MAGNETIZATION REVERSAL IN A THIN FERROMAGNETIC FILM WEAKLY EXCHANGE-COUPLED TO A BULK FERROMAGNET

The static magnetization distribution in a thin film and in a bulk magnet has been calculated using a simplified model in which the ferromagnetic thin film is coupled to the bulk magnet through a contact exchange interaction. The lowest lying frequencies of oscillation of the magnetization around this equilibrium configuration have also been calculated. It has been shown that for weak antiferromagnetic coupling, and assuming that the system adopts a configuration corresponding to a state of lowest free energy, there exists a range of applied magnetic field strengths for which the magnetization orientation in the thin film deviates from the applied field direction. In the state characterized by this non-parallel thin film magnetization, the magnetization orientations both in the thin film and in the bulk magnet become position dependent. For a 20 monolayer iron Nm the twist in magnetization orientation is similar to 1-3 degrees for exchange coupling strengths ranging from 0.5-1.5 erg/cm(2). The magnetization in the bulk is twisted a few degrees from the applied field direction at the bulk surface, and the angle through which the magnetization is turned decays towards zero for distances progressively deeper into the bulk magnet. The decay length is similar to 100 Angstrom for iron. Calculated magnetic field dependencies for the thin film frequency are found to be in reasonable agreement with data reported for 20 ML Fe(001) films exchange coupled through Cr(001) to a Fe(001) whisker.