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

Abstract

The chemical structure of epitaxial ␥ 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 Tϳ2 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 ␤ Ce, which has commensurate transverse modulation with a propagation wave vector ͓1/2 0 0͔ and moments along the hexagonal a direction. These two types of magnetic order appear to be mutually exclusive. ␥ Ce is the high-temperature fcc phase of Ce, our singlephase epitaxial sample is observed to go through a new, but partial, structural transition not previously seen in the bulk material.

The puzzle of the $ \gamma\to\alpha$ and other phase transitions in cerium

“…Of much interest are also experimental data on the epitaxial -phase of Ce grown on niobium surface [171]. On cooling instead of transforming to -Ce this epitaxial -phase undergoes a transition to samarium-type crystal structure characteristic of 5d-metals.…”

“…Because of the nature of the -phase, the growth of a -Се single crystal seems impossible. Nevertheless, it has turned out that a single crystal -phase can be obtained by the molecular epitaxy method on Nd surface, which stabilizes it down to low temperatures (Т = 2 К) [171]. In Ref.…”

“…In Ref. [171] such epitaxial -phase containing 60 layers of Ce 30 /Nd 10 was studied in detail. The lattice parameters are a=3.664(3) Å, c=2.970(3) Å (in niobium c=2.949(3) Å) implying a small compression within the hexagonal planes (0.5%) and a small expansion between the planes (0.2%) in comparison with the bulk -phase [16].…”

New magnetic phase and magnetic coherence in Nd/Sm(001) superlattices

“…The Nd L II edge (6.722 keV) being only 6 eV above the Sm L III edge (6.716 keV), the search was performed at the Nd L III edge (6.208 keV), shown later in superlattices with thick Nd layers to give resonant intensities about an order of magnitude smaller (see the next section). The fact that no magnetic signal was detected can be attributed either to its weakness, or to the fact that the magnetic structure is different from both the Sm and Nd magnetic structures, or finally to the lack of Nd magnetic ordering, as already reported in Ce/Nd superlattices [14]. This will be further discussed in section 5.…”

“…Recently, studies have been carried out on superlattices combining two light rare-earth metals (Nd/La [11], Nd/Pr [12,13] and Ce/Nd [14]), where both constituents have a dhcp structure, with sublattices of two crystallographic sites with hexagonal and cubic local symmetry. In Nd/Pr and Nd/La, the magnetic ordering of Nd is qualitatively similar to that of bulk Nd metal.…”