Direct Observation of Magnetic Relaxation in a Small Permalloy Disk by Time-Resolved Scanning Kerr Microscopy

Magnetic oscillations were studied in a single, lithographically patterned disk ( $8\mu \mathrm{m}\mathrm{diam}\times 0.1\mu \mathrm{m}\mathrm{thick}$) of permalloy placed in an in-plane biasing magnetic field, excited by subnanosecond pulses of perpendicular field. Local changes of the perpendicular magnetization component were detected by time-resolved scanning magneto-optical Kerr microscopy, allowing direct observation of the nonuniform spatial evolution of the magnetization. It was found that the modal frequency of the oscillations is very close to the ferromagnetic resonance frequency. The Landau-Lifshitz-Gilbert equation with damping constant equal to 0.008 describes broad features of the data, although departures due to the nonuniform response are also evident.