Unveiling the impact of Ni doping on the structural, electronic, and magnetic properties of nanocrystalline FeCo2O4spinel oxide: a combined experimental andab initioinvestigations.

Abstract

In the present work, nanocrystalline samples of compositionsNixFe1-xCo2O4(x= 0.0, 0.25, 0.50, and 0.75) have been synthesized via co-precipitation method by annealing at 900 °C. The nanocrystalline samples of compositions Ni_0.25Fe_0.75Co₂O₄(D_0.25) and Ni_0.5Fe_0.5Co₂O₄(D_0.5) crystallizes in a pure spinel phase, whereas Ni_0.75Fe_0.25Co₂O₄(D_0.75) show the existence of secondary phase of NiO, as confirmed by the x-ray diffraction analysis. The particle size and lattice strain in the samples both decrease as Ni substitution has increased. The field-dependent dc magnetizationM(H) virgin curve measured at 5 K for sample D_0.75shows the existence of field-induced metamagnetic transition, while this behavior is absent in samples D_0.5and D_0.25. Dynamic magnetic properties have been investigated by ac susceptibility measurement, which shows a strong frequency dependence behavior resulting from the blocking/spin-glass freezing states depending upon the amount of Ni substitution and the range of measurement temperature. High-resolution x-ray photoelectron spectroscopy analysis reveals the presence of mixed valence states of Fe²⁺/Fe³⁺, Co²⁺/Co⁺³, and Ni²⁺/Ni⁺³in all samples. Using first principles-based density functional theory calculations with HSE06 exchange-correlation functional predicts the correct description of the ground state, which is ferrimagnetic and insulating in their inverse spinel case for D_0.25, D_0.5, and D_0.75samples that agreed well with our experimental observations. A closer look at the electronic structure near the Fermi level (E_F) of Ni-doped samples suggests a typical Mott-Hubbard insulating state while it is found to be a mixture of charge-transfer and Mott-Hubbard insulating state for parent compound FeCo₂O₄. The obtained spin-dependent gap hierarchy can have possible applications in spintronics. We have studied a detailed correlation between experiment and theory.