. Kaiser, A. Panchula, K. Roche, and M. Samant, Proc. IEEE 91, 661 (2003).2S. S. P. Parkin, C. Kaiser, A. Panchula, P. Rice, B. Hughes, M. Samant, and S.-H. Yang, Nature Mater. 3, 862-867 (2004).3W. H. Butler, X.-G. Zhang, T. C. Schulthess, and J. M. MacLaren, Phys. Rev. B 63, 054416 (2001).4C. Kaiser, S. van Dijken, S.-H. Yang, H. Yang, and S. S. P. Parkin, Phys. Rev. Lett. 94, 247203 (2005).5C. Kaiser, A. F. Panchula, and S. S. P. Parkin, Phys. Rev. Lett. 95, 047202 (2005).
10:00 AM - Q1.2
How Many Crystalline Interface Layers are Necessary to Create High TMR?
Christian Heiliger 1 , Peter Zahn 1 , Ingrid Mertig 1
1 Department of Physics, Martin-Luther-University Halle-Wittenberg, Halle Germany
Show AbstractRecent experiments [1,2,3] based on epitaxially grown Fe/MgO/Fe samples shed light on the subject of tunneling magnetoresistance (TMR). First of all, the obtained TMR ratios exceed the predictions by Julliere's model [4]. Second, experimentally obtained bias voltage characteristics show features which could be related to the electronic structure of the system in the ballistic limit of tunneling. The high crystallinity of the samples [1,2,3] seemed to be the reason. New experiments [5], however, demonstrate that even amorphous electrodes attached to a crystalline MgO barrier show a TMR of more than 130%. The question that is addressed in this talk is: How many crystalline metal layers close to the interface are necessary to obtain high TMR.A screened Korringa-Kohn-Rostoker (KKR) method based on density functional theory was applied to calculate the electronic and magnetic structure of the different junctions self-consistently. The Landauer conductance of planar junctions was calculated using the Baranger-Stone scheme by means of Green's functions in the limit of coherent tunneling.[1] J. Faure-Vincent, C. Tiusan, E. Jouguelet, F. Canet, M. Sajieddine, C. Bellouard, E. Popova, M. Hehn, F. Montaigne, and A. Schuhl, Appl. Phys. Lett. 82, 4507 (2003).[2] S. Yuasa, T. Nagahama, A. Fukushima, Y. Suzuki, and K. Ando, Nature Materials 3, 868 (2004).[3] S.S.P. Parkin, C. Kaiser, A. Panchula, P.M. Rice, B. Hughes, M. Samant, and S.-H. Yang, Nature Materials 3, 862 (2004).[4] M. Julliere, Phys. Lett. 54A, 225 (1975)[5] K. Tsunekawa, D. D. Djayaprawira, M. Nagai, H. Maehara, S. Yamagata, N. Watanabe, S. Yuasa, Y. Suzuki, and K. Ando, Appl. Phys. Lett. 87, 072503 (2005)
10:15 AM - Q1.3
Finite Bias Calculations on Electronic Transport Through Fe/MgO/FeTunneling Junctions
Ivan Rungger 1 , Oleg Mryasov 2 , Alexandre Rocha 1 , Olle Heinonen 2 , Stefano Sanvito 1
1 School of Physics, Trinity College Dublin, Dublin Ireland, 2 , Seagate Research, Pittsburgh, Pennsylvania, United States
Show AbstractIn this work we calculate the I-V characteristics of Fe/MgO/Fe(100) tunneling junctions using our recently developed code Smeagol [1]. Smeagol interfaces the non-equilibrium Green's functions (NEGF) method with density functional theory (DFT) using the numerical implementation contained in the SIESTA code [2], which is based on a linear combination of atomic orbitals basis set. Smeagol calculates self-consistently the current for a finite applied bias, and it has been tailored for simulations of magnetic materials.First we investigate the zero-bias transmission coefficients T(E) as a function of the in-plane k-vector and the energy for either collinear parallel or antiparallel configurations of the magnetic leads, as well as for non-collinear orientations. Then the zero bias calculations are compared to the self-consistent calculations at finite bias, and the charging behavior of the interface is analyzed. It has been found that for small biases the minority current in the parallel configuration is mainly carried by surface states, but that it saturates for voltages above about 20 meV. In contrast, for the other spin channels of the different leads configurations the zero-bias predictions are found to be a good approximation of the self-consistent finite bias calculations.Furthermore we analyze the effect of geometrical asymmetry due to either partial asymmetric interface oxidation or due to asymmetric lattice distortions. It has been shown that these asymmetries are important for the understanding of the observed I-V curves. Finally we investigate how above results are sensitive to the inclusion of a pseudo self-interaction correction for the standard LDA exchange and correlation potential. This correction increases the bandgap of MgO compared to the LDA result, bringing it very close to the experimental value.References[1] A. R. Rocha, V. M. Garcia-Suarez, S. W. Bailey, C. J. Lambert, J. Ferrer and S. Sanvito, cond-mat/0510083 (2005)[2] J. M. Soler, E. Artacho, J. D. Gale, A. Garcia, J. Junquera, P. Ordejon and D. Sanchez-Portal, J. Phys.: Condens. Matter 14, 2745 (2002)
10:30 AM - **Q1.4
Tunnel Conductance Oscillation Induced by the Quantum Well in fully Epitaxial Double Barrier Magnetic Tunnel Junctions.
Koichiro Inomata 1 2 , Takayuki Nozaki 1 , Nobuki Tezuka 1 2
1 Materials Science, Tohoku University, Sendai, Miyagi, Japan, 2 CREST, JST, Kawaguchi, Saitama, Japan
Show AbstractQuantum oscillation of the tunneling conductance was observed in fully epitaxial double barrier magnetic tunnel junctions (DMTJs) consisting of a MgO seed layer (10)/Fe (50)/MgO(2)/Fe(t)/MgO(2)/Fe(15)(the numbers are film thickness in nm), deposited on a single crystal MgO(001) substrate using the molecular beam epitaxy (MBE)[1]. The designed thickness of a middle Fe layer was varied from 1.0 nm to 1.5 nm. All layers were evaporated by electron-beam bombardment at room temperature (RT) and annealed at 300 C for the bottom Fe electrode and 200 C for the middle and top Fe electrodes for 20 min. in order to improve the morphology of the layer surface. The epitaxial growth of each layer was confirmed by observing the reflection high-energy electron diffraction (RHEED) patterns during deposition and after the annealing treatment. The cross-sectional transmission electron microscopy (TEM) exhibits the layered isolated islands of 10 ~ 40 nm in diameter and 5 nm in height for the middle Fe layer, which is about three times thicker than the designed thickness. We confirmed that the Fe islands grow epitaxially on the MgO barrier with very flat interface, demonstrating parallel-connected tunneling for the DMTJs. We have observed the tunnel magnetoresistance (TMR) of 110% and 135% at 5K and RT, respectively, which are comparable to that for the single magnetic tunnel junctions (SMTJs). The large TMR in the DMTJs over the value estimated by the Julliere model using the Fe spin polarization is consistent with the coherent tunneling as in the SMTJs using a MgO barrier. The conductance of the DMTJs oscillates as a function of the bias voltage in the positive bias direction for various middle Fe layer thicknesses. The bias voltage where the conductance shows maximum changes into a higher value with decreasing the middle layer thickness. We can observe the oscillatory feature even at RT for the DMTJ with t = 1.2 nm (actual thickness is estimated to be about 2 nm from the cross-sectional TEM analysis) in the positive bias direction, although the oscillation amplitude attenuates with increasing temperature. For the case of t = 1.5 nm, on the other hand, the oscillatory feature disappears at RT due to the small energy gap between the quantum levels. These characteristics demonstrate the creation of the QW states in the middle Fe layer. The conductance oscillation was observed only in the parallel magnetization configuration. This result can be explained by the QW states formed in only one of the two spin bands, because in our structure only up spin electrons of Δ1 band in the Fe(001) electrode is expected to form the QW states due to the absence of the down spin Δ1 states at the Fermi energy. Further improvement of the quality of the quantum well layer will provides us the realization of the spin-dependent resonant tunneling effect.Reference[1]T. Nozaki, S. Nakamura, N. Tezuka and K. Inomata, Phys. Rev. Lett. submitted
11:30 AM - **Q1.5
Magnetic Materials for High-Performance Toggle MRAM
Jon Slaughter 1 , R. Dave 1 , S. Pietambaram 1 , J. Sun 1 , G. Grynkewich 1 , M. DeHerrera 1 , K. Smith 1 , N. Rizzo 1 , S. Tehrani 1
1 Technology Solutions Organization, Freescale Semiconductor, Inc., Chandler, Arizona, United States
Show AbstractMagnetoresistive random access memory (MRAM) employs a magnetoresistive device integrated with standard silicon-based microelectronics, resulting in a combination of qualities not found in other memory technologies. For example, MRAM is non-volatile, has unlimited read and write endurance, and has demonstrated high-speed read and write operations. Here we present an overview of the characteristics of our 4Mb Toggle-MRAM circuit based on magnetic tunnel junction (MTJ) devices, and outline paths for improving performance and scaling to higher densities. Specific technology demonstrations for improving performance and scalability of both the read and write operations will be presented. For example, high-MR, MgO-based tunnel junction material has been integrated with 180nm and 90nm CMOS circuitry to improve the read performance. The high MR values obtainable with MgO-based MTJ devices can enable significant read access time improvements, but are useful only if all the other requirements that enable fully functional MRAM circuits are met. These requirements include: narrow bit-to-bit resistance distributions, free-layer magnetic properties that produce narrow switching distributions, low interlayer coupling, good bias dependence, reliability, and thermal endurance. Results for MgO tunnel barriers grown with different processes show how the barrier structure affects the growth of the subsequently deposited magnetic material. Tests comparing properties of the 4Mb arrays with MgO-based and AlOx-based MTJ material show improved useable signal with MgO, although this polycrystalline barrier results in wider distributions for several array parameters. Paths to reduced power consumption that employ novel magnetic materials also will be presented.
12:00 PM - **Q1.6
STM, XPS and Spin-Torque Studies of Magnetic Tunnel Junctions
Robert Buhrman 1
1 School of Applied and Engineering Physics, Cornell University, Ithaca, New York, United States
Show AbstractThe recent advances in magnetic tunnel junction (MTJ) technology together with the demonstration of the ability of spin-polarized currents to excite and switch nanomagnets have opened up a wide range of new opportunities regarding the study and technological applications of nanomagnetism and metallic spintronics. To further advance these prospects we need new and more detailed understanding and enhanced control of these MTJ materials systems, and also of the details of spin transport through these tunnel barriers. In this presentation I will discuss results from recent STM and XPS studies in our laboratory of the chemical and electronic structure of half-formed and fully-formed magnetic tunnel junctions, which identify the nature of the electronic defects in these ultra-high oxide barrier layers and point to possible pathways for further improvement. I will also report on spin-torque experiments with MTJs which have provided a new approach to the measurement and understanding of the bias dependence of spin transport and TMR. The spin torque results demonstrate that the polarization of the tunnel current does not, at least in the systems we have studied, decrease strongly with bias, contrary to the bias behavior of the TMR. This result is quite positive for spin-torque applications of MTJs and is also consistent with the electronic structure of ultra-thin tunnel barriers as revealed by the STS and XPS studies.
12:30 PM - **Q1.7
Novel Transport Phenomena in Ballistic Conductors and Tunnel Junctions.
Evgeny Tsymbal 1
1 Department of Physics and Astronomy, University of Nebraska-Lincoln, Lincoln, Nebraska, United States
Show AbstractIn this talk we overview our recent results on modeling transport phenomena in ballistic conductors and tunnel junctions. For ferromagnetic ballistic conductors we predict that electronic transport exhibits ballistic anisotropic magnetoresistance (BAMR) – a change in the ballistic conductance with the direction of magnetization [1]. This phenomenon originates from the effect of the spin-orbit interaction on the electronic band structure which leads to a change in the number of bands crossing the Fermi energy when the magnetization direction changes. For epitaxial Co/SrTiO3/Co magnetic tunnel junctions with bcc Co(001) electrodes we predict a very large tunneling magnetoresistance(TMR), originating from a mismatch of majority- and minority-spin states contributing to the conductance [2]. In agreement with the experimental data, we find that the spin polarization of the tunneling current across the Co/SrTiO3 is negative. We attribute this property to the complex band structure of SrTiO3 which is formed from localized 3d states of Ti and, unlike MgO- and Al2O3-based MTJs, allows efficient tunneling of the minority d electrons of Co. Stimulated by experimental observations of ferroelectricity in thin films of a nanometer thickness, we consider a new type of tunnel barriers based on ferroelectric materials. We predict that in such ferroelectric tunnel junctions (FTJs) the conductance depends strongly on the direction of the electric polarization [3]. This giant electroresistance effect is the consequence of a different potential profile seen by transport electrons for the two opposite polarization orientations. Using a ferroelectric barrier in a magnetic tunnel junction makes it multiferroic, where ferromagnetic electrodes are separated by a ferroelectric barrier. Multiferroic tunnel junctions (MFTJs) have the potential to provide an additional degree of freedom in controlling the conductance. We discuss possible implications following from the interplay between ferroelectric and ferromagnetic properties of the two ferroic constituents in these junctions. This work is supported by NSF and Nebraska Research Initiative. [1] J. Velev, R. Sabirianov, S. S. Jaswal, and E. Y. Tsymbal, Phys. Rev. Lett. 94, 127203 (2005).[2] J. Velev, K. D. Belashchenko, D. Stewart, M. van Schilfgaarde, S. S. Jaswal, and E. Y. Tsymbal, Phys. Rev. Lett. 95 (2005), in press.[3] M. Ye. Zhuravlev, R. F. Sabirianov, S. S. Jaswal, and E. Y. Tsymbal, Phys. Rev. Lett. 94, 246802 (2005).
Q2: Ultrafast Magnetization Dynamics and Damping
Session Chairs
Tuesday PM, April 18, 2006
Room 3020 (Moscone West)
2:30 PM - **Q2.1
Spin Torques and Pumping in Itinerant Ferromagnets: From Heterostructures to Bulk Dynamics.
Yaroslav Tserkovnyak 1 , Arne Brataas 2 , Gerrit Bauer 3
1 Physics Department, Harvard University, Cambridge, Massachusetts, United States, 2 Department of Physics, Norwegian University of Science and Technology, Trondheim Norway, 3 Kavli Institute of NanoScience, Delft University of Technology, Delft Netherlands
Show Abstract3:00 PM - Q2.2
Modeling The Common Origin Of Fs Demagnetization And Gilbert Damping.
Francesco Dalla Longa 1 , Bert Koopmans 1 , Wim de Jonge 1
1 Dept. of Applied Physics, Eindhoven University of Technology, Eindhoven Netherlands
Show AbstractAll-optical techniques exploiting femtosecond laser pulses have opened the way towards the exploration of the ultimate limits of magnetization dynamics. It has been found that magnetic order in ferromagnetic transition metals can be quenched within a few hundred femtoseconds after laser heating. However, the microscopic interpretation of the phenomena at the sub-ps level have been a mystery until recently. In this presentation we present a microscopic model that successfully explains the ultrafast equilibration of magnetic order in ferromagnetic metals at a time scale τM of only a few hundred femtoseconds after pulsed laser excitation. We found that τM can be directly related to the so-called Gilbert damping factor α that describes damping of GHz precessional motion of the magnetization vector, thereby unifying two apparently unrelated issues in applied magnetism. The crucial ingredient in our approach is the inclusion of spin-flip processes accompanying momentum scattering with impurities or phonons. A simple model Hamiltonian is used to derive analytical expressions for both the Gilbert damping and the demagnetization. Independent of the spin-scattering mechanism, an appealingly simple equation relating the two key parameters via the Curie temperature TC is derived, τM ~ c0h / 2πkTC α, with h and k the Planck and Boltzmann constant, resp., and the prefactor c0 ~ 1/4. This readily predicts a demagnetization time of ~ 100 fs for reasonable values of Gilbert damping in ferromagnetic nickel. A comparison with experiments, in which both parameters are measured on the same sample, will be discussed.Results from numerical extensions of the model, incorporating an applied magnetic field and heat diffusion out of the irradiated volume, will be presented as well. In particular, this approach allows us to simulate ultrafast laser-induced reversal of magnetization in a reverse magnetic field. Our results demonstrate that phonon-mediated spin-flip scattering is a good candidate to explain the sub-ps magnetic response. However, although the lattice is an essential ingredient, it is found that the spin temperature is following the electron temperature (rather than the lattice temperature), and that reversal can be achieved even though the lattice temperature stays below the Curie temperature. Having established this crucial insight, a wide range of future experimental investigations can be envisioned – as presently being implemented.
3:15 PM - Q2.3
Reduced Spin-lattice Coupling and Gilbert Damping in Epitaxial Fe1-xVx (50nm) Thin Films.
Lili Cheng 1 , Christian Scheck 1 , Yong Feng Guan 1 , William Bailey 1
1 APAM, Columbia University, New York, New York, United States
Show AbstractThe reduction of Gilbert damping in magnetic thin films is an important materials goal for several applications in >1 GHz spin electronics, improving the signal to noise ratio (SNR) in nanoscale GMR sensors and the efficiency in spin momentum transfer (SMT) devices. Fe-low Z alloys, with average composition near Z = 25, are of potential interest for reduced damping due to an expected reduction in spin-lattice coupling, according to previous Einstein-de Haas experiments. We have deposited epitaxial MgO(100)/Fe1-xVx thin films by ultra high vacuum (UHV) cosputtering at a base pressure of 2×10-9 Torr, with Vanadium concentrations x up to 52%, spanning the Z = 25 composition at x = 33%. The GHz relaxation rate (damping) and anisotropy constants were measured using broadband (0–40 GHz) and angle-dependent X–band (10GHz) ferromagnetic resonance (FMR). Relaxation rates were measured directly by the frequency-swept FMR linewidth Df; these drop from ~ 270 MHz for pure Fe to 200 MHz for Fe67V33 over the range of 4 to 14 GHz. 10 GHz field-swept linewidths for the pure Fe films are measured as low as DHpp = 29 Oe, close to the lowest literature measured values, indicating a minor role for inhomogenous (extrinsic) broadening/relaxation. In angle-dependent measurements, we find a dramatic reduction in the cubic magnetocrystalline anisotropy constant K1 up to 42% V; K1 drops by nearly two orders of magnitude from 4.8×105 erg/cm3 for pure Fe to 1.5×104 erg/cm3 for Fe58V42. The concurrent reduction of relaxation rate and magnetocrystalline anisotropy is suggestive of a common origin in reduced spin-lattice coupling for the alloy.
3:30 PM - Q2.4
Spin-torque Effects in Single-crystalline Fe Nanomagnets and Nanopillars.
