Modeling the magnetic properties of DyFe2/YFe2 superlattices

The Stoner–Wohlfarth model has proved reasonably successful in describing the coercivities of antiferromagnetically coupled DyFe2/YFe2 hard/soft superlattices in the absence of magnetic exchange springs. In particular, the coercivity rises sharply as the net magnetic moment of the superlattice approaches zero. However the situation becomes more complicated as the thickness of the YFe2 layers is increased. Two distinct “instability fields” can be identified: the bending field BB, signifying the onset of a magnetic exchange spring, and the irreversible switching field BIS associated with magnetic reversal. We have developed a computational model to address this problem. In particular, it is shown that the two instability fields in question are characterized by vanishing eigenvalues in the matrix formed by the double energy derivatives ∂2E/∂θi∂θj, where E is the total energy and θi the angle of each individual monolayer. It is shown that the model provides a very good description of the M–Bapp loops of DyFe2/YFe2 multilayer films. In particular, the coercivity of a nearly magnetically compensated multilayer (75 ÅDyFe2/150 ÅYFe2) is much reduced below the prediction of the Stoner–Wolhfarth model, in accord with experiment.