Field-induced spin orders in monoclinic CoV₂O₆

Spin-ordered structures of monoclinic brannerite-type CoV₂O₆ have been determined from neutron diffraction data recorded at 2 K in applied magnetic fields of 0, 2.5, and 5.0 T. Three collinear magnetic phases were observed as field increases: an antiferromagnetic state with propagation vector (0 1 1/2), a ferrimagnetic (−1/3 1 1/3) phase, and a (0 0 0) ferromagnetic order. Co²⁺ moments of 4.4–5.0 μ_B have a large orbital component and are aligned close to the c-axis direction in all cases. Spin-lattice coupling leads to a magnetostriction and volume expansion as field increases. The ferrimagnetic phase accounts for the previously reported 1/3-magnetization plateau, and demonstrates that monoclinic CoV₂O₆ behaves as an accidental triangular antiferromagnetic lattice in which further frustrated orders may be accessible.