Controlling skyrmion bubble confinement by dipolar interactions

Abstract

Large skyrmion bubbles in confined geometries of various sizes and shapes are investigated, typically in the range of several micrometers. Two fundamentally different cases are studied to address the role of dipole-dipole interactions: (I) when there is no magnetic material present outside the small geometries and (II) when the geometries are embedded in films with a uniform magnetization. It is found that the preferential position of the skyrmion bubbles can be controlled by the geometrical shape, which turns out to be a stronger influence than local variations in material parameters. In addition, independent switching of the direction of the magnetization outside the small geometries can be used to further manipulate these preferential positions, in particular with respect to the edges. We show by numerical calculations that the observed interactions between the skyrmion bubbles and structure edge including the overall positioning of the bubbles are fully controlled by dipole-dipole interactions. a) Electronic mail: t.lichtenberg@tue.nl 1 arXiv:1905.10304v1 [cond-mat.mes-hall] 24 May 2019Magnetic skyrmions are whirls in the magnetization in which neighbouring spins are rotated with respect to each other with a specific chirality. They cannot be removed by continuous deformation of the magnetization without creating a singularity, which provides a topological barrier that makes them robust against annihilation. They are less hindered by pinning sites or defects than magnetic domain walls (DWs), and their size can be in the order of nanometers. These properties make them suitable for data storage. For the envisioned skyrmion racetrack memory 1-3 , the skyrmions are required to be present in small geometrically confined structures, instead of infinite sheets of material. Therefore, the interaction between skyrmions and the edge of the magnetic structure is crucial. In fact, this interaction is necessary to prevent skyrmions from being expelled from the track, it can stabilize skyrmions in absence of an external magnetic field 4,5 , assist in their formation 6,7 , and by reducing the width of the track it could be possible to reduce the size of the skyrmion and hence to achieve larger data storage densities 8 .In the research field on skyrmions, usually a distinction is made between a 'compact skyrmion' and a 'skyrmion bubble'. These objects share many properties, but the latter has typically a much larger size and has a constant magnetization at its core 8 . Numerical and experimental work on compact skyrmion confinement show that there is indeed a repulsive interaction between skyrmions and sample edges that is a result of tilting of the magnetic moments at the edge, which is caused by the Dzyaloshinskii-Moriya interaction (DMI) 9,10 . For skyrmion bubbles, dipolar interactions are paramount in their stabilization, and because near the sample edge these stray fields will change, it is intuitively expected that the edges will influence the skyrmion bubbles via this mechanism. Though this has been realized before 1...

Skyrmion velocities in FIB irradiated W/CoFeB/MgO thin films

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“…19,20 Furthermore local He + irradaition has been used to create skyrmion tracks 14 and FIB Ga + irradiation to confine skyrmions. 21 To assess how areal Ga + ion irradiation can be used to engineer the trajectories of skyrmions, we evaluated skyrmion motion, driven by currents, in focused ion beam treated magnetic wires.…”

Precise Tuning of Skyrmion Density in a Controllable Manner by Ion Irradiation

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Stabilization and topological transformation of magnetic bubbles in disks of a kagome magnet

“…[1][2][3][4][5][8][9][10] Skyrmion-based magnetic devices can also be realized in bubble-hosting magnets because the closure cylinder domain wall in type-I magnetic bubble contributes to an integer topological charge as that of a magnetic skyrmion. [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30] Accordingly, type-I magnetic bubbles were also renamed skyrmion bubbles or skyrmions. 13,15,[28][29][30] Previous studies have established at least two types of bubbles.…”