Henning Dassow 1 2 , Ronald Lehndorff 1 2 , Daniel Buergler 1 2 , Matthias Buchmeier 1 2 , Peter Gruenberg 1 2 , Claus Schneider 1 2
1 Institut fuer Festkoerperforschung, Forschungszentrum Juelich GmbH , Juelich Germany, 2 cni - Center of Nanoelectronic Systems for Information Technology, Forschungszentrum Juelich GmbH , Juelich Germany
Show Abstract4:15 PM - **Q2.5
Ultrafast Magnetization Dynamics, a New Frontier in X-ray Science
Andreas Scholl 1
1 , Lawrence Berkeley National Laboratory, Berkeley, California, United States
Show AbstractUltrafast magnetic writing using field and optical pulses has caught the interest of scientists because of the fascinating physics underlying the exchange of energy and angular momentum in a magnetic material and because of possible applications in data storage. Experiments in ultrafast magnetism put stringent requirements onto the experimental technique, both in terms of temporal and of spatial resolution. The availability of pulsed x-ray sources and fast detectors have now opened the door to such studies using x rays as a probe.First, I will discuss time-resolved imaging of magnetic patterns with 100 nm spatial resolution using the Photoemission Electron Microscope PEEM-2 at beamline 7.3.1.1 of the Advanced Light Source [1]. The bunch length of the storage ring sets the time resolution of 80 ps. Magnetic vortices appear in soft-magnetic micron-size structures and are characterized by a curling magnetization. We observed that the chirality or handedness of the vortex, which is determined by the out-of-plane magnetization of the nanometer-size vortex core, governed the dynamics of the structure in response to fast field pulses. The field pulses were generated by a laser-triggered Auston switch.On a faster time scale, magnetization dynamics experiments often rely on direct laser excitation of the material. Experiments using the time-resolved magneto-optical Kerr effect (TR-MOKE) [2] demonstrated the possibility of manipulating magnetism using a femtosecond laser pulse. The origin and the implications of the fsec-laser-driven demagnetization of Ni have been widely debated. X-ray magnetic dichroism is an ideal tool to probe such dynamics in complex materials because x-ray sum rules quantify spin moment, orbital moment and magnetic anisotropy of each element in a multi-element system. Furthermore, x-rays are sensitive to both ferromagnetic and antiferromagnetic order. I will present measurements of the transient spin and orbital moment of Fe/Gd following a f-sec laser excitation. The experiments were conducted using an x-ray streak camera with a temporal resolution of 2 ps at BL 4.0.2 of the Advanced Light Source.This work was supported by the U.S. Department of Energy under Contract No. DE-AC03-76SF00098 at Lawrence Berkeley National Laboratory.[1] S.B. Choe et al., Science 304, 420 (2004)[2] E. Beaurepaire et al., Phys. Rev. Lett. 76, 4250 (1996)
4:45 PM - Q2.6
Measurement of Ferromagnetic Resonance (FMR) by Time-resolved XMCD: Element- and Layer-resolved Magnetization Dynamics at 2 ps and 0.1 deg. Resolution.
Yongfeng Guan 1 , William Bailey 2 1 , Chi-Chang Kao 3 , Elio Vescovo 3 , Dario Arena 3
1 Applied Physics Program, Department of Applied Physics, Columbia University, New York, New York, United States, 2 Materials Science Program, Department of Applied Physics, Columbia University, New York, New York, United States, 3 National Synchrotron Light Source, Brookhaven National Laboratory, Upton, New York, United States
Show AbstractWe demonstrate an x-ray magnetooptical measurement of ferromagnetic resonance (FMR), enabling highly sensitive studies of the coupled precession of elements in an alloy and layers in a heterostructure. Small-angle (< 1.0 deg.) precession could be measured magnetooptically for the first time due to the high magnetic contrast of time-resolved x-ray magnetic circular dichroism (tr-XMCD) in the transmission geometry. Basic expectations of driven FMR precession could be verified in Ni81Fe19, driven to ~ 0.7 deg. cone angles at 2.3 GHz. Weak ferromagnetic coupling (~ 5 Oe), on the verge of detectability in variable-frequency FMR measurements, could be revealed clearly in the phase and amplitude of the layer-resolved in a Ni81Fe19(25 nm)/Cu(20 nm)/Co93Zr7(25 nm) trilayer precession. The technique enables studies of the microscopic mechanisms in relaxation / damping, either in complex materials or heterostructures.
5:00 PM - Q2.7
Interface Magnetization Precession and Switching in Fe/AlGaAs (001).
Gunter Luepke 1 , Haibin Zhao 1 , Diyar Talbayev 1 , Aubrey Hanbicki 2 , Conny Li 2 , George Kioseoglou 2 , Berry Jonker 2
1 Applied Science, College of William and Mary, Williamsburg, Virginia, United States, 2 , Naval Research Laboratory, Washington, District of Columbia, United States
Show Abstract5:15 PM - Q2.8
Field-driven Crossover in Precesssional Dynamics of Lateral Ferromagnetic Heterostructures.
Nikolay Polushkin 1 , Steven Michalski 1 , Lanping Yue 1 , Roger Kirby 1
1 Physics and Astronomy, University of Nebraska-Lincoln, Lincoln, Nebraska, United States
Show Abstract5:30 PM - **Q2.9
Spin Dynamics in Magnetic Vortices
Jooho Park 1 , Robert Compton 1 , Mun Chan 1 , Paul Crowell 1
1 Physics, University of Minnesota, Minneapolis, Minnesota, United States
Show AbstractA vortex is the simplest topological defect that can form in a magnetic system. We have used time-resolved Kerr microscopy to study vortex dynamics in ferromagnetic disks with diameters from 500 nm to 2 microns and thicknesses from 20 nm to 50 nm. In addition to ordinary spin waves, these systems show a low-frequency excitation corresponding to the translational degree of freedom of the vortex core.[1] We show how the motion of the vortex core couples to the spin wave spectrum, breaking the degeneracies that exist in the case of cylindrical symmetry.[2] I will also discuss recent work on excitations in elliptical particles, focusing on the relation between spin dynamics and inhomogeneous magnetic microstructure. This work was supported by NSF DMR 04-06029 and the NSF MRSEC program under DMR 02-12032.1. J. P. Park, P. Eames, D. M. Engebretson, J. Berezovsky, and P. A. Crowell, Phys. Rev. B 67, 020403R (2003).2. J. P. Park and P. A. Crowell, Phys. Rev. Lett. 95, 167201 (2005).
Q3: Poster Session: Magnetic Materials, Mostly Metallic
Session Chairs
Wednesday AM, April 19, 2006
Salons 8-15 (Marriott)
9:00 PM - Q3.10
Magnetic Properties of Ni Nanoinclusion in Alumina.
Annika Pohl 1 , Claudio Sangregorio 1 , Claudia Innocenti 1
1 Dipartimento di Chimica, Universita' di Firenze, Sesto Fiorentino (Fi) Italy
Show Abstract9:00 PM - Q3.11
Self-Assembled Growth of Ferrimagnetic Spinel Nanometric Pyramids
Florencio Sanchez 1 , Nico Dix 1 , Ulrike Luders 2 , Jean Francois Bobo 2 , Josep Fontcuberta 1
1 , ICMAB-CSIC, Bellaterra Spain, 2 , Onera, Toulouse France
Show AbstractWhereas self-assembled growth of semiconductor materials is widely investigated, the possibilities of complex oxides to grow in such a way are much less explored. However, self-assembled growth of oxides could allow functionalities not attainable with the common device fabrication technology of multilayer deposition and lateral patterning. An outstanding example is the fabrication by a self-assembly technique of arrays of ferrimagnetic spinel CoFe2O4 nanopillars embedded in a ferroelectric BaTiO3 matrix [1]. The nanostructured hybrid system displayed a remarkable coupling of magnetic and ferroelectric properties not observed in ordinary multilayer structures. Understanding the growth mechanisms of these nanocomposites is essential for its controlled fabrication and optimization of properties.Here we report on the self-assembled growth of three-dimensional pyramidal objects in ferrimagnetic spinel, CoCr2O4 and NiFe2O4, epitaxial films. The spinel islands [2] have a shape very similar to the SiGe pyramids and hut clusters, but detailed characterization reveals important dissimilarities. They are {111} faceted pyramids and hut clusters, and thus have a very high aspect ratio, and moreover, they maintain the shape during growth. The objects are perfectly oriented along the <110> directions and have a certain positional order along the same direction. The growth progression was investigated: at early stages strained (dislocation free) small nanometric pyramids form, which is followed by a structural relaxation and a spectacular growth of some of the objects. We demonstrate that the size of the objects (from the nanometer to the micrometer) and area coverage (from isolated pyramids to fully faceted surfaces) can be controlled by the deposition time, growth temperature and substrate used. The driving forces for the observed {111} faceted objects and surfaces, and bi-modal or single-modal size distribution are discussed.Our demonstration of self-organized growth of ferrimagnetic spinel pyramids and hut clusters of controlled size and on different surfaces may open the possibility of controlled fabrication of hybrid systems combining these ferrimagnetic structures with other functional materials. References[1] H. Zheng et al., Science 303, 601 (2004); F. Zavaliche et al., Nano Letters 5, 1793 (2005)[2] U. Lüders et al., Phys. Rev. B, 70, 045403 (2004); Nanotechnology 16, S190 (2005)
9:00 PM - Q3.12
Alignment-Sensitive Reversal Mechanisms of Epitaxial-FeF2/Polycrystalline-Ni Exchange Biased Thin Films
Justin Olamit 1 , Kai Liu 1 , Zhi-Pan Li 2 , Ivan Schuller 2
1 Physics Department, University of California, Davis, California, United States, 2 Physics Department, University of California - San Diego, La Jolla, California, United States
Show AbstractMagnetization reversal mechanisms of epitaxial-FeF2/polycrystalline-Ni exchange biased thin films have been investigated with vector magnetometry on a Vibrating Sample Magnetometer (VSM). The samples have been exchange biased by field cooling along the FeF2 spin axis, the [001] direction. The transverse hysteresis loops – sensitive to magnetic moments perpendicular to the applied field - show that when the applied field is misaligned with the FeF2 spin axis, the reversal is predominantly by rotation [1, 2]. When the applied field is aligned with the spin axis, the transverse loop is flat, indicating predominantly domain formation and motion. We have employed a First Order Reversal Curve (FORC) [3, 4] technique to further investigate the reversal mechanisms. When the applied field is aligned with the spin axis, FORC analysis shows that the magnetization switching is highly irreversible (82%), indicating that domain nucleation and motion is likely the reversal mechanism. With a misalignment of 5°, the FORC shows that majority of the magnetic switching (57%) was by a reversible mechanism, consistent with the rotation seen with vector magnetometry. These results show that the magnetization reversal mechanisms are extremely sensitive to the alignment of the applied field with the antiferromagnet spin axis and the exchange bias direction.*Work supported by ACS-PRF, Alfred P. Sloan Foundation, and DOE.1.Olamit, Arenholz, Li, Petracic, Roshchin, Morales, Batlle, Schuller, and Liu , Phys. Rev. B 72, 012408 (2005).2.Olamit, Li, Schuller, and Liu, Phys. Rev. B, in press.3. Davies, Hellwig, Fullerton, Denbeaux, Kortright and Liu, Phys. Rev. B 70, 224434 (2004). 4. Davies, Hellwig, Fullerton, Jiang, Bader, Zimanyi, and Liu, Appl. Phys. Lett. 86, 262503 (2005).
9:00 PM - Q3.13
Computational and Experimental Evidences for Asymmetric Interfacial Mixing of Co-Al system
Sang-Pil Kim 1 2 , Jae-Young Park 3 , Kwang-Ryeol Lee 1 , Chung-Nam Whang 3 , Yong-Chae Chung 2
1 Future Technology Research Division, Korea Institute of Science and Technology, Seoul Korea (the Republic of), 2 CPRC, Department of Ceramic Engineering, Hanyang University, Seoul Korea (the Republic of), 3 Institute of Physics & Applied Physics, Yonsei University, Seoul Korea (the Republic of)
Show AbstractFor the development of thin film multiplayer devices such as MRAM or GMR/TMR sensors, atomic scale intermixing at the interface should be controlled in a systematic manner. In the present work, we employed classical molecular dynamics simulation to investigate the atomic scale intermixing at room temperature during thin film deposition in Co-Al binary system. When Co atom was deposited on Al(001) surface, atomic intermixing was easily achieved at the interface, and highly ordered CoAl compound phase of B2 structure was formed spontaneously. On the other hand, when Al atom was deposited on Co surface, atomically sharp interface was formed between the deposited Al layer and Co substrate. Using a coaxial impact collision ion scattering spectroscopy (CAICISS) and a magneto-optical Kerr effect (MOKE) measurement, these asymmetric intermixing phenomena were confirmed. Experimentally observed interfacial structures were in good agreement with the simulation results.
9:00 PM - Q3.14
Quadrupole Magnetic Force Microscopy Tip and its Imaging Performance.
Gang Han 2 1 , Yihong Wu 2 , Yuankai Zheng 1
2 Electrical and Computer Engineering, National University of Singapore, Singapore Singapore, 1 , Data storage Institute, Singapore Singapore
Show AbstractMagnetic force microscopy (MFM) has become a standard tool for investigating magnetic nanostructures by detecting the magnetic stray field distribution of a magnetic sample. The research on MFM has been centered on several aspects which include but are not limited to resolution enhancement, stabilization improvement, quantitative image interpretation and tip-sample interaction reduction. So far, many efforts had been made to improve the resolution of MFM through sharpening the tips using different approaches such as attaching carbon nanotubes to the original tips, trimming the tips by focus ion beam, electron beam lithography, and ion beam etching, selective deposition by self-field emission, electron beam irradiation, and focused electron beam decomposition and deposition. We have previously demonstrated that the resolution of the MFM tips can be further improved by using an FM/Ru/FM trilayer as the magnetic coating. The improvement was attributed to the formation of a point-dipole tip because of the antiferromagnetic coupling between the two FM layers, which form a relative smaller effective volume interacting with the sample than that of the single layer tip. In order to further improve the performance of the tip, in this work we reported on the fabrication and testing of a double synthetic tip in which a central thicker FM layer is antiferromagnetically coupled with two thinner FM layers at both sides via an ultrathin Ru layer. The tip was prepared by coating one side of a bare tip with a structure of Ta ( 10nm) / NiFe (2nm) / IrMn (10 nm) / CoFe (4nm) / Ru (0.8nm) / CoFe (10nm) / Ru (0.8nm) /CoFe (4nm) / IrMn (10nm) / Ta (10nm). In this design, it is expected that a quadrupole will be formed at the apex of the tip from the two synthetic structures. Theoretically this kind of tip should have a better resolution because the dipoles due to the two synthetic layers are oriented opposite with each other, leading to a reduction of DC response of the overall tip. Our preliminary results showed that the performance of the tip is comparable to that of the conventional tips. Further experiments are being carried out to optimize the structure of the tips so as to improve its performance.
9:00 PM - Q3.2
Electrodeposition and Characterisation of Fe-Based Oxides for Magnetic Tunnel Junctions.
Chien-Lung Teng 1 , Mary Ryan 1
1 Department of Materials, Imperial College London, London United Kingdom
Show AbstractMagnetite (Fe3O4) is of interest because of fascinating magnetic properties for use in spintronic devices. In this work, polycrystalline and epitaxial Fe3O4 have been electrochemically deposited at low growth temperature (<90o) upon nickel oxides formed on Single-phase rolling-assisted biaxial textured substrates (RABiTS) of pure nickel with {100}<001> crystallographic orientation. The formation of nickel oxides is carried out by means of air-formed and surface oxidation epitaxy (SOE), respectively. XRD pattern shows there is no prefered orientation as Fe3O4 thin films grow on native nickel oxide; Moreover, with SOE it becomes highly out-plane and in-plane oriented due to close lattice matching between epitaxial NiO (a=0.417nm) and Fe3O4 (a=0.840nm). Structurally, AFM and FEGSEM images reveal Fe3O4 thin films are grown continueously and very flat for both cases, apart from loose clusters due to nucleation effect. These clusters agglomerate and become bigger as higher current density applied to substrates, and EDS analysis indicateds stoichiometric composition of clusters is identical with Fe3O4 thin films. In order to form sandwich structure (FM/I/FM), here we propose Fe-based oxides, α-FeOOH (Néel Temperature= 400K) and α-Fe2O3, as insulating/spacer layers. Magnetic properties (TMR vs. H, Resisitivity vs. H and Ms vs. H) for these two magnetic tunnel junctions (MJT's) are examined.
9:00 PM - Q3.3
Thickness and Temperature Ddependent Mmagnetic Properties of Uultrathin Fe/Al Nanostructures.
Ranjeet Brajpuriya 1 , Shilpa Tripathi 1 , Anupam Sharma 1 , T. Shripathi 1 , S.M. Chaudhari 1
1 , UDCSR, Indore India
Show AbstractStudies of magnetic interactions between ferromagnetic films separated by non-magnetic metallic films have been a subject of extensive investigations from both theoretical and experimental point of view. These kinds of artificial structures are expected to provide understanding of surface magnetism and transport phenomena, such as interlayer magnetic-coupling, surface anisotropy, magneto-optical effect and giant magneto-resistance, etc. In this respect, recently Fe/Al multilayer systems have been studied extensively because of their attractive soft magnetic properties such as low coercivity, high remanence and low saturation field, shown to be a good candidate for the possible above applications. Therefore, in this paper, the magnetic properties of electron beam evaporated ultra thin Fe-Al multilayers are studied as a function of Fe layer thickness keeping Al layer thickness constant. To investigate layer structure and surface morphology Grazing incidence X-ray diffraction (GIXRD) and Atomic force microscopy (AFM) techniques have been employed. Structural measurements carried out on structures having lower Fe thickness (≤ 2 nm) show substantial intermixing between Fe and Al layers during deposition. The resulting structures in these cases show loss of periodicity and resemble mostly a single layer composite film consisting of Fe and Al clusters. The magnetic measurement carried out using Vibrating sample magnetometer (VSM) at 300K and100K indicates that all the multilayer samples exhibit soft magnetic properties, having in-plane easy direction of the magnetization. We have also found that coercivity increases at lower temperature for lower Fe thickness (≤ 2 nm) multilayer samples. The observed soft magnetic behaviour in these multilayer samples is explained in terms of a) weak magnetic interactions between crystal grains and their sizes b) morphological changes occurred due to Fe thickness variation in deposited bilayer films and c) formation of non- magnetic Fe-Al phase at the interfaces.
9:00 PM - Q3.4
Alloy Synthesis by Atomic Layer Laminations for Read Sensor Applications.
Chih-Ling Lee 1 , Adrian Devasahayam 2 , Ming Mao 1 , Chih-Ching Hu 2 , Vicent Ip 2 , Piero Sferlazzo 2
1 , Veeco Instruments Inc, Fremont, California, United States, 2 , Veeco Instruments Inc., Plainview, New York, United States
Show Abstract9:00 PM - Q3.5
How Does the Interface Structure Influence the Thickness Dependence of Tunneling Magnetoresistance?
