Surface magnetic phase transitions in Dy Lu superlattices

Dy/Lu superlattices comprising ferromagnetic Dy blocks coupled antiferromag netically 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 applie d 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 pi/3. Furthermore, the effects of surface reconstruction an determined for finite chains. A [DY20Lu12](20) superlatt ice has been studied using bulk magnetization and polarized neutron reflect ivity. The (1, 5...) phase has been identified and the results provide dire ct evidence in support of the theoretical predictions. Dipolar forces are s hown to account for the magnitude of the observed exchange coupling. (C) 19 99 Elsevier Science B.V. All rights reserved.