Structural transition from a cubic to a tetragonal phase in magnetically frustrated ${AB}_2{\mathrm{O}}_4$ spinel compounds is generally found indispensable to enable long-range magnetic ordering. Intriguingly, ${\mathrm{MnCr}}_2{\mathrm{O}}_4$ and ${\mathrm{CoCr}}_2{\mathrm{O}}_4$, 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 ${\mathrm{MnCr}}_2{\mathrm{O}}_4$ (and ${\mathrm{CoCr}}_2{\mathrm{O}}_4$) undergo a hitherto undetected partial glassy magnetic ordering of the spiral-spin components at a temperature (${\mathrm{T}}_{SP}$) higher than the long-range FIM transition. The spin-glass transition at ${\mathrm{T}}_{SP}$ 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 ${\mathrm{MnCr}}_2{\mathrm{O}}_4$, the corresponding magnetic ordering is found to be glassy instead of being a long-ranged FIM ordering. In magnetic spinel oxides, like ${\mathrm{CdCr}}_2{\mathrm{O}}_4$ and ${\mathrm{FeCr}}_2{\mathrm{O}}_4$, 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 ${\mathrm{T}}_{SP}$, aided with presence of magnetic $A$-site ions, play a pivotal role in releasing the geometric-magnetic-frustration in ${\mathrm{MnCr}}_2{\mathrm{O}}_4$ (and also ${\mathrm{CoCr}}_2{\mathrm{O}}_4$) to enable long-range FIM ordering in these systems.

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• Ferrimagnetism
• Frustrated magnetism
• Jahn-Teller effect
• Crystal structures
• Spin glasses
• Spinels
• Magnetization measurements
• Neutron diffraction