Christian Heiliger 1 , Peter Zahn 1 , Bogdan Yavorsky 1 , Ingrid Mertig 1
1 Department of Physics, Martin-Luther-University Halle-Wittenberg, Halle Germany
Show AbstractNew experiments [1,2,3] based on epitaxially grown Fe/MgO/Fe samples obtained TMR ratios which exceed the predictions by Julliere's model [4]. In addition an oscillating behaviour of the tunneling magnetoresistance (TMR) depending on the barrier thickness was found [1]. The measured bias voltage characteristics show features which could be related to the electronic structure of the system. The aim of our work is to demonstrate the influence of different interface geometries on the thickness dependence of the TMR ratio. One of the considered interface structures was with a mixed Fe-oxide layer on both interfaces, experimentally proven at the bottom and the top Fe electrode [5,6]. A second configuration was an ideal one, where no intermixed layer occurs, which can be produced under oxygen deficiency [2]. To complete this investigation a junction with a mixed Fe-oxide layer and an ideal interface was considered.A screened Korringa-Kohn-Rostoker (KKR) method based on density functional theory was applied to calculate the electronic and magnetic structure of the different junctions self-consistently. The Landauer conductance of planar junctions was calculated using the Baranger-Stone scheme by means of Green's functions in the limit of coherent tunneling. Positive and negative TMR ratios are obtained as a function of interface structure, independent on barrier thickness.[1] S. Yuasa, T. Nagahama, A. Fukushima, Y. Suzuki, and K. Ando, Nature Materials 3, 868 (2004).[2] J. Faure-Vincent, C. Tiusan, E. Jouguelet, F. Canet, M. Sajieddine, C. Bellouard, E. Popova, M. Hehn, F. Montaigne, and A. Schuhl, Appl. Phys. Lett. 82, 4507 (2003).[3] S.S.P. Parkin, C. Kaiser, A. Panchula, P.M. Rice, B. Hughes, M. Samant, and S.-H. Yang, Nature Materials 3, 862 (2004).[4] M. Julliere, Phys. Lett. 54A, 225 (1975)[5] H. L. Meyerheim, R. Popescu, J. Kirschner, N. Jedrecy, M. Sauvage-Simkin, B. Heinrich, and R. Pinchaux, Phys. Rev. Lett. 87, 076102 (2001). [6] C. Tusche, H. L. Meyerheim, N. Jedrecy, G. Renaud, A. Ernst, J. Henk, P. Bruno, and J. Kirschner, Phys. Rev. Lett. 95, 176101 (2005)
9:00 PM - Q3.6
Epitaxial NiFe/GaAs via Electrochemistry.
Z.L. Bao 1 , A.A. Talin 2 , A. Arrott 1 , Karen Kavanagh 1
1 Physics, Simon Fraser University, Burnaby , British Columbia, Canada, 2 , Sandia National Laboratories, Livermore, California, United States
Show AbstractWe have recently shown that α-Fe films can be grown epitaxially by electrodeposition on (111), (110) as well as (001) GaAs substrates. In this work we report the epitaxial growth of single crystalline FexNi(1-x) films by electrodeposition on n-GaAs (001) oriented substrates from aqueous electrolytes. Iron, nickel, and ammonia sulphate (FeSO4, NiSO4 and (NH4)2SO4) electrolyte mixtures were used at room temperature, under galvanostatic conditions. The films nucleate as islands and relax the mismatch strain before coalescing into continuous layers. Pure Ni (f = 12 % when rotated about the normal by 45°) forms (011) oriented films that are also remarkable in their purity and narrow rocking curve widths. The film compositions, as determined by energy dispersive x-ray and Auger spectroscopies, are a function of the ratio of Fe++ to Ni++ in the electrolyte, consistent with previous literature reports on electrodeposited films. In the presence of FeSO4, Ni-rich FexNi(1-x), nucleates as face-centered cubic (fcc) (001) oriented films. The films remain single phase, fcc, up to an Fe composition of at least 17 at. %. With increasing additions of FeSO4 the films nucleate in the body-centered cubic (bcc) phase forming two-phase solid solutions. For Fe concentrations above 60 at. % the films are single phase, bcc Fe-rich FexNi(1-x) films, and (001) oriented. The resulting FexNi(1-x) /GaAs interfaces are atomically abrupt exhibit close to ideal diode behaviour (barrier height 0.82 eV, n = 1.10). The strain relaxed, electrodeposited Fe films are magnetic with 2-fold and 4-fold magnetic anisotropies aligned with in-plane <100> and <110> directions. Transmission electron microscopy, SQUID, and electrical investigations of the NiFe films will be reported. ZB and KK acknowledge funding support from the BC Advanced Systems Institute and NSERC Canada. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company for the United States Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000
9:00 PM - Q3.7
Annealing Induced Changes at Co/GaAs (001) Interface.
Anupam Sharma 1 , R. Brajpuriya 1 , Shilpa Tripathi 1 , T. Shripathi 1 , S.M. Chaudhari 1
1 , UGC-DAE Consortium FOr Scientific Reserach, Indore, M.P., India
Show AbstractIn recent years, the study of magnetic thin films and their interfaces with semiconductor surface has been received considerable attention due to their potential technological applications as non-volatile memories and in the newly developing field of spintronics. In this respect, Co/GaAs system has attracted much attention of the researchers due to its applications in spintronics, Giant Magneto Resistive (GMR) devices because of the high spin polarization of the carriers at the Fermi level. However, all these properties are very susceptible to the degradation with temperature, resulting from the significant intermixing between the substrate and the overlayer. Therefore, understanding the growth and microstructural evolution of Co on GaAs substrate and how the annealing affects the structure, magnetic and electronic properties, is of great technological interest. Present study is therefore, aimed to investigate the structural, magnetic, electronic and transport properties of ion beam sputter deposited Co (40 nm) thin film on GaAs substrate followed by annealing at different temperatures. The X-ray diffraction measurements show oriented growth of as-deposited Co film in hcp (002) direction. However, the sample annealed at higher temperatures shows formation of ternary Co2GaAs phase at the interface. The associated magnetization and resistivity measurements show decrement in magnetization and resistivity with increasing annealing temperature. Corresponding, XPS measurements also show modifications in core level as well as in valence band spectrum further supporting the formation of ternary Co2GaAs phase at the interface. The observed results are mainly attributed to the changes in electronic structure at the Co/GaAs interface.
9:00 PM - Q3.8
Characterization of a Continuous Flow RESS Apparatus for Growth of Magnetic Thin Films
Silvia De Dea 1 , Dominic Graziani 2 , David Miller 1 , Robert Continetti 2
1 Mechanical and Aerospace Engineering, University of California, San Diego, La Jolla, California, United States, 2 Chemistry and Biochemistry, University of California, San Diego, La Jolla, California, United States
Show AbstractA continuous flow Rapid Expansion of Supercritical Solution (RESS) apparatus has been developed to grow iron oxide thin films with particles in the range of 100nm to 1000nm. The magnetic thin film is produced by rapidly expanding a high pressure supercritical solution of ferric acetylacetonate (Fe(acac)3) and CO2 and directing the resulting supersonic jet onto both hot and cold silicon wafers. We have moved from a batch syringe pump type RESS source to a continuous source for better RESS/film control and better characterization of the impinging jet with a time-of-flight mass spectrometer (TOF). In the same RESS apparatus, the expansion can be made into ambient pressure conditions, passing through a shock wave, with controlled background composition, or into vacuum. While thermal decomposition of Fe(acac)3, with subsequent formation of α-Fe3O3, is expected to happen only at the hot surface, we observed a weak ferromagnetic phase in the cold sample which indicates an unexpected change in the magnetic properties of the original compound, likely occurring upon interaction with the solid surface. We present SQUID, Mossbauer, and SEM magnetic and structural data for these thin particle films and discuss the energetics of the decomposition of Fe(acac)3 on the hot and cold surfaces. We describe and characterize our continuous flow RESS apparatus, which allows us to control particle morphology and composition by adjusting process parameters such as pre-expansion temperature, pressure and composition of the initial solution, temperature of the substrate, nozzle to substrate distance, and expansion chamber pressure and composition. We also present some combined thermodynamic, kinetic, and fluid mechanic models to describe the properties of the RESS free jet expansion and of the solubility of Fe(acac)3 in the jet. We discuss the details of the interface of the RESS source to the TOF mass spectrometer, designed to directly probe the RESS expansion and to help elucidate the nature of the particles that strike the surface during film growth.
Symposium Organizers
Thomas Ambrose Seagate Research
William Bailey Columbia University
David Keavney Argonne National Laboratory
Y. Daniel Park Seoul National University
Q4: Switching of Devices and Heterostructures
Session Chairs
Wednesday AM, April 19, 2006
Room 3020 (Moscone West)
9:30 AM - **Q4.1
Switching Issues for High Density MRAM
Taewan Kim 1 , Keewon Kim 1 , Youngjin Cho 2 , Injun Hwang 2 , Jangeun Lee 4 , Wonchul Jeong 3
1 Nano Devices Lab., Samsung Advanced Institute of Technology, Suwon Korea (the Republic of), 2 Nano Fabrication Center, Samsung Advanced Institute of Technology, Suwon Korea (the Republic of), 4 Process Development Team, Semiconductor R&D Division, Suwon Korea (the Republic of), 3 Advanced Technology Development Team, Semiconductor R&D Division, Suwon Korea (the Republic of)
Show AbstractSuccessful demonstrations by the previous studies ensure that MRAM technology is a strong candidate of universal memory among the other new memory technologies from the viewpoints of power consumption, speed, scalability, retention, endurance, and density. However there are still some fundamental issues to be solved to realize density requirement that is attributted to small switching margin in the core array consisting of sub-micron or deep submicron magnetic tunnel junction (MTJ) cells[1]. In a MRAM array, the conventional writing operation uses a half selection scheme that induces a magnetic field simultaneously by two orthogonal line currents on a specific cell. However some cells selected by only one current line, Digit line (D/L) or Bit line (B/L), are partially or fully switched, which cells acts as a fail bit in the array. This writing scheme is directly related to the low writing current margin due to asteroid distribution.In this study, we consider the technological issues to improve the writing margin in submicron MRAM array, such as low Ms free layer, Synthetic anti-ferromagnetic (SAF) free layer and ultra-smooth roughness of bottom electrode. A new switching architecture without digit line current, which uses a local magnetic field generated directly by the current flowing at bottom electrode, is also proposed. 1. Taewan Kim, et al, J. of Magnetism. and Magnetic. Materials. 282, 232-236 (2004)
10:00 AM - **Q4.2
Giant Magnetoresistance in Multilayer Magnetic Rings
Caroline Ross 1 , F.J. Castano 1 , D. Morecroft 1 , W. Jung 1
1 Materials Science and Engineering, MIT, Cambridge, Massachusetts, United States
Show AbstractThin film magnetic multilayer rings exhibit a variety of interesting magnetic states and have applications in memory, logic or sensing devices. We have prepared circular and elliptical rings with diameters of 500 nm – 20 µm from from pseudo-spin-valve Co/Cu/NiFe films and spin-valve IrMn/Co/Cu/NiFe films. Each ring has 4 – 6 non-magnetic electrical contacts, and has been characterised by magnetoresistance measurements. Major loops show three well-defined resistance states corresponding to the NiFe and Co in parallel onion states (lowest resistance), the NiFe in an onion and the Co in a vortex state (intermediate resistance) and the NiFe and Co in antiparallel onion states (highest resistance). The range of stability of these configurations depends on the ring geometry and ellipticity and on the layer thicknesses. Minor loops, in which the NiFe is cycled without disturbing the Co, were also measured. The NiFe behavior differs depending on whether the Co is in a vortex state, in which case it weakly interacts with the NiFe, or whether the Co is in an onion state, in which case there are strong magnetostatic interactions between the layers. The details of reversal, for example the chirality of the vortex state in the Co, and the propagation of domains in the NiFe, can be deduced from these measurements. Moreover, measurements using more than one contact geometry enable the behavior of specific segments of the rings to be followed. This rich behavior makes the rings suitable for multiple-bit storage, and possibly for programmable logic devices.
10:30 AM - Q4.3
Control of Vortex Chirality in Magnetic Ring Elements*
Vitali Metlushko 1 , P. Vavassori 2 , M. Grimsditch 3 , U. Welp 3 , N. Zaluzec 3 , G. Crabtree 3 , J. Unguris 4 , B. Ilic 5 , A. Imre 6 , L. Ji 6 , W. Porod 6 , Xiaobin Zhu 7
1 , UIC, Chicago, Illinois, United States, 2 , University of Ferrara, Ferrara Italy, 3 , MSD ANL, Argonne, Illinois, United States, 4 , NIST, Gaithersburg, Maryland, United States, 5 , Cornel University, Ithaca, New York, United States, 6 , University of Notre Dame, Notre Dame, Indiana, United States, 7 , University of Alberta, Edmonton, Alberta, Canada
Show AbstractRecent studies on Co, Ni and permalloy rings have shown that a totally flux-closed magnetic vortex structure is stable at remanence. The two chiralities of the vortex, clock-wise and anti-clock-wise, have been proposed as the carriers for the stored information that could be read in a magneto-resistance-based device. To visualize the magnetization reversal process in individual rings we employed several different imaging techniques: magnetic force icroscopy (MFM), scanning Hall microscopy, magneto-optical (MO) imaging, Lorentz STEM (LSTEM) and scanning electron microscopy with polarization analysis (SEMPA). We found that MO, LSTEM and SEMPA allow a direct determination of magnetic vortex chiralities and that by controlling the shape of the nanoscale magnetic ring and the direction of applied magnetic field we can precisely tune the switching mechanism and reliably predict the chirality of the vortex states. The experimental results were compared with detailed micromagnetic simulations.*Supported by the US NSF under contract DMR-0210519. Work at ANL was supported by US Department of Energy, BES Materials Sciences under contract W-31-109-ENG-38.
10:45 AM - Q4.4
Domain Structures and Current-controlled Switching Characteristics of Micron Sized Permalloy Structures with Varying Aspect Ratios.
S. C. Seah 1 , Y. S. Soh 1 , V. Ng 1
1 Information Storage Materials Laboratory, Electrical and Computer Engineering Department, National University of Singapore, Singapore Singapore
Show AbstractIn this paper, we shall explore the different domain configurations observed during the switching process of micron sized permalloy structures with aspect ratios of 2, 3 and 4 as well as their current-controlled switching characteristics. The shape is a rectangle with a semicircle attached at each end. Permalloy 40nm thick round-ended rectangles of size 4μm x 1μm and 2μm x 1μm were fabricated in an array using electron beam lithography and deposited using thermal evaporation. The array was placed on a 50μm wide 200nm thick gold conductor fabricated using photolithography and thermal evaporation. Using magnetic force microscopy, we examined the remnant states of the 4x1um structures after attempting to switch the domain configuration of the structures with the magnetic field generated by passing a current through the conductors. The current value was varied to determine the switching current of the structures. Equivalent field strength was calculated using Finite Element Method Magnetics (FEMM) simulation of the 50um gold conductor. MFM images were used to observe the remnant state of saturation state as well as the remnant states of the intermediate fields of the switching process in both the easy and hard axis. M-H loops of the structures were obtained using the vibrating sample magnetometer. At the same time, using Object Oriented Micro-magnetic Framework (OOMMF), we simulated the M-H loops and the images of the domain configurations from -1T to 1T. We relaxed the various states observed at different fields to determine the switching field required. The MFM images as well as the M-H loops were compared with simulation results.The MFM images of the 4μm x 1μm structures showed a quasi-single domain configuration when saturated along the easy axis. Switching along the hard axis was unstable and the array showed different states even after attempted saturation at high current of 1A. From the simulations, the quasi-single domain was observed in the easy axis switching, confirming the MFM images. The 3-diamond state was dominant when saturated along the hard axis while 2-diamonds and multi-diamond states were also observed.The as-deposited images of 2μm x 1μm structures showed dominantly single-diamond domain configurations and some displaying chess-board configurations. OOMMF simulations showed that easy axis saturation resulted in 1-diamond structure when relaxed after applying a low field and chess-board structure after a high field application. Hard axis saturation resulted in single-vortex structure at low field, triangle structure at intermediate fields and 1-diamond structure at high fields. We will report on these sizes as well as the 3μm x 1μm structures to complete the characterization of different aspect ratios for this shape.
11:30 AM - **Q4.5
Fingerprinting Magnetization Reversal in Magnetic Nanostructures.
Kai Liu 1
1 Physics Department, University of California, Davis, California, United States
Show Abstract12:00 PM - Q4.6
Onset of Exchange Anisotropy in Tetragonal Distortedfcc-Cobalt(001)/γ-Manganese(001)Bilayers.
Harm Wieldraaijer 1 , Wim de Jonge 1 , Juergen Kohlhepp 1
1 Applied Physics, Eindhoven University of Technology, Eindhoven Netherlands
Show AbstractWe use well-prepared ultrathin fct-Co(001) films as templates for the stabilization of tetragonal distorted γ-Mn(001) films by Molecular Beam Epitaxy (MBE) methods. The Mn overlayers can be grown with a very high crystalline quality and extremely flat in a layer-by-layer mode up to a thickness of roughly 20-25 monolayers (ML). Above this thickness, the Mn films slightly roughen but still keep their face-centered-tetragonal (fct) structure, until above roughly 60 ML a transformation to the complex body-centered-cubic α-Mn bulk structure occurs.X-ray Photoelectron Spectroscopy (XPS), Auger Electron Spectroscopy (AES), as well as Auger/Photoelectron Diffraction (AED/XPD) measurements show that extensive interdiffusion and/or CoMn alloy formation at the Co/Mn interface can be excluded. With a combination of LEED, AED/XPD, and X-ray Diffraction (XRD) results, the tetragonal distortion of the Mn films is determined to c/a = 1.06. Recent ab-initio calculations predict an in-plane (001) antiferromagnetic (AFM) state for such a c-axis expanded γ-Mn phase.Indeed, room temperature (RT) antiferromagnetism of the fct-Mn is evidenced by the observation of a shifted magnetization loop and an enhanced coercive field for fct-Mn covered ferromagnetic (FM) Co(001) buffer layers, which is explained by a unidirectional exchange anisotropy induced by an exchange interaction at the FM-Co/AFM-Mn interface. Temperature dependent magnetization measurements for samples with >20ML Mn indicate blocking temperatures around 400 K and Néel temperatures well above 450 K.A closer MOKE investigation of the thickness dependence of the coercitivity (HC) and the shift field (HE) as a function of the Mn thickness using wedge shaped sample structures, shows two interesting features: (1) An enhanced HC is observed for Mn thicknesses as small as 1.3 ML (at 10 K) and 3 ML (RT), respectively, clearly evidencing the AFM state in ultrathin Mn films. (2) At 10 K a small negative HE is already observed at a thickness lower than 2 ML Mn, keeping its negative sign for thicker films, whereas at RT a small positive HE first sets in around 3-4 ML Mn, which is thereupon changing at 5-6 ML Mn into a negative HE. This change of sign in HE is also observed as a function of temperature for Mn-thicknesses between 2 and 5 ML. In the course of the presentation, this striking behaviour will be explained by the interplay of the growth mode of the Mn on the Co(001)-buffer layer and the thickness dependent magnetic properties of the ultrathin Mn overlayers.
