Femtosecond Control of the Magnetization in Ferromagnetic Semiconductors

TL;DR

This paper presents a theoretical model of femtosecond-scale magnetization dynamics in ferromagnetic semiconductors triggered by ultrafast optical pulses, revealing controllable light-induced magnetization tilts and nonlinear effects.

Contribution

The study introduces a comprehensive theoretical framework including density matrix equations and a Landau-Gilbert-like equation to describe ultrafast spin dynamics and magnetic anisotropy effects.

Findings

Prediction of light-induced magnetization tilt during ultrashort pulses

Derivation of a Landau-Gilbert-like equation for collective spin

Control of magnetization dynamics via optical pulse parameters

Abstract

We develop a theory of collective spin dynamics triggered by ultrafast optical excitation of ferromagnetic semiconductors. Using the density matrix equations of motion in the mean field approximation and including magnetic anisotropy and hole spin dephasing effects, we predict the development of a light--induced magnetization tilt during ultra--short time intervals comparable to the pulse duration. This femtosecond dynamics in the coherent temporal regime is governed by the interband nonlinear optical polarizations and is followed by a second temporal regime governed by the magnetic anisotropy of the Fermi sea. We interpret our numerical results by deriving a Landau--Gilbert--like equation for the collective spin, which demonstrates an ultrafast correction to the magnetic anisotropy effective field due to second order coherent nonlinear optical processes. Using the Lindblad semigroup method, we also derive a contribution to the interband polarization dephasing determined by the Mn spin and the hole spin dephasing. Our predicted magnetization tilt and subsequent nonlinear dynamics due to the magnetic anisotropy can be controlled by varying the optical pulse intensity, duration, and helicity and can be observed with pump--probe magneto--optical spectroscopy.