arXiv subjects

Stephen R. Boona

Publications and source records attributed to Stephen R. Boona.

4 recordsLinked to original sources

arXiv2021

Magnon-induced Giant Anomalous Nernst Effect in Single Crystal MnBi

Thermoelectric modules are a promising approach to energy harvesting and efficient cooling. In addition to the longitudinal Seebeck effect, recently transverse devices utilizing the anomalous Nernst effect (ANE) have attracted interest. For high conversion efficiency, it is required that the material should have a large ANE thermoelectric power and low electrical resistance, the product of which is the ANE conductivity. ANE is usually explained in terms of intrinsic contributions from Berry curvature. Our observations suggest that extrinsic contributions also matter. Studying single-crystal MnBi, we find a very high ANE thermopower (~10 $μ$V/K) under 0.6 T at 80 K, and a transverse thermoelectric conductivity of over 40 A/Km. With insight from theoretical calculations, we attribute this large ANE predominantly to a new advective magnon contribution arising from magnon-electron spin-angular momentum transfer. We propose that introducing large spin-orbit coupling into ferromagnetic materials may enhance the ANE through the extrinsic contribution of magnons.

cond-mat.mtrl-sci↗

arXiv2016

Observation of spin Seebeck contribution to the transverse thermopower in Ni-Pt and MnBi-Au bulk nanocomposites

Transverse thermoelectric devices produce electric fields perpendicular to an incident heat flux. Classically, this process is driven by the Nernst effect in bulk solids, wherein a magnetic field generates a Lorentz force on thermally excited electrons. The spin Seebeck effect (SSE) also produces magnetization-dependent transverse electric fields. SSE is traditionally observed in thin metallic films deposited on electrically insulating ferromagnets, but the films' high resistance limits thermoelectric conversion efficiency. Combining Nernst and SSE in bulk materials would enable devices with simultaneously large transverse thermopower and low electrical resistance. Here we demonstrate experimentally this is possible in composites of conducting ferromagnets (Ni or MnBi) containing metallic nanoparticles with strong spin-orbit interactions (Pt or Au). These materials display positive shifts in transverse thermopower attributable to inverse spin Hall electric fields in the nanoparticles. This more than doubles the power output of the Ni-Pt materials, establishing proof-of-principle that SSE persists in bulk nanocomposites.

cond-mat.mtrl-sci↗

arXiv2016

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.

cond-mat.mtrl-sci↗

arXiv2015

The effect of the magnon dispersion on the longitudinal spin Seebeck effect in yttrium iron garnets (YIG)

We study the temperature dependence of the longitudinal spin-Seebeck effect (LSSE) in a yttrium iron garnet Y3Fe5O12 (YIG) / Pt system for samples of different thicknesses. In this system, the thermal spin torque is magnon-driven. The LSSE signal peaks at a specific temperature that depends on the YIG sample thickness. We also observe freeze-out of the LSSE signal at high magnetic fields, which we attribute to the opening of an energy gap in the magnon dispersion. We observe partial freeze-out of the LSSE signal even at room temperature, where kBT is much larger than the gap. This suggests that a subset of the magnon population with an energy below kB x TC (TC about 40 K) contribute disproportionately to the LSSE; at temperatures below TC, we label these magnons subthermal magnons. The T-dependence of the LSSE at temperatures below the maximum is interpreted in terms of a new empirical model that ascribes most of the temperature dependence to that of the thermally driven magnon flux.

cond-mat.mtrl-sci↗