Spin-lattice coupling induced phase transition in the S=2 frustrated antiferromagnet CuMnO2

The crystal and magnetic structures of the layered oxide crednerite CuMnO2 have been studied by neutron powder diffraction. In this system, which is topologically representative of a frustrated square lattice with nearest (J(1)) and diagonal next-nearest- (J(2)) neighbor interactions, three-dimensional antiferromagnetic ordering is observed below T-N = 65 K, with propagation vector k(1) = (-1/2 1/2 1/2). Frustration is relieved through a strong magnetoelastic coupling to the lattice, evidenced by a structural phase transition from a monoclinic (C2/m) to a strained triclinic C (1) over bar phase as magnetic order sets in. The magnetic order observed, which is described as spins antiferromagnetically aligned along [1-1 0] and ferromagnetically aligned along [1 1 0], corresponds to the so-called collinear antiferromagnetic order predicted for a frustrated two-dimensional antiferromagnetic Heisenberg model on a square lattice for J(2)/J(1) > 0.5 in the presence of spin-lattice coupling.