Borynskiy V. Magnetic and resonant properties of multilayered nanostructures with antiferromagnetic components. – Qualifying scientific work on the manuscript rights.

The dissertation is devoted to the study of magnetostatic and resonant properties of multilayered structures with synthetic and natural antiferromagnetic components when their linear sizes reach nanoscale. A systematic analysis of temperature- and geometry-dependent changes in key magnetic parameters is carried out for the continuous-film heterostructures Fe/[Cr/FeCr/Cr]n/Fe, Py/FeMn/[Cu/Py], as well as the three-layer nanoelements Py/NiCu/Py. New reported physical effects can be utilized to adjust functional characteristics of electronic devices involving such components. In the Introduction (the First chapter), the relevance of the chosen topic of the doctoral research is substantiated and a general description of this work is provided. The Second chapter of the work acquaints the reader with general problems of the chosen research topic, the existing scientific achievements in the field of synthetic antiferromagnets (SAF) and further prospects for their use as integral components in magnonic metamaterials, neuromorphic junctions and other modern nanoelectronic devices. The Third chapter introduces the physical principles of the main methods used for the samples fabrication, magnetic parameters measurements and analysis of the results, namely magnetron sputtering of multilayered structures, ferromagnetic resonance spectroscopy (FMR) and micromagnetic modelling. Technical specifications of all the samples studied in this work are also provided. The Fourth chapter presents the studies of the changes in magnetostatic properties and the nature of the temperature-controlled SAF state switching in the context of the transition from two-dimensional thin-film structures to laterally confined nanoelements. The first part of the chapter focuses on the continuous multilayered SAF films Fe/[Cr/FeCr/Cr]n/Fe. The competition between direct and indirect exchange interaction, as well as between the indirect interaction of antiferromagnetic (AFM) and ferromagnetic (FM) types, which arise due to the ferromagnet-to-paramagnet phase transition of the FeCr alloy, are investigated. It is shown that the use of the composite spacer [Cr/FeCr/Cr]2 narrows the temperature range of the SAF state switching from ≥100 K, inherent to structures with a homogeneous FeCr spacer, to 15 K. The second part of the chapter considers the arrays of round planar SAF nanodisks Py/NiCu/Py, for which a similar mechanism of temperature-controlled magnetization reversal is demonstrated without the indirect exchange interaction. It is shown that the dome-like shape of the nanoelements leads to a significant remnant magnetic moment and a double hysteresis loop, when the SAF is in the dipole-coupled AFM state. The study presented in the Fifth chapter of the work focuses on investigating the peculiar magnetic state of the bilayer film system Py/FeMn. In the first part of the chapter the temperature- and thickness-dependent dissipation processes of spin-pumping current in Py/FeMn(3–7 nm)/Cu/Py structures are examined. Apart from the traditional contribution of spin current absorption in the volume of the AFM layer, an additional inhomogeneous contribution is revealed, which is explained as due to the formation of an exchange spring in the Py/FeMn interface region, caused by the competition between FM proximity effect and AFM exchange in FeMn. The separate FMR study for the bilayer film structures Py/FeMn(3–7 nm) is also presented. The possibility of an almost tenfold isotropic increase in the FMR frequency at room temperature has been demonstrated and prescribed to the strong rotatable anisotropy acting in thin AFM layer FeMn(3 nm). The calculations, performed using the modified Kittel equation, allow one to compare alternative mechanisms for the acceleration of the adjacent FM layer magnetic dynamics. The Sixth chapter of the dissertation is devoted to the systematic study of the features of spin-wave dynamics in spatially confined nanoscale SAF elements, the lateral sizes of which approach the order of exchange length. Within the framework of the micromagnetic model we reproduce the experimental conditions under which the oscillations of a homogeneous mode and two selected higher order modes are established in a single perpendicularly magnetized nano-SAF. The second part of the chapter investigates resonant behavior of the in-plane magnetized arrays of Py/NiCu/Py SAF nanodisks with the nominal diameter of 150 nm. It is shown that the asymmetry of the intra-element dipolar interaction in the dome-shaped nanoelements causes the splitting of the edge spin-wave mode into oscillations of acoustic and optical types – an effect that can be controlled via the phase transition of the NiCu spacer. The third part of the chapter addresses the analogous arrays of SAF nanoelements with, however, twice the smaller size. The spatial structure of a single degenerate mode, inherent to such nanoelements, is discussed.