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We consider magnetization reversal and decay in thin film and particulate magnetic systems where thermally activated processes are important. We present measurements on the activation volumes associated with magnetization reversal in CoCrPt thin films. We find that the measured activation volume V* goes through a minimum as the film thickness is decreased from 17.5 to 5.5 nm. We have modeled this behavior qualitatively by assuming that the energy barrier to thermally activated reversal varies as 1/H, as has been predicted for incoherent nucleation processes and thermally activated domain–wall motion. This model also predicts that V* varies as T2 at high temperature in qualitative agreement with measurements on several systems. This behavior is contrasted with that predicted on the basis of coherent rotation of the magnetization in single-domain particles.