Special features of magnetic susceptibility dispersion in nanocrystalline magnets

Abstract

It is demonstrated theoretically that the initial (both elastic and viscous) magnetic susceptibility components for nanocrystalline magnets caused by the processes of rotations (in the region of linear response) have resonant rather than relaxation character typical already for the susceptibility component caused by displacements of domain boundaries. Magnetic susceptibility of magnets and ferrites is a very structurally sensitive parameter [1, 2]. According to S. V. Vonsovskii and V. K. Arkad'ev, this is the case for its elastic ( ′ χ ) and viscous components ( ′′ χ ). Each of them, in turn, has a rotational component ( r χ ) and a component ( v χ ) caused by displacements of domain boundaries. In this case, ( ) r ′ χ ω first remains virtually unchanged with increasing frequency and then fast decreases to zero, and r ′′ χ , on the contrary, first increases from zero to a maximum and then decreases to zero. The dependence ( ) v ′ χ ω has resonant character: at the beginning it is maximal like ( ) r ′ χ ω , then passes through zero and, remaining negative, vanishes. The v ′′ χ value, like ( ) r ′′ χ ω , has a maximum. Similar dependences ( ) χ ω , correlating with the experimental data [1, 2], were obtained based on the macroscopic approach [3, 4] that allows the prehistory of the system and the orientation and frequency of the variable magnetic field Н to be considered in ample detail. Analogous situation is observed for ferroelectrics [5]: ( ) v χ ω has the same resonant character of the dispersion, and ( ) rχ ω has the relaxation character. However, all this refers to conventional, that is, non-nanodimensional materials. In nanocrystalline materials, the dispersion of the magnetic susceptibility is rather specific and has a number of special features [6]. We first dwell on a model description of one of them caused by the presence of internal stresses in nanocrystalline materials formed during their manufacture. We consider triaxial nanocrystalline materials that, in addition to the interphase component that does not have even near order [6], have a crystalline component. In nanocrystalline materials with grain sizes k d greater than or equal to the domain boundary width δ, magnetic phases with spontaneous magnetization vectors S

On direct and reverse magnetically operated acoustic effect in nanocomposites

“…Let us consider the reverse acoustic effect: generation of an acoustic signal by a variable magnetic field in the composite. Using a macroscopic approach, we described similar and accompanying effects not only in crystalline magnetic [7], ferroelectric and ferroelectromagnetic materials [8,9], but also in nanocrystalline materials [10].…”