Interplay of magnetic and hydrogen ordering in the hexagonal Laves hydrides

Abstract

We have studied the magnetic and crystal structures of the hexagonal Laves phases $R{\mathrm{Mn}}_{2}{\mathrm{H}}_{x}$ $(R=\mathrm{E}\mathrm{r},\mathrm{T}\mathrm{m},\mathrm{L}\mathrm{u};$ $x=4.2,4.6)$ by powder neutron diffraction. Hydrogen occupies interstitial sites of the metal lattice and forms ordered superstructures. Hydrogen stabilizes localized magnetic moments on the Mn sites by expanding the metal lattice. We have found a very strong coupling between the magnetic and structural properties of the hydrides. Very small modifications in the hydrogen sublattice result in drastic changes in the magnetic properties. The samples $R{\mathrm{Mn}}_{2}{\mathrm{H}}_{4.6}$ show a long-range antiferromagnetic ordering (propagation vector $\mathbf{k}=1/3$ 1/3 0), whereas the samples $R{\mathrm{Mn}}_{2}{\mathrm{H}}_{4.2}$ exhibit short-range magnetic correlations. We discuss our results in the framework of a model assuming that the magnetic ordering is driven by hydrogen superstructure which changes the local symmetries of the magnetic ions and releases the topological frustration in the Mn sublattice.