The lithium-ortho-phosphates LiMPO₄ (M = Mn, Co, Fe or Ni) are isostructural compounds that all exhibit a strong ME effect in their low-temperature commensurate (C) antiferromagnetic (AFM) phases. LiNiPO₄ stands out among the lithium-ortho-phosphates by exhibiting incommensurate (IC) magnetism, which is due to the presence of competing magnetic interactions.

The LiNiPO₄ compound has orthorhombic symmetry with space group Pnma (no. 62) and lattice parameters a = 10.02 Å, b = 5.83 Å and c = 4.66 Å. There are four magnetic Ni²⁺-ions in each unit cell with spin S = 1. At zero magnetic field the system displays long-range AFM order at temperatures below T_N = 21.7 K, while short-range IC magnetic order exists in the temperature range T_N < T < 40 K. Between T_N and T_NC = 20.8 K, LiNiPO₄ exhibits spontaneous IC magnetic order with ordering wave vector k_ic = (0,k_ic,0). Below TNC, the AFM structure is commensurate with the ordered moments almost parallel to the crystallographic c axis with a slight canting along the a axis. The small canting is responsible for the ME effect observed in the commensurate phase when a field is applied along the a or c axes. A magnetic field applied along the c axis induces a first order C to IC phase transition at a field strength μ₀H varying between 12 and 13.5 T depending on temperature. The low-temperature C--IC phase transition is the first out of the five magnetic-field induced transitions observed as peaks in the differential magnetic susceptibility dM/dH derived from magnetization measurements in pulse magnetic fields extending up to 27 T along the c axis. A second transition at around 16 T gives rise to a much smaller peak in dM/dH than the C--IC transition.

Neutron-diffraction studies as a function of temperature and magnetic fields up to 17.3 T - undertaken at FLEX - clarifies the structures of the IC magnetic phases of LiNiPO₄. The high field, low-temperature IC magnetic structure is shown to be a canted, elliptical spiral with the spin-components rotating in the crystallographic ac plane. This structure undergoes a temperature-driven transition to a canted, linearly polarized structure, where all moments are nearly parallel or antiparallel to the c axis. At fields larger than 16 T and temperatures below 10 K, the low-temperature IC spiral conforms to a commensurate quintupling of the crystallographic unit cell along the b axis corresponding to an ordering wave vector (0, 1/5, 0) (see figure 1 and 2). The complete phase diagram as measured on FLEX and RITA-II at the PSI is shown on figure 3.