THE CHEMICAL AND MAGNETIC-STRUCTURES OF HOLMIUM YTTRIUM AND HOLMIUM LUTETIUM SUPERLATTICES

We present the results of a study of the chemical and magnetic structu res of Ho/Y and Ho/Lu superlattices, all grown by molecular beam epita xy. By combining the results of high-resolution X-ray diffraction with detailed modelling we show that the superlattices have high crystallo graphic integrity: the average structural coherence length in the grow th direction is approximately 2000 angstrom, while the interfaces betw een the two elements are well defined, extending over approximately fo ur lattice planes. The magnetic structures were determined using neutr on scattering techniques. In the case of the Ho/Y superlattices, the H o3+ moments form a basal-plane helix at all temperatures in the ordere d phase, with an apparent suppression of the low-temperature cone phas e. Instead spin-slip structures are formed, including some not observe d in bulk Ho. For the Ho/Lu superlattices the magnetic structure was f ound to depend on n(Ho), the number of atomic planes in the Ho block. If n(Ho) > 20, then the Ho3+ Moments form a basal-plane helix down to the lowest temperatures. For n(Ho) less-than-or-equal-to 20, the Ho3moments undergo a phase transition at low temperatures from a helix to a state where they are ferromagnetically coupled within one block. In this phase, successive Ho blocks are antiferromagnetically coupled, a nd there is a small component of the moment along the c axis.