Finite-frequency spin conductance of a ferro-/ferrimagnetic-insulator|normal-metal interface

TL;DR

This paper extends the theory of longitudinal spin transport at magnetic insulator|metal interfaces by calculating the spin conductance non-perturbatively and at finite frequencies, revealing significant effects at low temperatures.

Contribution

It provides a non-perturbative and finite-frequency analysis of longitudinal spin conductance, improving understanding of spin transport in YIG|Pt interfaces.

Findings

Longitudinal spin conductance is about 40% smaller than perturbative predictions.

Finite-frequency effects are significant only below 100 K.

Low-temperature corrections involve few thermally occupied magnon modes.

Abstract

The interface between a ferro-/ferrimagnetic insulator and a normal metal can support spin currents polarized collinear with and perpendicular to the magnetization direction. The flow of angular momentum perpendicular to the magnetization direction ("transverse" spin current) takes place via spin torque and spin pumping. The flow of angular momentum collinear with the magnetization ("longitudinal" spin current) requires the excitation of magnons. In this article we extend the existing theory of longitudinal spin transport [Bender and Tserkovnyak, Phys. Rev. B 91, 140402(R) (2015)] in the zero-frequency weak-coupling limit in two directions: We calculate the longitudinal spin conductance non-perturbatively (but in the low-frequency limit) and at finite frequency (but in the limit of low interface transparency). For the paradigmatic spintronic material system YIG|Pt, we find that non-perturbative effects lead to a longitudinal spin conductance that is ca. 40% smaller than the perturbative limit, whereas finite-frequency corrections are relevant at low temperatures < 100 K only, when only few magnon modes are thermally occupied.