Current-induced skyrmion generation and dynamics in symmetric bilayers

Abstract

Magnetic skyrmions are quasiparticle-like textures which are topologically different from other states. Their discovery in systems with broken inversion symmetry sparked the search for materials containing such magnetic phase at room temperature. Their topological properties combined with the chirality-related spin–orbit torques make them interesting objects to control the magnetization at nanoscale. Here we show that a pair of coupled skyrmions of opposite chiralities can be stabilized in a symmetric magnetic bilayer system by combining Dzyaloshinskii–Moriya interaction (DMI) and dipolar coupling effects. This opens a path for skyrmion stabilization with lower DMI. We demonstrate in a device with asymmetric electrodes that such skyrmions can be independently written and shifted by electric current at large velocities. The skyrmionic nature of the observed quasiparticles is confirmed by the gyrotropic force. These results set the ground for emerging spintronic technologies where issues concerning skyrmion stability, nucleation and propagation are paramount.

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

“…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.…”

Bilayer skyrmion dynamics on a magnetic anisotropy gradient

“…In the implementation of a magnetic skyrmion racetrack memory, the transport techniques explored thus far have been mainly limited to the use of spin-polarized electrical current injection which utilizes spin transfer torque (STT) and spin-orbit torque (SOT). Experimental transport measurements based on such mechanisms showed that high current densities on the order of 10 12 A m −2 are required to achieve practical speeds of about 100 m s −1 [12,13]. The elevated temperatures at such high current densities can drastically reduce the propagation speed of skyrmions while the risk of skyrmion annihilation increases significantly [14][15][16].…”