Magnetic behavior of nanostructured NiTiO3 and NiO material: Anomalous increase in coercivity

In this paper we present an interesting magnetic property of ilmenite NiTiO3 material prepared by sol-gel mediated slow evaporation induced self-assembly (EISA) method using Pluronic F127. Two oxide samples were prepared using Ni:Ti 1:1 and 1:2 M ratios under similar reaction conditions. It is observed from powder XRD analysis that sample NTO11 (Ni:Ti = 1:1) is a predominating NiTiO3 phase with small percentage of NiO and TiO2, whereas NTO12 (Ni:Ti = 1:2) is a mixture of anatase TiO2 and NiO structures. Both materials reveal, presence of exchange bias (in hysteretic M-H loop) decreasing with increase in temperature. Both samples show an anomalous increase of magnetic coercivity with increase in temperature. The anomalous coercivity increase in NTO11 is much more pronounced than in NTO12 (by a factor of 6). We explain this large anomaly in NTO11, by invoking layer structure and weak interlayer exchange coupling between Ni moments in NiTiO3, which allows a transition near the surface of nanoparticles from antiferromagnetically coupled ferromagnetic planes of Ni moments to uncoupled set of ferromagnetic layers on the surface (each layer can be thought of as single ferromagnetic plane), blocked along different directions. The increased coercivity at higher temperature is due to excess coercivity of the nanosized ferromagnetic layers.