The term magnetoplasmon, or magnetoplasma surface wave, was first introduced in the early 70â?Ts, motivated by a renovated interest in surface plasmons in metals and degenerate semiconductors. Nowadays, the phenomenology associated to systems where plasmonic and magneto-optical (MO) properties coexist has become an active area of investigation. These magnetoplasmonic systems have opened new routes for the development of e.g. high performance gas and biosensing platforms as well as the exploitation of non-reciprocal effects in devices with potential applications in the telecom area. In magnetoplasmonic structures, MO and plasmonic properties are intertwined, allowing for plasmonic properties to become tunable upon application of a magnetic field (active plasmonics), or the MO effects to be largely increased by plasmon resonance excitation, as a consequence of the enhancement of the electromagnetic (EM) field in the MO active component of the structure. In this last case, the study of the enhanced MO activity in structures with subwavelength dimensions is especially interesting since the properties of these systems upon plasmon resonance excitation bring as a consequence an enhanced EM field in its interior, and more interestingly in the region where the MO active component is present. Unfortunately, it is not straightforward to experimentally determine the intensity of the EM field inside a nanostructure. Here we show how the EM profile related to the localized surface plasmon resonance can be probed locally inside the nanostructure by measuring the MO activity of the system as a function of the position a MO active probe (a Co nanolayer). At this stage, optimizing the EM field distribution within the structure by maximizing it in the MO components region while simultaneously minimizing it in all the other, non MO active, lossy components, will allow for the development of novel systems with high MO activity and reduced losses, becoming an alternative to state of the art dielectric MO materials, like garnets. We will show how the insertion of a dielectric layer in Au/Co/Au magnetoplasmonic nanodisks induces an EM field redistribution in such a way to concentrate it in the regions of interest of the nanostructure. The metallo-dielectric system exhibits large MO activity and low optical extinction in a specific wavelength range (around 780 nm). It will be demonstrated this is due to a EM field redistribution at this wavelength, controlled by the insertion of the dielectric layer. If time permits we will also show some examples in which the MO effect is used to control the plasmon properties, such as magnetoplasmon interferometry. The authors acknowledge funding support from the EU (NMP3-SL-2008-214107-Nanomagma), the Spanish MICINN (â?oFUNCOATâ? CONSOLIDER INGENIO 2010 CSD2008-00023, MAGPLAS MAT2008-06765-C02-01/NAN and PLASMAR MAT2010-10123-E) and the Comunidad de Madrid (â?oNANOBIOMAGNETâ?, S2009/MAT-1726 and â?oMICROSERES-CMâ?, S2009/ TIC-1476).

Hybrid magneto-plasmonic (MP) nanostructures are a new concept of nanomaterials composed by a plasmonic material and a magneto-optically (MO) active material. Noble metals exhibit strong plasmon resonances, but have no MO activity. A ferromagnetic material can indeed supply this MO activity, a coupling effect that is enhanced by nanoscaling. These materials are particularly promising for sensor devices because of the high sensitivity of plasmon to surface adsorption processes, an interaction that can be tailored now by an external magnetic field through the MO material to provide higher sensitivities. Here we report about the fabrication of different types of hybrid MP nanostructures which involve a MO active ferrite layer grown by atomic layer deposition. On the one hand, colloidal lithography is used to produce gold nanodisks by thermal evaporation. The nanodisks are grown on a previously deposited ferrite thin film or alternatively post-coated by the ferrite thin film, providing a different approach for the hybrid 0D-2D MP nanostructure. On the other hand, porous anodic alumina membranes are used as template for the fabrication of MP core-shell nanowires by a combination of atomic layer deposition and electrodeposition, resulting in hybrid 1D-1D MP nanostructures. Finally, we will present preliminary results about the characterization of the optical and MO properties which reveal promising about the viability of these new features.

In recent years, spin waves have attracted growing interest, especially due to their potential use in information technology. The possibility to perform logic operations using spin waves relies on the ability to manipulate spin-wave propagation. One well established ansatz to tailor spin-wave propagation employs patterning of thin ferromagnetic films, e.g. to obtain spin-wave resonators [1,2,3]. More recently, the anisotropy in the dispersion of spin waves [4] has been studied theoretically [5] and experimentally [6] as a novel way to manipulate spin-wave propagation in unpatterned ferromagnetic films. Here, we demonstrate for the first time perfect imaging with spin-waves [7]. We discuss time resolved scanning Kerr microscopy (TR-SKM) data on the diffraction of planar Damon-Eshbach spin waves on a one-dimensional grating. The grating is realized by a micrometer sized slit array in a Permalloy film. Behind the grating, we observe a unique diffraction pattern which produces images of the spin-wave field at the slits. The formation of these images relies on the anisotropic and linear shape of the iso-frequency line in k space (DIFL) of the spin-wave dispersion, which reflects the direction dependence of spin waves. As a consequence, the resolution of the spin-wave images is not limited by the wavelength of the incident spin wave, as it is the case in isotropic media [8]. Instead, deviations from a perfect image occur solely due to spin-wave damping and due to the finite curvature of the DIFL. In that sense, our spin-wave images represent a spin-wave analog to sub-wavelength resolution concepts with anisotropic media in optical metamaterials [9,10]. We demonstrate that the position of the images behind the slit array can be tuned by manipulating the DFIL via the excitation frequency and the external magnetic field. This opens up the fascinating possibility to create tailor made spin-wave fields in an unpatterned ferromagnetic film. We gratefully acknowledge support by the DFG via SFB 668, SFB 508, GrK 1286, and by the City of Hamburg via the Cluster of Excellence Nano-Spintronics. [1] Perzlmaier et al., Physical Review B 77, 054425 (2008) [2] Podbielski et al., Physical Review Letters 96, 167207 (2006) [3] Mendach et al., Applied Physics Letters 93, 262501 (2008) [4] Damon and Eshbach, Journal of Physics and Chemistry of Solids 19, 308 (1961) [5] Veerakumar and Camley, Physical Review B, 214401 (2006) [6] Schneider et al., Physical Review Letters 104, 197203 (2010) [7] Mansfeld et al., arXiv:1108.5883v1 (2011) [8] Abbe, Archiv fuer Mikroskopische Anatomie 9, 413 (1873) [9] Liu et al., Science 315, 1686 (2007) [10] Schwaiger et al., Physical Review Letters 102, 163903 (2009)

Nanostructured antidot lattices have generated large interest for applications in magnonics, i.e., the control and transmission of spin waves in magnetic devices [1]. We have fabricated different square-lattice antidot lattices by patterning circular holes of a diameter of 120 nm in 25 nm thick permalloy films by focused ion beam etching. From device to device we vary the lattice constant between 300 and 4000 nm. Using integrated coplanar wave guides we perform all-electrical spin-wave spectroscopy [2]. In short-period antidot lattices we observe the formation of allowed minibands supporting large-velocity spin waves. We attribute this to the coherent coupling of excitations at hole edges, i.e., so-called edge modes [3]. The coupling is controlled via an in-plane magnetic field. In a large-period lattice we observe transmission characteristics which are consistent with meta-materials properties for Damon-Eshbach-type spin waves [4]. The results are found to be relevant if one considers efficient injection of spin waves in periodically patterned antidot lattices. The findings open new perspectives for magnonic applications. Financial support by the German Excellence Cluster Nanosystems Initiative Munich (NIM) and the European Communityâ?Ts Seventh Framework Programme (FP7/2007-2013) under Grant Agreement no. 228673 MAGNONICS is gratefully acknowledged. We thank C.H. Back, H.G. Bauer, D. Berkov, G. Carlotti, G. Gubbiotti, M. Krawczyk, M. Madami, S. Mamica, M.L. Sokolovskyy, S. Tacchi, G. Woltersdorf, for experimental and theoretical support. References: [1] S. Neusser and D. Grundler, Adv. Mater. 21, 2927 (2009); [2] S. Neusser et al., Phys. Rev. Lett. 105, 067208 (2010); [3] S. Neusser et al., Phys. Rev. B. 84, 094454 (2011); [4] S. Neusser et al., Phys. Rev. B (in press).

In most magnetic applications the orientations of the magnetic elements are controlled by external magnetic fields. However, it is now well established that the orientations of nano-magnets can be controlled directly by the injection of spin polarized currents known as spin transfer torques. The ability of a spin-polarized current to reverse the magnetization orientation should enable a range of devices such as high performance random-access magnetic memories, spin-oscillators, and magnetic logic operations [1]. In this presentation I highlight recent research on spin-transfer effects in nano-elements having strong perpendicular magnetic anisotropy [2-11]. In such systems the shape demagnetization field is commensurate with the magneto-crystalline anisotropy axis and can be described as an effective uniaxial anisotropy. This geometry has a number of advantages including higher stability against thermal activation, efficient coupling of the spin-current to magnetic excitations, and higher magnetic resonance frequencies. We have studied spin-transfer effects in perpendicular anisotropy [Co/Pt]/[Co/Ni]/Cu/[Co/Ni], [Co/Pd]/[Co/Ni]/Cu/[Co/Ni], and [Co/Pd]/Ru/[Co/Pd]/[Co/Ni]/Cu/[Co/Ni] spin valve structures that are patterned as nanopillars. We have explored the influence of spin currents on the free layer switching fields [2], the distortion of the Stoner-Wohlfarth astroid under current [3], scaling of critical currents with the energy barrier height [4], ultra-fast spin-torque reversal [5], switching of the hard layer with current [6], the effectiveness of a antiferromagnetically-coupled reference layers [7], current-induced telegraph noise [8], and the role of thermal activation [9]. These results provide a detailed description of the role of anisotropy on spin-torque reversal in this class of devices. How these results compare to macro-spin and micro-magnetic simulations and their implications for new devices will also be addressed [10, 11]. This research was done in collaboration with S. Mangin, D. Ravelosona, J.A. Katine, Y. Henry, J. Cucchiara, D. Lacour, D. Bedau, H. Liu, J.-J. Bouzaglou, A. D. Kent, J. Z. Sun, J. Kan and I. Tudosa. This work is support by NSF Award # DMR-1008654 and by the Friends contract of the French National Research Agency (ANR). [1] J. A. Katine and E. E. Fullerton, J. Magn. Magn. Mater. 320, 1217 (2008). [2]S. Mangin, et al., Nat. Mater. 5, 210 (2006). [3] Y. Henry, et al., Phys. Rev. B 79, 214422 (2009) [4] S. Mangin, et al., Appl. Phys. Lett. 94, 012502 (2009). [5] D. Bedau, et al., Appl. Phys. Lett. 96, 022514 (2010). [6] I. Tudosa, et al., IEEE Trans. Mag. 46, 2328 (2010). [7] I. Tudosa, et al., Appl. Phys. Lett. 96, 212504 (2010). [8] J. Cucchiara, et al., Appl. Phys. Lett. 94, 102503 (2009). [9] D. Bedau, et al., Appl. Phys. Lett. 97, 262502 (2011). [10] K. Lee, et al., J. Appl. Phys. 109, 123910 (2011). [11] I. Yulaev, et al., Appl. Phys. Lett. 99, 132502 (2011).

As predicted by L. Berger and J. Slonczewski [1] when a spin-polarized current enters a ferromagnet the current exerts a torque on the ferromagnet magnetization. This torque can lead to magnetization switching between two stable configurations which was later demonstrated in nanopillar spin-valve structures [2]. The ability of a spin-polarized current to reverse the orientation of nanomagnets enables a range of magnetic devices such spin transfer torque magnetic random access memories (STT-MRAM). However, several advances are needed to realize practical devices [3]. One key point is the reduction of the currents required to switch magnetization while maintaining the thermal stability of the free layer. To address this, we study of the effect of both spin polarized current and thermal activation on magnetization dynamic in magnetic nanopillars with perpendicular magnetic anisotropy. Nanopillar spin valves 70 nm x 140 nm made of [Co/Ni] and [Co/Pd] multilayers showing perpendicular anisotropy were prepared. In such geometry one can observe that the critical current scales with the height of the anisotropy energy barrier and critical currents as low as 120 μA is achieved in quasi-static room-temperature measurements of a 45-nm diameter device [4]. Moreover fast switching is observed using short current pulses down to 300 ps [5,6] and thermally activated process give rise to telegraph noise [7]. Here we present results on the switching field distribution obtained by measuring the switching field of more that 1000 hysteresis loop for different injected current values. We observe a distribution of switching fields that depends on the current. The switching field distribution results are analyzed using the Kurkijärvi expression [8] derived from the Néel-Brown model [9] permits to deduce the switching field distribution and the spin-current-dependent energy barrier [10]. The study of the switching field distribution confirm that domain nucleation and growth need to be taken into account to fully explain the experimental observation [11]. This work is supported by NSF Award # DMR-1008654, the Friends contract of the French National Research Agency (ANR) and Partner University Fund of the Embassy of France. [1] L. Berger, Phys. Rev. B 54, 9353 (1996) J. Slonczewski, J. Magn. Magn. Mater. 159, L1 (1996) [2] J. A. Katine et al., Phys. Rev. Lett. 84, 3149 (2000)., E. B. Myers et al Science 285, 867 (1999). [3] J. A. Katine and E. E. Fullerton, J. Magn. Magn. Mater. 320, 1217 (2008). [4] ] S. Mangin et al., Appl. Phys. Lett. 94, 012502 (2009) [5] D. Bedau et al., Appl. Phys. Lett. 97, 262502 (2010) [6] D. Bedau et al., Appl. Phys. Lett. 96, 022514 (2010) [7] J. Cucchiara et al., Appl Phys. Lett 94 102503 (2009) [8] M. L. Néel, Ann. Geophys. 5, 99 (1949); W. F. Brown, Phys. Rev. B 130, 1677 (1963). [9] J. Kurkijärvi, Phys. Rev. B, 6, 832 (1972). [10] Z. Li et al., Phys. Rev. B 72.180405 (2005). [11] D. P. Bernstein et al., Phys rev B 83, 180410 (2011)

