MAGNETIC-STRUCTURE OF SUPERLATTICES WITH BIQUADRATIC INTERLAYER EXCHANGE COUPLING

We calculate the magnetic structure of an infinitely extended magnetic superlattice in the presence of interlayer biquadratic exchange coupling and a 4-fold in-plane anisotropy. An external magnetic field is also applied in the plane of the layers. We consider ground state configurations with unit cells involving 1, 2, or 4 magnetic layers. At low fields a two-layer unit cell structure is found to be the ground state. At higher fields the ground state has the spins in all layers aligned so the unit cell is one layer. Analytic expressions for the magnetic equilibrium structures are obtained in the small field limit, as are expressions for the critical fields for a transition from one state to another. Numerical results are presented for magnetic fields of arbitrary size.