Interaction effects in CrO2 audio tape: AC versus thermal demagnetization

The principal magnetizing and demagnetizing remanences of CrO₂ audio tape have been measured as a function of applied field, at various temperatures between room temperature and the ferromagnetic Curie temperature, under both ac and thermally demagnetized initial conditions. The Henkel plots of the thermally demagnetized data are all essentially linear and obey the Wohlfarth relation, while those for the ac demagnetized data all exhibit positive curvature, below the Wohlfarth line, which becomes progressively weaker with increasing temperature. Fits to the scalar Preisach model show that the interaction field distribution has no mean field component, and that thermal demagnetization produces a random initial state which is insensitive to fluctuations in the local interaction fields due to disorder, as long as there are no long range interactions