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.

We have discovered large MCE in a large class of perovskite-like structured materials (i.e. manganites and cobaltites). The excellent magnetocaloric properties in addition to the low-cost mataterial processing make them attractive for use in active magnetic refrigerators. We have discovered a reversible 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). The combination of the excellent thermoelectric and thermomagnetic properties makes it one of the best candidates for cryogenic applications. We have demonstrated the possibility of enhancing both the magnetocaloric effect and refrigerant capacity in nanostructured mixed phase manganites. 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.

Magnetic oxides exhibit rich complexity in their fundamental physical properties determined by the intricate interplay between structural, electronic and magnetic degrees of freedom. The common theme that is often present in many systems is the strong magnetostructural coupling and possible spin frustration induced by lattice geometry. In this talk, we will demonstrate the relatively unconventional experimental methods of RF transverse susceptibility (TS) and magnetocaloric effect (MCE) as being powerful probes of multiple magnetic transitions, glassy phenomena and ground state magnetic properties in three classes of oxides including Pr0:5Sr0:5CoO3, LuFe2O4 and mixed phase manganite (La,Pr,Ca)MnO3. These results point to the existence of an entirely new class of phenomena in the cobaltites due to the unique interplay between structure and magnetic anisotropy. In LuFe2O4, our experiments show the emergence of a complex phase diagram with ferrimagnetic clusters undergoing two glass transitions followed by kinetic arrest at low temperature. Finally, in LPCMO, we will discuss the subtle balance between coexistence of ferromagnetic metal (FMM), charge-ordered insulator (COI) and paramagnetic insulator (PMI) phases that are highly sensitive to strain and dimensionality.