Ultrafast demagnetization in the sp-d model: a theoretical study

TL;DR

This paper presents a theoretical model explaining ultrafast demagnetization in ferromagnetic semiconductors through energy and angular momentum exchange between localized spins and hot carriers, emphasizing the role of carrier spin relaxation.

Contribution

It introduces a comprehensive theoretical framework for ultrafast magnetization dynamics in sp-d coupled systems, specifically applied to ferromagnetic semiconductors, incorporating band structure effects.

Findings

Fast carrier spin relaxation facilitates demagnetization.

The model reproduces observed ultrafast demagnetization times.

Energy exchange dynamics are characterized within the sp-d interaction.

Abstract

We propose and analyze a theoretical model of ultrafast light-induced magnetization dynamics in systems of localized spins that are coupled to carriers' spins by sp-d exchange interaction. A prominent example of a class of materials falling into this category are ferromagnetic (III,Mn)V semiconductors, in which ultrafast demagnetization has been recently observed. In the proposed model light excitation heats up the population of carriers, taking it out of equilibrium with the localized spins. This triggers the process of energy and angular momentum exchange between the two spin systems, which lasts for the duration of the energy relaxation of the carriers. We derive the Master equation for the density matrix of a localized spin interacting with the hot carriers and couple it with a phenomenological treatment of the carrier dynamics. We develop a general theory within the sp-d model and we apply it to the ferromagnetic semiconductors, taking into account the valence band structure of these materials. We show that the fast spin relaxation of the carriers can sustain the flow of polarization between the localized and itinerant spins leading to significant demagnetization of the localized spin system, observed in (III,Mn)V materials.