We have studied the depinning behaviour of domain walls in bend V-shaped nanowires. In our experiments we use bent wires with an angle of 170° between the two arms. The width of the Permalloy wire is 350nm and the thickness 18nm. In the bent region domain walls can be easily nucleated using a magnetic field that is oriented along the line of intersection of the two arms. After nucleation of a domain wall the depinning behaviour is investigated via measuring the anisotropic magneto resistance (AMR) as a function of strength and orientation of the applied magnetic field. Additionally, dc electrical currents of high current density can be applied due to an efficient cooling of the wire [1]. When the substrate is cooled, current densities up to 4e12 A/m^2 can be applied. Influences of the current on the depinning is found above a current density of about 1e11 A/m^2. Besides the effect of Joule heating and a small influence of an Oersted field a strong impact of spin torque is found. We demonstrate a procedure that allows for separation of the different effects. The results are explained in the framework of the micromagnetic structures that evolve in the wire when applying magnetic fields. [1] Hankemeier S, Sachse K, Stark Y, Froemter R and Oepen H P, Ultrahigh current densities in permalloy nanowires on Diamond, Appl. Phys. Lett. 92, 242503 (2008)

The giant-magnetoresistance (GMR) effect found in multilayered structures composed by ferromagnetic (FM) layers separated by non-magnetic spacers has attracted sustained interest over the past decades for both fundamental and technological reasons [1,2]. Such effect consists in a significant change of the electrical resistance depending on the relative magnetization orientation of the FM layers, which could originate from spin-dependent scattering processes of the electrons traveling across the structure [3]. Even though it is commonly assumed that the MR depends on the magnetic anisotropy of multilayer structures, a comprehensive description of the magneto-resistive behavior related to the magnetization reversal is still lacking. Experiments just relies in either magnetization (usually parallel component) or MR curves measured independently for a given applied field angle, normally close to the easy axis (e.a.) direction. Here, we present a detailed study of the angular dependence of both magneto-resistive and magnetization reversal properties in a exchange-biased spin valve structure [4], by using a new experimental set-up that allows us to measure simultaneously magneto-resistance and vectorial-resolved Kerr [5] hysteresis loops, i.e., including MR and in-plane parallel and perpendicular magnetization components, at different applied field angles in the whole angular range. We advance towards a microscopic understanding of the MR properties by showing that their angular dependence leaves distinct fingerprints, which are directly related to their magnetization reversal processes. For instance, reversible and irreversible transitions are similar in both MR and vectorial-resolve magnetization curves. Well-defined MR-plateaus are observed around the e.a. direction whereas just reversible MR transitions are found around the hard axis (h.a.) direction. The MR-plateau value decreases as the magnetic field is misaligned with respect to the e.a. and the maximum of MR decreases approaching the h.a. The results directly show that the different magneto-resistive behaviors originate from the magnetic anisotropy of the structure, which ultimately depends on the relative magnetization orientation of the FM layers. References: [1] M.N. Baibich, J.M. Broto, A. Fert, F. Nguyen Van Dau, F. Petroff, P. Eitenne, G. Greuzet, A. Friederich, and J. Chazelas, Phys. Rev. Lett. 61, 2472 (1988); G. Binash, P. Grunberg, F. Saurenbach, and W. Zinn, Phys. Rev. B 39, 4828 (1989). [2] G. A. Prinz, Science 282, 1660 (1998). [3] A. Fert, Angew. Chem. Int. Ed. 47, 5956 (2008); C. Chappert, A. Fert and F. N. Van Dau, Nature Mater. 813, 6 (2007). [4] B. Dieny, V.S. Speriosu, S.S.P. Parkin, B.A. Gurney, D.R. Wilhoit, and D. Mauri, Phys. Rev. B 43, 1297 (1991). [5] D. Ecija, E. Jimenez, N. Mikuszeit, N. Sacristan, J. Camarero, J.M. Gallego, J. Vogel, and R. Miranda, Phys. Rev. B 77, 024426 (2008).

Spin dependent tunneling phenomena in small magnetic metal particles have attracted much attention. Magnetic granular thin films consisting of nanometer-sized magnetic metal particles (Fe, Co, Ni..) embedded in an insulating matrix exhibit giant magnetoresistance (MR) due to spin dependent tunneling. Carbon-based nanocomposite thin films have large application potential as spin transport materials because long spin relaxation time is expected due to its weak spin-orbit interaction [1]. The films of amorphous carbon (a-C) have a mixture of graphiteâ?"like sp2 and diamond-like sp3 bond characteristics. The ratio of sp2 and sp3 orbitals in an a-C film can be changed with the deposition condition. Therefore, interesting phenomena are expected in granular films with ferromagnetic metal (FM) nanoparticles distributed in an a-C matrix [2]. In this paper, a systematic investigation has been performed on magneto-transport properties of Co-C granular thin films with different deposition conditions. We studied the negative MR by varying the Co content and the carbon bonding state in Co-C granular films. Co-C granular thin films were fabricated on Si substrates by a co-sputtering technique. The base pressure was lower than 2Ã-10-5 Pa and the deposition was carried out in an Ar atmosphere of 2.1Ã-10-1 Pa at room temperature. The composition was controlled by the sputtering powers of Co and C targets, and was determined by electron probe microanalysis (EPMA). In this study, the carbon sputtering power was changed from 50 W to 200 W with keeping the same Co content (xCo). MR was measured by a physical property measurement system (PPMS) at 2-10 K under a magnetic field up to 90 kOe. Raman spectra were measured at room temperature by an argon ion laser with a wavelength of 488 nm. A large negative MR of -30.3% was obtained at 2 K for the sample prepared with the sputtering power of 50 W (C) and 4 W (Co). This MR was larger than that reported previously [2]. We studied structural properties of carbon thin films by Raman spectroscopy. Two bands (D and G modes) from carbon bonds were clearly observed, and the intensity ratio of two bands changed with the sputtering power, suggesting that the graphitization was promoted with the sputtering power. It was also revealed that the transport mechanism changed from tunneling to Mottâ?Ts variable range hopping and MR decreased with the promotion of graphitization. This is the first result that indicates the relation between the graphitization, transport mechanism and MR in carbon based granular films systematically. [1] S. Sakai, K. Yakushiji, S. Mitani, K. Takanashi, H. Naramoto, P. V. Avramov, K. Narumi, V. Lavrentiev, and Y. Maeda, Appl. Phys. Lett. 89, 113118 (2006). [2] R. Tang, M. Mizuguchi, H. Wang, R. Yu, and K. Takanashi, IEEE Trans. Magn. 46, 2144 (2010).

We report on an effect of finite size on the magnetoresistance (MR) in cobalt layers sandwiched by platinum. In current in-plane (CIP) geometry it is found that the resistivity depends on the magnetization orientation within the plane perpendicular to the current in contradiction to the anisotropic MR (AMR) in bulk materials [1]. The resistivity shows a symmetry adapted cosine square dependence on the angle to the surface normal, with the maximum along the surface normal. This behaviour was found for sputter deposited and electron-beam evaporated films in a temperature range from 4.2 to 300 K [1]. To investigate the origin of the MR anisotropy we varied the Co layer thickness of sputter-deposited Pt/Co/Pt sandwiches from 0.8 to 50 nm. The AMR, i.e., the resistivity difference for the magnetization oriented parallel and perpendicular to the current, while the latter is also oriented in plane, increases with Co thickness levelling off into a constant value. This behaviour can be explained with the decreasing Pt shunt on the resistivity with increasing Co thickness. In contrast the new anisotropic MR effect decreases with one over the Co thickness. The results are fitted utilizing the Fuchs-Sondheimer solution of the Boltzmann transport equation of electron motion, which includes phenomenologically diffusive and specular interface scattering [2]. The fit clearly points out that the mechanism behind this effect originates from the Co/Pt interface. In the thin film regime this anisotropic interface magnetoresistance (AIMR) can be as large as the AMR, while at large thicknesses the effect levels off into a thickness independent value, known as geometrical size effect [3]. The experiments are confirmed in terms of a fully relativistic spin-polarized ab initio-type approach by using layer-resolved resistivities and their dependence on the thickness of the Co layer [4]. The theoretical description shows in particular that the AIMR mainly originates in the vicinity of the Co/Pt interfaces.
[1] A. Kobs et al., Phys. Rev. Lett. 106, 217207 (2011)
[2] E. H. Sondheimer, Adv. Phys. 50, 499 (2001)
[3] W. Gil et al., Phys. Rev. B 72, 134401 (2005)
[4] A. Kobs et al., submitted for publication (2011)

The formation of rare earth magnetic nanoclusters with high energy product has attracted a great deal of attention¹. The desired structures are the L1₀ and β₂ ordered structures characterized by a high magnetocrystalline anisotropy. Our efforts have concentrated on indexing L1₀ and β₂ FePt, FeAu and MnAu clusters after inert gas condensation. High resolution transmission electron microscopy (HRTEM) revealed essentially single-crystalline clusters that lent themselves to transform indexing. Fast Fourier transforms (FFT) of HRTEM images, their analysis, and indexed crystal structures are presented.

Group IV dilute magnetic semiconductors (DMS) are candidates for the development of spin based devices due to their compatibility with the traditional semiconductor technology. Controlled self-assembly of DMS quantum dots with room temperature functionality offer novel possibilities for quantum computing. The fabrication of DMS Ge:Mn homoepitaxial alloy films was reported to require low growth temperature (50°C

L1₀-FePt is one of the most promising candidates for future 5-10 Tbits/in² recording density due to its ultrahigh magneto-anisotropy Ku of 7Ã-10⁷ erg/cc. However, writability is a big challenge for the application of L1₀-FePt because of its large coercivity - more than 3 T experimentally. The commercial writing head can only supply maximum 1.8 T writing field. To solve the writing problem, the most reliable solution is coupling the FePt hard magnetic phase to a soft magnetic phase for assisting the magnetization switch and hence reduces the coercivity. The idea is called exchange-coupled composite (ECC) media. Fabricating the ECC media with exact microstructure with the hard phase and soft phase in the same columnar grain by sputtering deposition is quite challenging. Ledge-typed FePt/Fe grown on MgO single-crystal substrate or doping oxide/C into the FePt/soft bilayers cannot give the structure as proposed. Hence to have an in-depth understanding of the magnetic properties of FePt/Fe ECC media, patterning the bilayer into the single-domain magnetic pillars is necessary. In our project, we patterned the FePt/Fe continuous bilayer into isolated nanodots using electron beam lithography. Each nanodot is a composite of FePt hard phase and Fe soft phase, which has the exact structure of ECC media. The effect of soft layer thickness on the magnetic properties of FePt/Fe ECC nanocomposites has been investigated. Due to the small patterned area (20µmÃ-20µm), common equipment which measures the magnetic moment vs. applied field cannot be used for the characterization due to the lack of signal. Therefore, we used Anomalous Hall Effect measurement to investigate the magnetic properties such as magnetization switching mechanism, thermal stability and switching field distribution. The Hall device is fabricated by photolithography. A mircomagnetic simulation based on Landau-Lifshitz-Bloch equation has been applied to study the magnetization and reversal properties of the FePt/Fe ECC nanodots.

We have successfully developed a technique to fabricate nanodots of variable size and varying magnetic properties based on self-organized assembling of diblock-copolymer micelles on Co/Pt multilayers with perpendicular anisotropy [1]. The micelles are filled with silica cores of variable size, which are used as shadow mask for subsequent Ar+ ion milling. Before ion etching the organic shells of the micelles are removed in an oxygen plasma. The mean distance between the silica cores that remain on the multilayer can be tuned by the length of the diblock-copolymers. Utilizing the anomalous Hall Effect (AHE) we have developed a method to study the magnetic properties of single Co/Pt-Nanodots with diameters of less than 30 nm. We use two different electron beam lithography (EBL) approaches to create Hall cross geometries. In the first approach a hall cross structure is written into positive PMMA. The Co/Pt multilayer is sputter-deposited on top of the resist. The lift-off gives a Hall cross that consists of the multilayer on which the micelles are deposited in the next step. Via Ar+ ion milling the ferromagnetic layer between the silica cores is erased. The remaining structure is a Hall cross consisting of the Pt seed layer with Co/Pt dots on top. In the second approach the magnetic dots are created first as described above. After that a negative resist is spin-coated on the sample and exposed. The developed resist acts as mask in a second Ar+ milling process in which the film is completely removed. In both ways Hall cross geometries with wire widths well below 100 nm are fabricated. With appropriate micelle sizes it is possible to create Hall bars that contain only a single dot. The magnetization behavior of single dots with diameters below 30 nm will be shown. [1] H. Stillrich et al., Adv. Funct. Mat. 18, 76 (2008)

Commercial ferrofluids usually consist of spherical, superparamagnetic nanoparticles suspended in a carrier liquid. The application of an external magnetic field, however, orients the magnetization of the particles, which can trigger network formation and a concomitant increase in the liquidâ?Ts viscosityâ?"its so-called magnetoviscosity. Permanent magnetic as well as superparamagnetic nanotube ferrofluids can be fabricated by depositing an iron oxide film sandwiched between two silica layers by atomic layer deposition (ALD) onto ultra-thin, hexagonally self-ordered alumina nanopore arrays. Subsequent reduction in an argon-hydrogen atmosphere converts the amorphous iron oxide layer into a polycrystalline, (superpara)magnetic magnetite phase. Release of the iron oxide nanotubesâ?"having diameters of 70 nm and lengths of 200 nmâ?"by wet chemical etching results then in the formation of a nanotube suspension. Such a ferrofluid shows a stronger magnetoviscous response than that of a conventional ferrofluid consisting of spherical nanoparticles, especially at low applied magnetic fields. Suspensions of superparamagnetic nanotubes exhibit a strongly enhanced stability against sedimentation, owing to a reduced magnetic interaction between individual nanotubes as compared to the ferromagnetic case. Furthermore, magnetoviscosity measurements performed on hybrid ferrofluids (mixtures of commercial ferrofluids with nanotubes) reveal a significant enhancement in the strength of the magnetoviscous effect in comparison to samples without nanotube additives, even at moderate magnetic fields (and very low nanotube concentrations below 0.05 vol%). Evidently, this ALD/template-based synthesis route allows the experimentalist not only to tailor all geometric tube parameters (length, diameter, wall thickness) but also to control the type of magnetism manifested by the nanotubes. The synthesis of stable, (superpara)magnetic (hybrid) nanotube suspensions might pave the way for new bio-medical imaging, drug delivery or engineering applications. We acknowledge the DFG Priority Program 1165 as well as the State of Hamburgâ?"through the excellence cluster â?oNanotechnolgy for Medicineâ?â?"for financial support.

