Epitaxial Ce and the magnetism of single-crystal Ce/Nd superlattices

Abstract

The chemical structure of epitaxial $\ensuremath{\gamma}$ cerium and the chemical and magnetic structures of cerium/neodymium superlattices have been studied using x-ray and neutron diffraction techniques. The samples were grown using molecular-beam epitaxy, optimized to yield the desired Ce allotropes. The x-ray measurements show that, in the superlattices, both constituents adopt the dhcp structure and that the stacking sequence remains intact down to $T\ensuremath{\sim}2\mathrm{K};$ these are the first measurements of magnetic ordering in single-crystal dhcp Ce. The magnetic structure of the superlattices with thicker Nd layers exhibit incommensurate order and ferromagnetism on separate sublattices in a similar manner to Nd under applied pressure. The sample with thickest Ce layers has a magnetic structure similar to bulk $\ensuremath{\beta}$ Ce, which has commensurate transverse modulation with a propagation wave vector [1/2 0 0] and moments along the hexagonal a direction. These two types of magnetic order appear to be mutually exclusive. $\ensuremath{\gamma}$ Ce is the high-temperature fcc phase of Ce, our single-phase epitaxial sample is observed to go through a new, but partial, structural transition not previously seen in the bulk material.