On the properties of a stochastic magnetic structure of low-dimensional ultradisperse ferromagnets

Analysis is given of the effect of the dimensionality of structural inhomogeneities on the properties of the magnetic structure of ultradisperse ferromagnetic materials. The coordinate dependence of the magnetization dispersion of magnets with one-dimensional and two-dimensional inhomogeneities of magnetic anisotropy has been calculated. The linear dimension of a magnetic block for one-and two-dimensional inhomogeneities of the anisotropy field has been estimated. A method for exact calculation of the distribution function of magnetization rotations in the block is suggested.     

Effect of the shape anisotropy and configurational anisotropy on the magnetic structure of ferromagnetic nanodots

Processes of magnetization reversal of ordered square arrays of “nanodots” of different geometry coupled by dipole-dipole interaction that were formed by a focused beam of Ga⁺ ions from continuous polycrystalline Co films have been investigated. It has been established that the magnetic structure upon the magnetization reversal is mainly determined by the shape anisotropy of the nanodots. The round and square nanodots undergo magnetization reversal via a magnetic vortex, whereas the rectangular nanodots demonstrate a finite set of single-domain states depending on the orientations of the external magnetic field and easy axis of magnetization. It has been shown that the magnitude of the switching field of arrays of round and square nanodots and the distribution of switching fields in the system of rectangular nanodots upon magnetization reversal along the easy axis is substantially affected by the configurational anisotropy.     

Forced motion of solitary domains and domain walls in the field of a nonlinear magnetization wave

For a ferromagnet with an easy-axis anisotropy, exact solutions to the Landau-Lifshitz equations have been obtained, which describe the interaction of a nonlinear wave of the precession of arbitrary amplitude with soliton-like objects such as breathers, solitary domains, and domain walls. Changes in the internal structure and physical parameters of solitons as a result of their interaction with a magnetization wave have been analyzed. It is shown that the solitary domains and domain walls move in the direction opposite to the magnetization wave. Conditions have been found under which the nonlinear wave of magnetization destroys solitons. It has been established that the destruction of breathers can be accompanied by the generation of magnetization self-oscillations.     

Analysis of the domain structure of submicrocrystalline ferromagnets by the method of Lorentz electron microscopy

Results are presented of an experimental study (by the method of Lorentz microscopy) of domain structure of ferromagnets with different values of the constant of magnetocrystalline anisotropy in the submicrocrystalline and coarse-grained structural states. It is shown that in the low-anisotropic pure ferromagnets Ni and Fe the domain structure does not virtually correlate with the crystal structure, and is determined by magnetostatic effects. In cobalt, which has a higher magnetocrystalline anisotropy, the stripe nature of the domain structure changes with a decrease in the size of crystallites. In the highly anisotropic alloy Mn_55.5Al_45.5C_0.5, the decrease in grain size results in not only to rtsults changes in the domain structure (DS), but also affects the basic mechanism of magnetization reversal. In the Fe₈₃Nd₁₃B₄ alloy, which has the maximum (among the materials investigated) value of the constant of magnetocrystalline anisotropy, the parameters of the DS are mainly determined by the average size of grains in the alloy. In a nanocrystalline structure, exchange-interaction domains are formed; in the submicrocrystalline structure, a single-domain state is realized; while in the microcrystalline structure, each grain becomes polydomain. A comparison of the domain structure of all materials under investigation makes it possible to conclude that the determining role belongs to the value of the energy of magnetocrystalline anisotropy. The parameters of the domain structure, in turn, determine the magnetization-reversal mechanisms of the ferromagnetic materials under investigation.     

快淬非晶合金制备态磁各向异性研究SCIEI

本文详细研究了铁基、铁镍基和钴基非晶合金条带制备态的平面磁各问异性。测定了退火前后试样中各向异性的变化及各向异性常数K_(us)随温度的变化,得出K_(us)(T)∝M_s^2(T)关系,由此得出平面磁各向异性由应力各向异性和表面形状各向异性两部分组成。     

Effect of the magnetic state on the damping properties of iron and nickel alloys

The dependences of the ΔEeffect and internal friction on the magnetic field have been investigated upon the magnetization and magnetization reversal of some polycrystalline ferromagnetic materials with different properties. It has been shown that for all the materials that have been investigated there is characteristic a nonmonotonic dependence of the magnitude of internal friction on the magnetic field both for the magnetization curves and for the major hysteresis loops. It is shown that in the sonic frequency range of elastic vibrations the basic mechanism of the formation of the magnetic peaks of internal friction in the materials investigated is magnetostrictive; the damping depends simultaneously on the instantaneous values of magnetostriction and magnetostrictive sensitivity.     

On the frequency dependence of magnetic losses for rotational magnetization reversal of Fe-3% Si single crystals

The frequency dependence of magnetic losses for the rotational magnetization reversal has been studied on Fe-3% Si single crystals in the range of frequencies of 20–250 Hz and amplitudes of induction of 0.25–1.8 T. The separation of the total magnetic losses into components has been carried out. It has been shown that the dependence of the total and eddy-current losses in the cycle of magnetization reversal on the frequency of the rotation field has a nonlinear character at all inductions investigated. The specific features of the frequency dependence of the magnetic losses of single crystals observed in the work are qualitatively explained by the dynamics of their domain structure.     

