The authors compare the results obtained from optical and magnetic measurements on TbFeCo magnetooptic thin films. Optical measurements were performed using a polar Kerr magnetometer, that is, the surface magnetization was probed. Bulk magnetic measurements were performed on an alternating gradient force magnetometer. Analysis of the dependence data from both techniques revealed similar activation volumes of reversal of approximately 1 × 10-18 cm3. As this is the volume required to nucleate a magnetic reversal, it may set a bound on the minimum domain size.