All-optical spin switching: A new frontier in femtomagnetism -- A short review and a simple theory

TL;DR

This paper reviews the emerging field of all-optical spin switching in femtomagnetism, highlighting experimental findings, mechanisms, and introducing a simple theoretical model that explains spin reversal under ultrafast laser pulses.

Contribution

It presents a new all-optical spin switching rule based on light helicity and introduces a harmonic oscillator model to simulate spin reversal, advancing understanding in femtomagnetism.

Findings

Circularly polarized light is more effective for spin switching.

The harmonic oscillator model accurately simulates spin reversal.

The inverse Faraday effect is explained through the model.

Abstract

Using an ultrafast laser pulse to manipulate the spin degree of freedom has broad technological appeal. It allows one to control the spin dynamics on a femtosecond time scale. The discipline, commonly called femtomagnetism, started with the pioneering experiment by Beaurepaire and coworkers in 1996, who showed subpicosecond demagnetization occurs in magnetic Ni thin films. This finding has motivated extensive research worldwide. All-optical helicity-dependent spin switching (AOS) represents a new frontier in femtomagnetism, where a single ultrafast laser pulse can permanently switch spin without any assistance from a magnetic field. This review summarizes some of the crucial aspects of this new discipline: key experimental findings, leading mechanisms, controversial issues, and possible future directions. The emphasis is on our latest investigation. We first develop the all-optical spin switching rule that determines how the switchability depends on the light helicity. This rule allows one to understand microscopically how the spin is reversed and why the circularly polarized light appears more powerful than the linearly polarized light. Then we invoke our latest spin-orbit coupled harmonic oscillator model to simulate single spin reversal. We consider both cw excitation and pulsed laser excitation. The results are in a good agreement with the experimental result. We then extend the code to include the exchange interaction among different spin sites. We show where the "inverse Faraday field" comes from and how the laser affects the spin reversal nonlinearly. Our hope is that this review will motivate new experimental and theoretical investigations and discussions.