Bilych I. Features of elastic, magnetoelastic and piezoelectric properties of magnetoelectrics TbFe3(BO3)4, HoFe3(BO3)4 and HoAl3(BO3)4 at low temperatures

The thesis is devoted to the study of elastic, magnetoelastic, piezoelectric and magnetopiezoelectric effects in single crystals of rare earth borates TbFe3(BO3)4, HoFe3(BO3)4 and HoAl3(BO3)4 at low temperatures. For the first time, with a high accuracy (the error of measurements 1-3 %) obtained absolute values of the sound velocities for single crystals TbFe3(BO3)4, HoFe3(BO3)4 and HoAl3(BO3)4. The main components of the elastic modulus tensors and the piezoelectric module of the investigated compounds are calculated. Significant anomalies in the elastic properties of the terbium ferroborate single crystal in the magnetic field dependence of the transverse sound velocities accompanying the reorientation phase transition induced by an external magnetic field directed along the trigonal axis of symmetry are revealed. Significant anomalies in the magnetic field dependence of the transverse sound velocities in terbium ferroborate accompanying the reorientation phase transition induced by an external magnetic field directed along the trigonal axis of symmetry are revealed. The magnetopiezoelectric effect is detected in a single crystal of holmium ferroborate. The parameters of magnetoelectric and magnetoelastic couplings in the easy-plane magnetically ordered phase in a single crystal of holmium ferroborate are determined. It is established that the constant of magnetoelastic interaction at T ~ 15-20 K changes its sign, probably leading to the onset of a helicoidal type of magnetic ordering. For the first time, on the example of the crystal HoAl3(BO3)4, a giant renormalization of the piezoelectric effect in paramagnets is detected; it is caused by the development of a nematic-like paramagnetic phase in the sample.