Structure- and Local Structure-Driven Magnetic Properties of Co_<i>x</i>Mn_3-xO₄ (<i>x</i> = 1.00 to 2.00) Spinel Nanoparticles

In the present paper, we have investigated the evolution of structure followed by local structure-driven magnetic properties in CoxMn3-xO4 nanoparticles synthesized through a facile coprecipitation technique. Rietveld refinement of the X-ray diffraction pattern reveals a structural phase transition from the coexistence of tetragonal and cubic to a pure cubic phase with increasing x from 1.00 to 2.00. Such transition is validated with local structure analysis where it is observed that while Mn3+ and Co2+ ions are distributed among B and A sites, respectively, in x = 1.00, with increase in x, Mn3+ in the B site is replaced by Co3+ ions. When x ≥ 1.75, the additional presence of Mn4+ is observed from extended X-ray absorption fine structure spectroscopy and confirmed by X-ray photoelectron spectroscopy. In accordance with two structural phases, for x = 1.00, two magnetic transitions Tc1 and Tc2 at 165 and 93 K, respectively, are observed. With an increase in x, Tc1 grows at the expense of Tc2 and finally, at x = 2.0, only Tc1 is detected. Unusual temperature-dependent Hc is observed for the coexistence of the cubic and tetragonal phase. Such magnetic behavior is discussed based on the competition between magnetic ions occupying the tetrahedral and octahedral sites.