Neutron diffraction study and temperature variation of magnetic anisotropy in Bi substituted nickel ferrite

Bismuth doped NiFe₂O₄ (NFO) polycrystalline samples: Ni_1-xBi_xFe₂O₄ with x = 0.00, 0.05 and 0.10, were synthesized using solid-state reaction method. The crystal structural, magnetic structure and magnetic properties of parent NFO and Bi-doped NFO samples were characterized using X-ray diffraction (XRD), neutron powder diffraction (NPD) and magnetization (M) versus magnetic field (H) isotherms, respectively. Rietveld refinement of XRD and NPD patterns confirms the formation of a single-phase cubic spinel crystal structure. No observable change in the lattice parameter was found (within the error limits) with the 5 at % Bi doping to NFO, whereas ≈0.15% change in the lattice parameter was observed in case of 10 at % Bi-doped NFO. Room temperature magnetic structure studies using NPD reveal a net magnetic moment of 1.55(3) μ_B/f. u., in the case of NiFe₂O₄. The net magnetic moment at room temperature changes to 1.61(4) μ_B/f.u. and 1.47(5) μ_B/f.u. for Ni_0.95Bi_0.05Fe₂O₄ and Ni_0.90Bi_0.10Fe₂O₄, respectively. The same observation was also inferred from the spontaneous magnetization (M_sp) obtained from approach-to-saturation (ATS) analysis of M-H isotherms. Temperature (T) dependence of M_sp follows a T² decline. Approximately 14% increase in magnetic anisotropy was observed in Ni_0.95Bi_0.05Fe₂O₄ compared to NiFe₂O₄ in the temperature range 5 K -300 K.