Dynamical Skyrmion State in a Spin Current Nano-Oscillator with Perpendicular Magnetic Anisotropy

Abstract

We study the spectral characteristics of spin current nano-oscillators based on the Pt/[Co/Ni] magnetic multilayer with perpendicular magnetic anisotropy. By varying the applied magnetic field and current, both localized and propagating spin wave modes of the oscillation are achieved. At small fields, we observe an abrupt onset of the modulation sidebands. We use micromagnetic simulations to identify this state as a dynamical magnetic skyrmion stabilized in the active device region by spin current injection, whose current-induced dynamics is accompanied by the gyrotropic motion of the core due to the skew deflection. Our results demonstrate a practical route for controllable skyrmion manipulation by spin current in magnetic thin films. The possibility to induce dynamical states of nanomagnets or change their static configuration by the currentinduced spin torque (ST) [1, 2] has stimulated intense research into current-induced phenomena in magnetic systems. While the early experiments utilized spin-polarized electric currents in magnetic multilayers [3,4], recent studies focused on the effects of spin current produced due to the spin-orbit interaction (SOI) in bilayers of ferromagnets (F) with heavy nonmagnetic metals (N) [5][6][7][8]. The spin-orbit effects that contribute to the currentinduced phenomena include the spin Hall effect (SHE) originating from SOI in N, the Rashba effect [9] and the Dzyaloshinskii-Moriya interaction (DMI) [10], both originating from the broken inversion symmetry at the F/N interface. The DMI produces chiral effective fields that can result in nontrivial topological magnetic structures such as spirals, skyrmions and chiral domain walls [11].

Magnetic droplet solitons in orthogonal spin valves

“…1(c), where the core is partially reversed and the spins both in the core and in the droplet perimeter precess inphase. Although droplet theory [40] was developed for fully perpendicular spin valves, reported experiments [41,44] have used STNOs where the remanent state of the fixed layer is in the film plane, which can promote droplet instability and auto-modulation [41,[47][48][49]. In this orthogonal geometry, the current-field phase diagram of the droplet qualitatively differs from the theoretical predictions.…”

Continuous Tuning of the Magnitude and Direction of Spin‐Orbit Torque Using Bilayer Heavy Metals

“…[9,14,15] SOTs have been recently demonstrated as an alternative approach to switch the magnetization efficiently, [1][2][3][4][5][6][7] drive fast chirality dependent domain wall motion, [8][9][10] as well as excite high-frequency magnetization oscillations. [11][12][13] The spin Hall effect (SHE) [16][17][18] and/or Rashba effect [1,19,20] arising from spin-orbit coupling (SOC), have been discussed as the mechanisms giving rise to SOTs, although their relative contributions are currently under debate. [1,3,7] On the other hand, SOTs were also discovered in a bare FM layer and Co/Pd multilayers.…”

“…Spin-orbit torques (SOTs) induced by in-plane currents have been intensively studied in heavy metal (HM)/ferromagnet (FM)/capping multilayers, in which a HM is adjacent to a thin FM capped by an oxide [1][2][3][4][5][6][7][8][9][10][11][12][13] or metal. [9,14,15] SOTs have been recently demonstrated as an alternative approach to switch the magnetization efficiently, [1][2][3][4][5][6][7] drive fast chirality dependent domain wall motion, [8][9][10] as well as excite high-frequency magnetization oscillations.…”

Interlayer interaction in multilayer [Co/Pt]n/Pt/Co structures

“…In particular, CoPt/F structures that consist of CoPt multilayer film with "easy axis" anisotropy and ferromagnetic layer (F) with "easy plane" anisotropy have been proposed to control hysteresis loop shift (exchange bias) of magnetic system [12][13][14] . Besides, patterned CoPt/F structures were recently used to create artificial magnetic skyrmion states [15][16][17][18][19][20][21] . Note that the effects of exchange bias and skyrmion nucleation are based on the existence of strong exchange interaction both between Co layers in CoPt film and between CoPt film and ferromagnetic layer F. However, as far as we know, there are no direct experimental studies of exchange coupling in such structures.…”