YRLG Workshop: Correlation and Topology in magnetic materials, July 16th - 18th 2024
Elizabeth M. Jefremovas
Skyrmions are topologically non-trivial spin textures that act as quasi-particles and have been extensively studied as promising candidates for information storage and processing technologies [1, 2]. First experimental observations of such textures were realized in bulk crystals at low temperatures [3]. Over the last years, magnetic multilayer films have replaced bulk materials for studying spin textures, since the fine tuning of the material parameters allows to obtain room temperature skyrmions, nucleated at relatively low magnetic fields (tens of mT), exhibiting even in some cases ultralow pinning [4].
Despite the role of the stray field in the energy minimization of an isolated skyrmion is relatively well understood [5], there are some common experimental findings that are still not well understood. It is the case, for instance, of the impact of dipolar skyrmion-skyrmion interaction in the stabilization of skyrmion lattices (size and density), temperature over the nucleation of topological structures, or even, the changes induced by confinement effects.
In this talk, I will present the main results of three systematic studies performed in a Ta/CoFeB-based stack. First, the role of magneto-dipolar interactions in the stabilization of skyrmion lattices will be discussed. For this purpose, the purely stray-field-coupled multilayer unit (n = 1) has been repeated, increasing thus progressively the dipolar coupling, remaining the remaining magnetic parameters (exchange, DMI, uniaxial anisotropy) unaffected. An analytical model for the skyrmion radius as a function of the number of repetitions has been derived, allowing to observe the transition from thin film to thick film regime [6].
Second, it will be shown how the temperature can be used to activate meta-stable topological spin textures. Its influence on diffusion will be also discussed [7].
Third, the impact of lateral confinement in the skyrmion stabilization will be shared. For this, a variety of confinement sizes (from hundreds of um to several um) and geometries (from high to low symmetry) have been studied. Both size and shape have a critical influence on the skyrmion stabilization, being even possible to establish a cut-off for topological textures to be energetically favourable below a certain size and/or at a particular geometry [8].
The presented work provides a comprehensive overview on the experimentally-accessible mechanisms and systems currently employed to study topologically-stabilized spin textures. The attained results enable the control over spin textures, opening the way for new applications.
References
- J Ping Liu et al. (Eds.) "Skyrmions: Topological Structures, Properties, and Applications" CRC Press, (2016).
- Fert et al. Nat. Rev. Mater. 2 (2017) 1-17.
- Mülbahuer et al. Science, 323 (2009) 915-919.
- Gruber et al. Nat. Commun. 13 (2022) 3144.
- Büttner et al. Sci. Rep. 8 (2018) 4464.
- , [7] and [8] E. M. Jefremovas et al. In preparation (2024).