12:15 PM - Q4.7
Planar Domain Walls in Exchange Biased Bilayers.
Elke Arenholz 1
1 Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California, United States
Show Abstract12:30 PM - Q4.8
Controlling Magnetism by Stacking Individual Atomic Monolayers of Magnetic- and Non-magnetic Materials.
Farid El Gabaly 1 , Silvia Gallego 2 , Christof Klein 4 , Carmen Munoz 2 , Laszlo Szunyogh 3 6 , Peter Weinberger 3 , Kevin McCarty 5 , Andreas Schmid 5 , Juan de la Figuera 1
1 Dpt. de Fisica de la Materia Condensada and Centro de Microanalisis de Materiales, Universidad Autonoma de Madrid, Madrid, Madrid, Spain, 2 , Instituto de Ciencia de Materiales de Madrid, Madrid, Madrid, Spain, 4 , Sandia National Laboratories, Livermore, California, United States, 3 Center for Computational Materials Sciencie, Technische Universitat Wien, Vienna Austria, 6 , Budapest University of Technology and Economics, Budapest Hungary, 5 , Lawrence Berkeley National Laboratory, Berkeley, California, United States
Show AbstractMagnetic properties of films are often a function of film thickness; for quite some time this has been a fruitful topic for research as well as for device applications. In this work we show just how rich these phenomena are, when measurements are made using microscopes that allow us to study essentially defect-free film regions in which thickness is perfectly homogeneous on the atomic scale coupled to fully relativistic ab-initio calculations.Spin-polarized low-energy electron microscopy (SPLEEM) simultaneously shows the detailed atomic-layer structure of ultrathin films during their deposition and provides with a detailed picture of magnetic properties. When we deposit films of Co onto Ru(0001) substrates in the thickness range of up to 3 atomic monolayers, SPLEEM reveals that the easy axis of magnetization switches twice in this range: both one-monolayer and three-monolayer thick regions are magnetized in a direction within the film plane, while two-monolayer thick Co/Ru(0001) regions are magnetized perpendicular to the film plane. By measuring the thickness-dependent relaxation of epitaxial strain in the Co layers and combining the experimental information with ab-initio computations of the magnetic anisotropy energy, we show how the unusual layer-by-layer double-spin-reorientation transition results can be understood in detail. Moreover, we find rather curious, additional possibilities to induce dramatic changes of the magnetism by adding atomic monolayers of Cu on top of the Co films. When we add just one single atomic Cu layer on top of in-plane magnetized Co/Ru(0001) films of three or four monolayer thickness, the magnetization axis switches to the direction perpendicular to the film plane. Adding just one additional Cu atomic layer flips the magnetization again to an in-plane configuration. Fully relativistic ab-initio calculations show that these spin reorientation transitions are the result of effects at the Cu/Co interface.
Q5: Novel Materials: High Frequency and Spin Injection
Session Chairs
Wednesday PM, April 19, 2006
Room 3020 (Moscone West)
2:30 PM - **Q5.1
Magnetostrictive Materials for High Frequency Applications.
R van Dover 1 , Noble Woo 2 , Jonathan Petrie 1
1 Materials Science and Engineering, Cornell University, Ithaca, New York, United States, 2 Chemistry and Chemical Biology, Cornell University, Ithaca, New York, United States
Show AbstractThe magnetostrictive properties of thin films at high frequencies (~ 100 MHz – 1 GHz) are of interest for a variety of applications, such as Magnetostrictively Transduced Surface Acoustic Wave Devices (MTSAWs) and strain-assisted switching in magnetic memory devices. We have studied MTSAW devices and demonstrated their feasibility, albeit with a high insertion loss due to incorporation of a magnetic layer with only a low magnetostriction. In order to improve performance we have employed various approaches to obtaining a larger magnetostrictive response. One approach involves engineering the Ferromagnetic Resonance (FMR) frequency of the magnetic material to coincide with an acoustic resonance, which leads to increased overall magnetoacoustic coupling at that resonance frequency. Experimental observation confirms the expected effect, although the response is complex. A second approach is to identify materials with a high response, dλ/dH, where λ(H) is the field-dependent magnetostriction. In order to evaluate the magnetostriction of a wide range of materials in an efficient manner, we prepare composition-spread films using a three-gun on-axis magnetron cosputtering system. Measurement of λ in uniform-composition thin films has typically been accomplished by measurement of (overall) substrate curvature as a function of field. We have developed a method to measure λ locally by depositing the composition-spread films on prefabricated 2D arrays (75 x 75) of small (typically 50 x 500 µm) cantilever beams prepared on a silicon substrate using MEMS techniques. A magnetic field is applied in two directions using orthogonal Helmholtz coils. The magnetostriction of the film results in differential strain of the thin film/cantilever system, and the resulting curvature is detected using an optical system and automated x-y scanning. The data are then analyzed to yield a quantitative measure of the quasistatic magnetostrictive response, λ(H), at each position. This “combinatorial” materials science study directly identifies promising thin film compositions, but the data must be interpreted with care, since incident-angle effects can lead to anomalous magnetic anisotropy in the films. In the past, the development of magnetostrictive materials has focused on development of bulk materials with high λ (thin film work has focused on finding materials with low λ) so this approach represents a novel strategy with unique potential.
3:00 PM - **Q5.2
Spin Waves and Dynamic Coupling in 2-D Magnetic Nanowire Arrays.
M. H. Kuok 1 , Z. K. Wang 1 , H. S. Lim 1 , S. C. Ng 1 , J. L. Goh 1 , H. L. Su 2 , S. L. Tang 2
1 Department of Physics, National University of Singapore, Singapore Singapore, 2 National Laboratory of Solid State Microstructure, Nanjing University, Nanjing, Jiangsu, China
Show Abstract3:30 PM - **Q5.3
Combinatorial Investigation of Magnetic Metallic Alloy Thin Films.
Ichiro Takeuchi 1 2
1 Materials Science and Engineering, University of Maryland, College Park, Maryland, United States, 2 Center for Superconductivity Research, University of Maryland, College Park, Maryland, United States
Show AbstractWe have developed a methodology for rapidly investigating large compositional phase space of magnetic metallic alloy thin films and heterostructures in order to optimize their properties for a variety of magnetic device applications. A number of deposition techniques have been implemented for synthesis of combinatorial thin film libraries of different designs. High vacuum three-gun magnetron co-sputtering systems are used to create composition spreads on 3-inch Si wafers, where natural compositional gradient across the wafers can cover large fractions of ternary phase diagrams. Wavelength dispersive spectroscopy is used to map the exact composition distribution of every wafer. A newly developed multi-gun combinatorial electron-beam deposition system has an automated 2-dimensional shutter system which allows fabrication of libraries with well-defined compositional variation across 1 cm x 1 cm chips. In order to quickly survey the composition-structure-magnetic property relationship across combinatorial samples and identify possible new compositions with enhanced physical properties, various rapid characterization techniques are employed together. Room-temperature scanning SQUID microscopy allows mapping of the magnetic field distribution. Obtained distribution is converted to quantitative remanent magnetization mapping using a numerical algorithm. Synchrotron x-ray magnetic circular dichroism and magneto-optical Kerr effect have been used to systematically measure hysteresis loops at different sites. Scanning x-ray microdiffraction is performed to map the structural phase distribution. I will discuss experiments where we used the co-sputtering technique to study ternary systems containing Heusler alloys and related compounds for screening of ferromagnetic shape memory alloys, magnetostrictive materials, and spintronic materials. Investigation of exchange coupling between hard and soft magnetic layers aimed at optimizing nanocomposite permanent magnets in bilayer libraries made by the electron-beam deposition system will also be discussed. This work is performed in collaboration with M. Wuttig, S. E. Lofland, F. C. Wellstood, L. Knauss, J. W. Freeland, M. Yu and J. P. Liu. This work is supported by ONR, NSF and DARPA.
4:30 PM - **Q5.4
Ferromagnetic Metal/Compound Semiconductor Heterostructures: Growth, Interfacial Reactions and Spin Transport.
C. Palmstrom 1 , C. Adelmann 1 , X. Lou 2 , X. Y. Dong 1 , B.D. Schultz 1 , J. L. Hilton 1 , J. Q. Xie 1 , J. Strand 2 , S. A. Crooker 3 , M. Furis 3 , D. L. Smith 4 , P. A. Crowell 2
1 Dept. of Chemical Eng. & Matl's Science, University of Minnesota, Minneapolis, Minnesota, United States, 2 School of Physics & Astronomy, University of Minnesota, Minneapolis, Minnesota, United States, 3 National High Magnetic Field Laboratory, Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 4 Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show AbstractControl of the interfacial properties of ferromagnetic metals and semiconductors is important for optimizing magnetic properties and spin dependent transport across these interfaces. Interfacial reaction studies have been used to determine metal-Ga-As thin film phase diagrams and for predicting the thermodynamic stability of ferromagnetic metallic compounds on GaAs. Molecular beam epitaxial (MBE) growth in combination with in-situ STM, XPS, Auger, RHEED and LEED and ex-situ XRD, RBS, TEM, magnetotransport and magnetic characterization have been used to develop ferromagnetic elemental and metallic compound/compound semiconductor tunneling contacts for spin injection. These have included epitaxial Fe, Fe3Ga, Co2MnGe, and δ-MnGa ferromagnetic contacts on Ga1-xAlxAs. The efficiency of the spin polarized current injected from the ferromagnetic contact was determined by measuring the electroluminescence polarization of the light emitted from Al1-xGaxAs light emitting diodes. Interfacial reactions and the semiconductor device band structure were found to have a dramatic influence on the measured spin injection. Lateral spin-transport devices with epitaxial Fe source and drain tunnel-barrier contacts were fabricated. Kerr microscopy was used to image the electrical spin injection and accumulation in the n-doped GaAs channel. The emission of spins from the source was observed, and a region of spin accumulation was imaged near the ferromagnetic drain contact. Both injected and accumulated spins have the same orientation (antiparallel to the contact magnetization), indicating that electron spins are polarized by reflection from the ferromagnetic drain contact. The electrical conductance could be modulated by controlling the spin orientation of optically injected electrons flowing through the drain contact.
5:00 PM - **Q5.5
Current-induced Macrospin vs. Spin-wave Excitations in Spin Valves.
Arne Brataas 1 , Yaroslav Tserkovnyak 2 , Gerrit Bauer 3
1 Department of Physics, Norwegian University of Science and Technology, Trondheim Norway, 2 Physics Department , Harvard University, Cambridge, Massachusetts, United States, 3 Kavli Institute of NanoScience, Delft University of Technology, Delft Netherlands
Show AbstractWe will discuss current-induced magnetic excitations in spin valves. We have found that the spin transfer torque exerted on the magnetization by transverse spin currents as well as the Gilbert damping constant depend strongly on the wave length of the excitation (spin wave). The onset of macrospin (zero wavelength) vs. finite wavelength spin-wave instabilities depends on the device parameters and the current direction, in agreement with recent experimental findings.
5:30 PM - Q5.6
Properties of Epitaxial Films of the Heusler Alloy Co{2}Cr{1-x}Fe{x}Al
Rajesh Kelekar 1 , Bruce Clemens 1
1 , Stanford University, Stanford, California, United States
Show AbstractWe have grown epitaxial thin films of the Heusler alloy Co{2}Cr{1-x}Fe{x}Al. Compounds with small x -- the level of Fe doping -- have been predicted to be half-metallic, where the conductions electrons at low temperature all have the same spin. In addition, bulk polycrystalline compacts of Co{2}Cr{.6}Fe{.4}Al displayed negative magnetoresistances of -30% at room temperature in magnetic fields of less than 1000 Oe. The properties of the single crystalline thin films we have grown for the first time show a sharp dependence on the level of Fe doping. Films with low Fe doping show large semiconductor-like resistivities, large reductions in the magnetic moment as compared to theory, and large values of the anomalous Hall resistivity. X-ray magnetic circular dichroism of these thin films shows that the magnetic moment on the Cr atom is virtually extinguished. Anomalous x-ray diffraction indicates that the films across the whole range of compositions contain a significant amount of selective transition metal disorder, where the transition metal atoms are disordered on the lattice sites while the Al atom is positioned on a single lattice site. These features indicate that Cr is much more sensitive to the local environment than is Fe. We have incorporated these Heusler alloys into epitaxial superlattices with Cr and spin valve-type trilayers. Though superlattices do not show giant magnetoresistances or any evidence of antiferromagnetic coupling, current-in-plane spin valves show large giant magnetoresistances of up to 7% at room temperature, the largest for any Heusler alloy or proposed half-metal.
5:45 PM - Q5.7
CrSb/GaSb Heterojunction - a Possible Robust Spin Current Injector.
Yuan Ping Feng 1 , Rongqin Wu 1 , Guowen Peng 1 , Lei Liu 1
1 Physics, National University of Singapore, Singapore Singapore
Show Abstract
Symposium Organizers
Thomas Ambrose Seagate Research
William Bailey Columbia University
David Keavney Argonne National Laboratory
Y. Daniel Park Seoul National University
Q6: Magnetic Nanoparticles and Nanowires
Session Chairs
Thursday AM, April 20, 2006
Room 3020 (Moscone West)
9:30 AM - Q6.1
Submicron Spherical Hollow Magnets: Synthesis, Dispersibility, Coercivity and Glassiness
Kunio Awaga 1 , Yasuharu Kozuka 1 , Mototaka Ohnishi 1 , Hirofumi Yoshikawa 1 , Shunji Bandow 2 , Sumio Iijima 2 , Motoyasu Kobayashi 3 , Atsushi Takahara 3
1 , Nagoya University, Nagoya Japan, 2 , Meijo University, Nagoya Japan, 3 , Kyushu University, Fukuoka Japan
Show AbstractThe preparation of sub-micron magnetic hollow spheres is a promising approach towards developing novel magnetic materials. Their comparatively large size means that the critical temperatures of magnetic ordering or magnetization blocking would be high enough for practical application. In addition, the spherical hollow structure is advantageous for various surface modifications, which would enable further applications. Further, the magnetic domains that exist on the hollow sphere are of great interest to fundamental science; various domain patterns are energetically allowed in this highly-symmetric shape, in contrast to a needle-shape magnet, which always exhibits magnetic polarization parallel to the needle axis. The existence of these energetically-degenerated domain structures should lead to materials that change their magnetic property under certain conditions. In the present work, we prepared spherical hollow magnets of ccp- and hcp-Co, Co3O4, Fe, Fe3O4, α-Fe2O3, etc. with a diameter of ca. 500 nm and a shell thickness of ca. 50 nm, using polystyrene-bead templates. These particles were characterized by SEM, TEM, ED, XRD, etc. The magnetic measurements on the Fe3O4 particles revealed a temperature-dependent coercive field that showed a significant decrease with an increase in temperature in the wide range 2-300 K. By a surface modification with hydrophilic organic polymers, the Fe3O4 spheres obtained good dispersibility in water. The separate syntheses of hollow spheres of ccp- and hcp-Co were successful under nearly identical calcination conditions, but from different precursors.
9:45 AM - Q6.2
Low-temperature Synthesis, Assembly, and Properties, of Monodisperse FePt-silica core-shell Nanomagnets of Tunable Size, Composition and Thermal Stability.
Qingyu Yan 1 , Arup Purkayastha 1 , Taegyun Kim 1 , Roland Kroger 2 , Arijit Bose 3 , Theodorian Theodorian Borca-Tasciuc 4 , Ganapathiraman Ramanath 1
1 Matierials Science, Rensselaer Polytechnic Institute, Troy, New York, United States, 2 Institut fur Festkorperphysik, Universitat Bremen, Otto-Hahn-Allee , Bremen, Germany, 3 Chemical Engineering Department, University of Rhode Island, Kingston, Rhode Island, United States, 4 Mechanical Engineering Department, Rensselaer Polytechnic Institute, Troy, New York, United States
Show Abstract10:00 AM - Q6.3
L10 Order In FePt Thin Films And X-Ray Rapid Thermal Annealing (XRTA).
Rosa Alejandra Lukaszew 1 , Jonathan Skuza 1 , Cesar Clavero 2 , Alfonso Cebollada 2 , Eric Dufresne 3
1 Physics and Astronomy, University of Toledo, Toledo, Ohio, United States, 2 , IMM, Marid Spain, 3 MHATT, APS, ANL, Argonne, Illinois, United States
Show AbstractHighly ordered L10 FePt thin films and nano-structures are important for magneto-recording applications because this ordered phase exhibits very large magnetic anisotropy.1 One possibility to achieve high degree of L10 order in epitaxial but chemically disordered films is to perform annealing treatments.2 One interesting variation of such treatments is rapid thermal annealing (RTA). RTA is widely used for electronic materials processing, from the activation of dopants to the healing of lattice defects caused by ion implantation.Here we describe an innovative application of x-ray undulator radiation to simultaneously perform RTA and to probe structural changes that occur during annealing. This is made possible by the high power-density of undulator beams (~100 W/mm2), combined with their excellent properties as a probe of crystal structure (especially collimation and high-brightness). XRTA is similar to laser annealing, but there is a unique advantage in that the x-ray energy can be tuned to enhance the coupling into the absorption edge of a particular species, thereby permitting selective annealing of buried layers and nanostructures. In our studies at the MHATT-CAT beam line we have used XRTA to enhance the degree of L10 chemical order in epitaxial (001) FePt thin films. Using a (111) Silicon crystal in the diffractometer analyzer arm, we were able to monitor the x-ray diffraction pattern in real time as the samples were heated by the undulator x-ray beam. The undulator beam (1st harmonic ~10 keV) heated a 1 mm2 spot in the sample to ~1000 C in a few seconds. We observed the enhancement of the fcc-fct transition with 30 msec temporal resolution in Bragg geometry. The results demonstrate that undulator radiation offers a unique possibilities for materials processing, particularly the ability to use the same beam for heating and as a structural probe. This work was partially supported by NSF-DMR (Grant #0355171), the American Chemical Society (PRF grant # 41319-AC) and the Research Corporation Cottrell Scholar award.1 A. Cebollada, R. F. C. Farrow, and M. F. Toney, Magnetic Nanostructures, edited by H. S. Nalwa (American Scientific Publishers, 2002), pp. 93 – 118.2 Xiao-Hong Xu, Hai-Shun Wu, Xiao-Li Li, Fang Wang and Jing-Fang Duan, Physica B 348, 436 (2004).