We have investigated the magnetic fine structure and the appearance of domain walls in V-shaped nanowires depending on their geometrical properties by means of scanning electron microscopy with polarization analysis (SEMPA). The wires were carved from 18 nm thick soft-magnetic Co39Fe54Si7 (at. %) film via Focused Ion Beam (FIB) milling. All three types of wall are found, i.e. transverse walls, asymmetric transverse walls and vortex walls. The type of domain wall that is found depends on the bending angle and the width of the wire as well as on the direction of the magnetic field that is used to seed the domain wall. With decreasing angle between the wire arms (bending angle) a transition from initial vortex walls to transverse walls are obtained. The orientation of the seeding field has a manifold impact on the micromagnetic structure of the walls. Recently it was demonstrated that the field orientation with respect to the symmetry plane, given by the intersection of the two arms determines the details of the vortex wall, i.e. the exact location of the core and the sense rotation [1]. Within the vortex wall regime a stronger deviation of the field orientation from the symmetry plane induces asymmetric transverse walls. Asymmetric transverse walls and vortex walls are competing structures with increasing probability for asymmetric transverse walls close to the transition point to the transverse wall [2]. Micromagnetic simulations allow for an understanding of the relaxation process within the seeding of asymmetric transverse and vortex wall. The relaxation is dominated by shape anisotropy which fixes the sense of rotation in the beginning of the relaxation process in case of vortex wall [1]. In asymmetric transverse walls the topological edge defect is not nucleating in the kink like in vortex walls in agreement with the experiments. [1] S. Hankemeier et al., Phys. Rev. B 82, 064414 (2010) [2] A. Kobs et al., in preparation

Size and shape of nanoparticles are fundamentally related to their structure-property relations that ultimately drive the development of novel applications, particularly those connected to the surfaces of the nanoparticles. Traditionally, external agents such as surfactants, reducing agents, and stabilizers have been used to enforce the preferential growth orientation, in order to develop tailor-made nanoparticles with size and shape control. However, these external agents cover the pristine surfaces of the nanoparticles and often modify not just the surface morphology but more importantly the surface activity. Here we use a surfactant-free, single-step method to control the shape of bimetallic FeNi alloy nanoparticles and produce even the thermodynamically unfavorable shape such as concave nanocubes. Both Scanning Electron Microscopy and Helium Ion Microscopy show that these FeNi alloy nanoparticles exhibit a homogeneous spatial uniformity on the Si substrate with a very narrow size distribution. Transmission Electron Microscopy confirms the shape of nanoparticles to be concave cubic, exposed with high-index facets. Using Superconducting Quantum Interference Device (SQUID) magnetometry and Magnetic Force Microscopy, we demonstrate how the shape of these nanoparticles could affect the total and local magnetic properties of FeNi nanoparticles. For instance, Saturation magnetization is higher in concave cubic alloy nanoparticles while the coercively is less compare to the spherical alloy nanoparticles. Furthermore, these FeNi nanoparticles are found to be extremely air-stable. In particular, both their X-ray diffraction and SQUID data show that their crystal structure and magnetic properties have not changed even after exposure to moist air for more than two months. X-ray Photoemission Spectroscopy reveals in-situ formation of a goethite (α-FeOOH) skin on top of the nanoparticles, which evidently passivates the surface of nanoparticles and results in this air stability. Our simple synthesis method therefore promises a powerful approach for producing novel bimetallic alloy nanoparticles with â?otunableâ? catalytic activities, magnetic properties, and enhanced air stability.

With the rising cost and demand for rare earth based magnets, a low cost replacement is a vital future permanent magnetic applications. Using exchange coupled nano-composite magnets could be the path to a viable RE magnet replacement. One of the biggest hurdles to this approach is combining proper hard and soft magnetic phases in composites, while retaining the nanocrystallinity that delivers enhanced properties. We present results on composites densified from nanocrystalline powders using Current Activate Pressure Assisted Densification (CAPAD). This technique has proven effective in proven effective for producing, large sized and intimately mixed magnetic nanocomposites. Results for Oxide-oxide and metal-oxide composites will be shown and the magnetic properties (remanance, coercivity etc) will be discussed in terms of nanocrystalline grain size and distribution of magnetic phases.

Since the advanced invention of ancient era â?" a compass, magnetic phenomena have been an essential facet of everyday life, most markedly in recent times with magnetic recording and information technologies. In the life sciences and in the field of health technologies, magnetic materials provide an unprecedented level of functionality. Life sciences applications include MRI contrast enhancement, targeted and triggered drug delivery and release, bioseparation, magnetofection, and magnetically induced hyperthermia [1, refs wherein].Innovative methods of magnetic actuation have recently been explored for the distant control and manipulation of cell adhesion, receptor clustering, and intercellular signaling. While the mainstream of reports on biological applications of magnetic materials deals with chemically synthesized superparamagnetic nanoparticles with sizes in the range of tens of nm, advanced systems based on ferromagnets are also of great interest. Instead of solution-based assembly, these materials are lithographically-defined using top-down techniques, such as micro- and nano-fabrication combined with physical vapor deposition. This approach allows the fabrication of monodisperse particles of virtually any shape, including multicomponent and multilayered materials, with tunable magnetic properties (including high saturation magnetization and zero remanence due to in-plane or our of plane flux closure), and sizes down to ~100s nm. Our objective is study of effects of magneto-mechanical stimulation of live cells by ferromagnetic disks with a spin vortex state. Recently we demonstrated that the disk-shaped ferromagnetic particles can serve as mediators of cellular mechanotransduction [2, 3]. Distinctive properties of the ferromagnetic disks allow utilizing unprecedentedly low frequency ac fields (~10-20 Hz) for triggering of cell apoptosis via direct energy transfer to a single cell, and the transformation of magnetic field-induced mechanical forces to ionic current signals. Similar approach can be utilized if a soft biopolymer scaffold is introduced into a ferromagnetic disks hybrid system for magneto-mechanically-induced controlled release of biologically active molecules (antibiotics, cofactors, dyes) [4]. 1.E Rozhkova, Adv Materials, 2011 2. H Kim, E Rozhkova, IV Ulasov, SD Bader, T Rajh, MS Lesniak, V Novosad, Nature Materials, 2010 3. Novosad&Rozhkova, Ferromagnets-Based Multifunctional Nanoplatforms. Biomed Eng, Trends in Materials Science, ISBN 978-953-307-513-6, ed A. N. Laskovski 2011 4. H Kim, P Karavayev, E Rozhkova, V Novosad, J Materials Chem 2011

Recent developments of single-molecule manipulation techniques have opened the way to single molecule biophysics, viz, the study of biomolecular interactions at the level of individual molecules. Besides application to biophysical research, such a control would open up new possibilities for medical and pharmaceutical applications, e.g., handling small volumes of bio-samples, performing sophisticated sorting of different molecules, or testing the effectiveness of a drug delivery system. The tools developed so far to achieve single-molecule manipulation are highly sophisticated, require accurate calibration and can produce substantial heating. We have recently demonstrated a vastly improved approach to confine and control the position and movement of an individual biofunctionalized magnetic nano-particle in liquid by remotely controlling the motion of magnetic domain walls (DW) in patterned nano-wires¹. The methodology has been used to trap, manipulate and release single viable cells on a surface². This novel methodology permits to overcome the limitations of standard methods employing magnetic beads and would open up new possibilities towards single molecule bio-medical applications as handling small volumes or performing sophisticated sorting of different targets attached to different particles on a chip device. Particularly relevant in view of medical applications, is the easy integration of such bio-manipulation system on a single chip with sensors able to detect the presence of magnetic nano-particles for â?olab-on-chipâ? diagnostic applications³. [1] M. Donolato, P. Vavassori, M.Gobbi, M. Deryabina, M.F.Hansen, V. Metlushko, B. Ilic, M. Cantoni, D. Petti, S. Brivio, and R. Bertacco, Adv. Mater. 22, 2706 (2010); P. Vavassori, V.Metlushko, B. Ilic, M.Gobbi, M.Donolato, M. Cantoni, and R. Bertacco, J. Appl. Phys. 107, 09B301 (2010). [2] M. Donolato, A. Torti, N. Kostesha, M. Deryabina, E. Sogne, P. Vavassori, M. F. Hansen, and R. Bertacco, Lab Chip 11, 2976 (2011). [3] P. Vavassori,V. Metlushko, B. Ilic, M. Gobbi, M. Donolato, M. Cantoni, R. Bertacco, Appl. Phys. Lett. 93, 203502 (2008); M. Donolato, M. Gobbi, P. Vavassori, M. Cantoni, M. Leone, V. Metlushko, B. Ilic, M. Zhang, S. X. Wang, and R. Bertacco, Nanotechnology 20, 385501 (2009).

Iron oxide nanoparticles have been extensively studied in targeted delivery, localized therapy, and as contrast agents for magnetic resonance imaging (MRI). In fact, sugar coated iron oxide NPs have been clinically used as the liver/spleen-specific contrast agents in MRI. This presentation will discuss how rationally designed iron oxide NPs can achieve highly effective MRI contrast agents. The focus will primarily focus on the shape control of NPs and the surface coatings. The formation of various shaped-iron oxides (e.g., cubes, nanoworms, nanoplates, and nanowires) will be elaborated. Further, strategies to achieve water soluble nanoparticles with great stability and surface functionally will be discussed. Finally, using spherical iron oxide NPs as a platform, the development of the magnetic-fluorescent dual imaging probe and the biological effects of these nanoparticles on fruit flies will be covered.

Issues related to the down size scalability of perpendicular STT-MRAM will be discussed. A first point concerns the thermal stability of the magnetization of the storage layer. In p-STT MRAM, a difficulty is to achieve a sufficiently large perpendicular anisotropy while maintaining a low damping since both are related to spin-orbit interactions. However, the use of the interfacial perpendicular anisotropy arising at magnetic metal/oxide barrier (e.g.CoFe/MgO) as first observed in Ref.1 may help circumventing this dilemma. Another point concerns the magnetic reversal mechanism i.e. nucleation/propagation versus coherent rotation. For CoFeB types of materials, coherent rotation is expected to occur below 20nm diameter. A third point is that a linear relationship exists between the total write current and the thermal stability factor in perpendicular STT MRAM. Since the latter must be larger than 70 to achieve a 10 years data retention in a 1Gbit chip, this sets a bottom limit for the selection transistor size of the order of 20nm assuming a damping factor of 0.01 and a current polarization of 80%. This means that pure p-STT MRAM would be limited in their downsize scalability to about 20nm. However, thanks to thermal assistance, this limit can be pushed forward. The idea is to use the current flowing through the tunnel junction to heat up the cell and induce an anisotropy crossover in the storage layer from out-of-plane to in-plane. The role of the spin transfer torque is then to slightly pull the magnetization out-of the sample plane. The full perpendicular anisotropy is recovered upon cooling to the standby temperature. This approach was demonstrated experimentally and offers the best Figure of merit (Thermal stability Factor/write current) ever reported. 1. S.Monso et al, Appl.Phys.Lett.80(2002), 4157

The conventional memories, such as NAND and DRAM, have faced the limitation of scalability. Among new memory candidates, STT RAM has good endurance, non-volatile and high speed. On the other hand, there are still some issues to be solved, such as high cell current drivability, high cost and thermal stability of magnetic tunneling junction. The patterning of MTJ stacks was executed by RIE process. After the etch process, the residues were formed on the edge of MTJ stacks, metal electrode, and ILD layer. These residues could cause the poor adhesion between the ILD oxide and the encapsulating Si3N4 and the increase of the film stress. Moreover, the edge of MTJ stacks was damaged during the etch process. Especially, the damage of MgO tunnel barrier and the storage magnetic layer could cause the increase of Rlow and the decrease of TMR. After the etch process, post treatments should be followed for removing the residues and the damaged part of MTJ stacks. The MgO and the magnetic storage layers are very vulnerable to the attack by wet clean chemicals such as HF, BOE, SPM, and APM, some solvents and even water-based rinse solution. When we tried to remove the residue, on the contrary, the MTJ stacks were easily damaged. In STT RAM, the etch post treatment will be one of the important issues. So we have investigated the solutions for removing the residue without the deformation of MTJ stacks. In this work, we used the in-plane type MTJ structure and low resistance W as the bottom and top electrode. We used the RIE process for MTJ patterning. Post wet treatments were conducted after patterning using various solutions. We have checked the cross-sectional images of the MTJ stacks after wet cleaning using several chemicals. The deformation of MTJ stacks was observed at the acid chemicals. However, the base chemicals did not deform the MTJ stacks. When wet cleaning, using the base chemical, was applied after etching process, the resistance of anti-parallel state and parallel state was lower and the TMR was higher than without cleaning. When the wet cleaning was not applied, the encapsulating silicon nitride layer was separated from the ILD oxide due to the adhesion and the film stress. However, the wet cleaning was applied after the etching, the delamination was cleared.. Furthermore, in case of the batch type cleaning process, the magnetic storage layer and the MgO layer were easily damaged by DI water during the rinse and drying steps. However, using the same chemical, at the single type cleaning process, those layers were not damaged and showed good electrical properties. These results indicate that the contact time between wafer and DIW should be critically controlled during the cleaning process. In conclusion, we investigated the wet cleaning after etching for MTJ stacks in order to remove the damaged layer and the residue. The proper wet cleaning could enhance the STT-RAM characteristics such the Rlow and TMR without the degradation of Hc.

