Off-Stoichiometric Nickel Cobaltite Nanoparticles: Thermal Stability, Magnetization, and Neutron Diffraction Studies

摘要

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_(0.75)Co_(2.25)O4 and NiCo2O4 cubic systems. A low temperature combustion method was utilized to synthesize stoichiometric (NiCo2O4) and off-stoichiometric (Ni_(0.75)Co_(2.25)O4) 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_(0.75)Co_(2.25)O4 as compared to that in the case of NiCo2O4 sample (~15-20%). The Ni 2p and Co 2p XPS spectra reveal the coexistence of Ni~(2+), Ni~(3+), Co~(2+), and Co~(3+) species on the surface of both the NiCo2O4 and Ni_(0.75)Co_(2.25)O4 samples in differing proportions. In addition to the basic magnetic characterizations using PPMS, these were also analyzed by neutron diffraction. The off-stoichiometric Ni_(0.75)Co_(2.25)O4 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_(0.75)Co_(2.25)O4 samples are further compared by neutron diffraction confirming stronger ordered moments and enhanced structural and thermal stability for the Ni_(0.75)Co_(2.25)O4 sample.