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

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.

arXiv comment4 pages, 4 figures. submitted to Phys. Rev. Lett

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