Being able to set a reference direction for the spin of conduction electrons constitutes a prerequisite in most spintronic devices. Exchange bias, which refers to the exchange interaction between a ferromagnet (FM) and an antiferromagnet (AFM) layers in direct contact, is commonly used for that particular purpose of shifting the hysteresis loop along the magnetic field axis. Coupling FM layers based on [Pt/Co]/NiFe with distinct anisotropy directions is an alternative solution with, important in view of potential applications, no need of applying a field cooling procedure in order to induce a loop shift.^1,2 By means of in-plane saturation, the formation of remanent domains in a [Pt/Co] multilayer with out-of-plane anisotropy leads to a shift of the hysteresis loop of an adjacent NiFe thin film with in-plane anisotropy. Controlling the geometry (size, shapeâ?¦) and the pinning of the above-mentioned domains and domain walls we are able of tuning the magnitude of the hysteresis loop shift.² This is demonstrated, first on continuous systems, by varying the thickness of an underlying Pt buffer layer,³ and introducing a Pt spacing layer with increasing thickness in between the [Pt/Co] and the NiFe bilayer. In a second part of the study, we will focus on [Pt/Co]/NiFe systems with geometrically reduced lateral sizes (patterned nanostructures consisting of stripes and dots). Experimental results in combination with micromagnetic simulations show that the magnetic configuration responsible of the measured loop shift is the result of the exchange coupling between the magnetization of the NiFe layer and the magnetization of vortices cores which form in the domain walls separating the upwards and downwards magnetized domains in the [Pt/Co] multilayer. In particular: (i) The formation of vortices cores after in-plane saturation of the [Pt/Co]/NiFe system marks the appearance of the exchange bias phenomenon. (ii) The resistance of vortices cores against magnetization reversal dictates the magnitude of the loop shift obtained through a cycling with an in-plane magnetic field. (iii) The vortices pinning strength is given by the effective perpendicular anisotropy of the [Pt/Co] multilayer and this latter can be effectively enhanced by increasing the thickness of the Pt buffer layer. Furthermore, an extension of the Pt/Co magnetic domains through the NiFe layer has been deduced from magnetometry results and corroborated by simulations. These results have been successfully applied to achieve a threefold increase of the exchange bias field in a patterned [Pt/Co]/NiFe system consisting of parallel long stripes as compared to a continuous film. ¹J. Sort et al., Phys. Rev. B 70, 174431 (2004). ²A. Bollero et al., Phys. Rev. B 73, 144407 (2006). ³A. Bollero et al., Phys. Rev. B 84, 094423 (2011).

Nanomagnetic devices currently rely on magnetic switching or controlled motion of domain walls by an external magnetic field or spin-polarized current. Achieving the same degree of magnetic controllability using an electric field has potential advantages including enhanced functionality and low power consumption. Here, an approach to electrically control local magnetic properties, including the writing and erasure of regular ferromagnetic domain patterns and the motion of magnetic domain walls, will be discussed [1-3]. The method is based on recurrent strain transfer from ferroelastic 90° stripe domains in ferroelectric media to continuous magnetostrictive films with negligible magnetocrystalline anisotropy. The dominance of the magnetoelastic anisotropy in these ferromagnetic-ferroelectric heterostructures causes full imprinting of the ferroelectric domain pattern into the ferromagnetic counterpart and strong pinning of ferromagnetic domain walls onto narrow ferroelastic boundaries. As a result, the width, spin rotation and resonance frequency of the ferromagnetic domain walls can be accurately tuned by a change in the direction or strength of the applied magnetic field. Moreover, optical polarization microscopy of both the ferromagnetic and ferroelectric domain structures reveals that domain correlations and strong inter-ferroic domain wall pinning are maintained in an applied electric field. This leads to unprecedented electric-field control over the formation of magnetic domains and lateral motion of ferromagnetic domain walls, an accomplishment that opens the way to electric-field driven nanomagnetism. The experiments in this work are complemented by static and dynamic micromagnetic simulations to further elucidate the physics of interacting ferromagnetic-ferroelectric domain walls. [1] T.H.E. Lahtinen, J.O. Tuomi, and S. van Dijken, Adv. Mater. 23, 3187 (2011) [2] T.H.E. Lahtinen, J.O. Tuomi, and S. van Dijken, IEEE Trans. Magn. 47, 3768 (2011) [3] T.H.E. Lahtinen, K.J.A. Franke, and S. van Dijken, arXiv:1109.5514v1

New application requirements have led to increased development efforts on base LTCC ferrite materials and compositions to meet the needs. The single greatest technology gap for these materials seems to be the properties of the existing ferrite composition and has led to limiting further reduction in size and cost of monolithic components. This work will present results on efforts to develop novel nanoparticle LTCC ferrite material(s) with significantly improved magnetic performance, allowing for a size and cost reduction compared to the existing â?ostate of the artâ? designs/materials used in these applications. Specifically, the objective is to provide new LTCC ferrite materials with an increased saturation magnetization and magnetic permeability as compared to the existing LTCC ferrite, which directly enables size reduction without sacrificing component performance. By enabling significant size and cost reduction, the newly developed LTCC ferrite material will be able to broadly address overall performance requirements and general needs for technology. Therefore, nanoparticle polycrystalline NiCuZn based ferrites have been prepared under controlled experimental conditions and magnetic and electrical properties have been measured. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

Spin current, a flow of electronsâ?T spins in a solid, is the key concept in spintronics that will allow the achievement of efficient magnetic memories, computing devices, and energy converters. To use spin currents, generation, manipulation, and detection of spin currents are necessary. Here, I review phenomena associated with spin currents recently discovered in insulator/metal hybrid systems [1]: inverse spin-Hall effect [2,4], spin pumping, and spin Seebeck effect [4-6]. We found that spin pumping and spin torque effects appear at an interface between Pt and an insulator YIG. This means that we can connect a spin current carried by conduction electrons and a spin-wave spin current flowing in insulators. We demonstrate electric signal transmission by using these effects and interconversion of the spin currents [1]. Seebeck effect (SSE) is the thermal spin pumping [5]. The SSE allows us to generate spin voltage, potential for driving nonequilibrium spin currents, by pl acing a ferromagnet in a temperature gradient. Using the inverse spin-Hall effect in Pt films, we measured the spin voltage generated from a temperature gradient in various ferromagnetic insulators. This research is collaboration with K. Ando, K. Uchida, Y. Kajiwara, S. Maekawa, G. E. W. Bauer, S. Takahashi, and J. Ieda. [1] Y. Kajiwara&E. Saitoh et al. Nature 464 (2010) 262. [2] E. Saitoh et al., Appl. Phys. Lett. 88 (2006) 182509. [3] A. Ando&E. Saitoh et al., Nature materials 10 (2011) 655 -659. [4] K. Uchida&E. Saitoh et al., Nature 455 (2008)778. [5] K. Uchida&E. Saitoh et al., Nature materials 9 (2010) 894 - 897. [6] K. Uchida&E. Saitoh et al., Nature materials 10 (2011) 737-741.

Spin Hall effects intermix spin and charge currents in non-magnetic materials and, therefore, offer the possibility to generate and detect spin currents without using ferromagnetic materials. In order to understand the underlying physical mechanism and to identify technologically relevant materials, it is important to quantify the spin Hall angle γ, which is a direct measure of the charge-to-spin (and vice versa) conversion efficiency. Towards this end we utilized non-local transport measurements with double Hall bars of gold and copper [1]. In principle, this geometry permits the study of spin currents both generated and detected via spin Hall effects. We observe an unusual non-local resistivity that changes sign with temperature. However, this result is quantitatively similar in gold and copper, indicating that the non-local signals are not due to spin transport. An analysis of the data based on a combination of diffusive and quasi-ballistic transport leads to an upper limit of γ0.027 for gold at room temperature. Therefore we developed an approach based on spin pumping, which enabled us to quantify even small spin Hall angles with high accuracy. Spin pumping utilizes the microwave excitation in a ferromagnetic layer adjacent to a non-magnetic metal to generate over a macroscopic area a homogeneous dc spin current, which can be quantified from the line-width of the ferromagnetic resonance. In this geometry voltages from spin Hall effects scale with the device dimension and therefore large signal-to-noise ratios can be obtained even for materials with small spin Hall angles. We integrated ferromagnet/normal metal bilayers into a co-planar waveguide and determined the spin Hall angle for a variety of non-magnetic materials (Pt, Pd, Au, and Mo) at room temperature. Of these materials Pt shows the largest spin Hall angle with γ = 0.013±0.002 [2,3]. Furthermore, we show how these spin Hall effects can be used to modify magnetization dynamics in an adjacent ferromagnetic insulator, such as yttrium iron garnet (YIG). By using a Pt/YIG bilayer we show how currents through the Pt can either reduce or increase the linewidth of the ferromagnetic resonance in YIG. Interestingly the current dependence of the line width shows a distinct threshold behavior. Work at Argonne supported by DOE BES under Contract No. DE-AC02-06CH11357.

During the last years, it became possible to measure the spin Hall effect (SHE) electronically in metallic devices which opens the opportunity to use the SHE for spin current generation in metal-based spintronics. This avoids the difficulties related to spin current injection from ferromagnets into semiconductors. With this perspective materials are requested which provide a strong effect, i.e. which show an effective conversion of charge currents into spin currents. The origin of the SHE is the spin orbit coupling (SOC). It is often expected that strong SOC leads automatically to large SHE, which is not generally the case. Furthermore, the spin diffusion length decreases with increasing SOC. Since the spin diffusion length restricts the distance over which the spin information is maintained it limits the dimension of a device in potential applications. Here we present an ab initio analysis for dilute alloys to optimize their potential for an effective charge to spin current conversion combined with a long spin diffusion length. We considered the extrinsic mechanism by calculating the skew-scattering cross section at impurities in an otherwise ideal host material. The calculations are based on density functional theory by means of the Korringa-Kohn-Rostoker Greens function method. The transport is described within the diffusive limit solving a linearized Boltzmann equation. The intrinsic contribution was calculated via the Berry curvature description of the anomalous velocity. The KKR Greens function method allows for a particularly efficient calculation of the Berry curvature. Combining all tools we were able to identify systems showing a charge to spin current conversion of nearly 10% in combination with a spin diffusion length as large as 100nm.

Creation and manipulation of spin currents is a key ingredient in spintronics, which has as a goal the use of the spin of the electron in addition to its charge. One way to create such spin currents is by using ferromagnetic (FM) / non-magnetic (NM) lateral spin valves (LSV), where due to their non-local geometry, it is possible to decouple a pure spin current from the charge current [1]. In a LSV, a spin-polarized current is injected from a FM electrode into a NM metal, generating a spin accumulation at the interface, which diffuses along the NM metal. Another FM electrode is used to measure the non-local voltage, which is proportional to the spin accumulation. In this work, LSV devices are fabricated by using a two-step electron-beam lithography process. In the first step, FM electrodes (cobalt or permalloy) are deposited and, in the second one, a NM metal, such as copper, gold or silver, is put on top. In these devices, the spin signal is measured at different temperatures, from which the spin polarization of the FM metal and the spin diffusion length of the NM metal are obtained. The behavior of the spin diffusion length is compared in different NM metals, showing the different role of surface spin-flip scattering events [2] in the various metals. In addition, the results for the spin polarization of the FM metals help understanding the contribution of the intrinsic polarization of the FM and the FM/NM interface in the electrical spin injection. Whereas the results obtained for the spin polarization of permalloy (around 40%) are consistent with literature [3], the obtained spin polarization of cobalt is higher (up to 14%) due to the achievement of a clean FM/NM interface. This is a fundamental issue for the creation of pure spin currents. The authors acknowledge funding from the Spanish MICINN (MAT2009-08494), the Basque Government (PI2011-1) and the European Commission (â?oITAMOSCINOMâ?, Marie Curie Actions). E.V. thanks the Basque Government for the Ph.D. fellowship â?oProgramas de ayudas para formación y perfeccionamiento de personal investigadorâ?. References [1] F. J. Jedema, M. S. Nijboer, A. T. Filip, B. J. van Wees, Physical Review B, 67 (2003) 085319. [2] T. Kimura, T. Sato, Y. Otani, Physical Review Letters, 100 (2008) 066602. [3] F. Casanova, A. Sharoni, M. Erekhinsky, I. K. Schuller, Physical Review B, 79 (2009) 184415

We have demonstrated that a heat current applied to the free layer of a spin valve can induce magnetization switching (Phys. Rev. Lett. 104, 146601 (2010)). Now we examine FMR (ferromagnetic resonance) in the presence of a heat current. In spin valves, we find that we can modulate the FMR by an alternating heat current. Using the Spin Seebeck geometry, we also find an effect of the heat current on the FMR.

