NSLS-II Friday Lunchtime Seminar

Nanometer-scale spin configurations such as magnetic domains walls (DW) or skyrmions are attractive as information entities for spintronic applications as they can be generated and manipulated by electrical spin-polarized currents. Naturally, the function of such devices is crucially determined by the thermal stability of the magnetic configuration used for encoding the information. In our study, we have investigated thermally activated magnetic DW dynamics on timescales ranging from sub-seconds to hours under equilibrium conditions in a thin-film magnetic multilayer material based on 15 repetitions of Pt/CoFeB/MgO trilayers. Such multilayers were already successfully used to demonstrate the basic operation of a skyrmion-based racetrack memory [1]. For our investigations, we developed a new experimental approach combining real-space imaging via Fourier-transform holography [2] and x-ray photon correlation spectroscopy [3]. Both methods rely on detecting coherent far-field diffraction from a disordered sample —a pattern of magnetic up and down domains in our case. Magnetic contrast is achieved by tuning the wavelength of circularly polarized x-rays to the Co L3 absorption edge (1.6 nm). For slow timescales (> 3 min), the analysis is based on the difference of scattering patterns recorded with opposite x-ray helicity (Fig. 1a). On one hand, the Fourier inversion of this difference results in a real-space image of the domains in the field of view (FOV) defined by a circular optics mask on the sample (Fig. 1b). On the other hand, we use the difference as input for an adapted temporal correlation analysis. Already at slightly elevated temperatures (310 K), the resulting two-time correlation function of the magnetic configuration at times t1 and t2 (Fig. 1c) reveals time periods of high correlation, i.e., high stability interrupted by sudden extensive domain rearrangements as witnessed by the related images.