The main goal of the research carried out in the dissertation was to determine the influence of the micro and mesoscopic crystal structure of epitaxial films of Heusler alloys on their magnetic and magnetodynamic characteristics, as well as to establish technological methods for their control.
The creation of magnetic nanoelectronics requires solution of a number of fundamental and technological problems. Among the fundamental ones, it should be noted the problem of controlling magnetic dynamics and magnetic parameters of nanosystems (magnetic anisotropy, damping coefficients, exchange constant, etc.). From an applied point of view, there is an urgent need to find materials having parameters that meet the requirements of specific systems and devices, and, on the other hand, to be highly technological and easily integrated into modern industrial cycles. The most attractive would be to use all-metal magnetic elements, which manufacture is well developed and allows using standard technologies. In addition, such elements can be easily incorporated into modern microchips. One of the most promising materials for this is considered to be Heusler alloys. Calculations show that some Heusler alloys can have one hundred percent spin polarization. This statement still remains a theoretical prediction, but the degree of spin polarization in these alloys at room temperatures is high enough, and they are already used in spintronic devices. In addition, Heusler alloys based on iron and cobalt are characterized by the lowest magnetic damping parameter among metallic ferromagnets, which opens the way for their application in magnonics elements. The main problem with Heusler alloys in spintronics and magnonics is the fact that their magnetic properties strongly depend on the crystal structure (degree of ordering, uniformity, crystallite size, twin structure, etc.), which, in turn, are determined by the
manufacturing and processing technology. Therefore, establishing the relationship between the crystal structure and magnetic parameters of these materials, as well as the development of magnetic structures with controlled parameters is a relevant task. This dissertation study investigated the physical principles of a control of magnetic characteristics of epitaxial films of Heusler alloys.
The dissertation consists of four sections. The Introduction sets out the main arguments regarding the relevance of the chosen topic, defines the main goal and objectives of the research, and outlines the object and subject of study. In addition, the key scientific results that are presented for defense and reflect the novelty of the results obtained are formulated, and the personal contribution of the applicant to the work performed is shown. The information on the approval of scientific results at professional scientific conferences and on the connection of the dissertation research with scientific research works is also provided.
The First section contains information on the current state of research on thin films of Heusler alloys and justification of the relevance and novelty of the results described in the following sections. Some main data on the structure and magnetic properties of Heusler alloys are presented with an emphasis on the features that occur in thin epitaxial films of these materials.
The Second section provides a brief information on the experimental methods used in this work to study the structural and magnetic properties of the samples (energy- dispersive X-ray spectroscopy, X-ray diffraction, atomic force microscopy, vibrational magnetometry, ferromagnetic resonance and Brillouin scattering), as well as the basic physical principles that underlie them.
The Third section is devoted to the study of the structure and magnetic properties of epitaxial films of Co2FeGe Heusler alloy deposited on single-crystal MgO(100) substrates. This study shows the significant role of heat treatment in optimizing the magnetic properties of these films for applications in spintronics and magnonics systems. It was shown how the magnetostatic and magnetodynamic properties are affected by the microstructure of the films, which, in turn, is affected by heat treatment. In particular, it was demonstrated that the effect of the thermal treatment is non-monotonic in nature and ways to optimize technological parameters to achieve the desired magnetic properties of the films are shown.
The Fourth section presents the results of the investigation of the influence of the mesoscopic spatially periodic twin structure, which is formed in epitaxial films of alloys with a magnetic shape memory effect, on their magnetic properties and magnetodynamic characteristics. The mechanisms of formation of a spatially periodic twin structure in thin films of alloys with a shape memory effect and the influence of the film thickness on the period of such a structure are discussed. The analogy between nanotwin structures and layered systems based on ferromagnets.
