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I am currently a member of the Functional Materials Laboratory – University of South Florida. My research interest spans a wide spectrum of materials, including smart structural magnetic composites, amorphous and nanocrystalline materials (e.g. wires, microwires and ribbons), magnetic thin films, complex oxides, intermetallic alloys, magnetic semiconductors, magnetic nanoparticles, ferrofluids, and multiferroic materials. Below are my major research interests:

1. Giant magnetoimpedance materials for magnetic sensors

2. Giant magnetocaloric materials for magnetic refrigerators

Magnetic refrigeration (MR) based on the magnetocaloric effect (MCE) offers several advantages over conventional gas compression refrigeration. MR has many potential applications in energy intensive industrial and commercial refrigerators systems, such as large-scale air conditioners, heat pumps, supermarket refrigeration units, waste separation, chemical processing, gas liquification, liquor distilling, sugar refining, grain drying, and so forth. MCE is defined as an isothermal change of magnetic entropy or an adiabatic temperature change of a magnetic material subject to a magnetic field. Currently there is a need for seeking materials that possess a large magnetic entropy change over a wide temperature range (i.e. the large refrigerant capacity). We have great interest in developing new magnetocaloric materials for active MR applications. We have been extensively studying MCE in a large class of perovskite-like structured materials (i.e. manganese oxides or manganites). We demonstrate that due to the absence of grains, single crystalline materials have superior magnetocaloric properties to their polycrystalline counterparts. These new findings will not only significantly advance our knowledge of magnetocaloric materials but also foster new technologies related to magnetic cooling. As compared with other candidates, such as Gd and its alloys, manganites are more convenient to prepare and exhibit higher chemical stability as well as higher resistivity that is favorable for lowering eddy current heating. Manganites are also cost-effective. As highlighted in our review paper, ferromagnetic manganites are attractive candidate materials for use in active magnetic refrigerators (click here to read more). Recently, we have discovered the giant magnetocaloric effect in a new type of material (the type VIII clathrate crystal structure, a material that is better known for its thermoelectric properties). Owing to the reversible giant magnetocaloric effect, this newly discovered material is considered to be one of the best candidates for cryogenic applications (click here to read more). We also demonstrate that reducing size of particles to a certain nanoscale led to an enhanced MCE in garnet nanoparticles relative to its bulk counterpart. It is demonstrated that while blocking is detrimental to achieving large MCE in garnet nanoparticles, surface spin disorder is found to enhance it under high applied fields. Our observations provide strong evidence for potential control of the surface spin ordering leading to further increase of MCE in nanoparticles and nanocomposites (click here to read more).

3. Magnetic thin films for magnetic recoding and sensor applications

Magnetic thin films are key elements in miniaturizing magnetic components of micromagnetic devices. For high-frequency device applications, they are required to have high electrical resistivity, to minimize energy loss due to eddy currents, a large saturation magnetization and a large hard-axis anisotropy field, to increase the magnetic switching capacity at high frequencies. Such requirements can be met by the use of a new class of recently developed nanostructured magnetic oxide thin films, namely, Co-Fe-T-O (T = Al, Zr, Hf), which can show a good high-frequency performance into the gigahertz range owing to an electrical resistivity approximately ten times higher than that of conventional metallic films. In order to design and produce Co-Fe-T-O (T = Al, Zr, Hf) thin films with the desired magnetic properties, it is important to understand the correlation between the chemistry, structure and magnetic properties of the films. We have been fabricating and characterizing a variety of magnetic thin films, such as Fe-B-N, Co-Fe-V, Co-Fe-Al-O, Co-Fe-V-Al-O-Nb, Co-Fe-Hf-O with the novel magnetic properties that meet the engineering demands. For the first time, we produced a novel laminate oxide nanostructure that well suits high-frequency micromagnetic devices, such as thin film inductors, transformers and flux-gate sensors (click here to read more). The laminate structure, which consists of nanocrystalline a-Fe(Co)-rich layers separated by amorphous HfO₂-rich layers, can be considered as a [Fe(Co)-rich/HfO₂-rich]n multilayer, where "exchange coupling" of Fe–Fe(Co) takes place between two neighbouring ferromagnetic layers through an insulating HfO2-rich amorphous layer. Therefore, CoFeHfO is itself a "tunneling multijunction", which could be developed for use in advanced spintronic applications, where the spin of the electron is exploited as well as its charge. Since the CoFeHfO thin films are insensitive to oxygen at interfaces, they can ideally be used as free layers for the fabrication of spin-dependent tunneling junctions and as electrode layers for tunneling magnetoresistive (TMR) junction applications. We also discovered the large magnetoimpedance effect in the CoFeHfO thin films, which makes them very promising for advanced magnetic sensing technology. Such an achievement has been featured in NanotechWeb: www.nanotechweb.org/articles/news) .

