Competing magnetic order in holmium-thulium superlattices

Abstract

Superlattices of holmium and thulium have been grown using molecular beam epitaxy and their magnetic structures determined using neutron scattering techniques. At all temperatures below the Néel temperature of Ho, , the holmium moments form a basal-plane helix. Above this helix is coherent across several bi-layer blocks even though there is no significant ordered moment in the intermediate thulium layers. Below approximately 50 K the thulium moments order, forming a longitudinally modulated structure similar to that of the bulk. For the and samples there is little coherence of the longitudinal thulium phase in successive blocks, while there is coherence of the holmium structure, although the magnetic coherence length decreases with decreasing temperature. In contrast, the sample has coherent magnetic order between the thulium blocks with a lack of coherence in the structure formed in successive holmium blocks. These results are similar to those reported for holmium-erbium superlattices which are also superlattices of two elements which have competing crystal-field anisotropies.