The influence of magnetic configurations on thermal transport in magnetic thin-film and nanowire systems is barely investigated. This is in part due to experimental difficulties that arise from thermal leakage through the substrate. The 3 omega method can be used to overcome these difficulties. We report on field-dependent 3 omega measurements of the thermal conductivity on giant magnetoresistive (GMR) thinfilm systems and nickel nanowires. GMR thinfilms were deposited on glass substrates. The 3 omega measurement is performed on a gold micro-heater line fabricated on top of the thinfilm system with an insulating layer in between. As the thermal conductivity of the film is more than one order of magnitude larger than for glass, the measurement is sensitive to the in-plane thermal conductivity of the film. The observed giant magneto-thermal resistance (GMTR) ratio is about 12 % at room-temperature. Individual nickel wires, fabricated by template assisted electrochemical deposition with diameters between 150 nm and 350 nm, were suspended over a predefined trench and electrically contacted with four gold leads. In this case, a combination of 1 omega and 3 omega measurements is used to investigate the anisotropic magnetoresistive (AMR) effect and the anisotropic magneto-thermal resistive (AMTR) effect. Considering both effects, we observe an anisotropy in the Lorenz-number. We gratefully acknowledge the financial support by the German Research Foundation via German Priority Program SPP1538 â?oSpinCATâ?.

A recent exciting discovery is the spin Seebeck effect,^1,2,3 which is the spin analog to the charge Seebeck effect. As thermoelectric materials generate electricity from thermal power via the charge Seebeck effect, thermospin materials are able to generate electrical power via the spin Seebeck effect. Here we present, for the first time, experimental results of power generation current-voltage curves under load from Pt strips on a GaMnAs sample in the conventional spin Seebeck geometry. Efficiency, generated power, and outlook potential will be discussed. Work supported by the National Science Foundation, NSF-CBET-1133589. 1. K. Uchida, et al., Nature 455 778 (2008) 2. C.M. Jaworski et al., Nature Materials 8 898 (2010) 3. K. Uchida, et al., Nature Materials 8 893 (2010)

The study of spin-effects influence on the electrical currents flowing under an applied magnetic field is a well-known research topic that was intensively investigated during the last few decades. With the recent interest in thermoelectric effects in combination with spintronics and magnetism the so called spin caloritronics also attracted considerable attention. This work compares the magnetic field dependent effects on the Seebeck coefficient and the electric conductivity in magnetic nanowires which exhibit an anisotropic and giant magnetoresistance behavior. We synthesize Co-Ni alloy nanowires and Co-Ni/Cu multilayered nanowires by electrochemical deposition into nanoporous anodic alumina membranes. After dissolving the template, hundreds of nanowires having diameters between 30 nm and 300 nm and lengths of ~20 µm are dispersed on a glass substrate. To study the thermoelectric effects of the nanowires under the influence of the applied magnetic field, we have employed a measurement technique introduced by E. Shapira et al. [Nanotechnology 18 (2007) 485703]. Applying laser lithography, we fabricate µm-sized metallic structures on selected individual nanowires. The structure includes a heater that enables us to create a temperature gradient. The temperature difference dT results in a thermal current, thus in a thermoelectric voltage V_th = -S * dT (S = Seebeck coefficient) which is measured between two thermometers along the nanowire. The relative change of this thermoelectric voltage when the magnetic field is applied is called â?omagneto thermoelectric powerâ?. Additionally, we use 4-point measurements to determine the anisotropic or the giant magnetoresistance of the nanowire. For each nanowire, we vary the temperature between 50 K and 450 K, magnetic field up to 30 kOe and temperature gradient between 1 K and 30 K. This experimental setup allows us to measure all the above mentioned properties on a distinct section of the nanowire without changing the measurement configuration. Lastly the temperature dependence of the Seebeck coefficient can be calculated. The authors gratefully acknowledge financial support via the DAAD, Spanish MICINN, the SPP 1538 Spintronic funded by the German Science Foundation (DFG) and the Cluster of Excellence â?~â?~Nanospintronicsâ?Tâ?T funded by the State of Hamburg.

An outstanding question in nanomagnetism is the quantitative understanding of magnetization reversal of nanoparticles. A priori it is not clear which reversal mechanism applies for a given nanostructure. Coherent rotation, domain nucleation and growth and other reversal mechanisms are conceivable. In order to elucidate the physics of the reversal, we extract the energy barrier of magnetization reversal from a quantitative analysis of the switching field of individual, nanometer small Co islands. We use spin-polarized scanning tunneling microscopy at low temperature in external magnetic fields, aligned perpendicularly to the sample surface, to extract the switching field of single well defined, two atomic layer thin Co islands from hysteresis loops of the differential conductance of islands with 700 to 18000 atoms. We find that the switching field changes in a non-monotonous manner with island size. The switching field increasing with size up to 5500 atoms, where a maximum switching field of 2.4 T is observed at 8 K. The switching field decreases for larger islands. We extract the energy barrier for magnetization reversal as a function of island size, and we identify a crossover of the reversal from an exchange spring behavior to domain wall formation with increasing size around 7500 atoms. I shall discuss the inhomogeneous electronic properties of the Co nanoislands [1, 2, 3] in view of their impact on the magnetic anisotropy. [1] O. Pietzsch, S. Okatov, A. Kubetzka, M. Bode, S. Heinze, A. Lichtenstein, and R. Wiesendanger, Phys. Rev. Lett. 96, 237203 (2006). [2] H. Oka, P. A. Ignatiev, S. Wedekind, G. Rodary, L. Niebergall, V. S. Stepanyuk, D. Sander, and J. Kirschner, Science 327, 843 (2010). [3] H. Oka, K. Tao, S. Wedekind, G. Rodary, V. Stepanyuk, D. Sander, J. Kirschner, Phys. Rev. Lett. 107, 187201 (2011).

We have investigated the temperature dependence of the magnetic switching of single Co/Pt nanodots. The dots were fabricated out of thin Co/Pt multilayers via Ar⁺ ion milling at 150eV utilizing SiO₂ particles as a shadow mask. Diblockcopolymeres that contain a SiO₂ core are deposited on Co/Pt via spin coating. In an oxygen plasma the organic shell is removed while the SiO₂ particles remain on the surface. The organic shell that can be varied on purpose determines the mean distance between the particles, here about 100nm [1]. The Co/Pt multilayer is tuned to give perpendicular anisotropy which is preserved in nanodots fabricated by our method. To obtain single dot sensitivity Hall-crosses with wire width below 100nm are created via electron beam lithography (EBL) on which the nanodots are fabricated. The magnetic behavior of Co/Pt nanodots with diameters below 30nm are presented.
We studied the temperature dependence of the coercive field and the time dependent switching behavior in the range of 80K to 300K. The magnetic volume V of the nanodots is determined directly from images taken by scanning electron microscopy. We used Sharrocks formula to determine the anisotropy constant K and the saturation magnetization M_S from the coercivities [2]. Additionally, the anisotropy constant is determined from the temperature dependent switching behavior [3]. Both fittings give results that are identical within the error margin and slightly smaller than the film value. The time constant Ï"₀ is considerably smaller than expected for a macro-spin behavior.

[1] H. Stillrich et al., Adv. Funct. Mat. 18, p76-81, (2008).
[2] M. P. Sharrock, IEEE Trans. Magn. 26, p193-197, (1990)
[3] Bean and Livingston, J. Appl. Phys. 30, 120S, (1959)

NiCo alloys display soft or hard magnetic behavior, respectively, depending on the Ni:Co ratio and thus are promising candidates for technological applications substituting the widely investigated Permalloy or e.g. transition metal oxides. We performed a systematic study of the switching fields in single NiCo nanowires and in NiCo nanowire ensembles. Ni_xCo_1-x nanowires with x varying between 0.05 and 0.77 have been synthesized by potentiostatic electro-deposition into self ordered Al₂O₃ Membranes (AAO) by hard anodization. The variation of the Ni:Co composition in the alloyed nanowires of appr. 20µm length, 150nm diameter and 305 nm spacing allows for a tuning of the switching field along the wire axes between 190 Oe and 400 Oe. The coercive fields were determined by two types of measurements along the wire axes. The hysteresis curves of the nanowire ensembles in the AAO templates were recorded using a vibrating sample magnetometer, VSM (Quantum Design VersaLab). Hysteresis measurements on individual nanowires were taken measuring the magnetooptical Kerr (MOKE) effect utilizing a NanoMOKE2^TM (Durham Magneto Optics Ltd). In order to compare the results of both measurement types we performed First Order Reversal Curve (FORC) measurements [e.g. see C. R. Pike et al., Phys. Rev. B 71, 134407 (2005)] on the nanowire ensembles in the VSM. The FORC analysis yields the distribution of the coercive fields for all nanowires in the membrane and the distribution of their interaction fields. Additionally, parts of the AAOs were used to dissolve the nanowires from the templates and cast it on a substrate. Then MOKE analyses were performed on up to 100 singular wires in order to have adequate statistics for the distribution of coercivities ranging from 150 Oe to 450 Oe. The obtained coercivity distributions with widths of appr. 18 % from the FORC and MOKE measurements are discussed and compared revealing the impact of different interaction fields in the investigated templates.

The magneto-static interaction of submicron Ni80Fe20 rectangles with an aspect ratio of two has been investigated by means of magnetotransport measurements using anisotropic magneto resistance (AMR). The structures were carved into a Cr(10nm)/Ni80Fe20(20nm)/Pt(2,5nm) trilayer utilizing a highly focused ion beam (FIB). The material around the rectangles has been rendered paramagnetic via ion milling by applying a Ga+-ion dose of 6000 µC/cm^2. The ion etching causes an intermixing of Ni80Fe20 and Cr that destroys verifiably ferromagnetic order. The remaining metallic layer around the ferromagnetic structure guarantees a good electric conductivity which is necessary for electrical connection. Microcircuits are milled by FIB that allow to contact electrically an individual element via a micromanipulator [1]. For this work structure sizes down to 300 nm x 600 nm have been investigated. This one-step structuring approach in combination with a magneto-resistive measuring setup with a sensitivity of Î"R/R=10^-6 was successfully utilized to measure the magnetic energy of micro-magnetic states of single, isolated rectangle obtaining flux closure structures, like Landau state [2]. Recently, it has been shown that rectangles with flux closure structure can magneto statically interact [3]. To quantify the interaction we have used the same method. We have systematically varied the distance between rectangles in an array and measured the energy change of a single element. The strength of the magneto-static interaction between rectangles revealing flux closure domain pattern has been quantified and will be discussed in the talk. [1] Daniel Stickler et al., Rev. Sci. Instr. 79, 103901 (2008) [2] André Kobs et al., Phy. Rev. B 80, 134415 (2009) [3] Sebastian Hankemeier et al., PRL 103, 147204 (2009)

Multi-segmented, magnetic nanowires with a diameter of 40 nm are synthesized by pulsed electrodeposition of alternating segments of nickel and copper into porous alumina membranes from a single electrolyte. During this procedure, the lengths of the nickel and copper phases can be tuned individually. Following selective etching of the supporting alumina matrix and the copper phase, only the short Ni nanorods remain, which are finally resuspended into a carrier liquid, resulting in a novel ferrofluid. The magnetic behavior of the short nanorods (aspect ratio 2â?"10) is characterized at various temperatures in the solid state as well as in suspension. Furthermore, it is proven that subjecting the nanorod suspension to an external magnetic field orients the long axes of the elongated particles along the direction of the applied field. Such alignment increases the liquidâ?Ts viscosityâ?"its so-called magnetoviscosityâ?"as a function of the magnetic field. Moreover, field-dependent viscosity measurements performed on hybrid ferrofluids (mixtures of the Ni nanorod suspension with a commercial ferrofluid consisting of superparamagnetic particles) reveal a dramatic enhancement of the magnetoviscous effect by two orders of magnitude up to a gel-like behavior. Evidently, the stacking of Ni nanorods between segments of a sacrificial phase enables the obstacle of limited template area to be overcome for this template-based preparation route. Consequently, the large-batch synthesis of identical, elongated magnetic nanorods becomes feasible, which might pave the way toward the realization of new magnetic fluids having novel properties. The authors are grateful to the DAAD RISE program and the State of Hamburgâ?"through the Excellence Cluster â?oNanotechnolgy for Medicineâ?â?"for financial support.

Multi-segmented, magnetic nanowires with a diameter of 40 nm are synthesized by pulsed electrodeposition of alternating segments of nickel and copper into porous alumina membranes from a single electrolyte. During this procedure, the lengths of the nickel and copper phases can be tuned individually. Following selective etching of the supporting alumina matrix and the copper phase, only the short Ni nanorods remain, which are finally resuspended into a carrier liquid, resulting in a novel ferrofluid. The magnetic behavior of the short nanorods (aspect ratio 2â?"10) is characterized at various temperatures in the solid state as well as in suspension. Furthermore, it is proven that subjecting the nanorod suspension to an external magnetic field orients the long axes of the elongated particles along the direction of the applied field. Such alignment increases the liquidâ?Ts viscosityâ?"its so-called magnetoviscosityâ?"as a function of the magnetic field. Moreover, field-dependent viscosity measurements performed on hybrid ferrofluids (mixtures of the Ni nanorod suspension with a commercial ferrofluid consisting of superparamagnetic particles) reveal a dramatic enhancement of the magnetoviscous effect by two orders of magnitude up to a gel-like behavior. Evidently, the stacking of Ni nanorods between segments of a sacrificial phase enables the obstacle of limited template area to be overcome for this template-based preparation route. Consequently, the large-batch synthesis of identical, elongated magnetic nanorods becomes feasible, which might pave the way toward the realization of new magnetic fluids having novel properties. The authors are grateful to the DAAD RISE program and the State of Hamburgâ?"through the Excellence Cluster â?oNanotechnolgy for Medicineâ?â?"for financial support.

