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Information × Registration Number 0209U006080, 0106U003119 , R & D reports Title Local magnetic fields on nucleus 57Fe in volume and on surface of hexaferrite crystals of different scale popup.stage_title Head Kuncevich Stanislav Petrovich, Registration Date 21-01-2009 Organization Kharkov Natuional university named after V.N.Karazin popup.description2 Object of research - hexagonal ferrite of structural type of M and other types in the form of single crystals and powder samples with the size of one-domain particles in nano- and micrometer range. Research methods - method of nuclear magnetic resonance, magnetostatic methods of research of the basic curves of magnetization, hysteresis loops in the fields up to 20 kOe in the temperature range 300-700 K with an engagement of modern theoretical models. The work purpose - to define temperature changes of local fields on 57Fe nuclei a by nuclear magnetic resonance method in macrocrystals of M-type hexaferrites. To test applicability of theoretical models used to describe of temperature changes of local fields. To define the contribution of surface anisotropy into an effective magnetic anisotropy of the system of barium hexaferrite nanocrystals. Based on the theory of spin waves, the analysis of temperature dependences of local fields on iron nuclei was carried out. It is established that intraboundary magnons do not make an essential contribution to temperature dependences of local fields on nuclei in the middle of domain walls. It is shown that spin dynamics in the ranges of 4.2-60 K and 60-295 K in barium hexaferrite is essentially different. In the range of 60-295 K it is caused by excitation of exchange magnons. An effective temperature of an energy gap of exchange magnons Te =58 K. The methodology of restoration of true temperature dependence of fields of effective magnetic anisotropy of a system of "small Stoner-Volfart particle" is developed. The methodology is tested on a system of nanocrystals of highly anisotropic hexagonal ferrite BaFe12O19. Distribution on fields of anisotropy in the range of 300-700 K is determined. Boundary values of Hamin (T) and Hamax (T) were corrected by the account of influence of thermal fluctuations for particles with the volumes close to superparamagnetic, and by use of the basic formula of higher order of approximation. By introduction of additional critical lines of fields of anisotropy Hamin (T), Hamax (T) and threshold fields , (Н-Т) the diagram of a magnetic condition of nanocrystal system was improved. For the first time Akulov's known method which was traditionally used only for polycrystalline samples is applied to research of the constant of magnetic anisotropy of a system of BaFe12O19 nanocrystals. Temperature dependence of the Kef constant is obtained. Local heterogeneities in near-surface layer of particles are displayed in change with temperature of an inclination of a linear segment of curve dependence ?? H2=f (H) according to the approach to saturation law. Product Description popup.authors popup.nrat_date 2020-04-02 Close