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

TL;DR

This study reveals that complex spin orders in ACr$_2$O$_4$ ferrimagnetic spinels induce lattice symmetry lowering, detectable through phonon splitting, with variations depending on the specific A-site ion and its orbital activity.

Contribution

It demonstrates how magnetic ordering causes crystal symmetry reduction in ACr$_2$O$_4$ spinels, highlighting the role of orbital activity and spin structure in lattice dynamics.

Findings

Spin orders lower lattice symmetry, evidenced by phonon splittings.

FeCr$_2$O$_4$ and CuCr$_2$O$_4$ show orthorhombic symmetry in magnetic phase.

CoCr$_2$O$_4$ remains nearly cubic, with no orbital degrees of freedom.

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.