Effects of Ni doping on the structural properties and collapse of magnetic ordering in NdFe_(1-x)Ni_xO_3(0.1 ≤x≤ 0.7) orthoferrites

Nd Fe_(1-x)Ni_xO_3(0.1 ≤ x ≤ 0.7) orthoferrites are synthesized by solid state reaction method, and the structural properties of these materials are investigated by employing x-ray diffraction(XRD), scanning electron microscopy(SEM) and M o¨ssbauer spectroscopy. The orthorhombic structure is observed in all systems; however, with the increase in Ni doping,the increase in tolerance factor and the decrease in the cell volume are observed. Orthorhombic distortion decreases with Ni content increasing up to 50%, while above 50% Ni doping it increases. SEM examination indicates the increases in grain size and intermixing of grains with increase in Ni concentration. Comparison between bulk and theoretical densities shows that in each of all samples porosity is less than 2%. M?ssbauer spectroscopic investigations are performed to explain local structure, Fe oxidation states and collapse of the magnetic ordering. In these samples the Fe oxidation state remains+3 and there is no considerable increase in hole states observed; however due to mismatch of the ionic radii between Fe~(3+) and Ni~(3+), octahedral distortions, sagging and distribution of hyperfine parameters increase with increase in Ni concentration. The major factors behind the collapse of magnetic ordering in the Ni-doped systems are the weakening of the super-exchange interactions, decrease in the Neel temperature, increase in spin–spin relaxation frequency and high spin to low spin transition.