Temperature dependent magnetic properties of NixCo1-xFe2O4: Single- and multidomain particles

In the present work, nanocrystalline NixCo1-xFe2O4 oxides with average crystallite size between 11 nm and 111 nm have been analyzed by Mossbauer spectroscopy, hysteresis loops, field cooled (FC) and zero field cooled (ZFC) magnetization measurements. A core-shell model has been proposed. Accordingly, the Mossbauer 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 similar to 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)(beta) where beta = 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 Ni0.1Co0.9Fe2O4 (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.