The vortex state, characterized by a curling magnetization, is one of the equilibrium configurations of soft magnetic materials and occurs in thin ferromagnetic square and disk-shaped elements of micrometre size and below. The interplay between the magnetostatic and the exchange energy favours an in-plane, closed flux domain structure. This curling magnetization turns out of the plane at the centre of the vortex structure, in an area with a radius of about 10 nanometres--the vortex core. The vortex state has a specific excitation mode: the in-plane gyration of the vortex structure about its equilibrium position. The sense of gyration is determined by the vortex core polarization. Here we report on the controlled manipulation of the vortex core polarization by excitation with small bursts of an alternating magnetic field. The vortex motion was imaged by time-resolved scanning transmission X-ray microscopy. We demonstrate that the sense of gyration of the vortex structure can be reversed by applying short bursts of the sinusoidal excitation field with amplitude of about 1.5 mT. This reversal unambiguously indicates a switching of the out-of-plane core polarization. The observed switching mechanism, which can be understood in the framework of micromagnetic theory, gives insights into basic magnetization dynamics and their possible application in data storage.

By combining magnetic transmission x-ray microscopy with a stroboscopic pump and probe technique using synchrotron radiation we are able to image the magnetization dynamics in micron sized magnetic particles on a sub-100 ps time scale with a lateral spatial resolution down to 21 nm. We report first observations in squared elements indicating locally varying precessional frequencies which are in agreement with micromagnetic simulations. The experiment opens a route towards a high spatiotemporal resolution of spin patterns which is needed to understand the microscopic origin of magnetization reversal of micron sized and nano-sized magnetic particles.

Time-resolved X-ray microscopy is used to image the influence of alternating high-density currents on the magnetization dynamics of ferromagnetic vortices. Spin-torque induced vortex gyration is observed in micrometer-sized permalloy squares. The phases of the gyration in structures with different chirality are compared to an analytical model and micromagnetic simulations, considering both alternating spin-polarized currents and the current's Oersted field. In our case the driving force due to spin-transfer torque is about 70% of the total excitation while the remainder originates from the current's Oersted field. This finding has implications to magnetic storage devices using spin-torque driven magnetization switching and domain-wall motion.PACS numbers: 68.37. Yz, 72.25.Ba , 75.25.+z, 75.40.Mg, The discovery that spin-polarized electrons traveling through ferromagnets apply a torque on the local magnetization 1 opened up a new field of research in solid state physics that could potentially result in new magnetic storage media. It is now understood that the spin-transfer torque acts on inhomogeneities in the magnetization, e.g., on interfaces between magnetic layers, 2 on domain walls, 3,4 i.e., interfaces between regions of uniform magnetization, or on magnetic vortices. 5,6,7,8 Magnetic domain walls, usually vortex walls, can be driven by spin-polarized currents to store information in bit registers. 10Vortices appear in laterally confined thin films when it is energetically favorable for the magnetization to point in-plane and parallel to the edges. In the center the magnetization is forced out-of-plane to avoid large angles between magnetic moments that would drastically increase the exchange energy. The region with a strong out-of-plane magnetization component is called the vortex core and is only a few nanometers in diameter. 11,12 The direction of the magnetization in the vortex core, also called the core polarization p, can only point out-ofor into-the-plane (p=+1 or p=−1, respectively). Hence ferromagnetic thin films containing vortex cores have been suggested as data storage elements. The chirality c = +1(−1) denotes the counterclockwise (clockwise) in-plane curling direction of the magnetization. It is known that vortices can be excited to gyrate around their equilibrium position by magnetic fields. 13,14 Recently it has been shown that field excitation can also switch the core polarization. 15,16,17,18,19,20 Micromagnetic simulations predict that spin-polarized currents can cause vortices both to gyrate 5,7 and to switch their polarization. 8,21,22 Both for field-and spin-torque-driven excitation, the direction of gyration is governed by the vortex polarization according to the right-hand rule (see Fig. 2 of Ref.14 ). The phase of field-driven gyration depends also on the chirality, while spin-torque driven gyration is independent of the chirality as the spin-transfer torque is proportional to the spatial derivative of the magnetization.7 Time-and spatially averaging experimental techniques indicate...

