Lifshitz point in the three-dimensional magnetic phase diagram of CoNb2O6

Neutron-diffraction and magnetization experiments have been performed on a single crystal of CoNb2O6 with an external magnetic field applied along dif ferent directions within the ac plane. Intermediate magnetic phases with lo ng 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 d iagrams provides a severe test of the applied model. The behavior of the pr opagation vector (0,k(y),0) at a transition point from an incommensurate ph ase to a commensurate one in zero field and near a Lifshitz paint with a ma gnetic field applied is discussed in the Light of theoretical consideration s on multicritical phenomena.