Existence of exchange bias and large coercivity in NiFe₂O₄/CoO core–shell structured nanoparticles

This article reports a systematic study on core–shell structured NiFe₂O₄/CoO nanocomposite systems synthesized by chemical co-precipitation route. Four samples with a generic chemical compound formula [(1 − x) NiFe₂O₄/x CoO: x = 0.00, 0.10, 0.20, 0.30] having different weight percentages of CoO component were prepared. The existence of cubic spinel nickel ferrite phase together with pure cobalt oxide phase was determined using x-ray diffraction patterns without any trace of impurity phases. The M–H response at 5 K exhibited an exchange bias in the range of 135–161 Oe for all nanocomposites together with large coercive field around 12.2 kOe. The nanocrystalline nickel ferrites exhibit low coercivity when capped with CoO of different weight percentages showed a massive coercivity along with exchange bias at 5 K due to strong interfacial spin coupling effect. This phenomenon pushed the superparamagnetic limit beyond room temperature even in smaller size. The exchange bias expectedly disappeared at room temperature hysteresis loops due to extremely weak interfacial spins coupling of both FiM/AFM components. It is observed that the saturation magnetization has decreased with increase of CoO weight percentage in nanocomposite systems.