Microscopic Theory of Spin Seebeck Effect in Antiferromagnets
Keisuke Masuda, Masahiro Sato
Abstract
We develop a microscopic theory for the spin Seebeck effect (SSE) in Néel and canted phases of antiferromagnetic insulators.We calculate DC spin current tunneling from an antiferromagnet to an attached metal, incorporating the spin-wave theory and the non-equilibrium Green's function approach.Our result shows a sign change of the spin current at the spin-flop phase transition between Néel and canted phases, which is in agreement with a recent experiment for the SSE on Cr2O3 in a semi-quantitative level.The sign change can be interpreted from the argument based on the density of states of up-spin and down-spin magnons, which is related to polarized-neutron-scattering spectra.The theory also demonstrates that the spin current in the Néel phase is governed by magnon correlation, whereas that in the canted phase consists of two parts: contributions from not only magnon dynamics but also static transverse magnetization.This result leads to a prediction that at sufficiently low temperature, the spin current non-monotonically changes as a function of magnetic field in the canted phase.Towards a more unified understanding of the SSE in antiferromagnets, we discuss some missing pieces in SSE theories: interface properties, effects of the transverse spin moment in the canted phase, spin-orbit coupling in metals, etc.Finally, we compare the SSE of antiferromagnets with that of different magnetic phases such as ferromagnets, ferrimagnets, a one-dimensional spin liquid, a spin-nematic liquid, and a spin-Peierls (dimerized) phase.