Bodnaruk A. Critical and nonlinear properties of magnetic nanopowders and magnetoactive elastomers.

The aim of this thesis is to determine the features of magnetization of the magnetic nanoparticle ensemble near the Curie point TC, as well as to establish the influence of matrix elasticity and induced magnetic anisotropy on the magnetic properties of magnetoactive elastomers during magnetization. This paper experimentally investigated magnetostatic properties of nanoparticles ensemble type manganites La1-xSrxMnO3 at different temperatures, also for the same ensemble of magnetic nanoparticles, studies were performed in an alternating magnetic field (AC). It is shown that ensembles of magnetic nanoparticles La1-xSrxMnO3 are not magnetized as an ideal system of superparamagnetic particles. Moreover, the value of the average relaxation time found by magnetostatic measurements turned out to be very high. Because estimates of the mean relaxation time of nanoparticles are often based on magnetostatic measurements, which assume that all ensemble particles are magnetized as superparamagnetic nanoparticles. It is shown that the hysteresis character of magnetic losses is decisive in the considered ensembles of magnetic nanoparticles La1-xSrxMnO3. For the first time, a procedure was proposed to separate the contributions to magnetization from the superparaprocess (related to the field-induced alignment of the magnetic moments of individual particles) and a ‘true’ paraprocess (related to the field-induced change of the modules of magnetic moments of individual particles). It was found that for an ensemble of superparamagnetic particles with moderate inhomogeneity of magnetic parameters, an effective Curie temperature can be introduced. In the vicinity of the effective Curie temperature, the nanoparticles are in the state which is a mixture of superparamagnetic and paramagnetic particles. It is proved that the behavior of the ensemble’s magnetization displays critical behavior at all temperatures lower than effective Curie temperature, with the exception of a narrow range of the dispersion of individual Curie temperatures. For the ensemble under study, the critical index β is almost two times greater than that for the corresponding bulk material. Magnetoactive elastomers are composites whose mechanical properties can be controlled by a magnetic field. The dissertation investigates a magnetoactive elastomer, which consists of an elastomeric matrix of polydimethylsiloxane and micron-sized carbonyl iron particles (so-called inclusions). The main difference between magnetoactive elastomers and conventional composites is that inclusions in the magnetoactive elastomer can move. In the dissertation a direct experiment was proposed, where for the same sample of magnetoactive elastomer, it is possible to obtain magnetization curves for cases when inclusions (filler particles) can change their positions and when inclusions do not have such a possibility, remaining stationary. It is shown that at low temperatures, the stiffness of the magnetoactive elastomer matrix increases significantly, as a result of which the filler particles, which are magneto-soft, cannot move relative to each other during their magnetization, and re-magnetization of the magnetoactive elastomer, in this case, occurs without hysteresis. It is also shown that this magnetic field-induced anisotropy is uniaxial, where the easy axis of magnetization is directed along the magnetic field. It is shown that the process of formation of magnetic anisotropy is nonlinear in the sense that the "constant" of the anisotropy nonlinearly depends on the magnitude of the external field.