Magnetic order, hysteresis, and phase coexistence in magnetoelectric LiCoPO4

The magnetic phase diagram of magnetoelectric LiCoPO4 is established using neutron diffraction and magnetometry in fields up to 25.9T applied along the crystallographic b axis. For fields greater than 11.9T, the magnetic unit cell triples in size with propagation vector Q=(0,13,0). A magnetized elliptic cycloid is formed with spins in the (b,c) plane and the major axis oriented along b. Such a structure allows for the magnetoelectric effect with an electric polarization along c induced by magnetic fields applied along b. Intriguingly, additional ordering vectors Q≈(0,14,0) and Q≈(0,12,0) appear for increasing fields in the hysteresis region below the transition field. Traces of this behavior are also observed in the magnetization. A simple model based on a mean-field approach is proposed to explain these additional ordering vectors. In the field interval 20.5–21.0T, the propagation vector Q=(0,13,0) remains but the spins orient differently compared to the cycloid phase. Above 21.0T and up until saturation, a commensurate magnetic structure exists with a ferromagnetic component along b and an antiferromagnetic component along c.