Dynamic Kerr Observations of High-Speed Flux Reversal and Relaxation Processes in Permalloy Thin Films

Abstract

Sequences of photographs with 10 μ resolution have been taken during high‐speed flux reversal and as the magnetization relaxes upon termination of the drive field. Equipment used consisted of a sampling Kerr magneto‐optic camera with a Q‐switched ruby laser for a light source. The 10 nsec exposure time was synchronized with a variable time delay between application of the magnetic field and actuation of the camera stable to within 3 nsec. The results show that, with zero transverse field, longitudinal propagation of diffuse transverse boundaries and nucleation of partially reversed regions are processes that are involved in intermediate and high‐speed switching, respectively. The velocity of the diffuse boundary varies approximately as H⁵, and the mobility is as large as 1.4 cm/Oe·μsec. With a transverse field, the low‐speed reversal is similar to diffuse boundary propagation and the higher‐speed processes are rotational in nature. The transition from propagational to rotational phenomenon is abrupt. Blocking is easily observable. If the reversing field is terminated prior to saturation, the flux either continues to reverse or relaxes to a less switched condition, in times of the order of 1 μsec, casting considerable doubt on conclusions drawn from interrupted pulse experiments.