MAGNETIZATION PROCESSES, HYSTERESIS AND FINITE-SIZE EFFECTS IN MODEL MULTILAYER SYSTEMS OF CUBIC OR UNIAXIAL ANISOTROPY WITH ANTIFERROMAGNETIC COUPLING BETWEEN ADJACENT FERROMAGNETIC LAYERS

Magnetisation processes have been investigated for model multilayer systems where antiferromagnetic interactions couple adjacent ferromagnetic layers. In this first study, only coherent rotations of the magnetisation of each layer are taken into account. Depending on the direction of the applied magnetic field, the initial moment configuration and the magnetocrystalline anisotropy, various first- or second-order magnetic transitions may be observed. The cases of cubic and uniaxial anisotropy bilayer systems are treated in detail. Spin-flop and spin-flip transitions are calculated to occur for both symmetries when starting from antiferromagnetic configurations that are parallel to the field axis. In the cubic case, various other transitions have been found. In particular, first-order transitions between symmetric and nonsymmetric states have been calculated, involving asymmetric behaviour of the magnetisation vectors of adjacent layers. Such transitions give rise to a transverse magnetisation. All the critical transition fields have been calculated as a function of the anisotropy and are reported in the various phase diagrams. Hysteresis loops have also been calculated. They generally consist of an upper and a lower part shifted symmetrically about the origin as in the case of bulk antiferromagnets. The influence of the number of layers has been investigated in some particular cases. It is shown that in most instances when the number of layers n becomes very large (i.e. when boundary effects disappear), the multilayer behaves like a bilayer but with different transition-field values. For small numbers of layers, whether or not the magnetism is compensated (n even or odd) has a very strong effect on the magnetisation processes.