Kondovych S. Influence of the magnetoelastic properties on magnetic dynamics of nano-magneto-mechanical systems with antiferromagnetic layers.

The dissertation is devoted to the theoretical study of the connection between the magnetoelastic properties of antiferromagnets and the magnetic properties of the multilayered structures with antiferromagnetic layers. It also covers the creation of phenomenological theory of the shape effects in antiferromagnetic nanoparticles. Theoretical interpretation of shape effects in nano-sized antiferromagnetic samples bases on the assumption that magnetic and mechanical subsystems interact in two ways: through local magnetoelasticity and shape-induced anisotropy. To model these effects, it is presumed that shape effects are caused by spontaneous magnetostriction during antiferromagnetic ordering (associated with "magnetoelastic charges"), and employed the concept of the destressing energy, which depends on geometry of the nanoparticle, its crystallographic orientation, and constants of magnetostriction. The main idea of proposed approach is to assume surface and bulk properties of the sample to be different. The resulting inconsistency between magnetoelastic strains acts as a source of magnetoelastic charges and contributes into magnetic energy of a sample. Due to phenomenological model developed in the thesis, shape-induced contribution to magnetic energy of antiferromagnetic nanoparticle affects the equilibrium magnetic structure in the sample, particularly the domain structure. Mechanism of domain structure formation in rectangular particles is discussed qualitatively, relying on the experimental evidence. Also, on the basis of developed theory effective anisotropy constants of elliptic-shaped nanoparticles of Cr2O3 are calculated and successfully compared to existing experimental data. The proposed approach and performed calculations allow to predict, design and control magnetic properties of nano-sized antiferromagnetic particles by choosing proper sample size and geometry during sample production. The influence of magnetoelastic properties of antiferromagnetic layers on the magnetic behavior of the multilayered structure is illustrated by the example of synthetic multiferroic particle of elliptic shape, which consists of antiferromagnetic nanopillar of an "easy plane" type on piezoelectric substrate. Taking into account shape-induced contribution to the anisotropy energy, the possible equilibrium distributions of antiferromagnetic vector are found. Calculations show that it is possible to control the magnetic state of the antiferromagnetic pillar varying the angle between the mechanical force and crystal axis, and thus switching processes in such two-layered structure are discussed. It is proposed to govern magnetic state of antiferromagnetic/piezoelectric system by applying external electric and magnetic fields simultaneously. It is shown that alternative electric field can be used to excite oscillations of antiferromagnetic vector in synthetic multiferroic that combine magnetoelastic and piezoelectric properties. In the framework of the Lagrange technique the oscillations spectra are calculated. The influence of the external magnetic field is taken into account. The case of parametric amplification of the magneto-mechanical oscillations is discussed separately. Analysis of the way in which the magnetoelastic properties affect the resonant frequencies and parametric resonance band width allows controlling these parameters by choosing proper sample size and geometry. New aspect of magnetoelastic interactions is considered in the thesis by proposing nano-magneto-mechanical system: nonmagnetic nanorod with antiferromagnetic and ferromagnetic layers, which twists under the action of spin-polarized current. The model is based on Lagrange-Rayleigh formalism, corresponding Lagrange and dissipative functions come out of general symmetry principles and account for both magnetic dynamics and mechanical one. The calculations demonstrate that alternative spin-polarized current can excite quasimechanical (torsional) as well as quasimagnetical modes, and thus the nanorod acts as a torsional oscillator, converting electromagnetic energy into mechanical motion. The modes and spectrum of current-induced coupled magnetomechanical oscillations are analyzed, extreme cases of low-frequency and high-frequency ranges are discussed. It is shown that the geometry of the sample affects the amplitude values of oscillation modes.