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 coupl ing and a 4-fold in-plane anisotropy. An external magnetic field is al so applied in the plane of the layers. We consider ground state config urations 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 aligne d 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 arbit rary size.