Very recently, an important class of magnetic solitons called magnetic skyrmions has been widely studied [1,2]. The stability of these topological defects has been analyzed in the presence of intrinsic dissipation both as a function of Dzyaloshinskii–Moriya interaction (DMI) and as a function of a spin-polarized current (SPC). Here, we show that in a spin-valve consisting of Pt(5 nm)/Co(0.6 nm)/Cu (4 nm)/CoPt(4 nm) a perpendicular spin current, in the presence of the DMI [3,4] and strong perpendicular anisotropy, induces the rotation of the spins from the hedgehog-like to the vortex-like texture in the topological droplet state and excites low-frequency topological modes. The SPC is injected locally in the Co giving rise to the local excitation of the magnetization. The Pt heavy metal adds a further degree of freedom, the interfacial DMI, to the effective field of the Co. The FL has a square cross section of 400 x 400 nm2, while the point contact has a diameter of 70 nm. Both FL and polarizer have out-ofplane magnetic state at zero bias field. The “topological droplet” state (integer skyrmion number) arises and its dynamical response, namely a “topological mode” (TM), can be excited and sustained by a SPC. The low-frequency TM is linked to a continual conversion from hedgehog-like (Néel skyrmion) to vortex-like (Bloch skyrmion) state, preserving the topology represented by the skyrmion number S = -1. A negative current (J < 0) flows from the free layer (Co) to the fixed (nanocontact, CoPt) layer. The topological character of these spin-wave excitations results from the synchronized dynamics between the 360o rotation of the spin of the outer droplet domain and the expansion/shrinking of the droplet core. A quantitative description of topological droplet modes is given according to an analytical model based on the linearization of the equations of motion including intrinsic positive Gilbert damping and negative damping related to the spin-transfer torque. The analytical frequency of the topological droplet mode is expressed as the solution of a second-order algebraic equation written in terms of the magnetic parameters including the intrinsic and extrinsic damping.

Topological skyrmion dynamics driven by spin-transfer torque

ZIVIERI, Roberto
2015

Abstract

Very recently, an important class of magnetic solitons called magnetic skyrmions has been widely studied [1,2]. The stability of these topological defects has been analyzed in the presence of intrinsic dissipation both as a function of Dzyaloshinskii–Moriya interaction (DMI) and as a function of a spin-polarized current (SPC). Here, we show that in a spin-valve consisting of Pt(5 nm)/Co(0.6 nm)/Cu (4 nm)/CoPt(4 nm) a perpendicular spin current, in the presence of the DMI [3,4] and strong perpendicular anisotropy, induces the rotation of the spins from the hedgehog-like to the vortex-like texture in the topological droplet state and excites low-frequency topological modes. The SPC is injected locally in the Co giving rise to the local excitation of the magnetization. The Pt heavy metal adds a further degree of freedom, the interfacial DMI, to the effective field of the Co. The FL has a square cross section of 400 x 400 nm2, while the point contact has a diameter of 70 nm. Both FL and polarizer have out-ofplane magnetic state at zero bias field. The “topological droplet” state (integer skyrmion number) arises and its dynamical response, namely a “topological mode” (TM), can be excited and sustained by a SPC. The low-frequency TM is linked to a continual conversion from hedgehog-like (Néel skyrmion) to vortex-like (Bloch skyrmion) state, preserving the topology represented by the skyrmion number S = -1. A negative current (J < 0) flows from the free layer (Co) to the fixed (nanocontact, CoPt) layer. The topological character of these spin-wave excitations results from the synchronized dynamics between the 360o rotation of the spin of the outer droplet domain and the expansion/shrinking of the droplet core. A quantitative description of topological droplet modes is given according to an analytical model based on the linearization of the equations of motion including intrinsic positive Gilbert damping and negative damping related to the spin-transfer torque. The analytical frequency of the topological droplet mode is expressed as the solution of a second-order algebraic equation written in terms of the magnetic parameters including the intrinsic and extrinsic damping.
2015
Topological defects, magnetic skyrmion, topological mode, spin-transfer torque
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11392/2337674
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