Spatially resolved ferromagnetic resonance: Imaging of ferromagnetic eigenmodes

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 × 1 μ m 2 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.