Universität zu Köln

Bezvershenko, Alla (2025). Pushing and Shaking of Skyrmion Lattices. PhD thesis, Universität zu Köln.

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Abstract

Magnetic skyrmions are topologically non-trivial spin textures that attract great interest, offering a possible avenue towards novel spintronics applications. One of the reasons for it is the small critical current density of about $10^6$ $A/m^2$ needed to depin the skyrmion lattice. Pinning by disorder remains arguably one of the most important obstacles for all skyrmion-based non-equilibrium experiments and the creation of useful skyrmion devices. In the presence of a current but in the absence of an oscillating magnetic field, only two phases describe the skyrmion lattice: a pinned phase, where the skyrmions are not moving and an unpinned phase, where skyrmions are moving. We study the unpinning process with the help of slowly oscillating tilted magnetic fields, so-called shaking fields. Strikingly, the depinning happens already at tiny transverse magnetic fields of only a few mT, which corresponds to a small tilting angle of the magnetic field of around 1 degree. In the first Part of this Thesis, we introduce an elastic model for skyrmion strings in the bulk of a skyrmion-hosting material in the presence of pinning forces under oscillating magnetic fields. We study the dynamics of this system using the Thiele-like approach and provide an analytic solution in the adiabatic limit. Using this model, we predict three phases and find the signatures of phase transitions in the Time-Involved Small Angle Neutron scattering Experiment (TISANE). We examine the unpinning process for different materials, including the metallic systems $\mathrm{Mn}_{1-x}\mathrm{Fe}_{x}\mathrm{Si}$, with x=0, 0.02, 0.04, 0.06 and the insulator $\mathrm{Cu}_{2}\mathrm{OSeO}_{3}$. We find a high degree of universality in the unpinning processes across all these materials. In the second Part, we study the dynamics within our model under the simultaneous application of a shaking field and an external current. A remarkably rich non-equilibrium phase diagram appears, which includes the so-called "walking" and "running" phases. Strikingly, the critical current density to depin the lattice drops to zero upon shaking the MnSi skyrmion lattice, starting from the critical magnetic field amplitude. Results obtained from this effective model qualitatively reproduce the experimental findings from the transverse susceptibility measurements on MnSi. In the third Part, we find, from the symmetry analysis of the skyrmion lattice, the periodic magnetic driving schemes under which a directed motion of skyrmion strings becomes possible. We are arguing that counterintuitively, pinning might facilitate a more efficient motion of skyrmion lines. This Thesis was conducted in close collaboration with Prof. Christian Pfleiderer's group, where most of the relevant experiments were performed. In this Thesis we aim to show that we have acquired important new insights into the nature of pinning in the bulk of skyrmion lattices.

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code Bezvershenko, Alla alla.bezvershenko@gmail.com UNSPECIFIED UNSPECIFIED
URN:	urn:nbn:de:hbz:38-752968
Date:	26 February 2025
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Faculty of Mathematics and Natural Sciences > Department of Physics > Institute for Theoretical Physics
Subjects:	Physics
Uncontrolled Keywords:	Keywords Language Skyrmions, driven systems, topological spin textures, shaking English
Date of oral exam:	11 December 2024
Referee:	Name Academic Title Rosch, Achim Prof. Dr. Trebst, Simon Prof. Dr. Hemberger, Joachim Prof. Dr.
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/75296