INTERPLAY OF MAGNETIC AND HYDROGEN ORDERS IN THE LAVES HYDRIDE YMN2H4.3

We have performed a neutron-diffraction study of the Laves hydride YMn 2H4.3 to investigate the interplay between the atomic order in the hyd rogen sublattice and the magnetic structure. We find that structural o rder in the H sublattice and antiferromagnetic order in the Mn sublatt ice have the same rhombohedral symmetry. At ambient pressure we observ e a first-order magnetostructural transition. where the magnetic subla ttice transforms form a paramagnetic state to an antiferromagnetic one , as the hydrogen atoms form the ordered superstructure. An applied pr essure decouples the two types of order, which then occur at different temperatures. It allows us to separate the effects of structural orde r from those related to tile average Mn-Mn distance.