Electrical behaviour of NdH2+x with regard to structural ordering, magnetism and approach to the metal-insulator transition

Abstract

Abstract Electrical resistivity measurements of NdH2+x show the effects of a structural order-disorder transition, related to the excess H concentration x, for which we give information on the ordering temperature, T s, and the stabilizing effects of quenching on the disordered H-sublattice. A second important feature concerns the magnetic ordering: for x ⩾ 0.32, the ferromagnetic ordering gives rise to a strong decrease of the spin-disorder resistivity, which is not much influenced by the structural order-disorder transition, but for x ⩾ 0.6, a sharp magnetic transition occurs essentially for the ordered structural state. For high x, a minimum of the resistivity is often observed when lowering the temperature towards the magnetic transition, which indicates either a localization of the conduction electrons or the occurrence of an incommensurate antiferromagnetic ordering with the opening of a gap. The fact that for x ⩾ 0.6 the resistivity over the whole temperature range is very high (attaining 6mω cm for x=0.65) is a sign that the γ-phase (trihydride) has nearly insulating behaviour.