10:15 AM - Q6.4
Reconstruction of the Magnetization Distribution of Small CoPt Patterns via Magneto-optic Imaging and Magnetic Force Microscopy.
Christian Jooss 1 , Sebastian Dreyer 1 , Sven Schnittger 1 , Jonas Norpoth 1 , Sibylle Sievers 2 , Martin Albrecht 2 , Uwe Siegner 2
1 Institute of Materials physics, University of Goettingen, Goettingen Germany, 2 , Physikalisch-Technische Bundesanstalt, Braunschweig Germany
Show Abstract10:30 AM - Q6.5
c-Axis Oriented, Isolated L10-FePt Nanoparticle Monolayers on TiN Underlayers with Controlled Grain Sizes.
Suguru Noda 1 , Yoshiko Tsuji 1
1 Department of Chemical System Engineering, The University of Tokyo, Tokyo Japan
Show Abstract10:45 AM - Q6.6
Indirect Exchange Interaction in Granular Permalloy Films.
Anatoli Pogorily 2 1 , Anatoli Kravets 2 , Yurii Dzedzerja 2 , Olena Shypil 1 , Chester Alexander 1
2 Physics of Thin Films, Institute of Magnetism, Kyiv, 03142 Ukraine, 1 MINT Center, University of Alabama, Tuscaloosa, Alabama, United States
Show Abstract11:30 AM - Q6.7
Ferromagnetism from a Co-porphyrin Supramolecular Assembly.
Anthony Caruso 1 , Doug Schulz 1 , Pam Jeppson 1 , Laura Jarabek 1 , Douglas Chrisey 1
1 , North Dakota State University, Fargo, North Dakota, United States
Show AbstractFerromagnetism from an air-stable supramolecular assembly of a cobalt porphyrin-based system has been observed above room temperature with saturation not observed up to 7 Tesla. Low temperature (5K) measurements indicate an extremely strong exchange mechanism from the ligand fields and this observation will be a focal point of this talk. We hypothesize that the organic assembly provides a scaffolding to hold the transition metal centers in place for direct exchange. In addition, two rather strange responses are observed for this system: (1) a metamagnetic transition occurs at 130K; and, (2) classical exchange bias is observed and is enhanced for field cooled samples. Thin films and chemical variations will also be discussed in the context of advantages for future spintronic devices that will use organic materials.
11:45 AM - Q6.8
Fabrication of Ordered Mesoporous Silica with Encapsulated Iron Oxide Particles via Doped Block Copolymer Templates in Supercritical Carbon Dioxide
David Hess 1 , James Watkins 1 , Rajesh Naik 2 , Melanie Tomczak 2
1 chemical engineering, University of Massachusetts-Amherst, Amherst, Massachusetts, United States, 2 Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright Patterson Air Force Base, Ohio, United States
Show AbstractEncapsulation of ferritin within mesoporous silica provides a potential route to devices that rely on isolated anti-ferromagnetic metal oxide clusters. Ferritin is a ubiquitous iron storage protein that is made up of a self-assembling protein shell 12 nm in diameter and with an 8 nm cavity that contains an iron oxide particle. Recently ferritin has been used to form ordered arrays of iron nanoparticles. Upon dipping a substrate into a solution, ferritin aligns in a hexagonal type monolayer arrangement. Upon heating, the shell is removed and the spherical iron oxide nanodot remains. Recently, our group has reported a new approach for fabricating mesoporous silicates involving the selective condensation of a silica precursor in the hydrophilic domain of a highly ordered amphiphilic block copolymer using supercritical carbon dioxide as a delivery medium. In this approach template organization silica network formation occur in discreet steps. Consequently the template can easily be doped with active components in specific sub-domains prior to silica infusion and removal of the template by calcination. In this work, a poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer was doped with horse spleen ferritin and a low concentration of a p-toluenesulfonic acid catalyst. The template was then spin-casted onto a silicon wafer. Upon drying the block copolymer undergoes microphase segregation and both the acid catalyst and ferritin partition into the hydrophilic domain. The doped template was then exposed to a solution of tetraethyl orthosilicate (TEOS) in supercritical carbon dioxide at 60 oC and 125 bar. TEOS selectively condensed in the PEO domain to yield a nanocomposite material, which upon calcination yielded an iron oxide-doped, well-ordered, mesoporous silica film. TEM and XRD analysis indicated that the structure of the mesoporous silica matrix was dependent upon the concentration of ferritin dopant. XRD revealed the presence of crystalline iron oxide within the mesoporous support. Samples were also characterized using a semiconducting quantum interference device (SQUID) and were found to possess interesting magnetic properties.
12:00 PM - Q6.9
A Novel Polymer Mediated Approach for the Synthesis of Magnetic Nanoparticles.
Suresh Valiyaveettil 2 3 , Swaminathan Sindhu 3 , Subbiah Jagadesan 2
2 Deaprtment of Chemistry, National University of Singapore, Singapore Singapore, 3 NUSNNI, National University of Singapore, Singapore Singapore
Show Abstract12:15 PM - Q6.10
Development of Block Co-Polymers as Self-Assembling Templates for Magnetic Media and Spin-Valves
Martin Bakker 1 2 , Vishal Warke 1 2 , David Nikles 1 2 , Jimmy Mays 3 , Phillip Britt 3
1 Center for Materials for Information Technology, The University of Alabama, Tuscaloosa, Alabama, United States, 2 Department of Chemistry, The University of Alabama, Tuscaloosa, Alabama, United States, 3 Center for Nanophase Materials Science, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractPoly(styrene)-Poly(methylmethacrylate) block co-polymers (PS-b-PMMA) of appropriate block length and PS to PMMA ratio self-assemble into a 2-D hexagonal phase in which the PS majority phase is continuous and surrounds cylinders of the minority, PMMA phase. By UV irradiation and washing with acetic acid it is possible to remove the minority phase to leave empty channels. It is also possible to rearrange the PMMA phase with acetic acid to leave somewhat smaller pores. For most substrates the interactions between the polymer and the substrate surface are such that one block is preferentially adsorbed to the substrate resulting in alignment of the PMMA domains parallel to the substrate surface. It is possible to orient the polymer perpendicular to the surface by first adding a thin film of a random PS-PMMA co-polymer before applying the PS-b-PMMA block co-polymer. However thin films of the random PS-PMMA do not give good surface coatings, and thicker films are generally too thick for the pores in the PS-b-PMMA block co-polymer to be propagated to the substrate surface. For a few substrates, thin PS-b-PMMA films naturally adopt a perpendicular orientation after annealing, washing with acetic acid produces arrays of pores of diameter as small as 3 nm. For a number of other substrates the interaction between the polymer blocks and the surface is such that upon annealing the polymer rearranges to form micron sized domains which are not polymer coated, surrounded a areas which have a thicker polymer coating. We have observed this behavior with both carbon coated substrates and with ITO glass substrates. In both cases the areas of polymer are perpendicularly oriented, and upon washing with acetic acid give rise to pores that extend completely through the polymer film. In some cases films on ITO glass are continuous even after annealing. After washing with acetic acid it was possible to electrodeposit nickel into the pores to give nickel nano-pillars of 18 nm diameter.
12:30 PM - Q6.11
Room Temperature Ferromagnetism in Ge-based Nanowires.
Olga Kazakova 1 , Jaideep Kulkarni 2 , Justin Holmes 2 , Sergej Demokritov 3
1 , NPL, Teddington, Middlesex, United Kingdom, 2 , UCC, Cork Ireland, 3 , WWU, Munster Germany
Show AbstractSpintronics requires fabrication of ferromagnetic nanostructures that can transport spin-polarized carriers at room temperature, be electrically tunable and easily compatible with existing silicon manufacturing technologies. The most direct method to induce spin-polarized electrons into a semiconductor is by introducing transition metal dopants, producing a dilute magnetic semiconductor (DMS) [1, 2]. Extensive research has been carried out in order to create DMS materials with well-established room temperature ferromagnetism. Despite several encouraging results, DMS materials are still struggling to reliably achieve the desired high Curie temperature.Here we report the first time observation of room temperature ferromagnetism in group-IV DMS materials in the form of 1D structures. We investigate magnetic properties of Ge nanowires (NWs) having a smallest diameter of 35 nm and length of 60 μm, synthesised within the pores of anodised aluminium oxide (AAO) templates [3] and doped with various transition metals (Mn, Co or Cr). Structural analysis of the NWs showed the existence of a highly crystalline germanium host lattice containing discrete dopant atoms. The dependences of the magnetic characteristics of the NWs on their diameter, transition metal concentration and preparation details were investigated by means of SQUID magnetometry. Of the all studied magnetic impurities, Mn-doped NWs are of particular interest, as these structures display well-pronounced ferromagnetic properties at room temperature [4], whereas a typical transition temperature for GeMn thin films is only about 110 K [5]. Ferromagnetic ordering reaches a maximum at intermediate Mn concentrations followed by a decay in the magnetic properties at x = 5 %. The effect of interface related anisotropy in NWs of different diameters was also studied. By eliminating the surface strain we were able to improve ferromagnetic performance of NWs at room temperature. We also show that room temperature ferromagnetism was preserved after post-annealing, meaning GeMn NWs are compatible with present CMOS technology. The magnetic properties of the NWs can be understood by considering the influence of co-dopant non-magnetic impurities, i.e. oxygen and carbon, as well as confinement effect at the interfaces.The observation of ferromagnetic properties in Mn-doped Ge NWs at 300 K as well as compatibility of germanium and silicon allow straightforward integration of Ge NWs into mainstream electronics and open the way for room-temperature spintronic devices. 1. A. H. Macdonald, et al., Nature Materials 4, 195 (2005). 2. S. J. Pearton, et al., Physica B 340-342, 39 (2003). 3. J. S. Kulkarni, et al., Chem. Mat. J. Mater. Chem. 17, 3615 (2005). 4. O. Kazakova, et al., Phys. Rev. B. 72, 0944415 (2005). 5. Y. D. Park, et al., Science 295, 651 (2002).
12:45 PM - Q6.12
Diluted Magnetic Semiconductor Zn(Mn)O Nanowire Array From A Self-Formed ZnO Substrate.
Jingjing Liu 1 , Minhui Yu 1 , Weilie Zhou 1
1 AMRI/Chemistry, University of New Orleans, New Orleans, Louisiana, United States
Show AbstractWell-aligned diluted magnetic semiconductor (DMS) Zn1-xMnxO nanowires have been successfully fabricated from a self-formed ZnO substrate using chemical vapor deposition (CVD) method. The as-synthesized Mn-doped ZnO nanowires were characterized by field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). The nanowires are well-aligned and are perpendicularly-grown along the c-aixs having single crystalline structure. Electron energy X-ray dispersive (EDS) analysis, X-ray diffraction (XRD) spectrometry and TEM analysis clearly showed that Mn was successfully doped in the ZnO nanowires and substrate. Ferromagnetic ordering of the as-synthesized Zn1-xMnxO nanowire arrays was observed at 5 K with Curie temperature of 44 K and room temperature by superconducting quantum interference device (SQUID) measurement due to different synthesis condition. The DMS nanowire arrays with room temperature ferromagnetic ordering have strong applications in spintronic nanodevices.
Q7: Dilute Magnetic Semiconductores, Oxide Heterostructures, and Multiferroics
Session Chairs
Thursday PM, April 20, 2006
Room 3020 (Moscone West)
2:30 PM - Q7.1
Exchange Biasing with Magnetoelectric YMnO3 Epitaxial Films
Xavier Marti 1 , Florencio Sanchez 1 , Nico Dix 1 , David Hrabovsky 1 , Vassil Skumryev 2 , Maria-Victoria Garcia-Cuenca 3 , Cesar Ferrater 3 , Manuel Varela 3 , Ulrike Luders 4 , Jean Francois Bobo 4 , Josep Fontcuberta 1
1 , ICMAB-CSIC, Bellaterra Spain, 2 , Institut Catala de Recerca i Estudis Avancats, Barcelona Spain, 3 Fisica Aplicada i Optica, Universitat de Barcelona, Barcelona Spain, 4 , Onera, Toulouse France
Show AbstractBiferroic materials are currently receiving much attention as they show the existence of ferroelectricity and magnetic order. Among oxides, the intriguing coexistence of ferroelectricity and antiferromagnetism in hexagonal YMnO3 has been much discussed. However, recently research has been also driven on the orthorhombic phase of this oxide (o-YMnO3) as it displays substantial changes on the dielectric constant close to the antiferromagnetic order temperature which make it very appealing for a new generation of magnetoelectric devices. Unfortunately, in bulk form o-YMnO3 can only be prepared under very high pressure thus facing severe problems of integration.Here, we report on growth of YMnO3 films. We will show first that by appropriate selection of substrates, the o-YMnO3 epitaxial films can be stabilized. Moreover, we will show that by appropriate choice of the substrate orientation, the texture of the YMnO3 can be tuned. Magnetic characterization of these films reveals the presence of a magnetic transition at about 40K from a paramagnetic state to a canted antiferromagnet. Aiming to integrate the magnetoelectric YMnO3 with ferromagnetic oxides in a new generation of magnetic devices, we have successfully grown epitaxial o-YMnO3 films on the SrTiO3 substrates buffered with either Pt or ferromagnetic SrRuO3 epitaxial film or capped with Permaloy. When measuring YMnO3/SrRuO3 bilayers a well defined exchange bias field appears. The exchange bias field decreases with temperature and vanishes at around 40 K, the Néel temperature of the o-YMnO3; the magnetoelectric character of YMnO3 offer the possibility of controlling the exchange bias and thus magnetic switching, by an electric field.
2:45 PM - Q7.2
Magneto-Electric Coupling in Multiferroic Oxide Films by Photoemission Electron Microscopy
Tong Zhao 1 , A. Scholl 2 , F. Zavaliche 1 , H. Zheng 1 , M. Barry 1 , M. Cruz 1 , Q. Zhan 1 , Y. H. Chu 1 , A. Doran 2 , L. Martin 1 , R. Ramesh 1
1 Department of Physics and Department of Materials Science and Engineering, University of California, Berkeley, California, United States, 2 Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California, United States
Show AbstractMultiferroic materials, showing simultaneous (anti-)ferromagnetic and ferroelectric ordering, have attracted much attention. The coupling between these two ordering parameters has great potential for novel electronic device applications. On the other hand, the new physics behind the materials and phenomena is interesting itself from the fundamental science point of view. To probe the magneto-electric coupling, therefore, is with great interests. In this work, a unique technique, namely Photoemission Electron Microscopy (PEEM) at the Advanced Light Source of the Lawrence Berkeley National Laboratory was utilized to probe the magneto-electric coupling in a multiferroic nanostructure comprised of ferrimagnetic CoFe2O4 pillars embedded in ferroelectric BiFeO3 matrix which was prepared by pulsed laser deposition through a self-assembly process. Piezoelectric force microscopy (PFM) was used to study the ferroelectric property, and to switch the ferroelectric polarization in the BiFeO3 matrix of the nanostructure by applying an electrical voltage through the PFM tip. The magnetic structure of the CoFe2O4 pillars in the nanostructure was measured by PEEM with a circularly-polarized X-Ray. The nanostructure was magnetized in a magnetic field to align the magnetization of the CoFe2O4 pillars. Then an electrical voltage was applied to a selected area to switch the ferroelectric polarization of the BiFeO3 matrix. A clear magnetic circular dirchroism was observed at both Co and Fe L edges of the CoFe2O4 pillars between the poled and unpoled areas. This is a strong evidence that the magnetization of the CoFe2O4 pillars were switched by switching the ferroelectric polarization of the BiFeO3 matrix. The magneto-electric coupling as a function of electric voltage and CoFe2O4 pillar size was studied. X-Ray absorption spectrum was also measured to study the cation site occupancy and valence state in the CoFe2O4 spinel structure. Linearly polarized X-Ray was used to probe the antiferromagnetic-ferroelectric coupling in a BiFeO3 thin film. A clear linear dichroism and a similar domain structure to the PFM measurement were observed before and after electrical poling on the BiFeO3 film, implying an antiferromagnetic-ferroelectric coupling in this material.This work is supported by an ONR-MURI and a LBL-LDRD.
3:00 PM - Q7.3
Multiferroic Characteristics of Highly Oriented ferrite-ferroelectric Multilayered and Composite Films Deposited by Laser Ablation.
Sarath Witanachchi 1 , H Nagaraja 1 , R Heindl 1 , H Srikanth 1 , Pritish Mukherjee 1
1 Department of Physics, University of South Florida, Tampa, Florida, United States
Show AbstractCoupling of ferroelectricity and ferromagnetism in a single system presents interesting applications in spintronics and sensors. Ferrimagnetic BaFe12O19 (BaM) and ferroelectric Ba0.5Sr0.5TiO3 (BSTO) are reasonably compatible materials to fabricate multilayered structures to investigate such coupling. In this study we have used a dual-laser ablation process to deposit single and multilayered films on various substrates that included A-plane and C-plane sapphire, 96% polycrystalline Al2O3 and MgO. Single and multilayer films deposited under optimum growth temperature and oxygen pressure showed high degree of orientation without any post deposition treatment. Low-angle x-ray diffraction and rocking curve measurements confirmed the epitaxial-like growth on sapphire substrates. While BST films were observed to grow epitaxially on MgO, films deposited from an ablation target that contained 50-50 mixture of BaM and BSTO showed the formation of separate phases with random orientation. However, when the composite film was deposited on MgO with a thin BSTO layer, the BaM and BSTO grains in the film became highly oriented. A versatile microwave probe station has been used to investigate the high-frequency characteristics of the films with applied parallel and perpendicular magnetic as well as electric fields. Magnetic hysteresis studies of multi layered structures at various field orientations showed large coercivities. Correlation between the microstructure of the films and the ferromagnetic and ferroelectric properties will be presented.
3:15 PM - Q7.4
Self-Assembled BiFeO3-CoFe2O4 Thin Film Nanostructure Growth Evolution
S. Y. Young 1 , L. Salamanca-Riba 1 , H. Zheng 2
1 Materials Science and Engineering Department, University of Maryland, College Park, College Park, Maryland, United States, 2 Materials Science and Engineering Department, University of California Berkeley, Berkeley, California, United States
Show AbstractWe report on the mechanism of the self-assembly of BiFeO3-CoFe2O4 (BFO-CFO) ferromagnetic thin film nanostructures observed by high-resolution transmission electron microscopy, and suggesting how the growth evolution of the CFO columnar structure forms. The BFO-CFO thin films were deposited on single crystal SrTiO3 (001) substrates using pulsed laser deposition at a substrate temperature and deposition rate of 700 degree C and ~5nm/min, respectively. In the early stages of growth CFO domains form with pyramidal-like shape covered by BFO. After approximately 5 mins of continuous deposition, the nanocomposite rearranges and diffusion takes place to form a self-assembly of CFO columns that extend to the surface of the film. These columns have a preferred, more stable faceted shape and are surrounded by a BFO matrix. There also exist a thin layer of a few atomic layers of BFO at the interface between the CFO columnar structure and the substrate. This layer helps relax the misfit strain between them. Magnetic properties of the nanocomposites samples will also be presented.
