COEXISTENCE OF LONG-RANGE AND SHORT-RANGE MAGNETIC CORRELATIONS IN HOLMIUM-ERBIUM SUPERLATTICES

The effect of competing crystal-field anisotropies on magnetic order h as been investigated in a series of Ho/Er superlattices using a combin ation of x-ray and neutron scattering techniques. For temperatures in the interval T-N(Er) less than or equal to T less than or equal to T-N (Ho) the Ho basal-plane order propagates coherently through the parama gnetic Er over a typical length scale of 1000 Angstrom. At low tempera tures the Ho moments retain their bulk-like helical configuration, whe reas the magnetic structure in the Er blocks has both basal-plane and c-axis components. Below T-N(Er), the coherence length of the basal-pl ane order decreases on cooling, while the longitudinal component of th e Er moments fails to order across the Ho block. It is argued that the se results require an extension of current models of indirect exchange in superlattices to explicitly include the superlattice band structur e.