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arXiv · 0904.3712

Magnetic order induced crystal symmetry lowering in ACr$_2$O$_4$ ferrimagnetic spinels

S. Bordacs·D. Varjas·I. Kezsmarki·G. Mihaly·L. Baldassarre·A. Abouelsayed·C. A. Kuntscher·K. Ohgushi·Y. Tokura
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Abstract

We demonstrate that the onset of complex spin orders in ACr$_2$O$_4$ spinels with magnetic A$=$Co, Fe and Cu ions lowers the lattice symmetry. This is clearly indicated by the emergence of anisotropic lattice dynamics -- as evidenced by the pronounced phonon splittings -- even when experiments probing static distortions fail. We show that the crystal symmetry in the magnetic phase is reduced from tetragonal to orthorhombic for FeCr$_2$O$_4$ and CuCr$_2$O$_4$ with Jahn-Teller active A-site ions. The conical spin structure in FeCr$_2$O$_4$ is also manifested in the phonon frequencies. In contrast, the multiferroic CoCr$_2$O$_4$ with no orbital degrees of freedom remains nearly cubic in its ground state.

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S. Bordacs, D. Varjas, I. Kezsmarki, G. Mihaly, L. Baldassarre, A. Abouelsayed, C. A. Kuntscher, K. Ohgushi, Y. Tokura. 2009-04-23. Magnetic order induced crystal symmetry lowering in ACr$_2$O$_4$ ferrimagnetic spinels. https://doi.org/10.1103/physrevlett.103.077205

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