The domain structure of small grain iron films at elevated temperatures

Observations have been undertaken with Lorentz microscopy on the domain structure, at elevated temperatures, of thin evaporated iron films. The grain size of the films was small, estimated at 20-40 Å. At temperatures well below the normal Curie point the magnetic contrast tended to disappear, but returned to its initial configuration when the temperature was lowered again. It is postulated that the films exhibit a quasi Curie point above which they behave as superparamagnetics under the influence of thermal activation. The value of the quasi Curie point is determined by the balance of the particle thermal energy and the restraining intercrystallite forces. A simple calculation indicates that these forces are magnetostatic in origin, and a value is obtained. This value is in reasonable agreement with that obtained using the Lorentz field approximation.