Ab-initio study of ultrafast spin dynamics in Gdx(FeCo){1-x} alloys

TL;DR

This study uses ab-initio simulations to investigate femtosecond spin dynamics in Gd-FeCo alloys under ultrashort laser pulses, revealing transient ferromagnetism, differential demagnetization rates, and spin reorientation phenomena.

Contribution

It provides new insights into ultrafast spin behavior in Gd-FeCo alloys, including predictions on transient ferromagnetism and spin reorientation that can be experimentally tested.

Findings

Ultrafast laser pulses induce charge transfer from Fe to Gd states.

Gd moments develop in-plane orientation within femtoseconds.

A transient ferromagnetic state emerges for certain compositions.

Abstract

Using an ultrashort laser pulse we explore {\it ab-initio} the spin dynamics of Gd$_x$(FeCo)$_{1-x}$ at femtosecond time scales. Optical excitations are found to drive charge from Fe majority $d$-states to the unoccupied Gd $f$-minority states, with $f$-electron character excited occupation lagging behind that of the $d$-electron character, leading to substantial demagnetisation of both species while leaving the global moment almost unchanged. For $x > 0.33$ this results in the creation of an ultrafast ferromagnetic (FM) transient by the end of the laser pulse, with the Gd demagnetization rate slower than that of Fe. For all concentrations the Gd moments begin to rotate from their ground state orientations developing in-plane moments of between 0.2-0.5~$\mu_B$. Thus, the ultrafast spin dynamics of the material captures three important ingredients of the all optical switching that occurs at much later (picosecond) times: (i) the development of a FM transient, (ii) the different rates of demagnetisation of Fe and Gd and, (iii), the breaking of the colinear symmetry of the ground state. Furthermore, several predictions are made about the behaviour of Fe-Gd alloys that can be experimentally tested and can lead to a spin-filtering device.