Temperature dependent magnetic properties of Ni_xCo_1-xFe₂O₄: Single- and multidomain particles

In the present work, nanocrystalline Ni_xCo_1-xFe₂O₄ oxides with average crystallite size between 11 nm and 111 nm have been analyzed by Mössbauer spectroscopy, hysteresis loops, field cooled (FC) and zero field cooled (ZFC) magnetization measurements. A core-shell model has been proposed. Accordingly, the Mössbauer spectra evidence a ferrimagnetic core and a disordered shell (spin-glass), the latter increasing with Ni concentration. Hysteresis curves reveal the ferromagnetic nature of the investigated compounds and transformation from single-to multi-domain behaviour at a critical particle size dependent on Ni²⁺ ion concentration. The magnetic properties of finest powders (average crystallite size ∼ 11 nm) are the most sensitive to the Ni²⁺ ions content. A general increase in the coercive field, H_C, with reducing temperature according to the modified Kneller's formula H_c(T) = H_c(0)[1-(T/T_B)^β where β = 0.45 occurs. A high saturation magnetization of about 90 emu/g and an increase in H_C from about 0.3 kOe at 300 K to 7 kOe at 10 K have been observed for the sample Ni_0.1Co_0.9Fe₂O₄ (x = 0.1). Increasing magnetization and coercive field with reducing temperature are also explained within the core shell model. FC and ZFC data show strong dependence of the magnetic properties on crystallite size and concentration of Ni²⁺ ions.