3:30 PM - Q7.5
Oxygen Thermodynamics and Rapid Growth of High Quality Epitaxial BiFeO3 Thin Films.
Meicheng Li 1 2 , Ahmed Kursumovic 1 , Xiaoding Qi 1 , Mark Blamire 1 , Judith MacManus-Driscoll 1
1 Dept. of Materials Science, University of Cambridge, Cambridge United Kingdom, 2 Department of Materials Physics and Chemistry, Harbin Institute of Technology, Harbin China
Show Abstract3:45 PM - Q7.6
Temperature and Bias Dependence of Epitaxial Magnetic Tunnel Junctions with Paramagnetic Barriers
Brittany Nelson-Cheeseman 1 , Lisa Alldredge 2 1 , Rajesh Chopdekar 2 1 , Yuri Suzuki 1
1 Materials Science and Engineering, University of California - Berkeley, Berkeley, California, United States, 2 Applied Physics, Cornell University, Ithaca, New York, United States
Show AbstractEpitaxial oxide magnetic tunnel junctions composed of La0.7Sr0.3MnO3 (LSMO) and Fe3O4 electrodes with paramagnetic NiMn2O4 barriers were fabricated in order to study how magnetic moments in the barrier layer affect the junction magnetoresistance (JMR). It has been shown that, due to an isostructural interface with Fe3O4, spinel barrier layers give rise to significant enhancement of the JMR in Fe3O4 based junctions by decreasing the spin scattering at the barrier-electrode interface. Previously, JMRs of up to -25% at 60K have been shown in junctions with paramagnetic CoCr2O4 barriers. In this study, we have synthesized epitaxial NiMn2O4 thin films whose TC of 55K is suppressed from the bulk value of 100K. Transitions in magnetization hysteresis loops coincide well with sharp and large transitions in magnetoresistance. Furthermore, the antiparallel LSMO-Fe3O4 magnetization configuration is the low resistance state, which confirms that Fe3O4 is a negatively spin-polarized material. We observed JMR up to -30% at 35K in LSMO / NiMn2O4 / Fe3O4 junctions. A peak in the JMR is observed as a function of temperature; this peak is attributed to the onset of the Verwey transition in the Fe3O4 electrode. Three different types of bias dependences of the JMR manifest themselves as a function of decreasing temperature, and occur with varying degrees in different junctions. In the high temperature regime, the JMR decreases as a function of increasing magnitude of the bias with slight asymmetry due to the difference in barrier heights at the two electrode-barrier interfaces.At intermediate temperature, a zero-bias anomaly appears with a maximum JMR at +/- 50-100mV. As the temperature is lowered, the zero-bias anomaly disappears and JMR again decreases monotonically as a function of increasing magnitude of the bias with little asymmetry. These different bias dependences can be attributed to the competition between direct tunneling and inelastic hopping transport mechanisms as well as the opening of a gap in the density of states in the Fe3O4 as the temperature is lowered below the Verwey transition.
4:30 PM - Q7.7
Design and Synthesis of a Novel Multilayer System for Low-field Giant Magnetoresistive Application
Ashutosh Tiwari 1
1 Materials Science & Engineering, University of Utah, Salt Lake City, Utah, United States
Show AbstractDuring the last few decades, extensive research has undergone to discover efficient materials and devices, which can exhibit high magnetoresistance. A tremendous boost to these efforts occurred by the discovery of colossal magnetoresistance in doped perovskite manganates La1-xAxMnO3 (A: Sr, Ba, Ca). These oxides undergo a Metal-insulator transition at TM-I closely followed by a Ferromagnetic-paramagnetic transition at Tc (TM-I~Tc) and exhibit giant magnetoresistance at the same temperature. However in order to realize giant magnetoresistance in these materials, very high magnetic field is required. High magnetic field requirement poses a serious problem in making any practical use of manganates in magnetic devices and sensors, which need to be operated at low fields. In this talk we will report the growth of a new class of superlattice structure, consisting of alternate layers of La0.7Sr0.3MnO3 (LSMO) and ZnO, which exhibits giant magnetoresistance at low fields. Giant magnetoresistance of >250% has been observed in these structures on the application of just ~400 Gauss of magnetic field over the broad temperature range 15-200 K with a maximum of about 1100% at around 100K. Observation of giant magnetoresistance at such low magnetic fields is a groundbreaking step in the field of novel magnetic materials and devices.
4:45 PM - Q7.8
Induced Ferromagnetism at the Interface of Paramagnetic CoCr2O4 Epitaxial Thin Films
Rajesh Chopdekar 1 2 , Marco Liberati 3 4 , Yayoi Takamura 2 , Elke Arenholz 4 , Andreas Scholl 4 , Andrew Doran 4 , Yves Idzerda 3 , Yuri Suzuki 2
1 Applied and Engineering Physics, Cornell University, Ithaca, New York, United States, 2 Materials Science and Engineering, UC Berkeley, Berkeley, California, United States, 3 Physics, Montana State University, Bozeman, Montana, United States, 4 Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California, United States
Show AbstractPrevious work on epitaxial manganite (La0.7Sr0.3MnO3)-magnetite (Fe3O4) magnetic tunnel junctions (MTJs) have shown large junction magnetoresistance (JMR) from the use of a spinel-structure paramagnetic insulator(1) as the tunnel barrier. However, the use of CoCr2O4 (CCO) as the barrier in an isostructural Fe3O4-CCO-Fe3O4 trilayer(2) yielded small JMR. In addition, the trilayer exhibited room-temperature exchange coupling between the ferromagnetic electrodes across the 6nm thickness of the barrier. Such behavior was puzzling in light of the CCO bulk Curie temperature (Tc=95K), and single layer CCO thin films that showed largely suppressed magnetization as measured by a SQUID magnetometer. We have grown 40nm thick layers of the ferromagnetic electrode materials, La0.7Sr0.3MnO3 and Fe3O4, as well as CCO by pulsed laser deposition on (110) SrTiO3 single crystalline substrates. Furthermore, 3nm thick CCO capping layers were grown on 40nm thick electrode films to study the magnetism and coupling at the paramagnet-ferromagnet interface. Soft x-ray magnetic circular dichroism (XMCD) spectroscopy at the Advanced Light Source indicated that octahedrally coordinated Cr atoms were strongly polarized by the underlying ferromagnetic oxide layers, but the tetrahedrally coordinated Co atoms were less strongly polarized. While the perovskite-structure manganite film weakly enhanced the ferromagnetism of the CCO capping layer, the commensurate spinel Fe3O4–CCO interface provided unexpectedly strong coupling to above room temperature. Domain images taken in the photoemission electron microscope confirmed that domains of magnetite ferromagnetically coupled to CCO capping layers with thicknesses of 3nm and 6nm. The domain structure in CCO persisted up to 500K on the Cr L3 edge, well above the Tc of CCO, but was not seen on the Co L3 edge even at room temperature.We attributed this strong inducement of ferromagnetic order in CCO to interface coupling to octahedrally coordinated Mn or Fe at electrode-barrier interfaces. Defects such as anti-phase boundaries at a perovskite-spinel interface suppressed the exchange coupling of the paramagnetic capping layer, but the high quality interface between spinel-structure oxides allowed for the persistence of the ferromagnetic state up to 6nm away from the ferromagnet-paramagnet interface. Thus the isostructural nature of the spinel-spinel CCO-Fe3O4 interface enhanced JMR in LSMO-CCO-Fe3O4 junctions by reducing structural disorder, but the strongly coupled Fe3O4-CCO-Fe3O4 junctions served as a spin-filter device, rather than a true MTJ with a non-magnetic barrier separating two distinct ferromagnetic electrodes.
5:15 PM - Q7.10
High Field Negative Magnetoresistance in Ni/oxide/InMnAs Tunnel Junctions.
Steven May 1 , Patrick Phillips 2 , Bruce Wessels 1 2
1 Materials Science and Engineering, Northwestern University, Evanston, Illinois, United States, 2 Materials Research Center, Northwestern University, Evanston, Illinois, United States
Show AbstractHigh field magnetoresistance of tunnel junctions consisting of a ferromagnetic metal, oxide barrier and a ferromagnetic semiconductor was investigated in order to determine its origin. We have fabricated and characterized junctions utilizing ferromagnetic Ni and p-InMnAs thin films and Al2O3 or SiO2 as tunnel barriers. The ferromagnetic InMnAs layers were deposited by metal-organic vapor phase epitaxy (MOVPE). I-V characteristics were measured in the tunnel junctions over a temperature range of 5 to 295 K. A conductance (dI/dV) well is observed for all samples, consistent with tunneling between a metal and a degenerate p-type semiconductor. Up to temperatures of 150 K, a negative junction magnetoresistance (~0.3%) was measured in fields to 0.45 T. The magnitude of the negative magnetoresistance is largest when the magnetic field is applied perpendicular to the plane of the junction. At high temperatures and fields, however, a positive magnetoresistance becomes dominant. In contrast, in Ni/oxide/p-InAs (non-magnetic) tunnel junctions and ohmic InMnAs/Ni junctions, a positive magnetoresistance is observed at all temperatures and applied fields. The negative magnetoresistance is tentatively attributed to a reduction of spin-flip scattering caused by Ni impurity ions in the oxide barrier layer. The total magnetoresistance can be fit to an equation consisting of a positive quadratic term and a negative term given by a Brillouin function. Tunnel magnetoresistance (TMR) arising from the antiparallel alignment of the ferromagnetic layers was not observed in the junctions. The absence of TMR is attributed to interlayer coupling between the Ni and InMnAs layers.
5:30 PM - Q7.11
Metal-insulator Transition and Magnetic Domain in GaMnAs Epilayers.
Alexandre Dourlat 1 , Catherine Gourdon 1 , Vincent Jeudy 1 , Frederic Bernardot 1 , Christophe Testelin 1 , Emmanuelle Lacaze 1 , Laura Thevenard 2 , Aristide Lemaitre 2 , Olivia Mauguin 2 , Ludovic Largeau 2 , Gilles Patriarche 2
1 , Institut des Nanosciences de Paris, Paris France, 2 , Laboratoire de Photonique et Nanostructures, Marcoussis France
Show AbstractA remarkable feature of the magnetic properties of GaMnAs layers is the persistence of the ferromagnetic phase even below the metal-insulator transition. This apparent contradiction with carrier-induced ferromagnetism can be lifted considering that the carrier localization remains sufficiently weak to mediate the interaction between the magnetic moments. However, strong modifications of the magnetic properties between these two regimes are expected, in particular the magnetic domain patterns. Here we investigated them by low-temperature Kerr microscopy. We show in particular the strong influence of defects on the domains in the insulating regime.GaMnAs epilayers (containing 5 to 7% Mn) with the magnetic easy axis perpendicular to the layer plane were grown by molecular beam epitaxy on a thick relaxed InGaAs buffer deposited on GaAs (001).In non-optimized samples (thick GaMnAs layers of 300 nm, no post-growth treatment) the Curie temperature is around 50-60 K while the resistivity increases strongly as reducing the temperature which indicates an insulating or poor metallic character. In this case, magnetic domain reversal occurs by multiple domain nucleation and propagation along crystallographic directions. Starting from the saturated magnetization state and decreasing the field, magnetic domains of opposite magnetization nucleate as narrow lines aligned along crystallographic <110> directions with a width smaller than the spatial resolution. They first grow by increasing their width which is revealed by an increased contrast. Then the magnetic domain growth proceeds by branching along <110> directions. AFM measurements reveal that they nucleate and propagate along some lines showing steps of height 5 to 10 nm. Theses lines are related to the crosshatch pattern, originating from bunches of dislocations propagating along the InGaAs/GaAs interface. We tentatively ascribe this behavior to weaker magnetic anisotropy along the lines of preferential nucleation. This may originate from a locally different strain and/or different carrier density. On the opposite, for optimized samples (thinner layers of 50 nm, with post-growth treatment) for which the Curie temperature reaches 135 K and the electrical resistivity remains finite at low temperature, the magnetic domain pattern changes drastically. Although the crosshatch pattern still exists it does not govern anymore domain shapes, nucleation and growth. Domains of opposite magnetization nucleate in the form of lamellae forming meanders. Their width increases in an irregular manner along their length. Large size homogeneous domains of several tens of microns are observed, which, together with the increase of Curie temperature, make these films promising for micro- and nanostructures patterning.
5:45 PM - Q7.12
Enhanced Room Temperature Ferromagnetism in Mn- and Co-Ion Implanted Silicon
Prabhakar Bandaru 1 2 , Joonsung Lee 1 2 , Jeongwon Park 1 2 , Yunjun Tang 5 , Sungho Jin 1 2 , Se Ahn Song 3 , James O'Brien 4
1 MAE, UC, San Diego, La Jolla, California, United States, 2 Materials Science program, UC, San Diego, La Jolla, California, United States, 5 Center for Magnetic Recording Research, UC, San Diego, La Jolla, California, United States, 3 Analytical Engineering Center, Samsung Advanced Institute of Technology, Suwon Korea (the Republic of), 4 , Quantum Design Inc, San Diego, California, United States
Show AbstractFurther progress in the rapidly advancing field of spintronics is critically dependent on the availability of room temperature magnetic semiconductors. We report here, for the first time, the occurrence of ferromagnetism at room temperature in cluster free, cobalt ion implanted crystalline silicon, along with evidence of manganese induced magnetism3. Through magnetic and structural analysis it is shown that the ion implanted Si actually consists of two layers of Co- and Mn-containing silicon: (1) an amorphous Si layer on the surface, and (2) single crystalline Si beneath. The amorphous layer shows very little magnetism by itself, but yet seems to be responsible for partially canceling out or masking the ferromagnetism present in the crystalline Si. Thus, contrary to the intuition that a removal of a portion of material decreases the net magnetic moment, an etching of the amorphous Si layer, dramatically enhances the measured magnetism, by as much as 400%. The soft-magnetic character of the ferromagnetic crystalline silicon could enable spintronics at relatively low magnetic fields.
Q8: Poster Session: Magnetic Materials, Mostly Non-metallic
Session Chairs
Friday AM, April 21, 2006
Salons 8-15 (Marriott)
9:00 PM - Q8.1
Effects of Annealing Temperature on Ferromagnetism of Rutile Co-TiO2 (100).
Jisheng Pan 1 , J. W. Chai 1 , S. J. Wang 1 , C. H. A. Huan 1 2
1 , Institute of Materials Research & Engineering, Singapore Singapore, 2 Division of Physics and Applied Physics, Nanyang Technology University, Singapore Singapore
Show AbstractDiluted magnetic semiconductors (DMS) are good candidates for spintronics because they can incorporate both the charge and spin degrees of freedom. Usually, ferromagnetic semiconductors are obtained by doping magnetic impurities into host semiconductors, and several III-V and II-VI semiconductors doped with magnetic transition metals were reported to be ferromagnetism. However, most of them exhibit Curie temperature below room temperature, which limits their application in real technology. Following the theoretical prediction that ZnO would become ferromagnetic by doping with transition metals, intensive experimental work has begun on dilute magnetic oxides (DMO). A recent discovery of room-temperature ferromagnetism in Co-doped anatase TiO2 film has motivated intensive studies on this material. In sharp contrast, there are relatively few reports in the literature on rutile Co-doped TiO2. Since the rutile phase is thermodynamically more stable than the anatase phase, it may lead to a higher potential for technological applications. In this study, Co overlayers of ~3 nm have been deposited at room temperature on rutile TiO2 (100) surfaces, followed by annealing to different temperatures. Ferromagnetic behaviour has been observed for all samples, but the saturation magnetic moment per Co atom is seen to decrease with increasing annealing temperature up to 530 oC. In-situ photoemission studies show the interfacial reaction between the Co overlayers and TiO2 (100) surfaces occurred upon annealing to temperatures above 400 oC. Above these temperatures, all metallic Co atoms were oxidized into the Co2+ state, while some Ti4+ were reduced to Ti3+ with increasing temperature. Therefore, the reduction of the saturation magnetic moment is accompanied by Co oxidation at high annealing temperature. However, an annealing temperature of 700 oC leads to an increase of the saturation magnetic moment. At this temperature, the formation of a new Co-Ti-O ternary compound phase is observed by high-resolution transmission electron microscopy. It is proposed that this new phase is responsible for the increase of the saturation magnetic moment. The origin of ferromagnetism is associated with the exchange interaction of magnetic ions via conduction electrons in the newly formed Co-Ti-O ternary compound rather than with the formation of Co clusters in the TiO2 substrate.
9:00 PM - Q8.10
Antiferromagnetic Structures in an fcc Lattice.
Felix Kassan-Ogly 1 , Boris Filippov 1
1 Department of Theoretical and Mathematical Physics, Institute of Metal Physics, Ekaterinburg Russian Federation
Show Abstract9:00 PM - Q8.12
Quasiferromagnetism in Semiconductors
Thierry Dubroca 1 , Jonathan Hack 1 , Rolf Hummel 1
1 MSE, university of florida, gainesville, Florida, United States
Show AbstractUntil now, electron spin has been a relatively unexplored degree of freedom that offers to assist in the continuing progress of the micro-electronics industry. Spin-transport electronics (“spintronics”) is a newly evolving device technology that functions using electron spin, either alone or in conjunction with, electron charge. Our research focuses on understanding how certain semiconductors can result in a ferromagnetic-like hysteresis loop. Ferromagnetic hysteresis has been observed at room temperature in materials not consisting of elements commonly associated with ferromagnetism, such as, Co, Ni, Fe, or Mn-containing alloys. In particular, we report on magnetic hysteresis seen in silicon prepared by different techniques: ion implantation, neutron irradiation, and spark processing of silicon. Because the material investigated contains no ferromagnetic elements, we name them “quasiferromagnetics”. The paramagnetic defects present in these materials were additionally investigated using Electron Paramagnetic Resonance (EPR). We present our understanding of the relationship between the defects at the electronic level and the macroscopic magnetic hysteresis loop. We suggest that some defects are one of the factors responsible for the observed macroscopic magnetic hysteresis loop.
9:00 PM - Q8.13
Ferromagnetic Properties of Clathrate Structured Eu8Ga16Ge30 Thin Films Deposited by Pulsed Laser Deposition.
