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Dr. Phan's research interests include the fundamental and applied physics of nanomagnetism and magnetic nanomaterials. He leads research focusing on the development of advanced nanomaterials with magnetocaloric and magnetoimpedance effects for energy-efficient refrigeration and smart sensor technologies. The research has attracted various funding agencies (read more). Below are his group's current research topics:

 
1. Giant magnetoimpedance materials for magnetic sensors
2. Giant magnetocaloric materials for magnetic refrigerators
3. Magnetic thin films for magnetic recoding and sensor applications
4. Magnetic nanoparticles, ferrofluids and nanocomposites
5. Magnetism and collective phenomena in complex oxides
 

1. Giant magnetoimpedance materials for smart sensors

Magnetic sensors find wide applications in high-density magnetic recording, navigation, military and security, target detection and tracking, anti-theft systems, non-destructive testing, magnetic marking and labelling, geomagnetic measurements, space research, measurements of magnetic fields onboard spacecrafts, and biomagnetic measurements in the human body. Current research is focused on developing inexpensive sensors with higher sensitivity and reduced size. The giant magneto-impedance (GMI) effect, which refers to a large change in the ac impedance of a magnetic conductor subject to a DC magnetic field, observed in a number of soft ferromagnetic materials, together with an ultrahigh sensitivity at low fields, lack of hysteresis and high temperature stability holds great promise in magnetic field sensing.

Our research aims to develop new materials and technqiues for GMI sensor applications (read more). We have developed a new concept of incorporating magnetic microwires with optimized GMI effect into fiber-reinforced composite as means of sensing stress and external magnetic fields through changes in impedance, which potentially offers an alternative to optical fibers for self-monitoring composites. The advantage of using these microwires that exhibit the best characteristics for sensing tend also to have strength, thus contributing to the structure integrity of the composites. Our research opens up new opportunities to develop smart structural composites with self-monitoring properties for a wide range of engineering and industrial applications (read more).

Our current efforts are to explore functional layered materials with improved GMI responses for advanced sensor applications and to develop GMI as gas, chemical, and biosensing probes. We have demonstrated that the presence of a thin magnetic metal coating layer (Co, Fe, FeNi, CoFe2O4) enhances both the GMI effect and field sensitivity of a soft ferromagnetic amorphous ribbon. This provides a simple, effective method for tailoring the GMI effect and field sensitivity in surface-modified soft ferromagnetic ribbons for use in highly sensitive magnetic sensors (read more). In addition to this we have recently discovered that pulsed laser depostion (PLD) is an effective method for the controlled growth of a thin magnetic layer on the surface of Co-based amorphous ribbons to create bilayer structures with improved GMI properties (read more).

Carbon nanotubes (CNTs) are ideal for gas molecule adsorption and storage, owning to their extremely high surface-to-volume ratio. CNT-based gas sensors with high sensitivity and selectivity (where sensing is achieved by the dc resistance change upon adsorption of analytic molecules) are important for leakage detections of explosive gases such as hydrogen, and for real-time detections of toxic or pathogenic gases in industries. However, these resistive sensors possess limited sensitivities (~2-10%). Therefore there is a need for developing alternative techniques, that allow detecting gases with a higher degree of sensitivity. We have proposed a new way for developing CNT-based gas sensors that operate based on the principle of the GMI effect (read more). These sensors have potential to detect the magnetoimpedance change of the sensing element (e.g. the magnetic ribbon) through variations in electrical resistance of CNTs when exposed to gases such as NO2, NH3, H2O, CO, Iodine, and ethanol.

We have developed a new method of using soft ferromagnetic glass-coated amorphous microwires as a microwave absorber to fabricate a novel class of microwave energy sensor based on Fiber Bragg Grating (FBG). The magnetic microwire absorbs microwave energy and heats up thus raising the temperature of the FBG. We find that the best sensitivity of the sensor to electromagnetic radiation corresponds to AC electric fields that have root mean square (RMS) amplitude of approximately 36 V/m. As compared to a similar sensor that uses gold to absorb electromagnetic radiation, this newly developed sensor shows a greater sensitivity (10 times at f ~ 3.25 GHz) relative to the perturbation of the microwave field. Since the sensor is physically very small and often only minimally perturbs the field being measured, it can be deployed as a distributed sensor (read more). We have also established a correlation between the magnetic softness, GMI and microwave absorption effects, which gives good guidance on improving the sensor sensitivity (read more). .

