Surprising insights from fluctuating magnetic textures: from topological phase transitions to a new form of pinning
Felix Büttner, Helmholtz-Zentrum Berlin
November 12, 2021
Fluctuations are ubiquitous at the nanometer scale. This general statement also applies to the emergent stripe domain and skyrmion states in thin-film perpendicular magnetic materials. Examples of known fluctuation-driven effects are domain wall creep motion, Barkhausen jumps, and skyrmion nucleation by an out-of-plane polarized spin current. However, this is just the tip of the iceberg – the majority of fluctuation-driven physics happen on length- and timescales that have been inaccessible to our existing instruments due to limited spatial and/or temporal resolution.
Here, I will present two surprising insights from fluctuating magnetic textures which we were able to resolve using advanced coherent x-ray scattering and imaging techniques. The first part of my talk will focus on the phenomenon of all-optical topological switching, i.e., on the nucleation of a large number of magnetic skyrmions from a single ultrafast laser pulse. This effect, first observed by Berruto et al.  and Je et al. , is surprising because of the enormous topological energy barrier of several hundred kT at room temperature for the nucleation of a single skyrmion, which one expects in typical multilayer materials based on micromagnetic models . Due to this energy barrier, ultrafast nucleation of an entire array of skyrmions appears to be impossible. Pump-probe small-angle resonant x-ray scattering at the new European x-ray free-electron laser (XFEL) reveals the solution to this puzzle : the laser pulses drive the magnetic multilayer into a fluctuation state, where exchange energy and the effective film thickness are largely reduced, such that the topological energy barrier is effectively eliminated. Atomistic models show that in this fluctuation state, skyrmion nuclei can form rapidly and in large quantities, even without their topological antiskyrmion counterparts. This form of switching constitutes the first example of an ultrafast phase transition which includes a change of the global topological charge.
In the second part of the talk, I will present a new technique to image such fluctuating states of matter in real space. The technique, which we call coherent correlation imaging (CCI) , is illustrated in the Figure. In brief, CCI is a form of coherent x-ray imaging where real-space images are numerically reconstructed from coherent scattering patterns. The novelty is that CCI uses speckle fingerprints in the scattering patterns to label them, i.e., to identify which patterns correspond to the same state. This is possible at 100 times less photons than required for a real-space image, which therefore improves the temporal resolution of imaging by a factor 100 and ultimately also enables non-destructive single-shot imaging at XFEL sources. Using this technique, we study thermally-activated transitions between stripe domain states in a Pt/CoFeB/MgO skyrmion material. We uncover an intricate state transition network of between 36 different stripe domain states. Remarkably, the transitions do not follow the expected Arrhenius behavior reported earlier for domain wall hopping. Instead, we find two new effects. First, long-range stray-field interactions provide a relevant dynamic contribution to the energy landscape, which dynamically changes the lifetime of a state, for example between milliseconds and hours. And second, we observe repulsive pinning sites on our material, which, unlike attractive pinning sites, have an area-shaped (2D) effect on domain walls and therefore induce conceptually different dynamics. These effects should be considered in domain-wall-based spintronics research and applications.
 Berruto, G. et al. Phys. Rev. Lett. 120, 117201 (2018).
 Je, S.-G. et al. Nano Letters 18, 7362–7371 (2018).
 Büttner, F. et al. Scientific Reports 8, 4464 (2018).
 Büttner, F. et al. Nature Materials 20, 30–37 (2021).
 Klose, C. et al., submitted.