Magnetic structures and quadratic magnetoelectric effect in LiNiPO₄ beyond 30 T

Neutron diffraction with static and pulsed magnetic fields is used to directly probe the magnetic structures in LiNiPO₄ up to 25 T and 42 T, respectively. By combining these results with magnetometry and electric polarization measurements under pulsed fields, the magnetic and magnetoelectric phases are investigated up to 56 T applied along the easy c axis. In addition to the already known transitions at lower fields, three new ones are reported at 37.6, 39.4, and 54 T. Ordering vectors are identified with Q_VI = (0, 1/3, 0) in the interval 37.6 - 39.4 T and Q_VII = (0, 0, 0) in the interval 39.4 - 54 T. A quadratic magnetoelectric effect is discovered in the Q_VII = (0, 0, 0) phase and the field dependence of the induced electric polarization is described using a simple mean-field model. The observed magnetic structure and magnetoelectric tensor elements point to a change in the lattice symmetry in this phase. We speculate on the possible physical mechanism responsible for the magnetoelectric effect in LiNiPO₄.