Kolodiazhna M. Magnetoelastic and piezomagnetoelectric effects in the multiferroics Nd0,9Dy0,1Fe3(BO3)4, SmFe3(BO3)4 and LiCoPO4

In the thesis magneto-elastic, piezoelectric, piezomagnetoelectric effect in single crystals Nd0,9Dy0,1Fe3(BO3)4, SmFe3(BO3)4 ferroborates and LiCoPO4 at low temperatures, have been discovered and analyzed. The sound velocities have been measured with high accuracy in the single crystal of rare earth ferroborate Nd0,9Dy0,1Fe3(BO3)4 for the first time. The components of the tensors of the elastic and the piezoelectric modules have been calculated. New spontaneous and induced by an external magnetic field phase transitions, which are realized in the magnetic subsystem of the ferroborate Nd0,9Dy0,1Fe3(BO3)4, have been discovered and analyzed. For the first time, H - T phase diagram of the single crystal Nd0,9Dy0,1Fe3(BO3)4, (H || C3) has been constructed. Piezoelectric response in the non-piezoactive configuration of SmFe3(BO3)4 ferroborate has been experimentally detected. It has been shown that the piezoelectric response is a manifestation of the piezomagnetoelectric effect in the antiferromagnetic ordered phase of the crystal, and the manifestation of the piezoelectric effect in the paramagnetic phase. For the first time, the piezomagnetoelectric effect theoretically predicted more than half a century ago has been discovered in the LiCoPO4 antiferromagnet. The intensity of the piezomagnetoelectric interaction has been estimated. Piezoelectric response has been detected in the paramagnetic phase of a LiCoPO4 single crystal. The analysis of the anisotropy of the manifestation of that response allows to classify the crystal as belonging to the pyroelectric class C2v or C2.