Magnetic order induced crystal symmetry lowering in ACr₂O₄ ferrimagnetic spinels

الأصل · EN

We demonstrate that the onset of complex spin orders in ACr₂O₄ 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 FeCr₂O₄ and CuCr₂O₄ with Jahn-Teller active A-site ions. The conical spin structure in FeCr₂O₄ is also manifested in the phonon frequencies. In contrast, the multiferroic CoCr₂O₄ with no orbital degrees of freedom remains nearly cubic in its ground state.