Non-equilibrium magnetic fields in ab initio spin dynamics

TL;DR

This paper derives an expression for effective magnetic fields in out-of-equilibrium spin dynamics and evaluates their role in ultrafast demagnetization in iron-based materials, revealing localized regions of enhanced demagnetization.

Contribution

It introduces a new theoretical framework for calculating kinetic magnetic fields in time-dependent spin systems and applies it to real materials.

Findings

Identification of spatial 'hot spots' with enhanced demagnetization

Demonstration of the role of kinetic magnetic fields in ultrafast spin dynamics

Application to both cluster and bulk iron-based systems

Abstract

Starting from the continuity equation for the magnetization in time-dependent spin-density functional theory, we derive an expression for the effective time-dependent magnetic fields driving the out-of-equilibrium spin dynamics in magnetic systems. We evaluate these, so called, kinetic magnetic fields in the ultrafast demagnetization response to optical pulse excitations of ferromagnetic iron-based materials, namely Fe$_6$ cluster and bulk bcc Fe. We identify spatial "hot spots" where the demagnetization is particularly enhanced as a result of the increased kinetic torque.