MAGNETIC-STRUCTURES AND INTERACTIONS IN HO Y, HO/LU, AND HO/ER SUPERLATTICES/

Ho/Y and Ho/Er superlattices have been grown by molecular-beam epitaxy using a Balzers UMS 630 instrument. The superlattices were grown on a sapphire substrate with an Nb buffer and Y seed layer. X-ray-diffract ion techniques were used to characterize the crystallographic structur e and neutron-diffraction techniques to determine the magnetic structu res. The results for the Ho/Y systems were consistent with long-range order being formed coherently through the whole superlattice. The mome nts in the Ho layers were aligned in the basal plane and most of the s tructures could be described by helical structures with a turn angle b etween holmium planes of Psi(H) and between nonmagnetic Y planes Psi(Y ). Psi(Y) is found to be largely independent of temperature or superla ttice, while Psi(H) decreases with decreasing temperatures and at low temperatures takes a commensurate value, so as to take advantage of th e basal plane anisotropy. The results for the Ho/Er superlattices diff er because the Er has a magnetic moment and the anisotropy favors alig nment along the c axis. Between the ordering temperature of bulk Ho an d bulk Er, the results are similar to those of the Ho/Y superlattices. The ordering propagates through the Er layers but the Er moments are not ordered. At lower temperatures the Er moments order in a cycloidal (a/c) structure with the basal plane components having fairly long-ra nge coherence with the Ho moments, but the c-axis components having no coherence from one Er layer to the next.