Coherent magnetic structures in terbium/holmium superlattices

Neutron-scattering techniques have been used to investigate the magnetic properties of three Tb/Ho superlattices grown by molecular-beam epitaxy. It is revealed that for temperatures in the range T = 10 to T-N(Ho)approximate to 130 K, there is a basal-plane ferromagnetic alignment of Tb moments within To blocks that is coherent with a basal-plane helical ordering of Ho moments. Between T approximate to T-N(Ho) and 200 K, the Tb moments remain ferromagnetically aligned within To blocks, with adjacent To blocks antiferromagnetically coupled. As the temperature is raised from T approximate to 200 to 230 K, two samples retain this magnetic structure while the third undergoes a transition first to a mixed phase of helically and ferromagnetically ordered Tb moments, then to a phase with only helically ordered To moments. Ln all cases, the magnetic ordering is found to be long ranged, with coherence lengths extending over three to six bilayers. The results are discussed with a consideration of previous rare-earth superlattice studies, and the possible mechanisms for interlayer coupling.