Skyrmion-based spin-torque nano-oscillator in synthetic antiferromagnetic nanodisks

Abstract

The skyrmion-based spin-torque nano-oscillator is a potential next-generation nano microwave signal generator. In this paper, the self-sustained oscillation dynamics of magnetic skyrmions are investigated in a nanodisk with synthetic antiferromagnetic (SAF) multilayer structure, in which the skyrmion Hall effect can be effectively suppressed. An analytical model based on the Thiele equation is developed to describe the dynamics of a pair of skyrmions formed in the SAF nanodisks. Combining the analytical solutions with the micromagnetic simulations, we demonstrate that circular rotations with opposite directions for a skyrmion pair could be suppressed by increasing the antiferromagnetic (AF) coupling in a nanopillar with dual spin polarizers. However, a stable circular rotation can be achieved in a nanopillar with a single spin polarizer, in which one skyrmion plays as a master whose rotation is driven by spin torque, while the other skyrmion is a slaver whose motion is dragged by the AF coupling between the two free layers. Moreover, we found that the effective mass factor in the SAF structure rather than the gyrotropic torque plays the dominant role in the circular rotation of skyrmions. The rotation orbit radius and frequency gradually increase with the decrease of damping factor and increase of applied current strength.

Magnetic anisotropy-controlled vortex nano-oscillator for neuromorphic computing

“…the STT current-to-spin conversion efficiency [52,53], the SOT spin current generated in Pt is negligible as compared to the spin current generated via STT and not considered in our structure. In order to quickly reach the steady precession state, a preset process is needed [54][55][56], which is done here by a applying a magnetic field of 5 mT parallel to the plane to preset the vortex core away from the geometry centre. The magnetic field is then removed.…”

“…As can be seen from the form, this second term zU(X) zX represents a conservative force acting on the vortex core that is derived from the total free energy of the system [13]. The edge-induced repulsive force together with the remanent gyrotropic force provide the centripetal force to sustain the circular motion of the core [54], as illustrated in Figure 1B. The last two terms in Eq.…”

“…The last two terms in Eq. (1) represent the damping force and spin transfer forces, respectively [54]. The spin polarised current perpendicular to the layers results in a torque force on the vortex core, which can be balanced by the dissipative force, and, in overall, they jointly support orbital motion of the vortex core [23].…”

Dynamics of orbital skyrmions in a circular nanodisk

“…Magnetic skyrmion, 1-3 a particle-like spin structure usually stabilized by Dzyaloshinskii-Moriya (DM) interaction, 4,5 has attracted much interest because of its rich physics [6][7][8] and possible implementations in various devices such as skyrmion diodes, 9,10 skyrmion ratchets, 11 racetrack memories, 12 logic gates, [13][14][15][16] and nano-oscillators. [17][18][19][20][21][22][23][24][25][26][27][28] Among them, nanooscillators based on skyrmions in circular nanodisks have been heavily studied because their high-frequency motion can be easily used for microwave generators and/or detectors. For a circulating skyrmion in a standard circular nanodisk, a radial force called Magnus force 29,30 always tends to move the skyrmion inward towards the center or outward toward the edge of the nanodisk, resulting in the skyrmion Hall effect (SkHE).…”

Suppression of the repulsion phenomenon of magnetic skyrmions at the end of synthetic antiferromagnetic racetracks

“…of spintronic devices, such as racetrack memory [10,11], logic devices [12][13][14], spin-transfer nano-oscillators [15][16][17] etc.…”