Local magnetic moment and hyperfine field in hydrogenated iron and iron-vanadium alloy

The local magnetic moment Μ and hyperfine field B_hf at Fe and V sites in hydrogenated iron and iron-vanadium were calculated using the discrete variational method. The variations in Μ and B_hf with H occupation of the octahedral (O) site were considered. It was found that when H occupies the O site neighbouring an Fe atom, both local moment and hyperfine field at this atom decrease linearly with increasing number of H atoms. The rate of decrease is larger for Fe in iron as compared to iron in vanadium. On the other hand, when H resides at an O site next neighbouring an Fe atom, whether in iron metal or in iron-vanadium, the Fe magnetic moment increases slowly, while the hyperfine field remains almost constant. The V moment in iron, which is negative (∼-0.83 Μ_B), becomes less negative (∼-0.30 Μ_B) as H occupies the neighboring O sites, whereas slight changes occur (∼-0.88 Μ_B) when H is at the next neighbouring O site. The net effect of H on Fe in iron is to decrease the average magnetic moment at a rate of ∼1.2 Μ_B per H/Fe for low H content. On the other hand, the average Fe moment in an iron-vanadium alloy increases if H resides at O sites which are immediate neigbours of V and next neighbours of Fe. This may explain the development of a magnetic state on hydrogenation of Fe-V alloys, which is exhibited by the specific heat and susceptibility measurements. The changes in the isomer shift were found to agree with experimental trends.