Structural, magnetic and Mossbauer studies on nickel-zinc ferrites synthesized via a precipitation route

Polycrystalline Ni-Zn ferrite, with a well-defined composition, $Ni_{0.8}Zn_{0.2}Fe_{2}O_4$, synthesized by a low temperature method showed the formation of a cubic ferrite phase with a crystallite size of 15 nm and saturation magnetization of Ms ≈ 44 emu/g at room temperature. When calcined at higher temperatures (up to 1200 oC) the magnetization increased continuously to Ms ≈ 67 emu/g. To understand the magnetic nature of the cubic ferrite phase formed at a temperature as low as 80 oC, 57 Fe Mossbauer spectra were recorded for samples annealed at three different temperatures, without any external magnetic field, and with an external field of $H_ext$ = 5 T, at 4.2 K. The spectral parameters at room temperature, namely, isomer shift, quadrupole splitting and hyperfine field, confirmed the presence of ultrafine superparamagnetic crystallites of Ni-Zn ferrite. The Mössbauer spectra at 4.2 K revealed spin relaxation effects resulting in very broad sextets, characteristic of ultrafine crystallites. The Mössbauer spectra recorded at 4.2 K and $H_ext$ = 5 T showed well resolved two sextet pattern with characteristic hyperfine interaction parameters of the cubic ferrite phase