Off-stoichiometric nickel cobaltite nanoparticles: thermal stability, magnetization, and neutron diffraction studies

Abstract

In the present investigation, we report a detailed examination of the effect of off-stoichiometry introduced in NiCo₂O₄ by adding excess cobalt. Thus, we compare and analyze the structural and magnetic properties of the Ni_0.75Co_2.25O₄ and NiCo₂O₄ cubic systems. A low temperature combustion method was utilized to synthesize stoichiometric (NiCo₂O₄) and off-stoichiometric (Ni_0.75Co_2.25O₄) nanoparticles on a large scale. The X-ray diffraction pattern for the sample annealed at high temperature (773 K) shows the presence of a much less intense NiO phase (∼2–5%) in Ni_0.75Co_2.25O₄ as compared to that in the case of NiCo₂O₄ sample (∼15–20%). The Ni 2p and Co 2p XPS spectra reveal the coexistence of Ni²⁺, Ni³⁺, Co²⁺, and Co³⁺ species on the surface of both the NiCo₂O₄ and Ni_0.75Co_2.25O₄ samples in differing proportions. In addition to the basic magnetic characterizations using PPMS, these were also analyzed by neutron diffraction. The off-stoichiometric Ni_0.75Co_2.25O₄ sample shows an interesting magnetic phase conversion from frustrated dipolar system to an enhanced magnetic ordering upon annealing. Local moments on the lattice sites of NiCo2O4 and Ni_0.75Co_2.25O₄ samples are further compared by neutron diffraction confirming stronger ordered moments and enhanced structural and thermal stability for the Ni_0.75Co_2.25O₄ sample.