Laser-induced ultrafast demagnetization time and spin moment in ferromagnets: First-principles calculation

TL;DR

This study uses first-principles simulations to explore how laser-induced demagnetization times in ferromagnets depend nonlinearly on the spin moment, revealing complex dynamics influenced by exchange interactions and spin-orbit coupling.

Contribution

It introduces a first-principles approach to model ultrafast demagnetization, linking spin moment variations to demagnetization times through an extended Heisenberg model.

Findings

Demagnetization time depends nonlinearly on spin moment.

Spin relaxes more slowly when the spin moment is small.

The model incorporates exchange interaction and spin-orbit coupling.

Abstract

When a laser pulse excites a ferromagnet, its spin undergoes a dramatic change. The initial demagnetization process is very fast. Experimentally, it is found that the demagnetization time is related to the spin moment in the sample. In this study, we employ the first-principles method to directly simulate such a process. We use the fixed spin moment method to change the spin moment in ferromagnetic nickel, and then we employ the Liouville equation to couple the laser pulse to the system. We find that in general the dependence of demagnetization time on the spin moment is nonlinear: It decreases with the spin moment up to a point, after which an increase with the spin moment is observed, followed by a second decrease. To understand this, we employ an extended Heisenberg model, which includes both the exchange interaction and spin-orbit coupling. The model directly links the demagnetization rate to the spin moment itself and demonstrates analytically that the spin relaxes more slowly with a small spin moment. A future experimental test of our predictions is needed.