Origin of the long-range ferrimagnetic ordering in cubic Mn(Co)Cr2O4 spinels

Structural transition from a cubic to a tetragonal phase in magnetically frustrated AB2O4 spinel compounds is generally found indispensable to enable long-range magnetic ordering. Intriguingly, MnCr2O4 and CoCr2O4, constitute exceptions to the above general rule, as they seem to undergo long-range ferrimagnetic (FIM) ordering without any structural involvement in their corresponding cubic phases. We find that MnCr2O4 (and CoCr2O4) undergo a hitherto undetected partial glassy magnetic ordering of the spiral-spin components at a temperature (TSP) higher than the long-range FIM transition. The spin-glass transition at TSP triggers the onset of structural modifications that helps to reduce the geometric-magnetic-frustration and thereby enable the long-range FIM ordering within an overall cubic phase. In the absence of the higher-temperature glassy transition in doped MnCr2O4, the corresponding magnetic ordering is found to be glassy instead of being a long-ranged FIM ordering. In magnetic spinel oxides, like CdCr2O4 and FeCr2O4, where cubic to tetragonal structural transition occurs from either a spin-Jahn-Teller effect or through Jahn-Teller distortions, such a higher-temperature glassy magnetic transition (i.e., above the long-range magnetic ordering) is absent. Our results, thus, clearly elucidate that the structural modifications associated with the glassy magnetic ordering at TSP, aided with presence of magnetic A-site ions, play a pivotal role in releasing the geometric-magnetic-frustration in MnCr2O4 (and also CoCr2O4) to enable long-range FIM ordering in these systems.