Micron-sized magnetic platelets in the flux-closed vortex state are characterized by an in-plane curling magnetization and a nanometer-sized perpendicularly magnetized vortex core. Having the simplest non-trivial configuration, these objects are of general interest to micromagnetics and may offer new routes for spintronics applications. Essential progress in the understanding of nonlinear vortex dynamics was achieved when low-field core toggling by excitation of the gyrotropic eigenmode at sub-GHz frequencies was established. At frequencies more than an order of magnitude higher vortex state structures possess spin wave eigenmodes arising from the magneto-static interaction. Here we demonstrate experimentally that the unidirectional vortex core reversal process also occurs when such azimuthal modes are excited. These results are confirmed by micromagnetic simulations, which clearly show the selection rules for this novel reversal mechanism. Our analysis reveals that for spin-wave excitation the concept of a critical velocity as the switching condition has to be modified.

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Magnetic thin-film square-or disc-shaped nanostructures with adequate dimensions exhibit a magnetic vortex state: the magnetization vectors lie in the film plane and curl around the structure centre. At the very centre of the vortex, a small, stable core exists where the magnetization points either up or down 1,2 . The discovery of an easy core reversal mechanism 3 did not only open the possibility of using such systems as magnetic memories, but also initiated the fundamental investigation of the core switching mechanism itself [4][5][6][7][8][9][10][11][12][13][14][15] . Theoretical modelling predicted that the reversal is mediated by the creation and annihilation of a vortex-antivortex pair 3,4,16 , but experimental support has been lacking until now. We used high-resolution time-resolved magnetic X-ray microscopy to experimentally reveal the first step of the reversal process: the dynamic deformation of the vortex core. In addition, we have measured a critical vortex velocity above which reversal must occur 5,17 . Both observations support the previously proposed reversal mechanism.Depending on the material type and thickness, the vortex core diameter is typically only 10 (ref. 18) to 25 nm (ref. 19). Although the core is very small, it significantly affects the dynamics as it gives rise to the so-called vortex gyration mode [20][21][22] . This mode corresponds to a circular motion of the vortex around the structure centre. It was recently shown that a low-field excitation of this mode can switch the out-of-plane polarization of the core 3 . This was experimentally observed by determining the vortex core polarization before and after the application of short bursts of an alternating magnetic field 3 . The dynamic process behind the switching could not be inferred from this experiment. However, micromagnetic modelling showed that near a moving vortex core, a region appears where the magnetization acquires an out-of-plane component opposing the vortex core polarization. If this so-called vortex core deformation becomes so strong that it points fully out of the sample plane, a vortex-antivortex pair is nucleated. At this point, the switching is initiated, as the antivortex rapidly annihilates with the original vortex, leaving behind only the newly created vortex core with an opposite polarization 3,4,16 . This annihilation process involves a magnetic singularity, which is necessary for the polarization reversal 23 . Apart from micromagnetic simulations, the dynamic core deformation had already been included in theoretical calculations by Novosad et al. 24 . Its origin and relevance for the switching process were investigated by Yamada et al. 17 and by Guslienko et al. 5 . These authors showed that near a moving vortex, a strong out-of-plane 'kinetic' term in the effective field appears. It is this field that pushes the magnetization towards the opposite direction of the core polarization, causing the dynamic deformation.As the effective out-of-plane field is proportional to the velocity of the vortex movement 17...

The response of magnetic vortex cores to subnanosecond in-plane magnetic field pulses was studied by time-resolved x-ray microscopy. Vortex core reversal was observed and the switching events were located in space and time. This revealed a mechanism of coherent excitation by the leading and trailing edges of the pulse, lowering the field amplitude required for switching. The mechanism was confirmed by micromagnetic simulations and can be understood in terms of gyration around the vortex equilibrium positions, displaced by the applied field. DOI: 10.1103/PhysRevLett.102.077201 PACS numbers: 75.40.Gb, 75.60.Jk, 75.75.+a The magnetic vortex is a typical ground state configuration of micron and submicron sized ferromagnetic thin film structures [1]. It minimizes the stray field energy by forming an in-plane curling magnetization. In order to avoid a singularity in the center of the structure, the magnetization turns out of plane, forming the vortex core which can point either up (vortex polarization p ¼ þ1) or down (p ¼ À1). This configuration is very stable; static out-ofplane magnetic fields of about 0.5 T are required to switch the core polarization [2]. The vortex also has a specific excitation mode, the so-called gyrotropic mode, which can be excited by an oscillating in-plane magnetic field [3,4]. It corresponds to a circular motion of the vortex around its equilibrium position.Recently, it was discovered that switching the polarization of the vortex core is not only possible by static fields, but also by excitation of the gyration mode [5]. In this case, only field strengths of a few millitesla are needed. Micromagnetic modeling of these experiments revealed that the vortex switching occurs by the creation and subsequent annihilation of a vortex-antivortex pair [5]. This discovery has triggered a variety of studies on vortex core switching by various excitation methods. In particular, it was found by micromagnetic simulations that the polarization may also be switched by very short in-plane magnetic field pulses. It was shown that switching times as short as 40 ps are possible [6]. Except for a brief report on switching events observed with 2 ns long spin-polarized current pulses [7], the possibilities of such excitations have not been explored experimentally so far.In this work, we have experimentally investigated vortex core switching by in-plane magnetic field pulses, using time-resolved magnetic x-ray microscopy. By taking advantage of the pulsed nature of synchrotron light, stroboscopic imaging was set up at scanning transmission x-ray microscopes [8] at beam line 11.0.2 of the Advanced Light Source and 10ID-1 of the Canadian Light Source. Using the x-ray magnetic circular dichroism [9] as a contrast mechanism, magnetization dynamics could be studied with 25 nm spatial and 70 ps temporal resolution.The samples studied in this work are 500 nm  500 nm and 1 m  1 m square-shaped magnetic Permalloy (Ni 80 Fe 20 ) elements with a thickness of 50 nm. These structures are defined on top of a 2:5 m wide, 1...

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...