Nanosecond magnetization reversal in high coercivity thin films

Abstract

We used a wide-field Kerr microscope to measure magnetization reversal in high coercivity thin film media that were subjected to nanosecond field pulses. Coplanar waveguides were used as a field source. Two different samples of CoCr/sub 10/Ta/sub 4/ were measured. Sample A had a coercivity of 83 kA/m and sample B had a coercivity of 167 kA/m. For sample A, we find that after a step change in H, the magnetization initially relaxes exponentially with a time constant of 5 ns, and then relaxes logarithmically. We interpret this result as indicating a transition from dynamic reversal to thermal relaxation. In higher fields, the exponential relaxation time decreases according to /spl tau/=S/sub w//(H-H/sub 0/), where S/sub w/=29.7 /spl mu/s/spl middot/A/spl middot/m/sup -1/ (373 ns/spl middot/Oe). For sample B, only logarithmic relaxation is observed, implying that the dynamic magnetization response time is subnanosecond. We observe correlated regions of reversed magnetization in our Kerr images of sample A with a typical correlation length of 1 /spl mu/m along the applied field direction. We propose a microscopic model of nucleation and growth of reversed regions by analogy to viscous domain wall motion.