Improving the sensitivity of existing biosensors for highly sensitive detection of magnetic nanoparticles as biomarkers in biological systems is an important and challenging task. We have developed a new method of combining the magneto-resistance (MR), magneto-reactance (MX), and magneto-impedance (MI) effects for making an integrated magnetic biosensor with tunable and enhanced sensitivity. This newly developed biosensor can detect low and various concentrations of superparamagnetic nanoparticles (below 10 nm in size), which is of practical importance in biosensing applications (read more). Recently, we have developed a novel approach to improving the detection sensitivity of a ribbon-based biosensor by patterning nanosized holes on the surface of the ribbon (read more).

2. Giant magnetocaloric materials for magnetic refrigerators

Air conditioners and refrigeration make a major contribution to the global energy consumption. They account for approximately 50% of the USA’s energy use during the summer months. Current refrigerators work based on traditional energy-guzzling gas-compression methods. However, these refrigeration systems have already reached out the upper limit of cooling efficiency and rely on hydrofluorocarbons that are greenhouse gasses that contribute to global climate change when they escape into the atmosphere.

Magnetic refrigeration is an environmentally friendly technology that uses magnetic fields to change a magnetic material’s temperature (i.e. the magnetocaloric effect - MCE) and allows the solid material to serve as a refrigerant. This technology provides a much higher cooling efficiency (about 20-30%) than a conventional gas compression technique. The majority of magnetic refrigeration is to find suitable magnetocaloric materials that are cost-effective and exhibit large MCE spanning over a wide temperature range from low to room temperatures.

Our research aims to develop new magnetocaloric materials for active magnetic refrigeration applications. We have discovered large MCEs in a large class of perovskite-like structured materials, especially in doped manganites, which are better known to show colossal magnetoresistive (CMR) effects. The tunable magnetocaloric properties together their cost-effectiveness indicate that these materials are attractive candidates for use in active magnetic refrigerators (read more).

Eu8Ga16Ge30 with the clathrate crystal structure is better known for its thermoelectric property. We have discovered a reversible giant magnetocaloric effect in the Type VIII clathrate (read more). A combination of the excellent thermoelectric and thermomagnetic properties makes it one of the best candidates for cryogenic applications. Recently we have successuflly designed and fabricated a new type of composite material based on the Type I clathrate (Eu8Ga16Ge30-EuO) showing the table-like magnetocaloric effect and enhanced refrigerant capacity (read more). The RC of this composite is the largest value ever achieved among existing magnetocaloric materials for magnetic refrigeration in the temperature range 10 K - 100 K. By adjusting the Eu8Ga16Ge30 to EuO ratio, it is possible to produce composites with table-like MCE, desirable for ideal Ericsson-cycle magnetic refrigeration. The excellent magnetocaloric properties of these Eu8Ga16Ge30–EuO composites make them attractive for active magnetic refrigeration in the liquid nitrogen temperature range.

Nanostructured magnetocaloric materials are desirable for advanced magnetic refrigeration technologies. While the conventional trend is reduction of magnetization and MCE with nanostructuring, we have demonstrated the possibility of enhancing both the magnetocaloric effect and refrigerant capacity in nanostructured mixed phase manganites (read more). This finding opens up a new way of exploring magnetic refrigerant materials at the nanometer scale for active magnetic refrigerators. We also demonstrate that while blocking is detrimental to achieving large MCE in magnetic 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 (read more).

Recently we have successfully developed a new class of magnetocaloric microwires using a modified precision melt-extraction method (read more). The enhanced MCE and refrigerant capacity (RC) have been achieved in these microwires relative to their bulk counterparts. The use of these microwires with increased surface areas will not only enhance heat transfer processes but also promote chemical reactions of solid refrigerant with liquid coolant used to transfer heat inside the system. We propose that the design and fabrication of a magnetic bed made of these parallel-arranged microwires are a very promising approach for active magnetic refrigeration for nitrogen liquefaction (read more). Since these microwires can easily be assembled as laminate structures, they have potential applications as a cooling device for micro electromechanical systems and nano electro mechanical systems.

3. Magnetic thin films for technological 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 fabricated and investigated the properties of various 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. In particular, we produced a novel laminate oxide nanostructure that well suits high-frequency micromagnetic devices, such as thin film inductors, transformers and flux-gate sensors (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. These findings have been featured in NanotechWeb and Cobalt 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. Our research is aimed at addressing these topical issues (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 (read more).

