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.

Magnetization dynamics in alloys of ferrimagnetic CoGd have been studied in the vicinity of the magnetization and angular momentum compensation point as a function of alloy composition and temperature. In agreement with standard mean-field treatments of the dynamics of the total magnetization we observe an increase of the precessional frequency and the effective damping parameter near the angular momentum compensation point. We demonstrate the consistency of the magnetization dynamics extracted from frequency domain methods such as ferromagnetic resonance and time resolved laser pump-probe measurements. DOI: 10.1103/PhysRevB.74.134404 PACS number͑s͒: 75.40.Gb, 75.50.Gg, 76.50.ϩg Transition metal ͑TM͒ rare earth ͑RE͒ ferrimagnets are ideal canonical systems to probe magnetization dynamics. Typically, TM-RE alloys are nearly amorphous materials. The TM sublattice is antiferromagnetically ͑AF͒ coupled to the RE sublattice. When the coupling is strong, as, e.g., in CoGd, there are two transition temperatures, the magnetization compensation temperature T M where M Gd = M Co , and the angular momentum compensation temperature T L , where M Gd / ␥ Gd = M Co / ␥ Gd , and ␥ is the gyromagnetic ratio. These temperatures are sensitive functions of the relative concentration. At the magnetic compensation temperature, applied magnetic fields cannot couple to the magnetization to alter its energy since M Gd − M Co = M eff = 0. Angular momentum is quenched at the angular momentum compensation point, where the AF coupled sublattices gyrate 180°out of phase about the magnetic field. Studying the dynamics in ferrimagnetic systems are complicated by these tightly coupled AF sublattices. As T L is approached from low temperatures, the phenomenological mean-field damping parameter ␣ eff which governs how fast the system as a whole dissipates energy increases quickly, and the gyromagnetic frequency changes sign as the angular momentum of the dominant sublattice changes from Gd to Co. An ideal ferrimagnet should dissipate angular momentum instantaneously at T L .1,2 CoGd was chosen for this study because T M and T L are very close to each other, and the intrinsic orbital moment of Gd is essentially zero, thereby eliminating additional loss channels due to spin-orbit coupling. 3 We compare experimental results obtained by a frequency domain method used to study the dynamics of the total magnetization of M eff -namely, ferromagnetic resonance ͑FMR͒-to time domain ultrafast laser pump/probe experiments.The most straight forward method to excite magnetization dynamics uses strong magnetic field pulses that couple directly to the magnetization ͑spin͒.4,5 These field pulses are typically produced by external sources. However, these methods cannot excite the magnetization at the magnetization compensation point in a ferrimagnet since there is no net magnetic moment one can couple to. Another method to excite spin-systems employs ultrashort laser pulses that alter the magnetic system by heating across a critical temperature ͑Curie, Néel, ...

We present vector network analyzer ferromagnetic resonance measurements of epitaxial Fe films having a thickness of 16 monolayers. Our objective is to test the reliability of this novel frequency domain technique with respect to frequency and damping. For this purpose we compare vector network analyzer ferromagnetic resonance to pulsed inductive microwave magnetometry, time resolved magnetooptic Kerr effect (both methods in the time domain), and conventional ferromagnetic resonance (measured in the field domain) in terms of position and width of the ferromagnetic resonance. In addition, we compare the various techniques with respect to the signal to noise ratio of the raw data. All data is obtained using the same well characterized ultrathin magnetic Fe/GaAs (0 0 1) film. Finally, we demonstrate the potential of the vector network analyzer ferromagnetic resonance technique for the investigation of nano-structured magnetic elements having nonuniform magnetization configuration. The absorption spectrum of Permalloy disks with a diameter of 200 nm and a thickness of 15 nm shows up to eight distinct resonance peaks. The spatial structure of the corresponding modes was derived from numerical calculations and reveals that azimuthal modes up to the fifth order have been observed inductively. r

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.

The spatially resolved eigenmode spectrum of micrometer-sized Co ring elements has been determined by means of combined vector network analyzer ferromagnetic resonance and time resolved magneto-optic Kerr effect measurements. Up to 5 resonant eigenmodes were observed in the frequency range from 45 MHz to 20 GHz as a function of an external magnetic bias field. A well-defined mode structure was found for the two equilibrium states (vortex and onion) which correspond to distinctive spatial modes. The effect of dynamic inter-ring coupling on the modes in the remanent states was evinced. The experimental results are found to be in good agreement with those of micromagnetic simulations. Our results demonstrate that, in analogy to the well-defined static equilibrium magnetic states of ring elements, the eigenmode spectra of this high symmetry geometry consist of a well-defined and simple mode structure.