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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 CoCr10Ta4 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 τ=Sw/(H-H0), where Sw=29.7 μs·A·m-1 (373 ns·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 μm along the applied field direction. We propose a microscopic model of nucleation and growth of reversed regions by analogy to viscous domain wall motion.