Origin of the long-range ferrimagnetic ordering in cubic <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>Mn</mml:mi><mml:mrow><mml:mo>(</mml:mo><mml:mi>Co</mml:mi><mml:mo>)</mml:mo></mml:mrow><mml:msub><mml:mi>Cr</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn>4</mml:mn></mml:msub></mml:mrow></mml:math> spinels
A. Das, Pratap Pal, Dheeraj Ranaut, Kazi Parvez Islam, Gourab Bhattacharya, Sakshi Mehta, Abhishake Mondal, A. Venimadhav, K. Mukherjee, A. Das, Debraj Choudhury
Abstract
Structural transition from a cubic to a tetragonal phase in magnetically frustrated ${\phantom{\rule{4pt}{0ex}}AB}_{2}{\mathrm{O}}_{4}$ 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.