December 1-6, 2013 | Boston
Meeting Chairs: Charles Black, Elisabetta Comini, Gitti Frey, Kristi Kiick, Loucas Tsakalakos
Functional composite materials with mechanical elasticity that possess substantial dielectric permittivity (ε), magnetic permeability (mu;), low dielectric and magnetic loss are needed for the fabrication and miniaturization of flexible electronic devices including radio-frequency (RF) antennas, sensors and electronic identification chips. We report on the fabrication of flexible magneto-dielectric composites with low magnetic and dielectric loss, achieved by dispersion of high saturation magnetization, low coercivity air-stable iron (Fe) nanoparticles and iron/silver (Fe/Ag) core-shell nanoparticles in polydimethylsiloxane (PDMS) matrices. The magneto-dielectric composites made of air-stable Fe nanoparticles are adapted for RF communication devices with broad operation frequency, since they possess high mu; (4.4) and low ε (16.4), while maintaining low magnetic (0.28) and dielectric (0.053) loss values. Additionally, the Fe nanoparticle composites allow tensile elongation of 15% before breaking, which demonstrates the flexible nature of the material. The magneto-dielectric composites fabricated using Fe/Ag core-shell nanoparticles can combine high mu; (3.68) and ε (26) with low magnetic (0.28) and dielectric (0.1) loss, which makes them suitable for the RF communication device miniaturization. The Fe/Ag core-shell nanoparticle composites can also deform plastically under tensile elongation as high as 70%.
Optimization of one-pot synthetic procedure for the control of the morphological features of magnetic hybrid materials (HNM) based on spinel-metal-oxide nanoparticles (SMON) and carboxymethyl-cellulose (CMC)/cetiltrimethyl-ammonium-bromide (CTAB) coordination complexes, is reported. The influence of their synthesis parameters (precursor ratio, stirring rate, temperature and synthesis time) on the particle size distribution and the size and morphology of CMC/CTAB templates was investigated. The synthesized HNM were characterized by transmission electron microscopy and their related techniques, such as bright field and high angular annular dark field (HAADF-STEM) imaging, as well as selected area electron diffraction (SAED), using a FEI Titan G2 80-300 microscope. The interactions among SMON, CMC and CTAB were investigated by Fourier transform infrared spectroscopy (FTIR) in a Thermo Scientific Nicolet spectrometer. Magnetic response of the synthesized HNM was evaluated in a Quantum Design MPMS3. The experimental evidence suggests that, the formation of these HNM occurs due to the competition between CTAB molecules and SMON to occupy CMC intermolecular sites nearby to its carboxylate functional groups. Thus, the morphology and size of CMC/CTAB templates can be tuned varying the CTAB:SMON ratio. Moreover, it was found that the magnetic response of the HNM depends on the confinement degree of the SMON into the CMC/CTAB template. Hence, its magnetic characteristics can be adjusted controlling the size of the template, the quantity and distribution of the SMO nanoparticles within the template and its sizes.
Electron holography is a unique electron microscopy technique to visualize electromagnetic fields on the nanometer scale. We developed the electron holography system on the basis of a JEM-3000F electron microscope by installing a Lorentz lens and a magnetizing stage for detailed study of magnetic nanostructures of various functional materials . Recently we have also utilized an HF-3300X electron microscope by which “split-illumination electron holography” can be carried out . In the split-illumination method, additional biprisms are inserted into a condenser lens system. This method is especially effective for electron holography study of specimens with strong stray fields around them. This is because a reference wave can be obtained from a region far from the specimen, being free from the stray fields. Thus, electron holography analysis with high precision can be carried out, and this system is expected to widen the application of electron holography extensively.One of the applications of electron holography study of functional materials is magnetic nanostructure analysis of Zn-doped Fe3O4 (FZO). It was reported that shape/size-controlled nanowires obtained from solid solutions based on Fe3O4, which was prepared by atomic force microscope lithography and pulsed laser deposition, show good functionalities, e.g., nonlinear I-V character and large magnetoresistance . Dark field microscope images of FZO obtained by an energy-filtered electron microscope revealed the microstructure consisting of fine antiphase domains (APDs) whose size is less than 100nm. On the other hand, electron holography study combined with Lorentz microscopy clarified small magnetic domains whose size corresponds to that of APDs. The complex magnetic domain structure was interpreted by the change in the local spin order being induced by the modulation of Fe-O-Fe bonding in the antiphase boundary region. Eventually the good functionalities are discussed in terms of the small magnetic domains clarified by electron holography. D. Shindo and Y. Murakami, J. Phys. D 41, 183002 (2008). T. Tanigaki, Y. Inada, S. Aizawa, T. Suzuki, H. S. Park, T. Matsuda, A. Taniyama, D. Shindo, A. Tonomura, Appl. Phys. Lett., 101, 043101 (2012). K. Goto, T. Kanki, T. Kawai, and H. Tanaka, Nano Lett., 10, 2772 (2010).