Exchange bias in nanostructures is of current interest because of its potential use in spin valves, MRAM circuits, magnetic tunnel junctions, and spintronic devices. An issue of fundamental interest is if the surface spin alignment in nanoparticles could be controllably influenced by forming interfaces with metals. If this is indeed possible, then it would provide an excellent mechanism to increase the strength of exchange coupling between the surface and core spins in individual nanoparticles, leading to novel magnetic properties. We have recently discovered a remarkable EB effect in novel coupled Au-Fe3O4 nanoparticles where one or more ferrimagnetic oxide nanoparticles (Fe3O4) are attached to a non-magnetic metallic core particle (Au). Our studies have revealed that the presence of Au strongly influences the alignment of surface spins in coupled Au-Fe3O4 nanoparticles (read more). It is demonstrated that magnetic interactions are very complex in these coupled Au-Fe3O4 nanoparticle systems (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. Our research aims to shed light on the nature of the collective magnetic phenomena and spin frustration in a wide range of complex oxide materials.

We have systematically studied the structural, magnetic and magnetotransport properties of a large class of doped manganites (i.e., ABO3 where A = La, Nd, Pr, etc., and B = Mn, Co, Ti, etc.) by means of DC and AC susceptibility, radio frequency transverse susceptibility (TS), electron paramagnetic resonance (EPR), and MCE. Our reseach reveals the complexity in the magnetic groud states in these systems. EPR studies have revealed the presence of ferromagnetic clusters even in the paramagnetic region (read more), which modifies the critical parameters near the ferromagnetic transition in these systems (read more). Recently, we have developed TS as a very useful research tool for studying magnetic anisotropy and phase transitions in doped manganites (read more) and doped cobaltites (read more).

La5/8Pr3/8-xCaxMnO3 (LPCMO) manganites exhibit a complex phase diagram due to coexisting and competing magnetic and electronic phases. Of particular interest is the charge-ordered (CO) phase that is unstable under various perturbations, such as carrier doping, strain, magnetic and electric field. Our magnetic, transverse susceptibility and magnetocaloric studies on LPCMO nanocrystalline and thin film materials reveal that the long-range CO is largely suppressed and the ferromagnetic (FM) order is established in these systems. We demonstrate that MCE is actually a very powerful probe of magnetic transitions and ground state magnetic properties in mixed-valent manganites like LPCMO (read more). Recently we have shown a significant impact of nanostructuring on the magnetic and magnetocaloric response in this system (read more). Magnetic and magnetocaloric measurements reveal that decreasing particle size affects the balance of competing phases in LPCMO and narrows the range of fields over which PM, FM, and CO phases coexist. The FM volume fraction increases with size reduction, until CO is suppressed below some critical size, ~100 nm. With size reduction, the saturation magnetization and field sensitivity first increase as long-range CO is inhibited, then decrease as surface effects become increasingly important. The trend that the FM phase is stabilized on the nanoscale is contrasted with the stabilization of the charge-disordered PM phase occurring on the microscale, demonstrating that in terms of the characteristic phase separation length, a few microns and several hundred nanometers represent very different regimes in LPCMO.

LuFe2O4 is a complex oxide of great current interest as ferroelectricity in this material arises from charge ordering and it also exhibits multiferroic behavior. A clear understanding of the magnetic phase diagram has remained elusive primarily due to the complexity of the system. We have studied multiferroic LuFe2O4 single crystals, revealing for the first time that this material undergoes multiple phase transitions, with the freezing of ferrimagnetic clusters at low temperature (read more). The origin of giant magnetic coercivity is attributed to collective freezing of ferrimagnetic clusters and enhanced domain wall pinning associated with a structural transition. Magnetocaloric effect measurements provide additional vital information about the multiple magnetic transitions and the glassy states. These findings lead to the emergence of a complex magnetic phase diagram in LuFe2O4 (read more).

Ca3Co2O6 (CCO) is a system exhibiting spin frustration and intrinsic low dimensionality due to the formation of 1D spin chains. It thus provides a very interesting case study of the cooperative phenomena of low-dimensional magnetism and topological magnetic frustration in a single material. CCO consists of face sharing CoO6 trigonal prisms and CoO6 octahedra chains running along the c-axis surrounded by six nearest neighbor chains, forming a triangular lattice in the ab-plane. Intrachain Co-Co separation is 2.59 A, while interchain Co-Co separation is roughly twice that, giving rise to large anisotropy and a quasi-1D structure. The Co ions are found in the 3+ oxidation state, with an alternating low-spin (LS) state for the octahedral configuration and high-spin (HS) state for the trigonal configuration. Due to the geometrical frustration and strong anisotropy, CCO has long been considered an Ising-like material, where each chain can be represented by a single spin in a 2D lattice. We have established for the first time a new magnetic phase diagram from the magnetic field and temperature dependence of magnetic entropy change. Our new findings are consistent with the spin-density wave description of the zero field order, pointing to the suppression of the modulated spin state at low field, the presence of short-range magnetic correlations coexisting with long-range order up to large fields, and the formation of intermediate spin configurations among the chains (read more).