Electrical probing of field-driven cascading quantized transitions of skyrmion cluster states in MnSi nanowires

Abstract

Magnetic skyrmions are topologically stable whirlpool-like spin textures that offer great promise as information carriers for future spintronic devices. To enable such applications, particular attention has been focused on the properties of skyrmions in highly confined geometries such as one-dimensional nanowires. Hitherto, it is still experimentally unclear what happens when the width of the nanowire is comparable to that of a single skyrmion. Here, we achieve this by measuring the magnetoresistance in ultra-narrow MnSi nanowires. We observe quantized jumps in magnetoresistance versus magnetic field curves. By tracking the size dependence of the jump number, we infer that skyrmions are assembled into cluster states with a tunable number of skyrmions, in agreement with the Monte Carlo simulations. Our results enable an electric reading of the number of skyrmions in the cluster states, thus laying a solid foundation to realize skyrmion-based memory devices.

Direct imaging of magnetic field-driven transitions of skyrmion cluster states in FeGe nanodisks

“…1Q). These results are consistent with recent experimental observations on thin MnSi nanowires by MR measurements (15), in which the magnetic field intervals of hosting maximum number of skyrmions N m s is always wider than those with N s < N m s if the same dynamical procedure is followed. By contrast, this observation is quite different from the theoretical calculation that the magnetic intervals of hosting different skyrmion clusters are comparable (8,13).…”

“…Since the discovery of magnetic skyrmion in 2009, quite a few theoretical and experimental works have been performed on the spin textures and magnetization process in confined helimagnets including nanodisks, nanowires with circular or parallelogram cross-sections (8,13,15); the elemental results lie in the emergent skyrmion cluster states and discrete transitions between them due to the confined geometry. These predictions were directly confirmed by our Lorentz TEM study.…”

“…The B20-type FeGe samples were synthesized with a cubic anvil-type high-pressure apparatus with the detailed description in ref. 15. The nanodisks for TEM observation were prepared by using an FIB and scanning electron microscope (SEM) dual beam system (Helios Nanolab 600i; FEI) combined with a gas injection system (GIS), and a micromanipulator (Omniprobe 200+; Oxford Instruments).…”

“…Because a single skyrmion is a twisted magnetic object with nontrivial topology, the creation or annihilation of an individual skyrmion is expected to be associated with a quantized transition (14). Recent measurement of magnetoresistance (MR) on an ultrathin MnSi nanowire provides such an indication of the quantized processes (15). However, direct real-space observations of these phase transitions in submicrometer helimagnets have not been reported and remain elusive.…”

Emergent geometric frustration of artificial magnetic skyrmion crystals

“…These nontrivial spin textures were theoretically predicted to exist in certain noncentrosymmetric magnetic materials [1] with Dzyaloshinskii-Moriya (DM) interactions [2,3] and were subsequently experimentally identified in the chiral magnet MnSi by neutron scattering [4], Lorentz transmission electron microscopy (LTEM) [5,6], topological Hall effect [7][8][9], and Dynamic Cantilever Magnetometry [10]. Interface-induced skyrmions have been experimentally observed in perfectly ordered, atomically thin layers by spin-resolved scanning tunneling microscopy (STM) [11,12] and time-resolved pump-probe X-ray holography [13].…”