Fast magnetization dynamics of ferromagnetic elements on sub-micron length scales is currently attracting substantial scientific interest. Studying the ferromagnetic eigenmodes in such systems provides valuable information in order to trace back the dynamical response to the underlying micromagnetic properties. The inherent time structure of third generation synchrotron sources allows for time-resolved imaging (time resolution: 70–100 ps) of magnetization dynamics at soft x-ray microscopes (lateral resolution down to 20 nm). Stroboscopic pump-and-probe experiments were performed on micron-sized Permalloy samples at a full-field magnetic transmission x-ray microscope (XM-1, beamline 6.1.2) at the ALS at Berkeley, CA. Complementary to these time-domain experiments a frequency-domain “spatially resolved ferromagnetic resonance” (SR-FMR) technique was applied to magnetic x-ray microscopy. In contrast to time-domain measurements which reflect a broadband excitation of the magnetization, the frequency-domain SR-FMR technique allows for detailed studies of specific ferromagnetic eigenmodes. First SR-FMR experiments at a scanning x-ray transmission microscope (STXM, ALS, BL 11.0.2) are reported. The sample, a 1×1μm2 Permalloy pattern, was excited by an alternating magnetic field with a frequency of 250 MHz. By varying the phase relation between the sine excitation and the x-ray flashes of the synchrotron, the dynamics of a vortex motion eigenmode was investigated in time and space.

Square-shaped thin film structures with a single magnetic vortex were investigated using a scanning transmission x-ray microscope. The authors report on the direct observation of the vortex core in 500ϫ 500 nm 2 , 40 nm thick soft magnetic Ni-Fe samples. The static configuration of the vortex core was imaged as well as the gyrotropic motion of the core under excitation with an in-plane alternating magnetic field. This enabled them to directly visualize the direction of the out-of-plane magnetization in the vortex core ͑up or down͒. The reversal of the core was effected by short bursts of an alternating magnetic field. An asymmetry appears in the core's trajectory for its orientation pointing up and down, respectively. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2738186͔The magnetic properties of patterned ferromagnetic thin film structures are recently attracting considerable attention. The arrangement of magnetic moments in micro-and nanostructures and their excitations are key subjects to be investigated. Micromagnetic calculations were employed to predict the magnetic equilibrium state of such systems, and have been verified experimentally. The dynamics of the magnetization in these small elements, on the other hand, is much more challenging. Such investigations are not only interesting for modern magnetism theory but are also important for developing high density magnetic recording media where fast switching speeds are necessary.Micron-or submicron-sized magnetic patterns minimize their stray field energy by forming regions of inhomogeneous magnetization, e.g., domain walls. In thin film ferromagnetic structures, the competing contributions from the exchange energy between neighboring spins and long-range dipoledipole interactions can result in a very stable magnetic vortex configuration, 1 also called Landau structure in squares. The stability of such structures has already been investigated and is well understood. [4][5][6][7][8] The uniformly magnetized domains in a Landau pattern are separated by 90°Néel walls and form an in-plane flux closure ͓yellow arrows in Fig. 1, panel ͑a͔͒. The curling magnetization at the center of the element turns out of the plane avoiding a singularity and forming in this region the vortex core ͓red arrow in Fig. 1, panel ͑a͔͒, which plays a key role in the magnetization dynamics. 2,3 For the experimental study of magnetic vortex structures magnetic force microscopy, 9 Lorentz microscopy, 9 spin-polarized scanning tunneling microscopy, 10 magnetic x-ray microscopy, 11 and magneto-optical techniques 5,6,12,13 can be deployed. Study of the details in the dynamic response of a vortex structure to externally applied magnetic field pulses and continuous excitations was only possible with the advent of time-resolved magnetic transmission x-ray microscopy 14,15,17 and photoemission electron microscopy. 16 In the current work we report on the direct observation of a magnetic vortex core and its dynamic behavior under influence of an in-plane alternating magnetic field. Squar...