Theoretical analysis of electronic processes occurring during ultrafast demagnetization of cobalt triggered by X-ray photons tuned to Co L3_3 resonance

Magnetization dynamics triggered with ultrashort laser pulses has been attracting significant attention, with strong focus on the dynamics excited by VIS/NIR pulses. Only recently, strong magnetic response in solid materials induced by intense X-ray pulses from free-electron lasers (FELs) has been observed. The exact mechanisms that trigger the X-ray induced demagnetization are not yet fully understood. They are subject of on-going experimental and theoretical investigations. Here, we present a theoretical analysis of electronic processes occurring during demagnetization of Co multilayer system irradiated by X-ray pulses tuned to L$_3$-absorption edge of cobalt. We show that, similarly as in the case of X-ray induced demagnetization at M-edge of Co, electronic processes play a predominant role in the demagnetization until the pulse fluence does not exceed the structural damage threshold. The impact of electronic processes can reasonably well explain the available experimental data, without a need to introduce the mechanism of stimulated elastic forward scattering.Comment: 10 pages, 4 figures (7 panels), 57 references; pdfRevTeX class; double column formatting; two appendices and 18 references added; author-created version submitted to and accepted in Physical Review B journal. arXiv admin note: text overlap with arXiv:2202.1384

Enabling time-resolved 2D spatial-coherence measurements using the Fourier-analysis method with an integrated curved-grating beam monitor

Direct 2D spatial-coherence measurements are increasingly gaining importance at synchrotron beamlines, especially due to present and future upgrades of synchrotron facilities to diffraction-limited storage rings. We present a method to determine the 2D spatial coherence of synchrotron radiation in a direct and particularly simple way by using the Fourier-analysis method in conjunction with curved gratings. Direct photon-beam monitoring provided by a curved grating circumvents the otherwise necessary separate determination of the illuminating intensity distribution required for the Fourier-analysis method. Hence, combining these two methods allows for time-resolved spatial-coherence measurements. As a consequence, spatial-coherence degradation effects caused by beamline optics vibrations, which is one of the key issues of state-of-the-art X-ray imaging and scattering beamlines, can be identified and analyzed. © 2020 Optical Society of America

Thermal conductance of interfaces with amorphous SiO2SiO_{2} measured by time-resolved magneto-optic Kerr-effect thermometry

We use time-resolved magneto-optic Kerr effect and ultrathin Co/Pt transducer films to perform thermal-transport experiments with higher sensitivity and greater time resolution than typically available in studies of interfacial thermal transport by time-domain thermoreflectance. We measure the interface conductance between Pt and amorphous SiO$_2$ using Pt/Co/Pt ferromagnetic transducer films with thicknesses between 4.2 and 8.2 nm and find an average value of $G_{Pt} ≈$ 0.3 GW m$^{−2} K^{−1}$. This result demonstrates that interfaces between metals and amorphous dielectrics can have a conductance corresponding to Kapitza lengths of the order of 4 nm, and are thus of relevance when engineering nanoscale devices. For thin SiO$_2$ layers, our method also provides sensitivity to the interface conductance between SiO$_2$ and Si and we find $G_{Si} = 0.6 GW m^{−2} K^{−1}$ as the lower limit

Impact of Symmetry on Anisotropic Magnetoresistance in Textured Ferromagnetic Thin Films

We report on the magnetoresistance of textured films consisting of 3d-ferromagnetic layers sandwiched by Pt. While the conventional cos2φ behavior of the anisotropic magnetoresistance (AMR) is found when the magnetization M is varied in the film plane, cos2nθ contributions (2n≤6) exist for rotating M in the plane perpendicular to the current. This finding is explained by the symmetry-adapted modeling of AMR of textured films demonstrating that the cos2θ behavior cannot be used as a fingerprint for the presence of spin Hall magnetoresistance (SMR). Further, the interfacial MR contributions for Pt/Ni/Pt contradict the SMR behavior confirming the dominant role of AMR in all-metallic systems

Electronic processes occurring during ultrafast demagnetization of cobalt, triggered by X-ray photons tuned to Co L3 resonance

Magnetization dynamics triggered by ultrashort laser pulses has been attracting significant attention, with a strong focus on the dynamics excited by visible/near-infrared pulses. Only recently has a strong magnetic response in solid materials induced by intense x-ray pulses from free-electron lasers been observed. The exact mechanisms that trigger the x-ray-induced demagnetization are not yet fully understood. They are the subject of ongoing experimental and theoretical investigations. Here, we present a theoretical analysis of electronic processes occurring during demagnetization of a Co multilayer system irradiated by x-ray pulses tuned to the $L_3$ absorption edge of cobalt. We show that, like in the case of x-ray-induced demagnetization at the $M$ edge of Co, electronic processes play a predominant role in the demagnetization until the pulse fluence does not exceed the structural damage threshold. The impact of electronic processes can explain reasonably well the available experimental data, without a need to introduce the mechanism of stimulated elastic forward scattering

Quenching of the Resonant Magnetic Scattering by Ultra-Short Free-Electron Laser Pulses

With the advent of free-electron lasers (FELs), new opportunities haveemerged for studying dynamics in matter on ultra-fast time and ultra-shortlength scales simultaneously. As one of the forefront topics within contemporaryresearch on magnetism, studies of ultrafast laser-induced magnetizationdynamics [1] have benefited from these recent developments – revealinga nanoscale spatial response in a domain system coupled to the demagnetizationprocess [2] or transfer of angular momentum between two magneticcompounds in an inhomogeneous magnetic alloy [3]. Such studies of magnetizationdynamics rely on achieving magnetic scattering contrast through theX-ray magnetic circular dichroism (XMCD) effect by tuning the incidentphoton energy resonantly to one of the dichroic M or L absorption edges ofthe magnetic element. However, at extreme FEL fluences, a quenching ofthe resonant magnetic scattering signal was recently observed [4], indicatingthat the FEL radiation does not only act as a probe but also strongly interactswith the sample, effectively altering it already on a time scale shorter thanthe pulse duration of 70 fs. Here, we report on resonant magnetic small-angleX-ray scattering (mSAXS) experiments at the cobalt M3-edge, performed onCo/Pt multilayers with perpendicular magnetic anisotropy showing 100-nmscale magnetic domain patterns, with the purpose of investigating the FELfluence dependence of the resonant magnetic scattering signal. Our results(Figure 1) show a significant reduction (quenching) of the magnetic scatteringstrength with increasing FEL fluence. The intra-pulse quenchingeffect already sets in at much lower fluences than previously expected,revealing the violation of the credo ‘diffract before destruct’ already forthe low fluence regime where the sample is not destroyed by a single-pulseirradiation