MAGNETIC-STRUCTURE OF DYSPROSIUM IN EPITAXIAL DY FILMS AND IN DY ER SUPERLATTICES/

We present a magnetization and neutron-diffraction study of the basal plane magnetic structure of Dy epitaxial films and Dy/Er superlattices . The thermal evolution of the magnetic phases, the stability of the h elical phase under a magnetic field, the thermal variation of the dysp rosium in-plane and c parameters, and of the dysprosium turn angle are successively shown. In Dy/Er superlattices, the dysprosium helix prop agates coherently through paramagnetic erbium; at low temperature, ind ividual dysprosium layers undergo a ferromagnetic transition and are c oupled antiferromagnetically to each other for erbium layers thickness es larger than 20 Angstrom. In dysprosium films, as expected from the epitaxy effect, the Curie temperature of dysprosium is reduced if dysp rosium is grown on yttrium and increased if it is grown on erbium, whe reas it is unexpectedly close to the bulk value in Dy/Er superlattices . This amazing value of the Curie temperature in superlattices is corr elated to two main experimentally observed effects: (i) the magnetoela stic driving force is reduced compared to bulk dysprosium because of t he clamped gamma distortion; (ii) the difference between the exchange energies in the helical and the ferromagnetic phases is increased comp ared to the bulk value.