At the interface between classical and quantum mechanical description, single molecular magnets (SMM) offer novel approaches in information storage, spintronics, and quantum computation. The high intrinsic spin of S=10 and the long spin relaxation time makes Manganese-12-acetate (Mn12) an archetypal SMM. While these characteristics have been measured on bulk samples, for nanoscale applications it is important to establish that the magnetic properties replicate themselves in isolated molecules, ideally immobilized on surfaces, where the can be addressed individually. The fragile nature and thermal instability of the Mn12 molecule, however, hinders in-vacuo or other highly controlled processing such that the individual molecule its magnetic properties could not be addressed. Electrospray ion beam deposition (ES-IBD) offers an alternative gentle approach to a fully controlled vacuum deposition of even the most fragile molecules.[] Here we demonstrate that ES-IBD facilitates grafting of intact Mn12 on metal as well as ultra-thin insulating surfaces enabling sub-molecular resolution imaging by scanning tunneling microscopy (STM). Using high resolution scanning tunneling spectroscopy at a temperature of 1K, we detect spin excitations of the magnetic ground state of the molecule when immobilized at an ultra-thin boron nitride decoupling layer. We find that the high spin system, that is Mn12 shows behavior typical for both, a quantum and a classical system. Our results are supported by density functional theory based calculations and establish that individual Mn12 molecules retain their intrinsic spin on a well chosen solid support.

Organic-based materials represent an alternative route to explore novel magnetic phenomena and have revealed interesting properties and ferromagnetism at room temperature. Famous examples are the charge transfer salts of metal-7,7,8,8-tetracyanoquinodimethane (M-TCNQ) assembled in bulk coordination networks where the magnetic properties depend strongly on the nature of the transition metal ion and the stoichiometry of the components. In this study, we report on the self-assembly and magnetic properties of two-dimensional Mn- and Ni-TCNQ charge-transfer networks on noble metal surfaces investigated by means of scanning tunneling microscopy and x-ray magnetic circular dichroism. Single Ni impurities were found to be in a spin-quenched mixed valence state on the surface. Upon coordination to TCNQ molecules the Ni atoms recover their magnetic moment and show ferromagnetic behavior. The strength of the ferromagnetic coupling and the valence state of the Ni centers depends strongly on the underlying substrate. Ni atoms embedded in the organic matrix remain in a mixed-valence state while the electronic structure exhibits features of a more localized atomic system.

Fulvalene diruthenium is a molecule which consists of two 5 member carbon rings called cyclopentadienes connected with a carbon-carbon bond. Each ring accommodates one Ru atom located at the axis of the ring. At the ground state, Ru atoms are lined up parallel to each other pointing in the same direction (parallel conformation). When the molecule absorbs light, one of the rings rotate around the common bond so that Ru atoms point in opposite directions (anti-parallel conformation). This molecule has been studied as a candidate to capture and store solar energy ["Catching the Sun's Heat," MIT News, October 2010, accessed at: web.mit.edu/newsoffice/2010/solar-storage-1026.html]. In this research, Ru is replaced by a magnetic atom to create magnetic coupling between the two halves of the fulvalene molecule. Magnetic interaction can be utilized to increase the energy storage capability of the molecule, as well as a mechanism to control the molecular transformation. Cobalt is chosen for ferromagnetic coupling and chromium for anti-ferromagnetic coupling. Lithium (Li) and strontium (Sr) are also included in the study as non-magnetic atoms with one and two electrons in the outer shell, respectively. Properties of the fulvalene molecule with the above elements were calculated using the DFT method in conjunction with the BLYP functional and Pople type basis sets augmented with polarization functions. Li atom of fulvalene dilithium use the only electron in the outer shell to bond with the carbon ring. There is no magnetic interaction and no bonding between the two Li atoms located at each half of the molecule. Calculated energy difference between the anti-parallel and parallel conformations is nearly zero. Sr atom of fulvalene distrontium can share one of its valance electrons with the carbon ring and the other one with the other Sr. This case represents zero magnetic interaction but possible bonding between the two Sr atoms. Calculated energy difference between the anti-parallel and parallel conformations is 0.8 eV. Co atoms of fulvalene dicobalt can magnetically interact with each other and also bond with each other. A scan of various spin values from 1 to 4 indicate that spin=2 is energetically the most favorable for this molecule. Calculated energy difference between the anti-parallel and parallel conformations is 1.3 eV at that spin. Cr atoms of fulvalene dichromium have the largest number of electrons which can magnetically interact. Calculated energy difference between the anti-parallel and parallel conformations is 2.4 eV at spin=0. Fulvalene dichromium turns out to be the most favorable option among those studied in terms of energy storage capability. Also anti-ferromagnetic coupling of Cr is more desirable than ferromagnetic coupling of Co in order to minimize magnetic inter-molecular interactions in an ensemble of such molecules.

The theoretical understanding of exchange interactions in organics provides a key foundation for quantum molecular magnetism, molecular spintronics, and even quantum information processing in molecules. Recent SQUID magnetometry of a well know organic semiconductor, copper-phthalocyanine [1,2] (CuPc) shows that it could form quasi-one-dimensional ½-spin chains. Greenâ?Ts function perturbation theory (GFPT) calculation [3] is used to find the mechanism of exchange interactions. Hybrid density functional theory simulations for one-dimensional molecular chain [4] are performed to have a quantitative insight to exchange interactions and electronic structures of metal-phthalocyanines. Both kinds of calculations are performed for a set of geometries with different stacking and sliding angles for lithium-, cobalt-, chromium-, and copper-Pc. The calculated exchange interactions depend strongly on stacking angles, but weakly on sliding angles. In order to extract the exchange interaction from the experiments, a global fit has been done for the magnetization data by using the Heisenberg model of a finite spin-1/2 chain. Our theoretical results qualitatively agree with the fitted exchange parameters based on the related experiments. Remarkably a quantitative agreement is obtained for α-CuPc, and α-cobalt-Pc has a very large exchange above liquid-Nitrogen temperature. Our theoretical predictions on the exchange interactions can guide experimentalists to design novel organic semiconductors for the realization of molecular spintronics and organic-based quantum information processing. [1] S. Heutz, et. al., Adv. Mat., 19, 3618 (2007) [2] Hai Wang, et. al., ACS Nano, 4, 3921 (2010) [3] Wei Wu, et. al., Phys. Rev. B 77, 184403 (2008) [4] Wei Wu, et. al., Phys. Rev. B 84, 024427 (2011)

Non-equilibrium processing of materials offers the opportunity to create and control materials with novel structures and properties. Specifically, inert-gas condensation (IGC) produces nanoscale materials, and the cluster formation has been found to be critically dependent on processing conditions, notably the sputtering power when dc magnetron sputtering is used to create the gas phase, and the structure formation is also cluster-size dependent. In this study, IGC has been utilized to synthesize non-equilibrium Co-W clusters. Clusters were produced by varying IGC processing parameters, in particular the condensation temperature and the power applied to the dc magnetron sputtering target. Transmission electron microscopy (TEM) revealed clusters consistently in the 10 nm size range for all deposition conditions, as well as a consistent composition for a given target composition. Both selected-area electron diffraction (SAD) patterns and x-ray diffraction (XRD) showed that with liquid-nitrogen cooling, the clusters were primarily amorphous regardless of sputtering power. However, with water cooling the structure was dependent on sputtering power. The SAD patterns with low sputtering power showed diffuse rings together with a few sharp diffraction spots, indicating that the samples were predominantly amorphous but with a low degree of crystallinity. With increasing power the degree of crystallinity increased, as evidenced by both electron and x-ray diffraction. While the SAD pattern revealed an HCP Co structure with lattice parameters a~0.262 and c~0.408 nm, the x-ray diffraction results suggested the presence of both Co and Co₃W. High-resolution TEM indicated that the clusters are single-crystalline, and high-angle annular dark field STEM images show internal compositional variations in larger clusters (>15 nm) suggesting the co-existence of both HCP Co and Co₃W. The magnetic behavior was observed to be dependent on the degree of crystallinity and the temperature. With increasing sputtering power (thus increasing degree of crystallinity), the coercivity increased in the water-cooled clusters, from 30 Oe to 120 Oe at 300K and up to 800 Oe at 10K. For the liquid nitrogen-cooled clusters, the coercivity of the clusters showed was independent of sputtering power. Research supported by the Department of Energy-Energy Efficiency and Renewable Energy, Vehicles Technology Office, PEEM program, under Contract No. DE-AC02-07CH11358 for the operation of Ames Laboratory (USDOE) and sub-contract no. SC-10-343 to the University of Nebraska-Lincoln.

Fe-Au nanoclusters have gained considerable interest as a potential candidate for biomedical applications. In the present work, inert gas condensation (IGC) has been used to produce Au-Fe nanoclusters of varied compositions with a mean size between 5-10 nm. New structural phases have been obtained for both as-deposited and annealed (equilibrium cooling) clusters that are inconsistent with the equilibrium phase diagram. The as-deposited structures were either fcc or bcc, depending on composition, with lattice parameters expanded relative to elemental and â?orule of mixturesâ? values, suggesting the presence of self-interstitials. The as-deposited clusters are single-crystalline but crystallinity is poor with significant disorder. The as-deposited clusters were ferromagnetic at room temperature. Heat treatment at 6000C for 15 minutes followed by furnace cooling resulted in the transformation of the clusters into additional, non-equilibrium structures that depended on cluster composition. At Fe-rich compositions, clusters transformed to a well-ordered, single fcc phase with a lattice parameter of 0.364 nm, whereas the phase diagram predicted two-phase equilibrium. The stabilization of a single fcc phase was explained by a thermodynamic analysis. This analysis suggests that the single-phase stability in the Fe-Au nanoparticles arises from the fact that the introduction of a phase boundary is energetically opposed. Near the 1:1 stoichiometry, the as-deposited fcc structure transformed to a an L10 structure, the first observation of this structure naturally forming in the Fe-Au system. Finally, at a ~1:3 Fe:Au ratio ordering in the Cu3Au-type structure may exist.

Probing the atomic composition, electronic structure, and magnetism in the buried layers and interfaces of multilayer magnetic nanostructures is a challenging yet crucial goal in spintronics research. Our group has addressed this challenge with two newly-developed aspects of photoemission: the use of hard x-rays in the multi-keV range for excitation to permit probing more deeply below the surface and of soft and hard x-ray standing waves to permit selectively looking at a given depth below the surface [1-7]. The depth resolution is achieved by setting-up an x-ray standing wave (SW) field in the sample by growing the sample as, or on, a synthetic periodic multilayer mirror which in first-order Bragg reflection acts as a nm-scale SW-generator. The SW is then moved vertically through the sample, thus highlighting different depths, by varying the photon energy or the incidence angle through the Bragg condition, or by growing one layer of the sample as a wedge and scanning the x-ray spot along the wedge. These methods have permitted determining concentration and magnetization profiles through structures relevant to giant magnetoresistive (GMR) - Cr/Fe-[1] and tunnel magnetoresistance (TMR) - MgO/Fe-[1,2,4], as well as individual layer densities of states near the Fermi level in a tunnel junction - Al2O3/CoFeB-[1,2]. The SW method has also been coupled with photoelectron emission microscopy (PEEM) to provide 3D images of multilayer structures [3]. For a LaNiO3/SrTiO3 multilayer, changes in electronic structure with LNO thickness and near the LNO/STO interface have been determined [5,6]. Combined soft and hard x-ray SW data have been used to study a SrTiO3/La0.67Sr0.33MnO3 superlattice, revealing detailed interface concentration profiles, as well as a crystal-field induced Mn core-level shift near the STO/LSMO interface [7]. Also for STO/LSMO, standing-wave angle-resolved photoemission (SWARPES)has been carried, so as to determine layer- and interface- specific band structures [7]. Work supported by the Department of Energy under Contract No. DE-AC02-05CH11231 and by Army Research Office MURI Grant W911-NF-09-1-0398. References: 1) S.-H. Yang, B.C. Sell, and C. S. Fadley, review in J. Appl. Phys. 103, 07C519 (2008). 2) C.S. Fadley, review in Journal of Electron Spectroscopy and Related Phenomena 178â?"179, 2 (2010). 3) C. Papp et al., Appl. Phys. Lett. 97, 062503 (2010). 4) S. Döring et al., Phys. Rev. B 83, 165444 (2011); S.H. Yang et al., Phys. Rev. B, to appear. 5) A. X. Gray et al., Phys. Rev. B 84, 075104 (2011). 6) A. M. Kaiser et al., Phys. Rev. Letters 107, 116402 (2011). 7) A.X. Gray et al., Phys. Rev. B 82, 205116 (2010), and to be published.

A new imaging strategy using atomic force microscopy (AFM) for detecting magnetic nanomaterials with much higher spatial resolution and sensitivity than the traditional magnetic force microscopy (MFM) technique is developed.1,2 This AFM-based imaging mode is referred to as magnetic sample modulation (MSM), since the flux of an AC-generated electromagnetic field is used to induce physical movement of magnetic nanomaterials on surfaces during imaging. The AFM is operated in contact mode using a soft, nonmagnetic tip to detect the physical motion of the sample. By slowly scanning an AFM probe across a vibrating area of the sample, the frequency and amplitude of vibration induced by the magnetic field is tracked by changes in tip deflection. Thus, the AFM tip serves as a force and motion sensor for mapping the vibrational response of magnetic nanomaterials. Essentially, MSM is a hybrid of contact mode AFM combined with selective modulation of magnetic domains. The positional feedback loop for MSM imaging is the same as that used for force modulation and contact mode AFM; however, the vibration of the sample is analyzed using channels of a lock-in amplifier. The investigations are facilitated by nanofabrication methods combining particle lithography with organic vapor deposition and electroless deposition of iron oxide, to prepare designed test platforms of magnetic materials at nanometer length scales. Examples of detecting magnetic nanoparticles and magnetic biospecies at single molecular level will be presented. 3,4 Reference: 1. Li et al. Detecting the Magnetic Response of Iron Oxide Capped Organosilane Nanostructures Using Magnetic Sample Modulation and Atomic Force. Analytical Chemistry, 2009, 81, 4792-4802. 2. Englade et al. Dynamic Magnetic Characterizations at the Nanoscale: A New Mode for AFM Imaging with Magnetic Sample Modulation (MSM/AFM), Invited chapter for Scanning Probe Microscopy in Nanoscience and Nanotechnology, Vol. III, Edited by B. Bhushan, Springer-Verlag Heidelberg, 2011. 3. Daniels et al. Investigation of the magnetic properties of ferritin by AFM imaging with magnetic sample modulation. Analytical and Bioanalytical Chemistry, 2009, 394, 215-223. 4. Kelley et al. Synthesis and AFM characterization of FeNi3 nanoparticles: Comparison of conventional oven heating versus microwave synthesis, Chemistry of Materials, 2011, submitted.

