From antiferromagnetic to ferromagnetic ordering induced by hydrogenation of the compounds NdCoSi and NdCoGe

NdCoSi and NdCoGe order antiferromagnetically at TN = 7.5 and 8.3 K respectively. These ternary compounds absorb easily hydrogen at 523 K under a hydrogen pressure of 4 MPa; the resulting hydrides are stable in ambient conditions and crystallize in the tetragonal ZrCuSiAs-type structure. Investigation of the hydrides by magnetization and electrical resistivity measurements and also by neutron powder diffraction reveals their ferromagnetic behaviours appearing below TC = 20.5 and 15.9 K respectively for NdCoSiH and NdCoGeH. Their magnetic structures are collinear with Nd-moments (2.28 and 2.35 μB at 1.5 K for NdCoSiH and NdCoGeH respectively) aligned along the c-axis as observed previously for NdFeSi. The antiferromagnetic-ferromagnetic transition evidenced during the hydrogenation can be explained by a change of the number of conduction electrons linked to the H-insertion as observed by electronic structure determination.