Photoemission electron microscopy combined with x-ray magnetic circular dichroism (XMCD-PEEM) is a powerful synchrotron radiation technique that can be used to observe magnetic configurations of nano-objects with high spatial resolution. Combined with time resolution, using stroboscopic detection, the magnetisation dynamics of such objects can be studied with sub-nanosecond resolution.

We will describe the study of domain wall (DW) dynamics and magnetic skyrmions in multilayers of composition Pt/Co/MOx (M=Al, Mg), characterised by a large perpendicular magnetic anisotropy (PMA) and anti-symmetric exchange interaction (Dzyaloshinskii-Moriya interaction, DMI). Such interaction favours non-collinear magnetic textures such as chiral Néel walls and magnetic skyrmions.

In systems with large DMI, such as Pt/Co/AlOx with ultrathin Co, domain walls acquire a Néel chiral structure and move at large speeds when driven by current pulses. Most microscopic studies of domain wall dynamics have been performed using quasi-static measurements, where the DW position and structure are imaged before and after the excitation pulse. In order to better understand the details of the interaction between current and DW magnetisation, we have observed DWs during the application of the current pulses, by synchronising the current pulses with the x-ray pulses, in stroboscopic mode. The measurements reveal that the DWs move without inertia, showing a practically vanishing mass. The negligible inertial effects can be explained taking into account  the narrow domain wall width induced by the large PMA, the large damping parameter and the stabilising effect of the longitudinal field associated to the Dzyaloshinskii-Moriya interaction.

Magnetic skyrmions are nanometer scale whirling spin configurations, predicted in the 80’s but only recently observed with high resolution magnetic microscopies. Their small size, topological protection and the fact that they can be moved by very small current densities call for applications to novel memory and logic devices where skyrmions are the information carriers. Here we report on the experimental observation of magnetic skyrmions in Pt/Co/MgO sputtered ultrathin magnetic nanostructures, stable at room temperature without applied magnetic field. XMCD-PEEM measurements allowed us to observe such skyrmions and to demonstrate their chiral Néel structure. The skyrmion sizes are typically of the order of 120 nm. Our experimental observations are well reproduced by micromagnetic simulations and numerical modelling. This allows the identification of the physical mechanisms governing the size and stability of the skyrmions, which are keys for the design of devices based on their manipulation.

REFERENCES

J. Vogel at al. Phys. Rev. Lett. 108, 247202 (2012)

O.Boulle et al., Nature Nanotechn. 11, 449 (2016)

Figures:

Fig. 1 : (Left) : time-resolved XMCD-PEEM image illustrating the instantaneous domain wall positions within 500nm wide Pt/Co/AlOx nanostripes, during the application of current pulses. (Right) : The domain wall displacements, averaged over the three stripes on the right of the image, as a function of time (black squares, left axis). The driving pulse is also shown (continuous line, right axis). The dashed line is a linear fit to the data points taken during the current pulse.

Fig. 2 : Experimental XMCD-PEEM image (left) and micromagnetic simulation (right) of a chiral Néel skyrmion in a 400 nm dot of Pt/Co/MgO.

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EMC Abstracts - https://emc-proceedings.com/abstract/dynamics-of-magnetic-domain-walls-and-skyrmions-studied-by-high-resolution-xmcd-peem-microscopy/