Magnons versus electrons in thermal spin transport through metallic interfaces

We develop a theory for spin transport in magnetic metals that treats the contribution of magnons and electrons on equal footing. As an application we consider thermally-driven spin injection across an interface between a magnetic metal and a normal metal, i.e., the spin-dependent Seebeck effect. We show that the ratio between magnonic and electronic contribution scales as $\sqrt{T/T_C}T_F/T_C$, with the Fermi temperature $T_F$ and the Curie temperature $T_C$. Since, typically, $T_C \ll T_F$, the magnonic contribution may dominate the thermal spin injection, even though the interface is more transparent for electronic spin current.Comment: Contribution to the Special issue on Spincaloritronics in Journal of Physics D: Applied Physic

Giant Magnetothermal Conductivity Switching in Semimetallic WSi2_{2} Single Crystals

Materials able to rapidly switch between thermally conductive states by external stimuli such as electric or magnetic fields can be used as all-solid-state thermal switches and open a myriad of applications in heat management, power generation and cooling. Here, we show that the large magnetoresistance that occurs in the highly conducting semimetal $\alpha$-WSi$_{2}$ single crystals leads to dramatically large changes in thermal conductivity at temperatures <100 K. At temperatures <20 K, where electron-phonon scattering is minimized, the thermal conductivity switching ratio between zero field and a 9T applied field can be >7. We extract the electronic and lattice components of the from the thermal conductivity measurements and show that the Lorenz number for this material approximates the theoretical value of L$_{0}$. From the heat capacity and thermal diffusivity, the speed of thermal conductivity switching is estimated to range from 1 x 10$^{-4}$ seconds at 5 K to 0.2 seconds at 100 K for a 5-mm long sample. This work shows that WSi$_{2}$, a highly conducting multi-carrier semimetal, is a promising thermal switch component for low-temperature applications such cyclical adiabatic demagnetization cooling, a technique that would enable replacing $^{3}$He-based refrigerators.Comment: 20 pages, 6 figure

Magnon-drag thermopower and Nernst coefficient in Fe, Co, and Ni

Magnon-drag is shown to dominate the thermopower of elemental Fe from 2 to 80 K and of elemental Co from 150 to 600 K; it is also shown to contribute to the thermopower of elemental Ni from 50 to 500 K. Two theoretical models are presented for magnon-drag thermopower. One is a hydrodynamic theory based purely on non-relativistic, Galilean, spin-preserving electron-magnon scattering. The second is based on spin-motive forces, where the thermopower results from the electric current pumped by the dynamic magnetization associated with a magnon heat flux. In spite of their very different microscopic origins, the two give similar predictions for pure metals at low temperature, allowing us to semi-quantitatively explain the observed thermopower of elemental Fe and Co without adjustable parameters. We also find that magnon-drag may contribute to the thermopower of Ni. A spin-mixing model is presented that describes the magnon-drag contribution to the Anomalous Nernst Effect in Fe, again enabling a semi-quantitative match to the experimental data without fitting parameters. Our work suggests that particle non-conserving processes may play an important role in other types of drag phenomena, and also gives a predicative theory for improving metals as thermoelectric materials.Comment: main text plus 7 figures; accepted in PRB September 201