Tuning the ferromagnetism of LaCoO3 by doping bulk samples with smaller Sr atoms or straining thin film sample have been previously shown experimentally. In this work, we will use atomic-resolution Z-contrast imaging, annular bright field (ABF) imaging and electron energy-loss spectroscopy (EELS) in the aberration-corrected JEOL JEM-ARM200CF in combination with in-situ cooling experiments to examine the magnetic and spin-state transitions in La(1-x)SrxCoO3 (x=0-0.3) at liquid nitrogen and room temperature. Using energy-loss magnetic circular dichroism method, we confirm the magnetic ordering transition at room temperature with increasing doping concentrations. A magnetic transition is observed in 5% doped sample at liquid temperature. EELS mapping is, also utilized to determine the presence of nanoscale islands of different spin states using both the Co L3/L2 ratio and Oxygen prepeak intensities. Additionally, with increasing doping concentration, a change in crystal structure is measured using ABF imaging, more specifically areas with different distortions of the CoO6 octahedral within the same doping concentrations and change in distortion in different Sr doping concentrations.Acknowledgment:The authors acknowledge funding from the National Science Foundation [DMR-0846784] and DMR-0959470 for the acquisition of the UIC JEOL JEM-ARM200CF.
The use of magnetoresistive thin films based on layers with perpendicular magnetic anisotropy (PMA) is very interesting for many applications such as high density magnetic recording media, MRAM and magnetic logic. Such films can exhibit good thermal stability in nanoscale dimensions and it is easier to define an easy axis than the case for thin films with in-plane anisotropy. Basic magnetic characterization for these thin films includes measuring hysteresis loops by using magnetometry such as vibrating sample magnetometry (VSM) and superconductor quantum interference device (SQUID), or using extraordinary Hall Effect (EHE). However, these techniques can only measure average magnetization behavior. It is hard to image the magnetic domain reversal directly. Magneto-optical Kerr effect (MOKE) microscopy and magnetic force microscopy (MFM) are the two most used tools to image the magnetic domains of these thin films. The lateral resolution for MOKE is limited to micron-size, too low to image magnetic domains on the nanoscale. MFM has high resolution, and can easily image nanoscale magnetic domains. However, most MFM images are taken in zero magnetic field and cannot image magnetic domain structure during magnetization reversal.In this work, we present non-contact MFM images taken in magnetic fields on Co/Pd-based pseudo spin valves. The multilayer sample has the following composition: Ta 5/Pd 5/(Co 0.2/Pd 1)8/Co 0.4/Cu 2.9/Co 0.4/(Pd 1/Co 0.4)3/Pd 5 (nm). All layers were grown on thermally oxidized Si wafers at 300 K by DC-magnetron sputtering in a shamrock sputtering tool. The initial magnetization curve and hysteresis loop with applied magnetic field perpendicular to the films was first measured by SQUID at 300 K. It clearly shows two step sharp switching, which indicate good PMA of our multilayers. The multilayers with thicker Co layers (0.4 nm) which lie above the Cu spacer layer have smaller coercivity. The non-contact MFM images were taken on a NanoScan hr-MFM in a constant average height mode. The height was regulated using the electrostatic force due to a DC tip-sample bias. The MFM images were taken in increasing magnetic fie