Neutron-diffraction and magnetization experiments have been performed on a single crystal of ${\mathrm{CoNb}}_2{\mathrm{O}}_6$ with an external magnetic field applied along different directions within the $\mathrm{ac}$ plane. Intermediate magnetic phases with long periodicities along the b axis are observed. Exchange and crystal-field parameters in a proposed model are determined by fitting the magnetization curves, and the comparison between calculated and observed magnetic phase diagrams provides a severe test of the applied model. The behavior of the propagation vector ${(0,k}_y,0)$ at a transition point from an incommensurate phase to a commensurate one in zero field and near a Lifshitz point with a magnetic field applied is discussed in the light of theoretical considerations on multicritical phenomena.

• Received 29 March 2000

DOI:https://doi.org/10.1103/PhysRevB.62.12146