S. Srinath

Exchange bias effect in Au-Fe 3 O 4 nanocomposites

We report exchange bias (EB) effect in the Au-Fe3O4 composite nanoparticle system, where one or more Fe3O4 nanoparticles are attached to an Au seed particle forming dimer and cluster morphologies, with the clusters showing much stronger EB in comparison with the dimers. The EB effect develops due to the presence of stress at the Au-Fe3O4 interface which leads to the generation of highly disordered, anisotropic surface spins in the Fe3O4 particle. The EB effect is lost with the removal of the interfacial stress. Our atomistic Monte Carlo studies are in excellent agreement with the experimental results. These results show a new path towards tuning EB in nanostructures, namely controllably creating interfacial stress, and opens up the possibility of tuning the anisotropic properties of biocompatible nanoparticles via a controllable exchange coupling mechanism.

Growth and characterization of sputtered BSTO∕BaM multilayers

Multilayers of Ba0.5Sr0.5TiO3 (BSTO) and BaFe12O19 (BaM), with tunable permeability and permittivity are attractive systems for radio frequency and microwave applications. We have grown multilayers of BSTO and BaM using magnetron sputtering on Al2O3 substrates. Film growth conditions such as sputtering parameters were optimized to obtain high quality multilayers. X-ray diffraction established that both BSTO and BaM were formed and cross-sectional SEM studies showed sharp interfaces between BSTO and BaM layers. Magnetization showed a large coercivity (∼2000Oe) consistent with the hexaferrite component. The hysteresis loops also revealed the distinct influence of magnetocrystalline and shape anisotropies at different temperatures.

Static and Dynamic Magnetic Properties of Composite Au-Fe

Fe3O4 (9 nm) grown epitaxially on one or several facets of a seed Au particle (8 nm) leads to two distinct composite nanoparticle structures dubbed dumbbell- and flower-like. These particles exhibit novel magnetic behavior such as large magnetic anisotropy compared to the single component Fe3O4. We have studied the static and dynamic magnetic properties using temperature and field-dependent dc magnetization and RF transverse susceptibility. While the magnetic properties of dumbbell-like particles are relatively conventional, field-cooled M-H loops of flower-shaped particles as well as the transverse susceptibility show large shifts along the field axis indicative of exchange bias

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SrFeO3−δ belongs to the Ruddlesden-Popper class of systems exhibiting interesting electronic and magnetic properties. Ti4+-doped, oxygen-deficient SrFeO3−δ(SrFexTi1−xO3−δ; x=0.9 and 0.7) samples show canted antiferromagnetic spin ordering, in contrast to the undoped sample (x=1) which is known to exhibit a metallic behavior with a helical spin arrangement. ac susceptibility χac (f,T) at 10⩽T⩽300K shows a frequency (100Hz⩽f⩽10kHz)-dependent temperature maximum, reminiscent of a spin-glass behavior. Increasing the Ti4+ content reduces the irreversibility, indicating a decrease in the frustration, which results in a diluted helical spin structure. Hysteresis loops can be associated with a gradual reorientation of spins in the field direction, indicating a canted type of spin arrangement.

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We report an observation of large magnetocaloric effect in clathrate compounds. Systematic dc magnetization in polycrystalline Eu8Ga16Ge30 clathrates was studied in the temperature range of 5–60 K and over an applied field range of 0–3 T. Change in entropy (ΔSmag) was calculated using the Maxwell relation from the family of M‐H curves at different temperatures in the vicinity of the ferromagnetic Curie temperatures. Large entropy changes of 6 and 9.3J∕kgK were observed for the type-I and type-VIII phases, respectively. The larger effect in type-VIII samples can be associated with the sharper ferromagnetic transition. Overall, these results indicate that the clathrates known to have potential for excellent thermoelectric properties are also promising candidates as magnetic refrigerant materials.

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Magnetic anisotropy and spin polarization are fundamental parameters in ferromagnetic materials that have use in spintronic device applications. As the need for screening properties of new magnetic materials rises, it is important to have measurement probes for quantities such as anisotropy and spin polarization. We have developed two unconventional yet powerful techniques to study these parameters. A resonant RF transverse susceptibility method is used to map the characteristic anisotropy and switching fields over a wide range in temperature and magnetic fields. For studies of spin polarization, the phenomenon of Andreev reflection across ferromagnet-superconductor junctions is used to extract values of the transport spin polarization. The effectiveness of these approaches is demonstrated in candidate spintronic materials such as half-metallic CrO/sub 2/ thin films and arrays of monodisperse, single-domain Fe nanoparticles.

Nanoparticles

Magnetic refrigeration based on the magnetocaloric effect (MCE) has been proposed as an attractive alternative to gas compression technology. Some calculations suggest that MCE can be obtained with higher efficiencies than compressor driven refrigeration. We examine the MCE in a system of nickel ferrite nanoparticles with size range of 6 to 15 nm. A peak in the MCE at 55 K is observed that increases with higher magnetic fields. This gives this system a relatively high peak entropy change compared to other ferrite systems. A sensitive radio-frequency (RF) transverse susceptibility measurement has also been used to study the magnetic anisotropy. We show that the MCE peak is not associated with the blocking temperature and is likely a field-driven surface spin reorientation which also has a signature in the transverse susceptibility. Work supported by NSF-CTS-0408933 and ARO- W911NF-05-1-0354

Magnetization in insulating phases of Ti4+-doped SrFeO3−δ

We have investigated the exchange bias in CVD grown epitaxial CrO2/Cr2O3 bilayer thin films using hysteresis loops and resonant RF transverse susceptibility. M-H loops indicated an enhanced coercivity without appreciable loop shift and the transverse susceptibility in CrO2/Cr2O3 bilayers revealed features associated with both the ferromagnetic and antiferromagnetic phases. In addition, TS yielded large anisotropy constant (Keff) values depending on the fraction of Cr2O3 present. The large anisotropy fields observed cannot be accounted for by the variable thickness of CrO2 alone and are indicative of possible exchange coupling between CrO2 and Cr2O3 phases that significantly affects the effective magnetic anisotropy.