Robert Owings 1 , Susan Schima 1 , Brian Sales 2 , David Mandrus 2 , David Pappas 1
1 Quantum Electrical Metrology Division, NIST, Boulder, Colorado, United States, 2 Condensed Matter Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractBulk single crystal clathrate structured Eu8Ga16Ge30 exhibits ferromagnetism with a Curie temperature of 35 K. In this structure, the Eu atoms can tunnel between sites in a 4-fold potential well. These materials have interesting thermo-caloric properties. However, it is difficult to integrate them into devices in the bulk form. Therefore, in this work we explore the feasibility of depositing these materials as thin films directly from a bulk single crystal target. The films were grown on sapphire substrates at 300 degrees celsius using pulsed laser deposition. The samples were then cooled slowly. SQUID magnetometry of the samples showed two ferromagnetic transitions, one at 8 K and one at 35 K, indicating a mixed phase with approximately equal parts of the two phases. The phase with the 35 K Curie temperature is identified as the clathrate phase. The phase with the 8 K transition temperature is a known phase of non-clathrate material. This demonstrates that it is possible to grow the clathrate phase in thin films using PLD. We will present Auger spectroscopy results from these films and the bulk reference samples to determine the stoichiometry. This will indicate the direction needed to stabilize pure clathrate phase films.
9:00 PM - Q8.14
GaN Calcined with CuO in Air and N2: Qualitative and Quantitative X-ray Diffraction Studies
Lori Noice 1 , Bjoern Seipel 1 , Peter Moeck 1 , Amita Gupta 1 2 , V. K. Rao 2
1 Physics, Portland State University, Portland, Oregon, United States, 2 Materials Science, Tmfy-MSE, The Royal Institute of Technology, Stockholm Sweden
Show AbstractFollowing the theoretical prediction that certain wide band-gap semiconductors may attain Curie temperatures well above room temperature when doped with transition metals, such as manganese, cobalt, iron, etc.(Dietl T. et al., 287 (2000) 1019), much attention has been given to zinc oxide and gallium nitride to achieve dilute magnetic semiconductors (DMS). Despite the fact that ferromagnetism above room temperature was achieved for copper-doped zinc oxide, the potential of copper doped gallium nitride as a DMS remains largely unexplored. Although a well known impurity of gallium nitride, the role of oxygen in the magnetic properties of the semiconductor is not well understood. In order to address the structural characterization of copper and oxygen doped gallium nitride, several samples of (wurtzite) GaN calcined with copper oxide (CuO) in air or nitrogen at 500 °C for 40 hours were analyzed with X-ray diffraction. Qualitative x-ray diffractometry show that in all the treated samples, gallium nitride and copper oxide were present. Gallium oxide hydrate (GaO2H) was also detected in those samples calcined in air and in the untreated gallium nitride. The samples calcined in nitrogen did not contain GaO2H, but did contain cuprite (Cu2O), suggesting that a lack of oxygen (O2) during sample preparation caused a reduction of the copper. In all the samples, including the untreated GaN, several unknown peaks in the diffractograms indicate that additional low concentration phases such as gallium oxide (Ga2O3) or gallium oxide hydroxide (GaO(OH)) may be present. Quantitative phase analyses and the determination of the GaN lattice constants were performed using the Rietveld method. For selected samples, the GaN lattice constants were also obtained by analytical extrapolation from high Bragg angles close to 90 degrees. Trends in the GaN lattice constants with initial CuO contents are explained by incorporation of both Cu and O into the GaN lattice.
9:00 PM - Q8.15
Structural and Raman Spectroscopic Study of Fe doped SnO2.
Xavier Mathew 1 , Concepcion Mejia-Garcia 2 , Gerardo Contreras-Puente 2 , Jason Hays 3 , Alex Punnoose 3
1 , UNAM, Temixco, Morelos, Mexico, 2 , ESFM-IPN, Mexico D.F, D.F, Mexico, 3 Physics, Boise State University, Boise, Idaho, United States
Show Abstract9:00 PM - Q8.17
Ab Initio Study on The Magneto-Structural Properties of MnAs
Ivan Rungger 1 , Stefano Sanvito 1
1 School of Physics, Trinity College Dublin, Dublin Ireland
Show AbstractThe magnetic and structural properties of MnAs are studied by mapping ab initio total energies onto the Heisenberg model. This provides an explanation for both the first order phase transition at about 317 K and the second order phase transition at about 400 K. The stability of the different phases is found to depend mainly on the volume and on the amount of magnetic order, confirming previous experimental findings and phenomenological models. It is generally found that for large lattice constants the ferromagnetic state is favored, whereas for small lattice constants different antiferromagnetic states can be stabilized. In the ferromagnetic state the structure with minimal energy is always hexagonal, whereas it becomes orthorhombically distorted if there is an antiferromagnetic component in the hexagonal plane.For the paramagnetic state the stable cell is found to be orthorhombic up to a critical lattice constant of about 3.7 Å, above which it remains hexagonal. This leads to the second order structural phase transition between paramagnetic states at 400 K, where the lattice parameter increases above this critical value with rising temperature due to the thermal expansion. At 317 K the lattice constant for the paramagnetic state is smaller than this critical value and therefore it is stable in the orthorhombic structure. As MnAs becomes paramagnetic at this temperature the cell changes abruptly from hexagonal to orthorhombic, which corresponds to a first order phase transition. For the paramagnetic state an analytic approximation for the magnitude of the orthorhombic distortion as a function of the lattice constant is given.Within the mean field approximation the dependence of the Curie Temperature on the volume and on the orthorhombic distortion is calculated. For orthorhombically distorted cells the Curie temperature is much smaller than for hexagonal cells. This is mainly due to the fact that some of the exchange coupling constants in the hexagonal plane become negative for distorted cells. This is also the reason for the appearance of canted spin structures at low temperatures and high pressures, where the cell is orthorhombic. With these results a description of the susceptibility as function of temperature is given, where the temperature dependence enters via the dependence of the Curie Temperature on the lattice parameters.
9:00 PM - Q8.18
Room-temperature Ferromagnetism of Cu-implanted GaN.
Jong-Han Lee 1 3 , Sangwon Shin 2 3 , Jonghan Song 3 , Seung-Cheol Lee 4 , Jong-Hyeob Baek 5 , In-Hoon Choi 1 , Chungnam Whang 2
1 Materials Science and Engineering, korea univ., seoul Korea (the Republic of), 3 Advanced Analysis Center, Korea Institute of Science and Technology, Seoul Korea (the Republic of), 2 Physics and Applied Physics, Yonsei Univ., Seoul Korea (the Republic of), 4 Future Techonology Research Division, Korea Institute of Science and Technology, Seoul Korea (the Republic of), 5 LED Device Team, Korea Photonics Technology Institute, Gwangju Korea (the Republic of)
Show Abstract9:00 PM - Q8.19
Room-temperature Ferromagnetism of 1 MeV Cu-implanted ZnO/Ga-doped ZnO and ZnO Films.
Jong-Han Lee 1 3 , Sangwon Shin 2 3 , Jonghan Song 3 , Seung-Cheol Lee 4 , Won-Kook Choi 5 , In-Hoon Choi 1 , Chungnam Whang 2
1 Materials Science and Engineering, Korea Univ., seoul Korea (the Republic of), 3 Advanced Analysis Center, Korea Institute of Science and Technology, Seoul Korea (the Republic of), 2 Physics and Applied Physics, Yonsei Univ., Seoul Korea (the Republic of), 4 Future Technology Research Division, Korea Institute of Science and Technology, Seoul Korea (the Republic of), 5 Thin Films Materials Center, Korea Institute of Science and Technology, Seoul Korea (the Republic of)
Show Abstract9:00 PM - Q8.2
La2S3 Thin Films from Metal Organic Chemical Vapor Deposition of Single-source Precursor.
Lu Tian 1 , Jagadese Vittal 1 , Kian Ping Loh 1 , Ti Ouyang 1
1 chemistry, national university of singapore, Singapore Singapore
Show AbstractRare earth chalcogenides have been extensively studied because of their potential applications in electronic, optical, superconducting devices, cold cathode configurations, current controlled devices, switching devices, photoconducting cells and thermoelectric components. Also rare earth elements make them very attractive for the fabrication of new permanent magnets. Thin films of Lanthanum sulfide (La2S3) have been prepared from tris(N,N-diethyldithiocarbamato)(2,2’-bipyridyl) lanthanum (III) precursor ([La(bipy)(S2CNEt2)3]) by using metal organic chemical vapor deposition (MOCVD) on different substrates for the first time. The preparative parameters, such as substrate temperature and the nature of substrate, are optimized to get well-defined cubic phase (γ) lanthanum sulfide thin films. The optimized films are characterized by means of X-ray powder diffraction (XRPD) techniques, scanning electron microscopy (SEM), High Resolution Transmission Electron Microscopy (HRTEM) and in-situ X-ray photoelectron spectroscopy (XPS). Electrochemical impedance study indicates that the as-deposited La2S3 thin film shows n-type characteristics. This new route may open the way to the creation of nanostructures and avoid the use of high temperatures and toxic substances such as H2S, CS2.
9:00 PM - Q8.20
Photoinduced Phase-separation in Bi0.4Ca0.6MnO3 Thin Films.
Vera Smolyaninova 1 , Robert Kennedy 1 , Elena Talanova 1 , Luis Aldaco 1 , Rajeswari Kolagani 1 , Mason Overby 1
1 Dept. of Physics, Astronomy and Geosciences, Towson University, Towson, Maryland, United States
Show AbstractDoped rare-earth manganese oxides (manganites) exhibit a wide variety of physical phenomena due to complex interplay of electronic, magnetic, orbital, and structural degrees of freedom. One of the most intriguing properties of manganites is coexistence of two (or several) distinct electronic phases. A photoinduced insulator to conductor transition in charge-ordered (CO) manganites is especially interesting from the point of view of creating photonic devices [1]. We have observed a photoinduced sub-micron phase coexistence of CO insulating phase and conducting phase via optical contrast in Near-field Scanning Optical Microscope (NSOM) images. Such phase coexistence is possible because of the presence of two local energy minima corresponding to CO insulating and charge-disordered conducting phases in the energy landscape [2]. To better understand the physics of phase coexistence in manganites we studied the dynamics of photoinduced conductivity changes. The temperature dependence of this process will be presented. The energy barrier separating the CO insulating and conducting states will be discussed. This work is supported by the NSF under grants DMR-0348939 and DMR-04221141.[1] V. N. Smolyaninova at al., Appl. Phys. Lett. 86, 071922 (2005).[2] K. H. Ahn at el., Nature 428, 401 (2004).
9:00 PM - Q8.21
Room-Temperature Ferromagnetism of Co-Doped TiO2 Thin Films Grown by Plasma-Enhanced Metal-Organic Chemical Vapor Deposition
Eui-Tae Kim 1 , Nak-Jin Seong 1 , Soon-Gil Yoon 1 , Myung-Hwa Jung 2
1 Dept. of Materials Engineering, Chungnam National University, Daejeon Korea (the Republic of), 2 National Fusion R&D Center, Korea Basic Science Institute, Daejeon Korea (the Republic of)
Show Abstract Diluted magnetic semiconductors (DMS) have attracted considerable research interests because of their potential spintronic applications such as spin transistors, nonvolatile storage, logic devices, etc. Realizing a room-temperature ferromagnetism of DMS is central to exploiting their full potential. Recently, Co-doped TiO2 anatase, grown by pulsed laser ablation, has been demonstrated to be ferromagnetic and semiconducting for doping levels up to around 8 at.%, and temperatures of up to 400K [1]. Such Co-doped anatase TiO2 DMS thin films were prepared by oxygen plasma-assisted molecular beam epitaxy and sol-gel process as well. For increase of integration levels in Si process, a demand for thin film fabrication methods with precise composition control, conformal step-coverage, good uniformity, and high throughput is increasing. The chemical vapor deposition (CVD) technique is well-know as the best approach to fulfill these requirements. In this presentation, we report the systematic study of the microstructure and the magnetic characteristics of transition metal-doped TiO2 DMS thin films grown by metal-organic CVD (MOCVD) and plasma-enhanced MOCVD (PEMOCVD). Co-doped TiO2 DMS thin films are prepared onto SiO2/Si and R-Al2O3 (1102) substrates using (C11H19O2)2(C3H7O)2Ti and Co(C11H19O2)3 as the metal-organic sources. Using MOCVD, Ti1-xCoO2 thin films deposited on SiO2/Si at 400 oC showed a polycrystalline anatase phase while thin films prepared at 450 oC were a rutile structure, irrespective of Co doping levels. The microstructure and magnetic characteristic of Ti1-xCoO2 DMS thin films are strongly depends on Co doping levels. After annealing at 700 oC in vacuum ambient, Ti1-xCoO2 thin films with a composition less than x=0.05 had as uniform and smooth surface morphology as as-deposited ones. Samples having a composition above x=0.05, however, showed a severe Co-rich precipitation on the surface. We think that the Co solid solubility in Ti1-xCoO2 thin films prepared by MOCVD is approximately 5 at.%. The magnetization hysteresis loops were observed in the range of x=0.03 to 0.12, indicating anatase Ti1-xCoO2 thin films are ferromagnetic even at room temperature. As the Co doping level increased, the saturation magnetization abruptly increased and the coercive filed markedly decreased. Such a magnetic behavior can be explained by a microstructural change related to Co-rich precipitation and agglomeration. Above x=0.05, the Co-rich clusters having a soft magnetic property increases the saturation magnetic field and decreases the coercive field. We will further discuss the characteristics of Ti1-xCoO2 DMS thin films having high Co doping levels, meanwhile suppressed Co-rich precipitation by deposition at a low temperature using PEMOCVD.[1] Y. Matsumoto, M. Murakami, T. Shono, T. Hasegawa, T. Fukumura, M. Kawasaki, P. Ahmet, T. Chikyow, S.-Y. Koshihara, and H. Koninuma, Science 291, 854 (2001).
9:00 PM - Q8.22
Magnetic Rare Earth (Gd) implanted Tetrahedral Amorphous Carbon (ta-C).
Li Zeng 1 2 , Erik Helgren 1 , Frances Hellman 1 , Carsten Ronning 3
1 Physics, UC, Berkeley, Berkeley, California, United States, 2 Materials Science Program, UC, San Diego, La Jolla, California, United States, 3 Institute of Physics, Georg-August-Universitat Gottingen, Gottingen Germany
Show AbstractTetrahedral amorphous carbon (ta-C) thin films were prepared by Mass Selected Ion Beam Deposition (MSIB) using 100eV carbon ions at room temperature (RT). Amorphous carbon (a-C) films prepared under such conditions exhibit high sp3 fraction up to 80% and have diamond-like properties. They are under extensive study as a potential wide band gap semiconductor. 155Gd, a rare earth magnetic dopant, was implanted with varying energy and fluence at RT into these ta-C films. The purpose is to study the magnetic interactions between the magnetic centers as well as the correlation between the moments and the carriers (magneto-transport properties) in this wide band gap amorphous semiconductor matrix. Previous studies on Gd doped amorphous silicon (a-Si) have shown remarkable physics for compositions near the three-dimensional metal-insulator (MI) transition: many orders of magnitude negative magnetoresistance (MR) at low temperatures, and a high onset temperature (T*) where the effect of magnetic dopants “turns on”. Both MR and T* are significantly reduced when doping Gd into a-Ge and sp2 rich a-C:(H), which have narrow band gap and consequently larger dielectric constant. We believe this is due to the larger electron screening effect, thus larger MR and T* are expected for Gd doped in a true ta-C matrix. The influences of the Gd ion implantation (dose and energy) on the ta-C matrix are studied by Raman spectroscopy. Chemical and structural properties are studied by RBS and TEM. The temperature and magnetic field dependence of electrical conductivity and magnetic properties as a function of Gd concentration and materials structure have been determined. Results and comparison to previous work will be discussed and presented. Thanks for the NSF for support.
9:00 PM - Q8.23
Atomic Scale Characterization of Co and Nb-doped TiO2/LaAlO3 Interface
Lianfeng Fu 1 , Nigel Browning 1 2 , Shixiong Zhang 3 , Darshan Kundaliya 3 , Satish Ogale 3 , T. Venkatesan 3
1 Chemcial Engineering and materials science, University of California at Davis, Davis, California, United States, 2 National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 3 Center for Superconductivity Research, University of Maryland, College Park, College Park, Maryland, United States
Show AbstractDiluted magnetic semiconductor (DMS) materials, such as Co-doped TiO2 films, have aroused much attention due to their potential application in the rapidly developing area of spintronics. However, the fundamental issues in the DMS materials, such as the origin and the nature of FM, are still in controversy. Beside this, the magnetic properties of Co-doped TiO2 films are found to depend critically on the Co distribution, which is also strongly dependent on the growth process. In this regard, the structural qualities of the diluted magnetic semiconductors play an important role in maintaining the spin character of the electron and its magnetic properties in the devices. To fully develop the DMS materials for spintronics applications, it is therefore necessary to characterize and understand the properties of the DMS thin films as a function of the growth conditions.Thin films of anatase Ti1-xCoxNbyO2-δ(x= 0.03, y=0.0, 0.01) studied in this work were grown on (001) LaAlO3 (LAO) single crystalline substrates by a pulsed laser deposition method with an excimer laser at 875 degrees at an oxygen partial pressures of 1x10-5 torr. After the growth, the cross-section of thin film samples was characterized using high spatial resolution scanning transmission electron microscopy (STEM) Z-contrast imaging and electron energy loss spectroscopy (EELS) on a 200kV Schottky field-emission gun (FEG) FEI Tecnai F20. STEM Z-contrast imaging in the pure Co doped TiO2 thin film under this growth condition revealed the formation of nanoclusters mostly at the heterointerface. EELS measurements revealed that these clusters were Co-rich titanium oxide and excluded that they were metallic Co with the valence state measurements. However, in the Co and Nb co-doped TiO2 thin film, the interface was observed to smooth and epitaxial with no clustering. The same EELS measurements across the thin film showed the formation of a uniform Co-rich Ti1-x-yCoxNbyO2-δ surface phase enrichment without clusters. The experimental results showed that cobalt distribution in TiO2 thin films not co-doped with Nb was sensitive to the oxygen pressure. The Nb doping effect on the ferromagnetism and its chemical role on the microstructure of TiO2 thin films will be discussed here. This research was performed at the National Center for Electron Microscopy, LBNL supported by the U.S. DOE under Contract No. DE-AC02-05CH11231. The financial support is provided by NSF on Grant No. DMR-0335364 and NSF-MRSEC on grant No. DMR-00-80008.
9:00 PM - Q8.24
Synthesis and Characterization of Mixed Iron-Oxide Nanoparticles/Poly(styrene-co-carboxyalkylmaleimide) Composites.
