The lock-in and reorientational phase transitions in metallic superlattices

We show that in the metallic magnetic superlattices, along with the bilinear RKKY-like exchange, there is significant biquadratic contribution arising from intrinsic and fluctuational mechanisms. Within the phenomenological approach, we postulate a particular form of free energy density functional and study the thermodynamic behaviour of the system under consideration. The model permits a transparent interpretation of the phase transitions, observed in metallic multilayers, with multiple oscillation periods and biquadratic exchange effects taken into account.