Magnetocrystalline interactions inMnCr2O4spinel

Abstract

In this work we present a magnetic, structural, and electron spin resonance (ESR) study of the geometrically frustrated ${\text{MnCr}}_{2}{\text{O}}_{4}$ spinel in an extended range of $T$ (2--1000 K). At the lowest temperature $(Tl18\text{ }\text{K})$ the ESR lineshape is compatible with the coexistence of spiral and ferrimagnetic spin ordering. Above ${T}_{C}$ $(\ensuremath{\approx}41\text{ }\text{K})$, magnetic susceptibility $(\ensuremath{\chi})$ and ESR intensity coincide with each other showing the typical behavior of a ferrimagnet. From the $\ensuremath{\chi}(T)$ vs $T$ dependence, absolute values for the three exchange constants ${J}_{\text{MnCr}}$, ${J}_{\text{CrCr}}$, and ${J}_{\text{MnMn}}$ were determined. Our results indicate that (i) these values are approximately independent of $T$, (ii) the antiferromagnetic direct Cr-Cr exchange is the main interaction, and (iii) ${J}_{\text{MnMn}}$ is indeed not negligible as compared with ${J}_{\text{MnCr}}$ and ${J}_{\text{CrCr}}$. One noticeable anomaly in the temperature dependence of the ESR linewidth is observed at $T\ensuremath{\approx}450\text{ }\text{K}$. This behavior is accounted by a 30% variation in the extrapolated high-temperature linewidth and it is attributed to a crystalline distortion not reported previously. Experiments of high-temperature x-ray diffraction allowed us to associate this distortion to a deformation of the oxygen sublattice. ESR and x-ray results are both compatible with a more symmetric high-temperature structure.