Magnetoelastic coupling in the frustrated antiferromagnetic triangular lattice CuMnO2

The magnetic ordering transition coupled to a structural transition in the anisotropic triangular lattice ${\text{CuMnO}}_{2}$ is investigated by high-resolution neutron powder diffraction. Temperature dependence of the magnetic order parameter and relevant structural quantities, such as cell constants and atomic positions, and analysis of the Landau potential show that there is a unique critical temperature, as well as that the primary order parameter is magnetic. The structural phase transition, necessary to lift the magnetic degeneracy, is improper ferroelastic with a linear-quadratic coupling between strain and magnetization density.