Magnons and phonons are the elementary excitations, which describe the spin- and lattice-dynamics in solids. Of particular interest are the excitations in nanostructures or at the surfaces, since they reflect the dimensionality aspects and the exotic properties of this class of materials. At a magnetic surface both excitations coincide and possess similar energies. Hence, it has been an experimental challenge to probe and distinguish between these two kinds of excitations. We report on the first simultaneous detection and unambiguous identification of the elementary surface excitations. The experiments are performed by spin-polarized electron energy loss spectroscopy on a prototype magnetic surface with various magnon and phonon excitations: an oxygen passivated Fe(001)â?"p(1Ã-1) surface. Although both magnons and phonons are bosons, their spin nature is different. In our experiments, we use the spin degree of freedom of the incident electrons together with their extreme surface sensitivity to simultaneously probe and distinguish between different types of excitations at the surface. The dispersion relation, which connects the energy of the excitations to their propagating wave-vector, is probed over the whole surface Brillouin zone. A description of different phonon modes observed in our experiments is provided by means of ab initio calculations.

The L1₀-ordered alloy has attracted much attention in recent years for the application to perpendicular magnetic recording media because of high uniaxial magnetic anisotropy leading to the excellent thermal stability of magnetization even in a nanometer scale[1]. It is also regarded as a promising candidate of the materials for future spintronic devices; perpendicular magnetization is advantageous for large-scale integration of nanometer-sized magnets, because of an arbitrary aspect ratio and a low switching current for spin-torque-induced magnetization reversal in addition to the thermal stability of magnetization. However, there have been only a few studies on spintronic properties of the L1₀-ordered alloy. We have been working on the L1₀-ordered alloy as a spintronic material for several years, particularly focusing on L1₀-FePt that shows the largest anisotropy constant (K_u=7Ã-10⁷ erg/cm³) of all L1₀-ordered alloys. In the early stage, we fabricated current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) devices with perpendicularly magnetized L1₀-FePt layers, and spin-torque-induced magnetization reversal was successfully observed in the L1₀-FePt layer[2]. Perpendicularly magnetized L1₀-FePt was also used as a perpendicular spin injector/detector in a lateral device structure: a giant spin-Hall effect was observed in a multi-terminal device with a L1₀-FePt and a Au (or impurity-doped Au) Hall cross[3, 4]. Recently, furthermore, voltage-induced coercivity control has been demonstrated for a perpendicularly magnetized L1₀-FePt layer in a Hall device[5]. In the presentation, I will make a review on our research activities on the L1₀-ordered alloy as a material for spintronics. [1] For review, T. Shima and K. Takanashi, â?oHandbook of Magnetism and Advanced Magnetic Materialsâ? 4, 2306 (John Wiley&Sons Ltd, 2007). [2] T. Seki et al., Appl. Phys. Lett. 88, 172504 (2006); Phys. Rev. B 77, 214414 (2008). [3] T. Seki et al., Nature Mater. 7, 125 (2008). [4] B. Gu et al., Phys. Rev. Lett., 105, 216401 (2010). [5] T. Seki et al., Appl. Phys. Lett., 98, 212505 (2011).

Molecular spintronics is an emerging research field at the frontier between organic chemistry and the spintronics concept of adding non-volatility and spin degree of freedom to electronics. Compared to traditional inorganic materials molecules are flexible and can be easily tailored by chemical synthesis. However, due to their theoretically expected very long spin lifetime opportunity, they were first only seen as the ultimate media for spintronics devices[1] and it was only very recently that new spintronics tailoring opportunities, unachievable or unthinkable of with inorganic materials, and that could arise from the chemical versatility brought by molecules and molecular engineering were unveiled.[2] It has been shown that the molecular structure, the local geometry at the molecule-electrode interface and more importantly the ferromagnetic metal/molecule hybridization can strongly influence interfacial spin properties going from spin polarization enhancement to its sign control in spintronics devices.[2] Self-Assembled Monolayers (SAMs) are modular (can be easily engineered), nanometre thick (ideal as tunnel barriers) and auto-assemble into surfaces (the structure of the film is readily accessible). These properties make them perfect toy barriers to further test molecule interfacial spin tailoring properties in molecular magnetic tunnel junctions (MTJs). However up to now they have been scarcely studied as spintronic barriers.[3; 4] This fact is probably due to the difficulty in adapting SAMâ?Ts wet chemistry to air sensitive traditional ferromagnetic electrodes and the technological challenge that contacting nanometre thin film represents. Due to its very high spin polarization and air stability compared to easily oxidable ferromagnetic metals such as Co or Fe La_2/3Sr_1/3MnO₃ (LSMO), a half-metallic manganite with perovskite-type structure, has positioned itself as the electrode of choice in most of the organic spintronics devices. Despite being perfectly compatible with SAM grafting conditions reports on the necessary protocols for SAM functionalization of LSMO films are missing in the literature. We will present a missing building block for molecular spintronics tailoring: the grafting and film characterization of organic monofunctionalized long alkane chains over LSMO. Moreover, we will present how nanocontacts can be used to ease the integration of SAMs into Co/SAM//LSMO spintronic devices without short-circuiting. Tunnel magneto-resistances of up to 35% and its dependence on the temperature and bias voltage will be also shown. S.T wants to thank NANOCON (FP7/2007-2013-252757) project for its financial support. [1] V. Dediu, L. Hueso, I. Bergenti, et al., Nature Mater. 8 (2009) 707-716. [2] C. Barraud, P. Seneor, R. Mattana, et al., Nature Phys. 6 (2010) 615-620. [3] J. Petta, S. Slater, D. Ralph, Phys. Rev. Lett. 93 (2004) 136601. [4] W. Wang, C. Richter, Appl. Phys. Lett. 89 (2006) 153105.

Accenting semiconductor electronics with spin functionality is a major thrust of current spintronics research. Several approaches are currently pursued, among those, magnetic oxides are particularly interesting as they offer the unique combination of both generating almost fully spin polarized tunnel currents via a true spin filter effect, and facilitating a conductance matched magnetic tunnel contact to a semiconductor. We present an in-depth electronic structure study of a magnetic oxide/semiconductor model system, EuO on Silicon, which is dedicated to efficient spin injection and detection in silicon-based spintronics devices. We used hard x-ray photoelectron spectroscopy (HAXPES) to probe the electronic structure of EuO tunnel barriers and the EuO/Si interface. EuO is predicted to be chemically stable in direct contact with silicon, however, ferromagnetic EuO is very difficult to synthesize due to its instantaneous reactivity towards higher oxides which are not ferromagnetic. We succeeded in preparing high-quality EuO/Si(001) heterostructures with bulk-like magnetic properties. A combined electronic structure analysis of Eu 3d, 4s and 4d core levels and 4f valence bands was performed to quantify a nearly ideal stoichiometry and chemical homogeneity of ferromagnetic EuO thin films directly on silicon. Moreover, we could clearly identify the absence of silicon oxide and silicide formation at the EuO/Si interface. The results provide clear evidence for the successful integration of a magnetic oxide tunnel barrier into silicon, paving the way for the future integration of magnetic oxides into functional spintronics devices.

History of magnetic materials dates back to more than 2500 years when the magnetic phenomenon was observed in Loadstone, a naturally occurring mineral. At that time probably the only useful application of the magnets was in compass needles. Presently the magnetic materials find application in a much wider domain. Their use in traditional areas such as electric motors, generators, loudspeakers, magnetic separators etc. is well known. Lately they have found applications in more advanced fields such as biomedical imaging and drug delivery. However, the one field that has probably been influenced the most is the magnetic data storage technology. This technology has been advancing with almost the same pace as that of the semiconductor industry. A very exciting concept that is being explored currently is marrying the two technologies, that is, the semiconductor technology and the advanced magnetic data storage and processing technology, together. This emerging hybrid technology is known as spintronics. Spintronics requires the development of magnetic materials which can efficiently inject spin polarized carriers into the semiconductors. Despite considerable efforts, efficient injection of spins into nonmagnetic semiconductors continues to be a major hurdle in this field. Conventional ferro-magnets such as Fe and Ni are not efficient for the above application. This has led to an urgent need for developing new kinds of magnetic materials which are either semiconducting or insulating. In this talk, I will present some of our very exciting results about the observation of room temperature ferromagnetism in some unconventional highly insulating oxides. Particular focus will be on nanostructured Rare Earth Oxide based high-k dielectric materials. Observation of an entirely new magnetic phenomenon â?oDynamic superparamagnetismâ? will also be reported.

Mn-doping of group IV semiconductor materials is highly coveted for a seamless combination of spin- and charge driven electronics. The main goal of our research is to develop nanoscale building blocks such as Mn-doped quantum dots, and delta-doped Mn-layers. We aim to develop a fundamental understanding of relation between structure, bonding and magnetic signature. The nanostructures are analyzed with scanning tunneling microscopy (STM), magnetometry and x-ray magnetic circular dichroism (XMCD). We focus on the synthesis, geometric and electronic structure of the following building blocks: (A1) monoatomic Mn-wires on the Si(100)(2x1) surface, (A2) delta-doped Mn layers within a Si-matrix, and (B1) Mn-surface structures on Ge quantum dots (QD), and (B2) codeposition of Mn and Ge QDs. Recent experiments in the literature indicate that Mn-doped Ge QDs can form a dilute magnetic semiconductor with a high TC. The monoatomic Mn-wires self-assemble on the Si(100)(2x1) reconstructed surface, whereas the quality of the chains is controlled by the defect concentration, and their assembly is described with a Monte Carlo simulation. The bonding between Mn-chains and the Si-surface is still discussed controversially. The Mn chains remain intact when they are capped with an amorphous Si or Ge layer, thus forming a delta-doped Mn layer in a Si-matrix. The second set of nanostructures are Mn-doped Ge-quantum dots, which are grown by Stranski-Krastanov MBE. We present here two different pathways for Mn-doping of QDs, including surface doping (B1), and co-deposition (B2). The deposition of Mn on the QDs (B1) leads to roughening of the wetting layer surface, and formation of Mn-islands, whose orientation is controlled by reconstruction on the {105} QD facet. The Mn-islands on the QDs are stable up to 150C and we will discuss the feasibility of combining surface doping with conformal low T growth of a Ge-cap layer. The Mn-island are buried within the Ge-QD surface at elevated temperatures. In the high temperature (450C) co-deposition (B2) the formation of secondary phases sets in for Mn concentrations above 5%, and rod-like Mn-germanide structures are formed. Atomic resolution images have been obtained for these rods, but phase identification is still ambiguous. The competition between germanide formation and QD growth is controlled mainly by the long-range diffusion of Mn. The magnetic measurements with XMCD (X-ray magnetic circular dichroism) strongly indicate that Mn is bonded as Mn 2+ and the majority of Mn-atoms contribute to the magnetic signature. The saturation magnetization, spin and orbital contributions depend on Mn-concentration, Si- or Ge cap in the delta layers, and the quality of the interlayer (wires or clusters). This work is supported by NSF-Chemistry CHE-0828318, DMR-0907234

Multifunctional materials of reduced dimensionality have become an increasingly active area of research in nanoscale solid-state chemistry and physics. Spin-electronics (spintronics), for example, relies on the mutual interactions of electron spins and charges. Nanostructured diluted magnetic semiconductors (DMSs) having high ferromagnetic phase transition temperatures (TC) have been identified as promising materials for this emerging technology. Transparent conducting oxides (TCOs) are particularly interesting as host lattices for high-TC ferromagnetic semiconductors, due to their stability, high electrical conductivity, and transparency to visible light. I will describe the preparation and characterization of free-standing colloidal transition-metal-doped In2O3 NCs with unprecedented range of doping concentrations, and show that dopant impurities have a profound effect on the morphology, structure, and properties of In2O3 NCs. Magneto-optical studies of these materials (magnetic circular dichroism (MCD) and X-ray MCD) will be correlated with the results of the conventional magnetization measurements. Magnetic properties of transition-metal doped TCO NCs will be compared to those of III-V based DMS nanostructures. The opportunities and limitations of using these DMS nanostructures as building blocks for spintronic materials and devices will be discussed.

Understanding ultrafast magnetization dynamics on the nano-scale is a forefront problem in modern magnetism research with direct impact on the quest for faster and smaller storage devices. Probing the magnetization element-specifically and on the nanometer length-scale is a pre-requisite when probing technologically relevant material systems with complex composition. X-ray free electron laser (FEL) sources with their unique properties delivering ultrashort and super intense soft X-ray pulses allow for the first time to address magnetization dynamics on the relevant length scale. We present recent results obtained on multi-domain Co/Pt magnetic multilayer samples with perpendicular magnetic anisotropy. As a probe we use small angle X-ray scattering from the magnetic domains which, via X-ray magnetic circular dichroism at the Co M-edge, allows us to simultaneously obtain information on the magnitude of the local magnetization and the characteristic length scale of the domains. The FEL source FLASH at DESY (Hamburg) was tuned to deliver soft X-ray pulses with a length of about 80 fs and a wavelength of 20.8 nm corresponding to the Co M-edge.

