Zhekov K. Magneto-elastic effects in rare-earth borocarbides and ferroborates

In the thesis features of elastic properties as well as the magnetoelastic effects that are manifested in compounds of nickel borocarbides RNi2B2C (R = Y, Ho, Lu, Er, Gd)) and NdFe3(BO3)4 and Nd1-xDyxFe3(BO3)4 (х=0.25; 0.4) ferroborates at low temperatures, have been discovered and analyzed. The new method of high-precision measurements of absolute sound velocity has been proposed and hardware implemented, which allows working with samples of sub-millimeter sizes which are highly absorbing. For the first time, with the accuracy of above 1%, the sound velocities in single crystals of nickel borocarbides RNi2B2C, (R = Y, Lu, Ho, Gd, Er) have been measured with an accuracy of more than 1%. The components of the tensor modulus of elasticity have been calculated. The high sensitivity of the gadolinium borocarbide elastic modules to phase transformations of magnetic nature, as well as significant acoustic "non-reciprocity" have been experimentally detected. The effects are associated with the significant contribution of its interaction with the antiferromagnetic domain structure to the characteristics of the propagation of the sound. Significant anomalies of the elastic characteristics of a single-crystal of rare-earth ferroborate NdFe3(BO3)4 have been found out for spontaneous and magnetic field -induced phase transitions. It allowed constructing phase diagrams of the compound in the “magnetic field-temperature” plane for the first time. The transition to an incommensurate spiral phase has been classified as a first-order phase transition. New phase transitions in the magnetic subsystem of ferroborates with two types of rare-earth ions Nd1-xDyxFe3(BO3)4 (x=0.4; 0.25), which are realized spontaneously at temperature changes and induced by an external magnetic field, have been discovered and analyzed. For the first time H – T phase diagrams of the investigated compounds for different directions of the external magnetic field have been constructed.