Competing Anisotropies in Holmium-Erbium Superlattices

The effect of competing crystal-field anisotropies on magnetic order has been investigated in a series of Ho/Er superlattices. For temperatures in the interval $T_N\left(\mathrm{Er}\right)\le T\le T_N\left(\mathrm{Ho}\right)$ the Ho basal-plane order propagates coherently through the paramagnetic Er with a typical length scale of 1000 Å. Below $T_N\left(\mathrm{Er}\right)$ the coherence length of the basal-plane order decreases, while the longitudinal component of the Er moments fails to order across the Ho block. It is argued that these results require an extension of current models of indirect exchange in superlattices to explicitly include the superlattice band structure.