Temperature dependence of magnon transport*

Both ferromagnetic and antiferromagnetic materials can carry magnon currents at finite temperature. We present a generalized transport theory of non-equilibrium magnons in magnetic metals and insulators driven by either the electric field or thermal gradient. We find that the magnon transport resembles most of the electron spin transport as far as angular momentum currents are concerned. The key difference of magnon and electron spin transport is the temperature dependence because the number of carriers for magnons is strongly temperature-dependent, but not for electron spins. We apply our magnon transport theory to study several cases. First, we address the electron spin and magnon accumulation at metallic magnetic interfaces and find the magnon accumulation could be much larger than the electron spin accumulation. Second, we discuss the magnon transport in an all-insulator spin valve consisting of two ferromagnetic insulator sandwiched by an antiferromagnetic insulator. We find two profound effects when a thermal gradient is applied to the trilayer: the magneto spin-Seebeck effect and the magnon transfer torque. This work was in collaboration with K. Chen and Y. Cheng at the University of Arizona and with W. Lin and C. L. Chien at the Johns Hopkins University.