Temperature-dependant phase transformation of Ni_xFe_y−xO_z nanoferrites: their dielectric and magnetic properties

0–5 wt% nickel-doped iron oxide nanoparticles were synthesised using microwave-assisted method. Microwave-treated samples were annealed at 450 °C and 600 °C and named as Ni_xFe_y−xO_z-450 and Ni_xFe_2−xO₃-600 (x = 0. 0.1, 0.2, 0.3, 0.4 and 0.5), respectively. XRD, TGA, SEM and EDS were performed to study structural, thermal, morphological and compositional properties of the samples. Undoped sample has hematite phase at both 450 and 600°C, which is confirmed from both XRD and TGA. On nickel doping, an intermediate phase formed and there occurred two transition temperatures in TGA. Ni_xFe_2−xO₃-600 forms rhombohedral phase, while Ni_xFe_y−xO_z-450 forms a mixture of rhombohedral and inverse spinel structures till 2 wt% nickel concentration and pure inverse spinel structure above 2 wt% of nickel doping. M–H curves of samples from vibrating sample magnetometer (VSM) show the ferrimagnetic behaviour of both Ni_xFe_y−xO_z-450 and Ni_xFe_2−xO₃-600 except Fe₂O₃-600 which shows antiferromagnetic behaviour. In both set of samples, saturation magnetisation increases with increasing nickel concentration. Further, a two-probe method was used to study electrical and dielectric properties of the samples. AC conductivity (σ_ac), dielectric constant (ϵ′) and loss factor (tan δ) of Ni_xFe_y−xO_z-450 decreased with increase in 2 wt% nickel concentration and then increased with further increase in nickel, whereas these factors varied randomly with increasing nickel concentration for Ni_xFe_2−xO₃-600. σ_ac, ϵ′ and tan δ values of Ni_xFe_2−xO₃-600 are lower than those of Ni_xFe_y−xO_z-450, which can be attributed to the formation of hematite phase at 600 °C.