Semiconductors with the clathrate hydrate crystal structure have demonstrated interesting physical properties that are directly related to the fact that “guest” atoms reside inside “host” polyhedra that are formed by other species. These materials are well known for their excellent thermoelectric properties. One of the interesting “guests” in the clathrate structure is europium. Since the magnetic moment of Eu is large and the Eu moments order at low temperatures in Eu8Ga16Ge30 clathrates, these materials are expected to exhibit interesting magnetic and magnetocaloric properties. In this work, we report on the systematic studies of magnetic and magnetocaloric properties of Eu₈Ga₁₆Ge₃₀ clathrates. The magnetic entropy change was numerically calculated from the magnetization isotherms using the Maxwell relation and giant magnetocaloric effect (GMCE) was observed. Experimental results reveal a coherent correlation between the structure, magnetic property and the GMCE in Eu8Ga16Ge30 clathrates. The low-field GMCE, in addition to the absence of thermal hysteresis and field hysteresis, makes this material an attractive candidate for active magnetic refrigeration at low temperatures.

From a fundamental physics perspective, it is proposed that blocking of magnetic nanoparticles and freezing of a carrier fluid would affect the magnetization and relaxation processes in ferrofluids. To verify this hypothesis, we have conducted systematic DC magnetization and AC susceptibility studies in different ferrofluids composed of Fe₃O₄ and CoFe₂O₄ nanoparticles suspended in hexane and dodecane, which respectively have freezing temperatures below (178K) and above (264K) the blocking temperature of magnetic nanoparticles (~200K). Experimental results reveal that the particle blocking and carrier fluid freezing effects play key roles in the formation of glass-like relaxation peaks in ferrofluids, which remained largely unexplained in previous studies. It is also shown that the nature of these peaks is strongly affected by varying particle size and carrier fluid medium. Quantitative fits of the frequency dependent AC susceptibility to the Vogel-Fulcher model clearly indicate that the blocking of magnetic nanoparticles in the frozen state significantly affects the interparticle dipole-dipole interaction, causing characteristic spin-glass-like dynamics. A clear correlation between the blocking and freezing temperatures emerges from our studies for the first time.

  The ground state magnetic properties and relaxation mechanism in magnetically frustrated system of Gd₃Fe₅O₁₂ is of topical interest due to its complex magnetic structure. As a consequence of geometric and magnetic frustrations, the Gd₃Fe₅O₁₂ system is expected to show glassy magnetic behavior. Through a comprehensive study of DC magnetization, AC susceptibility, transverse susceptibility, and magnetocaloric effect in Gd₃Fe₅O₁₂ bulk and nanostructured materials, we provide physical insights into the glassy nature and magnetic relaxation mechanisms in the gadolinium iron garnet system. It is shown that bulk Gd₃Fe₅O₁₂ undergoes two different glassy states at temperatures below its compensation temperature with the low temperature glass properties strongly influenced by Gd ordering. However, the glassy nature is largely suppressed in Gd₃Fe₅O₁₂ nanoparticles in which the blocking phenomenon competes with the spin frustration effect. As particle size is decreased, the blocking effect is dominant over the spin frustration effect. As a result, the nanostructured system shows magnetic relaxation features arising mainly from superparamagnetism.

Magnetic nanoparticles embedded in polymer matrices are good examples of functional nanostructures with excellent potential in applications such as tunable microwave devices, EMI shielding, and flexible electronics. The challenge comes with evenly dispersing the nanoparticles once they are embedded in the polymer matrix. To avoid clustering of particles in the polymer nanocomposites and achieve excellent dispersion, competition between polymer-polymer and polymer-particle interactions must be balanced. In earlier work, we demonstrated the synthesis of 2µm thick, spin-coated nanocomposite PMMA films with Fe₃O₄ (mean size 15nm) nanoparticles embedded that displayed superparamagnetic behavior. In this work we will report on the successful extension of this strategy to 20 µm thick films that are needed for microwave applications. In addition to Fe₃O₄, we have also functionalized the films with other ferrite nanoparticles. Magnetic characterization and microstructural studies of the polymer nanocomposites will be presented and discussed. Microwave response of these films using a coplanar waveguide fixture will also be reported.

Magnetic oxides are an important class of materials from the perspectives of fundamental physics and technological applications. Advances in growth of high quality thin films and epitaxial oxide heterostructures over the years, have led to the realization of ideal condensed matter systems in which the complex and rich physics associated with cooperative phenomena can be explored. Examples of coupled phenomena in heterostructures include exchange bias effects, magnetoelectric coupling and interplay between magnetism and superconductivity. In this talk, I will focus on three classes of oxide heterostructures –PLD-grown M-type barium hexaferrite(BaM)/barium strontium titanate(BST), CVD-grown CrO₂/Cr₂O₃ bilayers and high-pressure sputtered LCMO/YBCO films. The common theme is the magnetic coupling across the interfaces. I will demonstrate that dynamic susceptibility and kinetic inductance experiments using a sensitive tunnel-diode oscillator (TDO) are effective probes of such coupled effects. In the case of CrO₂/Cr₂O₃ and LCMO/YBCO, the interface coupling results in anomalous anisotropy, exchange bias in the former and complex interaction between the LCMO magnetism and YBCO vortex lattice in the latter. In BaM/BST heterostructures, I will discuss how interfacial coupling influences the microwave response that is both electrically and magnetically tunable.