Room-Temperature Current-Induced Generation and Motion of sub-100 nm Skyrmions

Abstract

Magnetic skyrmions are nanoscale windings of the spin configuration that hold great promise for technology due to their topology-related properties and extremely reduced sizes. After the recent observation at room temperature of sub-100 nm skyrmions stabilized by interfacial chiral interaction in magnetic multilayers, several pending questions remain to be solved, notably about the means to nucleate individual compact skyrmions or the exact nature of their motion. In this study, a method leading to the formation of magnetic skyrmions in a micrometer-sized track using homogeneous current injection is evidenced. Spin-transfer-induced motion of these small electrical-current-generated skyrmions is then demonstrated and the role of the out-of-plane magnetic field in the stabilization of the moving skyrmions is also analyzed. The results of these experimental observations of spin torque induced motion are compared to micromagnetic simulations reproducing a granular type, nonuniform magnetic multilayer in order to address the particularly important role of the magnetic inhomogeneities on the current-induced motion of sub-100 nm skyrmions for which the material grains size is comparable to the skyrmion diameter.

Skyrmions in magnetic multilayers: chirality, electrical detection and current-induced motion

“…The effect is qualitatively the same for any grain-to-grain varying micromagnetic parameter that we studied: exchange, PMA, DMI and magnetization, or thickness (assuming purely interfacial PMA and DMI). 14,22 Note that similar results were also obtained for a particle-based model with pinning sites. 23 We can even find a quantitative match with experiments, but for sets of parameters (magnitude and dispersion of the grain-to-grain magnetic properties) that we cannot verify yet.…”

“…However, we do not observe a uniform velocity. On the contrary, the trajectories of moving skyrmions are meandering and no clear skyrmion Hall effect is observed, 14 unlike in other systems. 20 We also observe that some skyrmions do not move at all, some move when pushed by another, and finally, some are annihilated or nucleated.…”

Interplay between size and stability of magnetic skyrmions

“…For nanoscale skyrmions, thermal stability becomes an issue as thermal fluctuations can spontaneously destroy skyrmion states and, therefore, corrupt the stored data. Available experimental data on magnetic skyrmions in various materials and material combinations demonstrates inverse relationship between the skyrmion size and skyrmion stability: small skyrmions tend to be less stable compared to large ones [8,[11][12][13][14][15][16][17][18][19][20][21]. For example, roomtemperature skyrmions in a Pt/CoFeB/MgO heterostructure are roughly 100 nm in diameter [18], which is more than an order of magnitude larger than skyrmions observed in a Pd/Fe/Ir(111) system only at low temperatures by means of spin-polarized scanning tunneling microscopy [19,20].…”

Controlled Individual Skyrmion Nucleation at Artificial Defects Formed by Ion Irradiation

“…Both of these effects will likely increase the depinning field, as will the local reduction in M s predicted by alloying. [ 23,52 ]…”

“…One crucial step necessary for realizing a skyrmion‐based device lies in finding a reliable and controllable method of nucleating individual skyrmions. A multitude of nucleation methods have been proposed in recent years and these can be put into three categories based on: electrical currents, [ 10,19–24 ] laser pulses, [ 25–27 ] and locally applied electric fields. [ 28,29 ] Skyrmions can also nucleate at naturally occurring defects in the material; [ 30,31 ] theoretical studies, which consider nonmagnetic defects, find that defects both localize skyrmion nucleation and reduce the nucleation energy barrier.…”