Competition between Cubic and Tetragonal Phase Induced Unusual Vertical Magnetization Shift and Exchange Bias in CoxMn3-xO4 (1 ≤ x ≤ 2) Spinel Nanoparticles

CoxMn3-xO4 (1 <= x <= 2) spinel nanoparticles synthesized through conventional coprecipitation technique exhibit the coexistence of tetragonal and cubic phases in the range of 1 <= x <= 1.5, while a pure cubic phase is observed for 1.75 <= x <= 2 at room temperature. Reduction in tetragonal phase fraction from 92 % for x = 1 to 47% for x = 1.5 is attributed to diminution of Jahn-Teller (J-T) active Mn3+ ions occupying the octahedral site of spinel lattice. X-ray photoelectron spectroscopy confirms both +2 and +3 oxidation states for Co and Mn. Surprisingly, cubic and tetragonal phases exhibit magnetic transition, Tc-1 and Tc-2, corresponding to a paramagnetic to a high and low temperature ferrimagnetic state, respectively. Tetragonal phase induces high spontaneous (H-SEB) and conventional (H-CEB) exchange bias with unusually high vertical magnetization shift (VMS) than that of the pure cubic phase, shows maximum H-CEB of 4.062 kOe for x = 1.5 and a VMS of 2.5 emu g(-1) for x = 1. Such dependence of VMS and exchange bias on tetragonal to cubic phase ratio in CoxMn3-xO4 is demonstrated for the first time and interpreted based on the interaction between different arrangement of spins in tetrahedral and octahedral sublattice.