Surface magnetic phase transitions in Dy/Lu superlattices

Dy/Lu superlattices comprising ferromagnetic Dy blocks coupled antiferromagnetically across the Lu blocks may be modelled as a chain of XY spins with antiferromagnetic exchange and six-fold anisotropy. We have calculated the stable magnetic phases for the cases of large anisotropy and a field applied along an easy direction. For an infinite chain an intermediate phase 〈1, 5,…〉 is predicted, where the notation gives the angle between the moment and the applied field in units of π/3. Furthermore, the effects of surface reconstruction are determined for finite chains. A [Dy 20Lu 12] 20 superlattice has been studied using bulk magnetization and polarized neutron reflectivity. The 〈1, 5,…〉 phase has been identified and the results provide direct evidence in support of the theoretical predictions. Dipolar forces are shown to account for the magnitude of the observed exchange coupling.