Effect of elastic and plastic deformations on the remanent magnetization of an ensemble of nanoparticles

A theoretical investigation of the effect of mechanical stresses on the remanent magnetization has been performed in terms of the model of single-domain noninteracting nanoparticles. Relationships have been obtained which define two main types of remanence in the entire range of stresses. In the low-field approximation, the magnetization of the first type, whose mechanism of formation is similar to that of the normal remanence, is quadratic in both the magnetic field and stresses and only slightly changes with increasing stresses. Depending on the relationship between the magnetostriction constants, this magnetization can both increase and decrease with increasing stresses. The magnetization of the second type, which arises as a result of a nonmonotonic behavior of the critical fields of nanoparticles depending on mechanical stresses, is proportional to the magnetic field and mechanical stresses. It has been shown that the longitudinal remanence arising in the field of stresses parallel to the magnetic field is always greater than the transverse remanence. The behavior of the remanence with increasing mechanical stresses depends substantially on whether this magnetization is formed in a loaded state or in a state unloaded after plastic deformation. In the range of deformations where the anisotropy of the applied stresses is less than the magnetocrystalline anisotropy, the plastic tension should lead to a decrease in the magnetization as compared to that arising in the plastically undeformed state. Plastic compression can lead to both an increase and a decrease in the remanence.     

Strong stray fields in magnets with a nonuniform distribution of the magnetization

Stray fields produced by magnets in the form of a cylinder in which the vector of magnetization at each point is directed along the cylinder radius have been calculated. It was assumed that this distribution of magnetization was achieved due to the presence of a radial crystalline texture and the related magnetic anisotropy with a large field of the uniaxial anisotropy H _K . It has been shown that the greatest stray fields are reached on the surface of the magnet at points that are located near its axis, and the limiting value of the point field is equal to 4πM _S ln(a/r). It has been established that in magnets with a radial magnetization the region of localization of strong fields H > 4πM _S substantially exceeds that characteristic of the systems of magnets with a uniform magnetization.     

Quantum Model of Hysteresis in a Single-Domain Magnetically Soft Ferromagnet

A quantum model of a magnetically soft ferromagnet constructed based on a single domain of an α-Fe crystal magnetized to saturation and placed in an alternating magnetic field is suggested. Based on the method of an effective Hamiltonian, the model takes into account the Zeeman energy, the spin-orbital interaction, and the interaction with the crystal field. In order to take into account the magnetic anisotropy, an expansion of a trial single-electron wave function into a series in terms of a small parameter of the spin-orbital interaction is suggested. The nonlinear equations of motion for the magnetization and orbital moment of the domain have been determined within the Heisenberg representation. The parameters of the nonlinear equations have been determined by comparing with the experimental data on the magnetic anisotropy of iron. The equations were solved numerically using the Runge–Kutta method with taking into account the magnetic friction introduced phenomenologically. The dependence of the magnetization of a single domain on the strength of the applied magnetic field has been characterized by hysteresis. The main parameters of the hysteresis loop are in quantitative agreement with the experimentally measured magnetic properties of nanoparticles of iron and iron oxide. A method of simulating the magnetization dynamics of a multidomain ferromagnet in the approximation of a strong crystal field has been suggested.     

Magnetic anisotropy constants of a Fe-3.8% Si single crystal

Magnetic anisotropy constants of an Fe-3.8% Si single crystal have been determined from the condition of approximation of the magnetization curve in the [110] direction using the Akulov theory. It is shown that only two constants of the six magnetic anisotropy constants can be found, namely, those that are related to the first and second powers of one of the invariants of the symmetry group of the crystal in the expression for the free energy of magnetic anisotropy.     

Nonstationary Dynamics of Vortex-Like Domain Walls in Magnetic Films with In-Plane Anisotropy

The dynamic rearrangement of the structure of vortex domain walls (DWs) in films with uniaxial in-plane anisotropy has been examined based on two-dimensional micromagnetic model simulation. The optimum computational region has been chosen; grids with up to 270 × 75 cells 3 nm in size have been used. The scenarios of dynamic rearrangement of the DW structure in strong fields (up to 450 Oe) and in films with thicknesses exceeding 100 nm have been revealed for the first time. The emergence of multivortex instantaneous distributions of magnetization upon an increase in the film thickness and the external field intensity have been established. The dependence of the critical field on the film thickness has been refined.     

Magnetization of superparamagnets in the state of mechanical anisotropy

The internal energy of magnetic anisotropy in some particles dominates over the thermal energy, even at room temperature. The existence of strong magnetic anisotropy of nanoparticles can significantly affect the process of magnetization of superparamagnets. However, if the axes of magnetic anisotropy of nanoparticles are randomly oriented, then their presence does not affect the process of magnetization, which occurs according to the classical Langevin theory. However, if the axes of nanoparticles are polarized (mechanical anisotropy), then the magnetization curve of a superparamagnet under the conditions of mechanical anisotropy lies between the Langevin curve and the curve of hyperbolic tangent and with increasing anisotropy moves progressively farther from the Langevin curve and approaches the curve of hyperbolic tangent. It has also been shown that, in the case of powder superparamagnets, the presence of mechanical anisotropy leads to significant changes in the Curie constant.     

