Contributed by Pani Varanasi, Program Manager, Physical Properties of Materials, Materials Science Division, Army Research Office:
Thermal transport in materials by phonons and electrons is relatively well understood as compared to heat transport in materials by spin excitations (i.e., thermal conductivity in materials due to magnons). Although high thermal conductivity due to magnons has been observed in a few oxide materials, little progress has been made to date in applying them to practical applications as the processes are currently limited to low temperatures. A complete basic understanding of the mechanisms of interactions such as magnon-phonon, magnon-defect, magnon-magnon and other factors that influence magnon mean-free paths in the materials does not exist.
Basic research on these material systems could result in the discovery of unique compositions of advanced materials with enhanced thermal conductivity properties at high temperatures, which would expand potential areas of applications.
Recent experimental results indicate that extraordinary ballistic (dissipation-less) spin thermal transport is possible to occur in spin chain (SrCuO2) compounds. It is shown that by defect engineering (by reducing the number of impurities) thermal conductivity of high-purity SrCuO2 single-crystal samples can be enhanced ~4 times at 50K.
In addition, novel material compositions such as Ca9La5Cu24O41 spin ladder compounds also have been shown to achieve an order of magnitude improvement in thermal conductivity as compared to other spin ladder compounds such as Sr14Cu24O41 at room temperature. These new compounds show a high thermal conductivity of ~100 watts per meter kelvin at 300K as compared to Si (142 watts per meter kelvin) and Cu (400 watts per meter kelvin). Although these materials have anisotropic thermal conductivity properties (higher conductivity along the c-axis than along a-axis), the advantage is that these materials are electrically insulating as compared to Si and Cu.
Finally, researchers have demonstrated magnetic field tunable thermal conductivity in some anti-ferromagnetic materials such as K2V3O8.
Fundamental understanding of the magnon interactions and processing science related to these materials is expected to lead to the accelerated discovery of novel materials with unprecedented spin-mediated thermal properties at high temperatures, which will lead to many practical thermal management applications.
In addition to heat transport (thermal conductivity), a “spin-Seebeck” effect has also been observed in ferromagnetic materials. The spin-Seebeck effect refers to the generation of voltage in a ferromagnetic material when it is placed in a temperature gradient due to redistribution of spins. Uchida et al. used an inverse spin-Hall Effect measurement method to investigate the spin-Seebeck effect in ferromagnetic materials and reported for the first time experimental evidence for the effect in NiFe metallic alloy materials.
Although some theoretical studies are ongoing (e.g., influence of phonon drag on the spin-Seebeck effect, magnon driven spin-Seebeck effect, etc.), we also need more detailed studies to thoroughly understand the physics of the observed effect. In particular, we need this basic understanding to develop new materials and structures with enhanced figure of merit for practical cooling and power generation applications.
While spintronics deals with the charge transport and electron spin, the new emerging field of spin caloritronics deals with spins and thermal transport. Basic research in this area of spin-mediated thermal properties of materials offers unique opportunities for breakthrough research in physics and materials science, and could result in the discovery of materials with unprecedented thermal properties.
Long-term applications include thermoelectric power generators, temperature sensors, advanced spintronics devices, thermal management (e.g., cooling of electronics, IR detectors, etc) and switchable or tunable heat conduction (via magnetic field, light, microwaves, etc.).
For more information about ARO interests in these areas, please email me.