Magnetic and hydrogen, ordering in the frustrated Laves hydrides RMn2H4.5 (R = Y, Gd, Tb, Dy, Ho): A neutron-diffraction study

We have studied the magnetic and crystal structures of different Laves hydr ides RMn2H4.5 (R = Y, Gd, Tb, Dy, Ho), having the cubic C15 structure at hi gh temperature. We observe a strong coupling between the hydrogen and magne tic order in the frustrated Mn sublattice. The Neel temperature coincides w ith the ordering temperature in the hydrogen sublattice, resulting in a sin gle magnetostructural transition. In contrast to the RMn2 compounds, in the hydrides the Mn-Mn magnetic interaction dominates and it imposes the magne tic order in the rare-earth sublattice. On the other hand, the anisotropy o f the rare-earth ion strongly influences the orientation of the magnetic mo ments at low temperature. The Laves hydrides show a very unusual case where the structural and magnetic orders strongly interact with each other. They also offer many examples of the interplay between the localized Mn moments and the rare-earth moments. [S0163-1829(99)13213-2].