Magnetization reversal of a double-layer exchange-coupled ferromagnetic film

A theoretical investigation of magnetization reversal of a double-layer magnetic film with an easy-plane and easy-axis anisotropy has been performed. An analytical expression for the magnetization distribution in the layers in the presence of a magnetic field has been obtained. The calculations performed on the basis of the analytical relationships obtained show that when the thickness of the layers is close to the width of a magnetic inhomogeneity, it substantially affects the magnitude of the critical field for the magnetization reversal of the sample and the shape of the magnetization curve. It is also shown that a decrease in the exchange coupling between the layers leads to an increase in the magnitude of the critical field of magnetization reversal.     

Structure and dynamic properties of asymmetric vortexlike domain walls in inhomogeneous magnetic films with an in-plane anisotropy: I. Equilibrium structures. Nonlinear dynamics in two-layered films

Within the framework of a two-dimensional distribution of magnetization and exact allowance for the basic interactions, including dipole-dipole one, the static properties and nonlinear dynamic behavior of vortexlike domain walls in multilayer magnetically uniaxial films with an in-plane anisotropy have been studied. It has been shown that in such films, just as in single-layer ones, there exist asymmetric vortexlike walls; however, in this case the wall vortices prove to be moved toward the layers with a higher magnetization or toward the layers with a lower anisotropy. At some thicknesses of the layers and magnetization differences in them, the asymmetric Néel walls prove to be unstable. New scenarios of the dynamic transformation of the structure of the walls in multilayer films have been established, which in some cases differ significantly from the appropriate scenarios in the single-layer films.     

Sm_2Co_(17-x)Ti_x化合物的结构和磁性

利用 Ｘ射线衍射和磁性测量研究了 Ｓｍ２Ｃｏ１７－ｘＴｉｘ化合物的结构和磁性实验表明这些化合物都具有 Ｔｈ２Ｚｎ１７型 结构, Ｔｉ替代 Ｃｏ不改变化合物的晶体结构,但引起晶格膨胀所有化合物在 Ｃｕｒｉｅ温度以下都表现为单轴磁晶各向异性随 着 Ｔｉ含量的增加,化合物的 Ｃｕｒｉｅ温度和饱和磁化强度都单调降低,而磁晶各向异性场在 Ｔｉ含量为 １．０时出现一极大值     

Magnetic microstructure of amorphous,nanocrystalline, and nanophase ferromagnets

The magnetic microstructure of nanostructured ferromagnets is represented by an ensemble of stochastic magnetic domains—regions with dimensions of the length of magnetic orientation coherency. It is shown that the curves displaying the approach of magnetization to saturation make it possible to determine the dimension of the element of the micromagnetic structure, i.e., the size of the stochastic domain and the constant of the effective anisotropy in this element, the size of the element of the nanostructure and the constant its local anisotropy, as well as the dimensionality of the exchange-coupled ferromagnetic nanoparticles.     

Structure and dynamics of a domain wall in a ferromagnet with an inhomogeneous magnetoelectric interaction

Effect of an electric field parallel to the easy axis on the structure and dynamics of a domain wall (DW) in ferromagnets with a large quality factor with allowance for a small orthorhombic anisotropy has been studied. It has been shown that in an electric field there is formed a DW with a structure that is intermediate between the Bloch and Néel types. The critical field for the transition of the intermediate wall into a Néel one, as well as the dependence of the velocity of motion of the DW on the gradient electric field have been determined.     

Magnetization reversal of Tb-Co/Fe19Ni81 films with a unidirectional anisotropy

Conditions have been found for the realization of induced in-plane uniaxial anisotropy of the amorphous Tb₃₁Co₆₉ films and of unidirectional anisotropy in the Permalloy layers in the composite two-layered Tb₃₁Co₆₉/Fe19Ni81 films. The mechanisms of magnetization reversal in such film structures have been studied in a temperature range of 5–300 K. It has been found that a decrease in temperature leads to a transformation of hysteresis loops, a significant increase in the field of unidirectional anisotropy, and a nonmonotonic change in the coercive force of the Permalloy layer. The regularities found are interpreted taking into account the variation in the properties of the amorphous layer Tb₃₁Co₆₉ with temperature under the assumption on a temperature-induced change in the localization of the interlayer magnetic boundary, which is formed upon a layer-by-layer magnetization reversal of the film structure.     

Resonance modes of two-layer exchange-coupled ferromagnetic film

Ferromagnetic and spin-wave resonances in two-layer exchange-coupled ferromagnetic films have been investigated numerically at different intensities of a magnetic field when it is directed in parallel or perpendicular to the film plane. Layers of the film have finite thicknesses and possess anisotropy of the easy-plane and easyaxis types. It has been shown that at a nonzero parameter of interlayer exchange coupling the dynamic component of magnetization upon ferromagnetic resonance is distributed nonuniformly across the film thickness. Its change has been described when the external magnetic field decreases from the saturating field to zero.