A study of the magnetization of the antiferromagnetic magnetoelectric crystal LiCoPO4 as a function of temperature and the strength of a magnetic field oriented along the antiferromagnetic vector reveals features due to the presence of a weak ferromagnetic moment. The value of the magnetic moment along the b axis at 15 K is approximately 0.12 G. The existence of a ferromagnetic moment can account for the anomalous behavior of the magnetoelectric effect observed previously in this crystal.

The temperature dependence of the magnetization of single-crystal LiNiPO4 is measured for magnetic-field orientations along the a, b, and c crystallographic axes. It is found that the value of the magnetization depends on the magnetic prehistory of the sample. The magnetic behavior of the antiferromagnetic sample is explained by the presence of weak ferromagnetism in LiNiPO4. At a temperature of 5 K the value of the spontaneous magnetic moment along the c axis is around 0.005 G. When the sample is heated to 20.8 K the magnetic moment decreases monotonically to zero. All of the magnetic susceptibility curves M(T)/H exhibit two features: a jump and a kink at temperatures T1 and T2, respectively. At a magnetic field of 10 kOe these temperatures are close to 20.84 and 21.86 K. The observed features indicate that in the establishment of the main antiferromagnetic order in the LiNiPO4 crystal, an intermediate antiferromagnetic phase is spontaneously formed in the temperature interval from TN1=20.8(5) K to TN2=21.8(5). The sequence of continuous and abrupt transitions at the boundary temperatures of its existence region indicate that the intermediate phase is most likely an incommensurate antiferromagnetic state.

Specific heat, magnetic torque, and magnetization studies of LiCoPO4 olivine are presented. They show that a unique set of physical properties of LiCoPO4 leads to the appearance of features characteristic of two-dimensional Ising systems near the Ne´el temperature TN = 21.6 K and to the appearance of an uncommon effect of the influence of a magnetic field on the magnetocrystalline anisotropy. The latter effect manifests itself as a first-order transition, discovered at ∼9 K, induced by a magnetic field of 8 T. The physical nature of this transition was explained, and a model describing experimental dependences satisfactorily was proposed