Modeling of ultrafast X-ray induced magnetization dynamics in magnetic multilayer systems

Abstract

In this work, we report on modeling results obtained with our recently developed simulation tool enabling nanoscopic description of electronic processes in X-ray irradiated ferromagnetic materials. With this tool, we have studied the response of Co/Pt multilayer system irradiated by an ultrafast extreme ultraviolet pulse at the M-edge of Co (photon energy ~60 eV). It was previously investigated experimentally at the FERMI free-electron-laser facility, using the magnetic small-angle X-ray scattering technique. Our simulations show that the magnetic scattering signal from cobalt decreases on femtosecond timescales due to electronic excitation, relaxation, and transport processes both in the cobalt and in the platinum layers, following the trend observed in the experimental data. The confirmation of the predominant role of electronic processes for X-ray induced demagnetization in the regime below the structural damage threshold is a step toward quantitative control and manipulation of X-ray induced magnetic processes on femtosecond timescales.

Ultrafast demagnetization in bulk nickel induced by X-ray photons tuned to Ni M3 and L3 absorption edges

“…For comparison, the density of states for fcc bulk Co (with 25 ) used in Refs. 6 , 7 is also presented. The calculated magnetic moments of nickel and cobalt are and , respectively, i.e., with a good agreement with those from 9 .…”

“…This X-ray induced electronic excitation changes the population of spin-up and spin-down electrons in the band. As the electronic occupation in the d -band determines the magnetization of the material, this results in the decrease of the total magnetic moment in the material 6 – 8 . In our previous studies 6 , 7 , we modeled the experimentally observed ultrafast decrease of magnetic small-angle X-ray scattering (mSAXS) signal from X-ray irradiated cobalt which reflected a transient decrease of the cobalt magnetic moment.…”

“…As in our previous works 6 – 8 , we will use our recently developed XSPIN code to obtain predictions for the “magnetic signal” from the X-ray irradiated nickel. The electronic density of states is obtained from the density functional theory (DFT) calculations implemented in the Vienna Ab initio Simulation Package (VASP) 18 – 20 .…”

“…For the simulation, we use a simulation box with Ni atoms. We performed averaging over realizations in the Monte Carlo module of the XSPIN code 6 – 8 . The XFEL pulse is assumed to have a Gaussian temporal profile of the duration of 70 fs FWHM (full width at half maximum) for M -edge case ( , eV) and 50 fs FWHM for L -edge case ( , eV).…”

“…The XFEL pulse is assumed to have a Gaussian temporal profile of the duration of 70 fs FWHM (full width at half maximum) for M -edge case ( , eV) and 50 fs FWHM for L -edge case ( , eV). The specific pulse duration was chosen to compare the XSPIN predictions for nickel with our previous results for cobalt presented in 6 – 8 . For more details on the simulation parameters, see Table 1 .…”