Ultrafast Demagnetization Excited by Extreme Ultraviolet Light From a Free-Electron Laser

Abstract

Ultrashort and intense extreme ultraviolet (XUV) and X-ray pulses readily available at free-electron lasers (FELs) enable studying non-linear light−matter interactions on femtosecond timescales. Here, we report on the non-linear fluence dependence of magnetic scattering of Co/Pt multilayers, using FERMI FEL’s 70-fs-long single and double XUV pulses, the latter with a temporal separation of 200 fs, with a photon energy slightly detuned to the Co M2,3 absorption edge. We observe a quenching in magnetic scattering that sets-in already in the non-destructive fluence regime of a few mJ/cm² typically used for FEL-probe experiments on magnetic materials. Calculations of the transient electronic structure in tandem with a phenomenological modeling of the experimental data by means of ultrafast demagnetization unambiguously show that XUV-radiation-induced demagnetization is the dominant mechanism for the quenching in the investigated fluence regime of <50 mJ/cm², while light-induced changes of the electronic core levels are predicted to additionally occur at higher fluences. The modeling of the data further indicates that the demagnetization proceeds on the sub-20-fs timescale. This ultrashort timescale is consistent with non-coherent models for ultrafast demagnetization, considering the sub-femtosecond lifetime of hot electrons with energies of a few 10 eV generated by the XUV radiation.

Role of electronic excitation, relaxation and transport processes for X-ray induced ultrafast demagnetization within magnetic multilayer systems

“…The transient intensity of the resonant magnetic scattering signal (per unit surface), I magn (t), (cf. [10,22,42]) is:…”

“…Historically, since its discovery in 1996 [6], ultrafast demagnetization on sub-picosecond timescales was studied mostly with lasers working in the infrared wavelength regime [7][8][9]. X-ray FELs provide not only a unique opportunity to probe magnetic properties of solids on femtosecond timescales and at nanometer length scales but they also enable to study ultrafast demagnetization induced by photons of much higher energies than those accessible with optical lasers [10][11][12][13][14][15][16][17][18]. This is possible with resonant X-ray magnetic scattering [19][20][21].…”

“…In particular, for samples characterized by a strong perpendicular magnetic anisotropy, the latter scheme gives access to X-ray induced ultrafast changes within magnetic domains [24,25]. Let us emphasize that in such experiments [10][11][12] the X-ray pulse serves both as a pump and as a probe, exciting the material and simultaneously probing the excited state with magnetic scattering.…”

“…It depends, among others, on the difference between the incoming photon energy and the resonant energy, and on the actual material magnetization, M . As the resonant magnetic scattering strength, F m1 , is proportional to the magnetization of the sample [10,12], any changes of the magnetization within magnetic domains will be reflected by the change of the scattering signal. This, in particular, implies that any demagnetization of the sample will cause a decrease of the magnetic signal.…”

“…With the XSPIN tool, we analyze the results of a recent experiment on resonant magnetic scattering with ultrashort XUV pulses (tuned to the M-edge of cobalt) from Co/Pt multilayer system with perpendicular magnetic anisotropy [27]. The experiment was performed at the FERMI FEL facility [10]. In particular, we demonstrate that the processes of electronic excitation, relaxation and transport induced by XUV radiation predominantly affect the behaviour of the transient magnetization and hence the scattered magnetic signal.…”