Selene Sepulveda-Guzman 1 , Odilia Perez-Camacho 1 , Oliverio Rodriguez-Fernandez 1 , Amelia Olivas-Sarabia 2 , Roberto Escudero 3
1 , Centro de Investigacion en Quimica Aplicada, Saltillo , Coahuila, Mexico, 2 , Centro de Ciencias de la Materia Condensada (UNAM), Ensenada, Baja California, Mexico, 3 , Instituto de Investigacion en Materiales (UNAM), Mexico, DF, Mexico
Show AbstractHybrid materials composed of polymer and iron oxide nanoparticles are interesting because they combine organic functionality and processability with magnetic properties. These materials are used in novel separation systems and magnetic information storage technologies. In this work, maghemite (γ-Fe2O3) and gohethite (α-FeOOH) nanoparticles, were deposited within a styrene/carboxyalkylmaleimide copolymer matrix. Iron oxide deposition was done by repeated treatments with ferrous chloride followed by alkaline oxidation. X-ray diffraction (XRD) analysis was used to identify the inorganic phase and shown the formation of γ-Fe2O3 and α-FeOOH after each deposition cycle. The chemical composition of the composites was studied by X-ray photoelectron and Fourier transform infrared spectroscopy. Alkaline oxidation of the template results in the formation of sodium carboxylate groups, that promoted the adsorption of ferrous ion by the matrix. Iron oxide deposition showed an autocatalytic pathway leading to an exponential increment with number of depostion cycles. Morphology and particle size distribution were determined by high resolution transmission electron microscopy. These results are in agreement with particle size determination by means of wide angle X-ray diffraction, using the Scherrer equation. Mean particle size for γ-Fe2O3 phase was of 14 nm whereas for α-FeOOH phase was of 24 nm. The magnetic properties dependence on the number of deposition cycles was investigated by magnetometry. In addition, zero-field-cooled and field-cooled analysis were used to investigate the magnetic behavior of the synthesized composite. The poly(styrene-carboxylalkylmaleimide)/γ-Fe2O3/α-FeOOH/ composites showed a soft ferrimagnetic behavior, with a saturation magnetization of 8 emu/g, a coercitivity of 50 Oe, and a blocking temperature of 26 K
9:00 PM - Q8.25
An ab initio Study of the Giant Magnetocaloric Effect in MnAs.
HyeJung Kim 1 , Y.-C. Chang 2 1 , S. Chaieb 3
1 Department of Physics, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States, 2 , Research Center for Applied Science, Academia Sinica, Taipei Taiwan, 3 Department of Mechanical and Industrial Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States
Show AbstractMnAs is one of the materials which show a giant magnetocaloric effect. Electronic and magnetic properties of MnAs in NiAs structure and hypothetical zinc-blende structure are studied using a full potential linear augmented-Slater-type-orbital (LASTO) method1-3 within the local spin density approximation. Total energies and magnetic moments as a function of volume as well as band structures are in agreement with previous calculations employing different methods such as full potential linearized augmented plane wave method and a plane-wave pseudopotential implementation. Exchange coupling parameters are determined with a frozen magnon approach4. Using the exchange coupling parameters obtained, a mean-field theory is applied to calculate the quantities of interest such as the magnetic entropy and free energy as a function of temperature and the critical temperature5, to evaluate the giant magnetocaloric effect in MnAs. 1J.W.Davenport, Phys. Rev. B 29, 2896(1994)2J.W.Davenport, M.Weinert, and R.E.Watson, Phys. Rev. B 32, 4876(1994)3J.W.Davenport, R.E.Watson, and M.Weinert, Phys. Rev. B 32, 4883(1994)4M.Pajda, J.Kudunovsky, I. Turek, V. Drchal, and P. Bruno, Phys. Rev. B 64,174402(2001)5A.L.Lima, K.A.Gschneidner, Jr., and V.K.Pecharsky, J. Appl. Phys. 96 2164(2004)
9:00 PM - Q8.26
Deposition of Magnetic Materials on Organic Self Assembled Monolayers.
S. Ahmad 1 , S. Rao 2 1 , S. Shaheen 1
1 Physics & Center for Materials Research and Technology, and Center for nanomagnetics and Biotechnology, Florida State University, Tallahassee, Florida, United States, 2 Physics, Western Illinois University, Macomb, Illinois, United States
Show AbstractThe concept of exploiting soft materials for memory applications poses a great challenge and recent developments in organic self assembled monolayers may facilitate studying interactions of soft matter with conventional memory (magnetic) materials. We report for the first time the effect of depositing a magnetic material (permalloy) on the organic self assembled monolayers as well as on patterned self assembled monolayers surfaces. The interface between the organic monolayer and permalloy may be fundamentally different from the conventional metallic interfaces, because a chemical reaction may be occurring at the interface here. When permalloy is deposited on the patterned surfaces having different functional groups of self assembled monolayers, it is observed that the deposition is very uniform and smooth in the regions where the self assembled monolayer is polar in nature, and it is non-uniform and cluster like in the regions where the self assembled monolayer is non-polar in nature. Consequently, depending upon the functional group of the self assembled monolayer, magnetic properties are affected. We compare the magnetic properties of permalloy films deposited on self assembled monolayers with a variety of functional groups.
9:00 PM - Q8.27
Mn Doped Ge in Spintronics
Sriram Dixit 1 , SVS Nageswara Rao Sunkaranam 2 , An Ping Li 3 , Matthew Chisholm 3 , Hanno Weitering 4 3 , Leonard Feldman 2 1 3
1 Interdisciplinary Materials Science, Vanderbilt University, Nashville, Tennessee, United States, 2 Department of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee, United States, 3 Condensed Matter Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States, 4 Department of Physics and Astronomy, The University of Tennessee, Knoxville, Tennessee, United States
Show AbstractDevices that rely on electron spin form the foundation for spin-based electronics, also known as magnetoelectronics or spintronics. Mn doped Ge is one of the materials systems currently under study for this technology. A group IV based system, such as Ge, may be silicon compatible and hence meld smoothly with current technology.We present a combined ion beam/electron microscopy/magnetotransport study of the Ge(Mn) materials system in ferromagnetic MnxGe1-x epitaxial films. The materials structure supports the model associated with the unusual magnetotransport properties. Films were grown by Molecular Beam Epitaxy (MBE) at temperatures of 50-110°C at a growth rate of 2-4 Å/ min with a substrate temperature of 250°C with varying Mn concentrations of up to 9%. Nanoscale Mn clusters are revealed using transmission electron microscopy, electron energy loss spectroscopy, and channeling Rutherford backscattering spectroscopy. About 20% of total Mn is in the substitutional site of Ge as measured by channeling along the <100> and the <110> crystalline directions. The channeling and the TEM/EELS data are in strong agreement in the concentration values for the substitutional and the interstitial Mn in the MnxGe1-x amorphous clusters. The combination of clusters and the substitutional dopants leads to a model of magnetotransport described by extended hole states (substitutional component), which mediate the cluster driven magnetism.
9:00 PM - Q8.28
Magnetism in InP:Mn Quantum Dots.
Yudhisthira Sahoo 1 , Jerome Keister 1 , Pankaj Poddar 2 , Sanyadanam Srinath 2 , Srikanth Hariharan 2 , Paras Prasad 1
1 Chemistry, Institute for Lasers, Photonics and Biophotonics, SUNY at Buffalo, Buffalo, New York, United States, 2 Physics, Materials Physics Laboratory, University of South Florida, Tampa, New York, United States
Show AbstractQuantum Dots of diluted magnetic semiconductor (DMS) are zero dimensional counterparts of the two dimensional quantum well spintronics structures. We present the preparation of InP:Mn quantum dots from custom made precursors by hot colloidal nanochemistry. These quantum dots are uniform spheres with 3nm diameters, crystalline, photoluminescent and magnetic. The crystallographic and optical properties are similar to undoped InP nanocrystallites signifying that the lattice and photoluminescence characteristics of the host InP are preserved. Structural characterization and analysis confirm there are no segregated binary MnP or MnO phases. We observe robust onset of ferromagnetic order in magnetization measurements at around 25K with blocked state behavior below 15 K, a trend characteristic of magnetic nanocrystallites. The system shows strong frequency dependence of the susceptibility, similar to the behavior observed for spin glasses. Reversible transverse susceptibility experiments done using a resonant radio-frequency (RF) method reveal a strong temperature-dependent effective anisotropy.
9:00 PM - Q8.29
Magnetic Properties of Microcrystalline Si Thin films and Nickel Silicide Nanowires
Joondong Kim 1 , Seongjin Jang 2 , Bi-Ching Shih 2 , Hao Zeng 2 , Wayne Anderson 1
1 Electrical Engineering, University at Buffalo, Buffalo, New York, United States, 2 Department of Physics, University at Buffalo, Buffalo, New York, United States
Show AbstractThe silicides, such as NiSi2 and CoSi2, have been attractive materials to crystallize Si and grow an epitaxial Si film with a small lattice mismatch to Si of 0.46% and 1.23%, respectively. The silicides are popularly used in interconnection as well as device applications such as a transistor with Si/NiSi2 or Si/CoSi2 structures. Additionally silicide nanowires are promising entities in nanoscale technology due to the small scale contact area and low resistivity. The hybrid structure of the magnetic/semiconductor materials have a potential for combining semiconducting and magnetic behavior in one system. Although much research has been performed on silicide materials, very few studies have been reported for the magnetic properties of the silicides. We herein report the growth mechanisms of microsize polycrystalline (mc)-Si and Ni silicide nanowires (NWs) based on silicide formation and the magnetic behavior. A 200 nm SiO2 layer was first deposited by plasma enhanced chemical vapor deposition acting as a buffer layer against metal diffusion into the Si substrate. Ni was thermally evaporated to be a catalyst metal layer for both poly-Si and NW growth. Si was sputtered from a Si target by a dc magnetron. A higher power, up to 200W, was applied for mc-Si growth at 600 oC. Compared to the mc-Si growth procedures, a lower sputtering power (5 - 30W) was used in NW growth at a lower temperature of 575 oC. The surface morphological changes and cross sectional observation were investigated by a field emission electron microscope. The mc-Si was grown in columnar fashion to a 2.5 µm thickness. The grain size ranged from 0.5 to 1 µm in diameter. NWs were grown above 3 µm in length with 30 - 80 nm in diameter.X-ray diffraction was performed to analyze the mechanism of metal silicide formation of the mc-Si film and NW growth. In the mc-Si film, NiSi2 was formed as seeds of poly-Si growth with little lattice mismatch. The main peaks exhibited were Si (111), (220), and (311) at 28.7o, 47.6o, and 56.2o, respectively. The Si peaks are close to NiSi2 peaks, which indicate good epitaxial crystalline Si. The minor peaks of Ni rich phases (Ni3Si2 and Ni3Si) were also found. Otherwise, the NW grown sample showed the NiSi peaks (2 1 1), (1 2 1), and (0 0 4), Ni3Si2 (2 2 1), and Ni3Si (2 2 0). The magnetic properties of the mc-Si and NW grown samples were carried out at room temperature using a VSM magnetometer in the applied magnetic field ranging up to 5000 Oe. The NiSi2 induced mc-Si sample showed a saturated magnetization of 50.6 emu/cm3, The magnetization of the nanowire grown sample was estimated to 77 emu/cm3. The difference may originate from the silicide formation. This heterostructure of magnetic silicide/mc-Si has a potential for spintronics combined semiconductors. We will present the possibility of growing magnetic/semiconductor structures and discuss the magnetic properties of Ni induced mc-Si and NWs.
9:00 PM - Q8.3
Tunable Magnetic Properties of Metal Doped Gallium Oxide Nanoparticles.
Vannah Rahn 2 , Donny Magana 2 , Geoffrey Strouse 2
2 , Florida State University, Tallahassee, Florida, United States
Show AbstractOptical and magnetic properties of Gallium oxide has been an area of interest for potential magnetoresistive and spintronic applications. A novel synthetic route was employed in the synthesis of Gallium oxide nanoparticles using a single mode microwave and gallium acetylacetonate as a single source precursor. Gallium oxide was doped with varied concentrations of metal ions (Co, Fe) to engineer the varying magnetic properties of the alloyed nanocrystal. From powder X-ray Diffraction, we observe a single phase spinel crystal structure that remains constant despite the change in dopant concentration. From temperature dependant magnetic susceptibility analysis we observe a change in the Curie temperature with change in dopant concentration. This is expected due to an increase in exchange interations with increase in dopant concentration level. Promising applications of this magnetically tunable material include materials for use in spintronic devices.
9:00 PM - Q8.4
Three Dimensional Magnetophotonic Crystals on the Base of yttrium-iron-garnet Infiltrated Opals and Magnetization-Induced Second-harmonic Generation.
Oleg Aktsipetrov 1 , Tatyana Murzina 1 , Evgeniya Kim 1 , Ruslan Kapra 1 , Irina Moshnina 1 , Dmitriy Kurdyukov 2 , Saveliy Kaplan 2 , Valeriy Golubev 2
1 Physics Department, Moscow State University, Moscow Russian Federation, 2 , Ioffe Physico-Technical Institute, St. Petersburg Russian Federation
Show Abstract9:00 PM - Q8.6
Textured Growth of Ferromagnetic Metal Sulfide Films using Single Source Precursor.
Jia Mei Soon 1 , Yong Lim Foo 3 , Jun Ding 2 , Lai Yoong Goh 1 , Kian Ping Loh 1
1 Department of Chemistry, National University of Singapore, Singapore Singapore, 3 , Institute of Materials Research and Engineering, Singapore Singapore, 2 Department of Material Science and Engineering, National University of Singapore, Singapore Singapore
Show AbstractTransition metal sulfides of iron[1],[2],[3] and chromium[4] are interesting semiconducting materials which can exhibit ferromagnetism when the metal:sulfur atomic ratios are non-stoichiometric. When coupled with magnetic anisotropy character, they become potentially useful in the fields of magnetic semiconductors and field emission. Using the single source precursor iron diethyl-dithiocarbamate, we deposited highly textured nano-pillars of Fe0.975S via chemical vapor deposition. Nanocrystals of Cr1.89S3 are deposited using a similar method with a chromium diethyl-dithioxanthate precursor. Both materials exhibit preferential orientation in the c-axis with respect to the substrate as investigated by texture analysis using high-resolution x-ray diffraction. The growth morphology of the nanomaterials are sensitive to the orientation of the substrate: When deposited on Si(100), the Fe0.975S nanopillars are epitaxial i.e. the nanopillars exhibit in-plane registry as well as c-axis alignment, with a tilt of 14.5° off the normal. On Si(111), they exhibit fiber texture, with alignment in the c-axis. Similarly, Cr1.89S3 nanocrystals exhibit fiber texture when deposited on Si(100). Such assembly on the substrate occurs without the use of any template or catalyst. This unique morphology is due to the low lattice mismatch of less than 3% between the nanomaterials and substrate. In the case of Fe0.975S, the NiAs-type crystal packing is also responsible for the unique crystal morphology. The well-defined shape anisotropy imparts magnetic anisotropy to the material, causing it to exhibit differential in-plane: out-of-plane magnetic anisotropy ratio of 4:1, with saturation magnetization of 12emu/g.
9:00 PM - Q8.7
Spin Polarized Chalcogenide Thin Films Of CuCr2Se4.
Joanna Bettinger 1 , Rajesh Chopdekar 2 1 , Marco Liberati 3 , Janell Neulinger 4 , Lisa Alldredge 2 1 , Elke Arenholz 5 , William Butler 6 , Yves Idzerda 3 , Angelica Stacy 4 , Yuri Suzuki 1
1 Materials Science and Engineering, UC Berkeley, Berkeley, California, United States, 2 Applied Physics, Cornell University, Ithaca, New York, United States, 3 Department of Physics, Montana State University, Bozeman, Montana, United States, 4 Department of Chemistry, UC Berkeley, Berkeley, California, United States, 5 Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 6 Department of Physics and Astronomy, University of Alabama, Tuscaloosa, Alabama, United States
Show AbstractBulk forms of CuCr2Se4 have been known for many years to have a magnetization near 5μB per formula unit at low temperatures and have a relatively high Curie temperature of 460K [1]. Recent electronic structure calculations indicate that this material is nearly half metallic, and the magnetic moment is primarily due to the high spin polarized Cr density of states balanced by small contributions from the Cu and Se sites with opposite magnetization. Therefore, given successful growth of CuCr2Se4 in magnetic thin film form, it would serve as an ideal electrode material for magnetic tunnel junctions. We have grown CuCr2Se4 films by pulsed laser deposition and varied deposition temperature, partial pressures, and single crystal substrates. Additionally, we have looked at the effect of post growth Se anneal on the film’s properties, including magnetic and phase information. Depositions from room temperature to 650°C in a vacuum or selenium atmosphere exhibit magnetism on a variety of substrates, including MgO, MgAl2O4, and LaF3. X-ray diffraction has indicated that we are growing films in the [111] direction, which can be grown epitaxially on (0001) LaF3 with a 1.67% strain. Using a superconducting quantum interference device (SQUID) magnetometer, we have determined the magnetization value to be near the bulk value of 5μB per formula unit. Atomic force microscopy reveals a topographically smooth surface with a RMS value of 0.24 nm, while magnetic force microscopy has been used to characterize the domain structure of the thin film. The Curie temperature is above 390K, the detection limit of our SQUID. X-ray magnetic circular dichroism and x-ray absorption spectroscopy have been performed on these films showing that high growth temperatures and vacuum background produce films that closely resemble those of the bulk CuCr2Se4 phase.[1] F.K. Lotgering. Solid State Commun. 2 (1964) 55.
9:00 PM - Q8.8
Magnetoresistance and Transport-Magnetism correlations in Hole Doped Lanthanum Manganites Grown by Polymer Assisted Deposition
Menka Jain 1 , P. Shukla 2 , Y. Li 1 , B. Maiorov 1 , M. Hundley 1 , M. Hawley 1 , A. Burrell 2 , T. McCleskey 2 , L. Civale 1 , Q. Jia 1
1 Materials Science Technology Division, Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 2 Chemistry Division, Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show AbstractThe effect of colossal magnetoresistance (CMR) in hole-doped manganites (La1-xMxMnO3+y, where M=Sr, Ca, and Ba) has attracted significant interest in the past decade. In a range of doping, x~0.2-0.4, the ground state is ferromagnetic and the paramagnetic to ferromagnetic transition is accompanied by a sharp drop in resistivity (ρ), indicating that ρ is strongly influenced by the magnetic transition. An applied magnetic field (H) also suppresses ρ by aligning magnetic moments. In the present work, films of La0.67Sr0.33MnO3 (LSMO) and La0.67Ca0.33MnO3 (LCMO) were grown on single crystalline LaAlO3 substrates by polymer assisted deposition developed recently at the Los Alamos National Laboratory. Significant improvement in the electrical properties of these films was observed with the annealing temperature. Both the resistivity and the large negative magnetoresistance peak near the ferromagnetic ordering temperature, with Δρ/ρ0 = -50% and -88% (H= 50 kOe) in LSMO and LCMO films respectively. We have found a clear correlation between the microstructure, resistivity, and magnetization. The results of temperature and magnetic field dependent resistivity and magnetization measurements on the pure LCMO and LSMO films as well as in multilayered films will be presented in detail.