Magnetic Antivortex-Core Reversal by Circular-Rotational Spin Currents

Abstract

Topological singularities occur as antivortices in ferromagnetic thin-film microstructures. Antivortices behave as two-dimensional oscillators with a gyrotropic eigenmode which can be excited resonantly by spin currents and magnetic fields. We show that the two excitation types couple in an opposing sense of rotation in the case of resonant antivortex excitation with circular-rotational currents. If the sense of rotation of the current coincides with the intrinsic sense of gyration of the antivortex, the coupling to the Oersted fields is suppressed and only the spin-torque contribution locks into the gyrotropic eigenmode. We report on the experimental observation of purely spin-torque induced antivortex-core reversal. The dynamic response of an isolated antivortex is imaged by time-resolved scanning transmission x-ray microscopy on its genuine time and length scale. DOI: 10.1103/PhysRevLett.105.137204 PACS numbers: 75.60.Jk, 68.37.Yz, 72.25.Àb, 75.60.Ch Magnetization dynamics in microstructures is not only an aspect of basic research. Its investigation is also motivated by the vision of engineering new storage devices. Magnetic antivortices and vortices are capable of storing binary information [1][2][3] represented by their core polarization p ¼ AE1 and behave as two-dimensional oscillators with a gyrotropic eigenmode which can be excited resonantly by spin currents and magnetic fields [4][5][6][7]. With the perspective of possible applications in storage devices, spin-torque induced core switching is preferred because in this case single elements can be addressed, which is obviously important for storage cells of high density. Another motivator to investigate the current induced dynamics of magnetization is the patent for the magnetic race track memory [8,9], which is based on the unidirectional control of magnetic domain walls in a nanowire by spinpolarized currents [10]. Thereto, a detailed understanding of the coupling between the spin of the conduction electrons and the magnetization of ferromagnets, namely, the spin-transfer torque [11][12][13], has to be gathered. Nevertheless, experiments on spin-torque driven magnetization dynamics come along with a general difficulty: the discrimination between the spin-transfer torque and the torque from parasitic Oersted fields as driving force [4][5][6][14][15][16]. Both torques are generated simultaneously by the applied current. In this Letter we show that both excitation types couple in an opposing sense of rotation in the case of resonant antivortex excitation with circular-rotational currents. The magnetic coupling is suppressed and only the spin-torque contribution locks into the gyrotropic eigenmode if the sense of rotation of the applied current coincides with the intrinsic sense of gyration of the antivortex. We report on the first experimental observation of purely spin-torque induced antivortex-core reversal.Antivortices and vortices form in ferromagnetic thinfilm structures [17][18][19] by a compromise between the shape anisotropy, which prefers the...

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