4. Magnetic nanoparticles, ferrofluids and nanocomposites

Magnetic nanoparticles are technologically important for a variety of applications ranging from biomedicine, magnetic recording to refrigeration. Of an interesting feature in magnetic nanoparticles is strong modification of surface spins when the nanoparticle size is reduced below a certain value. This leads to anomalous magnetic properties, including enhanced surface anisotropy and exchange bias (EB). However, the origin of the spin misalignment and the magnetic disordering related phenomena observed in nanoparticle assemblies is still under discussion. From an experimental perspective, it is vital to distinguish the contributions of the dipolar interaction and particle-size effects to the formation of a collective state through systematic studies of the influences of particle size, shape and distribution on the static and dynamic magnetic properties of well-defined nanoparticle assemblies. From a theoretical viewpoint, it would be important to incorporate Ising spins into a model with Heisenberg spins with finite anisotropy to account for the surface anisotropy on the collective state and EB. We are very interested in addressing these topical issues (click here to read more).

Ferrofluids are colloidal mixtures composed of nanoscale ferromagnetic, or ferrimagnetic, particles suspended in a carrier fluid, usually an organic solvent or water. They have important applications in biomedicine, hydraulics and power generation. They are also model systems for the investigation of the magnetic characteristics and relaxation phenomena in magnetic nanoparticles. From a fundamental physics standpoint, it is proposed that blocking of magnetic nanoparticles and freezing of a carrier fluid would affect the magnetization and relaxation processes in ferrofluids. We demonstrate that the particle blocking and carrier fluid freezing effects play a key role in the formation of glass-like relaxation peaks in ferrofluids. The nature of the glass-like relaxation peaks is strongly affected by varying particle size and carrier fluid medium. 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 (click here to read more).

5. Magnetism and collective phenomena in complex oxides

Transition-metal oxides exhibit rich complexity in their fundamental physical properties determined by the intricate interplay between structural, electronic and magnetic degrees of freedom. Of particular current interest are systems that inherently favor charge and spin frustration due to the arrangement of transition-metal or rare-earth atoms on a triangular lattice. Research on geometrically frustrated materials has continued to yield new surprises in terms of cooperative phenomena such as the coexistence of ferrimagnetism, superconductivity and ferroelectric behavior. We have been extensively sudying the spin dynamics and collective phenomena in numerous exotic oxide systems by means of DC and AC susceptibility, radio frequency transverse susceptibility (TS), electron paramagnetic resonance (EPR) and MCE.

We have studied systematically the structural, magnetic and magnetotransport properties of a large class of manganites (i.e., ABO3 where A = La, Nd, Pr, etc., and B = Mn, Co, Ti, etc.) Our reseach reveals the complexity in the magnetic groud states in these systems. EPR studies have revealed that ferromagnetic clusters persist even in the paramagnetic region which strongly modify the critical magnetic behaviors in the vicinity of the paramagnetic to ferromagnetic transition in such systems. Recently, we have successfully used TS as a very useful research tool for studying magnetic anisotropy and phase transitions in charge-ordered manganites, such as Pr_0.5Sr_0.5MO₃ (click here to read more).