The influence of the excitation pulse length on ultrafast magnetization dynamics in nickel.

A Fognini,R Gort,T Michlmayr,A Vaterlaus,Y Acremann,G Salvatella

doi:10.1063/1.4914891

Abstract

The laser-induced demagnetization of a ferromagnet is caused by the temperature of the electron gas as well as the lattice temperature. For long excitation pulses, the two reservoirs are in thermal equilibrium. In contrast to a picosecond laser pulse, a femtosecond pulse causes a non-equilibrium between the electron gas and the lattice. By pump pulse length dependent optical measurements, we find that the magnetodynamics in Ni caused by a picosecond laser pulse can be reconstructed from the response to a femtosecond pulse. The mechanism responsible for demagnetization on the picosecond time scale is therefore contained in the femtosecond demagnetization experiment.

Highlights

For ultrafast demagnetization of a ferromagnet,1–4 spin angular momentum needs to be transported away from the spin system
By pump pulse length dependent optical measurements, we find that the magnetodynamics in Ni caused by a picosecond laser pulse can be reconstructed from the response to a femtosecond pulse
The desired effect of the laser pulse for heat assisted magnetic recording (HAMR) lies in the softening of the magnetic bit, it leads to a reduction of the magnetic moment simultaneously, a process investigated in this experiment

Summary

INTRODUCTION

For ultrafast demagnetization of a ferromagnet, spin angular momentum needs to be transported away from the spin system. If the ferromagnet is heated by a femtosecond laser pulse, the electron gas initially reaches a temperature, which exceeds the lattice temperature. This initial temperature rise in the electron gas is followed by thermalization with the lattice to a common temperature. In case of excitation by a picosecond laser pulse, the electron gas and the lattice stay close to thermal equilibrium during the whole demagnetization process. For the two main models of ultrafast demagnetization, the Elliot-Yafet spin flip scattering model as well as the superdiffusion model, the excited electron gas is the driving force for the femtosecond magnetization dynamics. In case of the Elliot-Yafet mechanism, the lattice temperature promotes the high spin flip probability necessary for demagnetization. Achieved by studying the ultrafast demagnetization dynamics of a Ni film as a function of the pump pulse length

EXPERIMENTAL SETUP

RESULTS

Findings

CONCLUSIONS