Effect of exchange-correlation spin-torque on spin dynamics

TL;DR

This paper investigates a new exchange-correlation functional in density functional theory that produces a non-zero internal spin-torque, significantly affecting spin dynamics in certain materials, especially when magneto crystalline anisotropy is small.

Contribution

The study extends a source-free exchange-correlation functional to the time domain and analyzes its impact on laser-induced spin dynamics in metals and interfaces.

Findings

Internal spin-torque is significant in bulk materials with small anisotropy.

At surfaces/interfaces with large anisotropy, spin-torque effects are minimal.

Spin-dynamics due to internal spin-torque are slower than other spin transfer mechanisms.

Abstract

A recently proposed exchange-correlation functional (Dewhurst et al. 10.1021/acs.jctc.7b01049) with in density functional theory, which ensures that the exchange-correlation magnetic field is source-free, is shown to give non-zero internal spin-torque. This spin-torque is identically zero for all conventional local and semi-local functionals. Extension of this source-free functional to the time domain is used to study the effect of the internal spin-torque on the laser induced spin-dynamics in bulk Co, Ni and interfaces of these metals with Pt. It is shown that the internal spin-torque contribute significantly to spin-dynamics only when the magneto crystalline anisotropy energy is small, as in the case of cubic bulk materials. For surfaces or interfaces, where the anisotropy energy is large, these torques are too small to cause any significant precession of spins in early times ($<$ 100fs). Further more it is shown that the spin-dynamics caused by the internal spin-torque is slow compared to the inter-site spin transfer and spin-orbit mediated spin-flips.