Khrustalov V. Peculiarities of the magnetic and magnetoelectric properties of the LiCoPO4 and LiNiPO4 crystals in high pulse magnetic field

The dissertation is devoted to the revealing of new phase transitions and changes in electric polarization induced by a strong magnetic field in antiferromagnets LiCoPO4 and LiNiPO4. It has been found that transition of LiCoPO4 from antiferromagnetic to saturated paramagnetic state under the pulse magnetic field, directed along the easy axis, occurs through three phase transitions. The high-field phase with electric polarization and the linear changes in magnetization have been revealed. Possible magnetic structures forming in LiCoPO4 in high-field phases have been proposed within a collinear model approximation, and the values of exchange parameters have been estimated. It has been found that in the field up to 285 kOe and T = 4.2 К the transformation of magnetic structure of LiNiPO4 occurs through the five phase transitions. The electric polarization has been revealed both in antiferromagnetic and in one of the high-field phases. In the antiferromagnetic phase the electric polarization in addition to the linear component also contains the cubic one. The phase (Н,Т)-diagrams of magnetic states for both compounds LiCoPO4 and LiNiPO4 under high magnetic field up to 285 kOe in the temperature range 1,6 – 20,8 К has been constructed.