Mössbauer and magnetic studies of nanocrystalline γ′-Fe4−xNixN (0.2⩽x⩽0.8) compounds

Nanocrystalline γ′-Fe4−xNixN (0.2⩽x⩽0.8) compounds were synthesized by using a citrate precursor route. It was observed that lattice constants for γ′-Fe4−xNixN (0.2⩽x⩽0.8), decrease with increasing Ni atom concentration. The atmospheric oxidation of γ′-Fe4−xNixN (0.2⩽x⩽0.8) compounds result in the formation of Fe-oxide layer at the surface of the ultrafine particles in addition to pure nitride phase. The local magnetic structures of the iron atoms in the nitride materials are quite similar to those found in the case of dilute alloy systems. The role of Ni substitution in γ′-Fe4N as γ-Fe4−xNixN is investigated with regard to crystal structure and magnetic properties of the ultrafine nitride materials. The results suggest that the average magnetic moment per iron atom in ultrafine γ′-Fe4−xNixN compounds is affected by the Ni atom concentrations, superparamagnetic relaxation, presence of oxide layer and randomly canted spin structure at the particle surface. The observed larger values of coercivities in the ultrafine materials are on account of the reversal of magnetization by spin rotation mechanism.