Understanding the physical limits of magnetization dynamics is of central importance for the development of magnetic data storage and spintronics. Of particular interest is the phenomenon of ultrafast demagnetization on the femtosecond regime [1]. It is expected to give insight into the energy and angular momentum transfer processes between the electronic and spin systems. We present results of a novel element-selective pump-probe approach involving table-top pulsed laser systems, which allows us to investigate the demagnetization behavior for the individual constituents in an alloy. This is achieved by probing the magnetic system with fs VUV light pulses from a HHG upconversion process reaching harmonics with photon energies of up to 70 eV. The resonant magnetic reflectivity at the transition metal M-edges in a T-MOKE geometry provides large signals [2], which enable us to study spindynamic processes with true element selectivity and a time resolution of better than 20 fs, i.e. approaching the time scale of the exchange interaction [3]. Experiments on pure Fe and Ni films produce demagnetization times of 100 fs (Fe) and 150 (fs). In Permalloy (Ni₈₀Fe₂₀) films we find an average demagnetization time constant of 240 fs, however, with a distinct and unexpected time delay between the demagnetization of Fe and Ni. Initially the Ni response is lagging behind the Fe response for about 20 fs, until both demagnetize with the same time constant. This time lag is found to increase to about 80 fs when alloying Cu into the Permalloy, thereby weakening the exchange interaction between Fe and Ni and reducing the Curie temperature. This findings demonstrate that although in the static case Fe and Ni are strongly exchange coupled, this coupling may be overcome by strong optical excitations on very short time scales. â?"â?"â?" [1] E. Beaurepaire, J. C. Merle, A. Daunois and J. Y. Bigot, Phys. Rev. Lett. 76, 4250 (1996). [2] P. Grychtol, R. Adam, S. Valencia, S. Cramm, D. E. Buergler and C. M. Schneider, Phys. Rev. B 82, 054433 (2010). [3] C. La-O-Vorakiat, M. Siemens, M. M. Murnane, H. C. Kapteyn, S. Mathias, M. Aeschlimann, P. Grychtol, R. Adam, C. M. Schneider, J. M. Shaw, H. Nembach and T. J. Silva, Phys. Rev. Lett. 103, 257402 (2009).

Employing femtosecond laser pulses, we have been able to demonstrate that an ultrafast photoexcitation is sufficient to induce deterministic magnetization reversal in GdFeCo nanostructures. This important finding is promising for technology application opening new opportunities for ultimate data storage devices. Using lithography and lift-off techniques we have fabricated GdFeCo nanostructures with sizes down to 100 nm [1]. To reveal the magnetic properties of the nanostructures, we use X-ray magnetic circular dichroism (XMCD) with the Photoemission Electron Microscope (PEEM) at the SIM beamline. We discovered that the magnetization direction of the material changes every time that a linearly polarized fs-laser pulse is applied. Our experimental results are supported by atomistic simulations and these results are the proof of an unambiguous demonstration that an ultrafast heat stimulus is sufficient to trigger deterministic magnetization switching in a ferrimagnet [2]. This mechanism of reversal is based on the discovery that in ferrimagnetic GdFeCo the Gd and the Fe sublattices demagnetize at different time scales leading to a ferromagnetic transient state [3]. We will report our results obtained in both static and timeresolved PEEM modes demonstrating this surprising new mechanism in nanomagnets. [1] Nanostructuring of GdFeCo thin films for laser induced magnetization switching applications, L. Le Guyader, S. El Moussaoui, E. Mengotti, L.J. Heyderman, F. Nolting, A. Tsukamoto, A. Itoh, A. Kirilyuk, Th. Rasing, A.V. Kimel, Journal of the Magnetics Society of Japan, accepted 2011. [2] Ultrafast Heating as a Sufficient Stimulus for Magnetization Reversal in a Ferrimagnet, T. A. Ostler, J. Barker, R. F. L. Evans, R. Chantrell, U. Atxitia, O. Chubykalo-Fesenko, S. El Moussaoui, L. Le Guyader, E. Mengotti, L. J. Heyderman, F. Nolting, A. Tsukamoto, A. Itoh, D. Afanasiev, B. A. Ivanov, A. M. Kalashnikova, K. Vahaplar, J. Mentink, A. Kirilyuk,Th. Rasing, and A. V. Kimel, manuscript submitted. [3] Transient ferromagnetic-like state mediating ultrafast reversal of antiferromagnetically coupled spins, I. Radu, K. Vahaplar, C. Stamm, T. Kachel, N. Pontius, H. A. Dürr, T. A. Ostler, J. Barker, R. F. L. Evans, R. W. Chantrell, A. Tsukamoto, A. Itoh, A. Kirilyuk, Th. Rasing&A. V. Kimel, Nature 472, 205â?"208 (2011)

Bit patterned media (BPM) has been studied as a candidate for high-density magnetic recording media. In BPM, a magnetic recording layer is cut into a size of one-bit in order to suppress the thermal fluctuation effect. One of key challenges is a fabrication process. The feature size, that is the bit size, will be less than 10nm in near future. We have developed a self-assembled diblock copolymer material for using an etching mask formation. This pattern is also controlled to form artificial patterns suitable for HDD operation by the directed self assembling (DSA) technique, in which resist grooves are used as alignment guides for the self assembled polymer dots[1]. A Ni mold with data tracks and servo patterns were fabricated using the electron beam lithography. Nano-imprint lithography using the Ni mold was used to form groove guides on a perpendicular magnetic recording medium (glass substrate / CoZrNb (70nm) / Pd (4nm) / Ru (20nm) / Co80Pt20 (7nm) / C (10nm)). A diblock copolymer [polystyrene (PS) - polydimethylsiloxane (PDMS)] was cast into the imprinted resist patterns and annealed to form 17nm-pitch (equivalent to 2.5 Tb/in²) self assembled dot pattern. Molecular weight of the polymer was 14.6 k and the volume fraction was 0.785(PS): 0.215(PDMS). After removing the PS matrix and the guide patterns by an oxygen RIE, the recording layer was etched through the PDMS mask by Ar ion milling. Carbon layer was then deposited on it for protecting and filling the pattern. Another key challenge is a reduction of the switching field distribution (SFD) of magnetic dots of the BPM. Possible origins of the SFD are the size distribution of the dot pattern, etching damage and microscopic distribution of magnetic properties. Magnetic dipole interaction between magnetic dots also causes the SFD. When the dot size is in the range of 10nm, the thermal fluctuation affects the SFD estimation. The magnetic switching properties under the recording condition (1-ns timescale range) may differ from the measurement timescale. Magnetic properties were estimated by a polar Kerr measurement under a static and pulse (150-200 μs) magnetic field. SFD was estimated by the delta-Hc method[2]. It was found that SFD was reduced at shorter timescale[3]. This result indicated that the thermal fluctuation increases Coercivity dispersion at long timescale. The thermal fluctuation may cause little change in the dipole interaction contribution of the SFD, since the magnetic moment is less sensitive to the time at shorter timescale compare to the magnetic anisotropy. A part of this work was funded by the New Energy and Industrial Technology Development Organization (NEDO) under the "Development of nanobit technology for ultra-high density magnetic recording (Green IT)" project. REFERENCES [1] K.Naito et al,: IEEE Trans. Magn. 38, 1949 (2002) [2] I. Tagawa et al.: IEEE Trans. Magn., 27, p. 4975 (1991) [3] A. Kikitsu et al., abstract of 55th MMM conference, CF-01 (2010)

Perpendicular magnetic recording media in excess of 2Tbit/in² faces thermal stability and/or writeability challenges that are currently driving intense research in alternative technologies. Bit patterned media (BPM), in which magnetic bits are lithographically patterned as individual islands, stands as a promising technology for thermally stable, writable media. One of the main challenges of BPM lies in its lithographic specifications which push dimensions beyond those established for the conventional semiconductor industry roadmap. A potential solution to the lithographic challenge can be found in directed self assembly of block copolymer films which has recently evolved as a viable lithographic technique to achieve large-area, high-density patterns in time for BPM technology. The challenges faced in the fabrication of templates for BPM using block copolymer self assembly have spurred a set of important innovations in nanofabrication techniques such as the formation of geometries that deviate from the typical shapes naturally formed by block copolymers, as well as advances in high-fidelity pattern transfer and progress in scalability towards higher feature densities. In this presentation, we discuss some of these innovations. We demonstrate a process that combines directed self assembly with nanoimprint lithography to achieve rectangular patterns mounted on circular tracks. We will also discuss recent advances in our understanding of the internal structure of block copolymer films and its consequences during pattern transfer. We also explore viable materials for densities above 2Tdot/in². Lastly, we will also discuss the impact of the nanofabrication process on the magnetic performance of the magnetic islands and the outlook for BPM technology.

In nanomasking, self-assembled nanoparticle monolayers are used as etch masks to transfer the nanopatterns into underlying thin films.¹ Here we apply this technique to prepare two types of magnetic nanodot arrays, and characterize their structural and magnetic properties. Large area nanoparticle arrays were fabricated by a new Langmuir film method, ² and then transferred onto a solid substrate. To make nanodots directly, the nanoparticle monolayer array was cured by electron irradiation, most of the surfactant was removed by oxygen plasma, and then the pattern was transferred into the film using methanol-based reactive ion etching (RIE). The RIE also removed the original particles. Five nanometer L10 FePt thin films were patterned by this process. By changing the size of nanoparticles, nanodot arrays with different feature size were obtained. Magnetic hysteresis loops were obtained for the unpatterned film, arrays of 26 nm dots with a 30 nm pitch, and arrays of 12 nm dots with a 14 nm pitch. Compared to the continuous film, a slight decrease in the coercivity is observed for the patterned film with 30 nm pitch, and there is a considerable decrease for the 14 nm pitch. A second strategy was used to prepare nanodot arrays by filling pit arrays with a magnetic material. To make pit arrays, two nanometers of aluminum is deposited on an array of bare nanoparticle cores. Particle removal leaves an alumina antidot mask on a silicon nitride film. CF4 based RIE was then used to generate an array of pits in the silicon nitride. The pits were overcoated with a deposit of permalloy or cobalt. The samples were then back-etched using the methanol-based RIE to transform them from continuous thin films into nanodot arrays. The evolution of the magnetic response was monitoered by SQUID and MOKE magnetometry. Changes in the structure were observed using SEM. [1]. C. R. Hogg, S. A. Majetich, and J. A. Bain, IEEE Trans. Magn. 46, 2307-2310 (2010) [2]. T. Wen and S. A. Majetich, Ultra-large-area self-assembled monolayers of nanoparticles, ACS Nano, Article ASAP (2011), DOI: 10.1021/nn2037048

Ferromagnetic fct-FePt alloys with large coercivity have shown great potentials for various magnetic applications [1-3]. As well known, exchange coupling between hard and soft magnets could result in a reasonable coercivity and a high magnetic moment, which is favorable for magnetic energy production. Previously our group produced L10FePt-Fe3Pt nanocomposites by annealing the binary composite assemblies of fcc-FePt and Fe3O4 nanoparticles in forming gas [4]. The L10FePt-Fe3Pt nanocomposites contain two distinct phase, namely hard magnetic fct-FePt phase and soft magnetic Fe3Pt phase which could be exchange coupling in the nanocomposites. It needs two steps to obtain L10FePt-Fe3Pt nanocomposites, which means that we should synthesize fcc-FePt and Fe3O4 nanoparticles, separately. Then these nanoparticles were mixed to get uniformly self-assembly to optimize the exchange coupling of hard magnetic fct-FePt phase and soft magnetic Fe3Pt phase in annealed nanocomposites and obtain high magnetic energy production. Recently, we developed a simple method to obtain exchange coupling L10FePt-Fe3Pt nanocomposites. We found that if the synthesis temperature of fcc-FePt particles was above 220 oC, FePt-Fe3O4 dumbbell structure would appear in the production. After annealing, pure FePt particles would transform into L10FePt phase and FePt-Fe3O4 dumbbell structure would become Fe3Pt phase. So we chose 260 oC to synthesize particles and got the mixture of pure FePt and FePt-Fe3O4 dumbbell structure. Then, we can directly anneal the mixture of pure FePt and FePt-Fe3O4 dumbbell structure at 600 oC for 1 hr in forming gas to get exchange coupling magnet. The results measured under highest 15 kOe magnetic field show that after annealing, the samples initially composed of pure FePt and FePt-Fe3O4 dumbbell structure have 5. 21 kOe coercivity which is lower than 12.15 kOe coercivity of annealed pure FePt particles and 73.48 emu/g saturated moment which is higher than 34.26 emu/g saturated moment of annealed pure FePt particles. And the hysteresis loop shows the single phase behavior, which means that nanocomposite is exchange coupled by L10FePt phase and Fe3Pt phase and could be used to fabricate microinductors and nanocomposite permanent magnets . This work was supported by DOE/Ames. [1] Shouheng Sun, C. B. Murray, Dieter Weller, Liesl Folks, Andreas Moser, Science 287, 1989 (2000) [2] Shouheng Sun, Adv. Mater. 18, 393 (2006) [3] Jaemin Kim, Chuanbing Rong, J. Ping Liu, and Shouheng Sun, Adv. Mater. 21, 906 (2009) [4] Hao Zeng, Jing Li, J. P. Liu, Zhong L. Wang, Shouheng Sun, Nature 420, 395 (2002)