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Metlov K.L.,Donetsk Institute for Physics and Technology NAS | Michels A.,University of Luxembourg
Physical Review B - Condensed Matter and Materials Physics | Year: 2015

Magnetic small-angle neutron scattering (SANS) is a powerful tool for investigating nonuniform magnetization structures inside magnetic materials. Here, we consider a ferromagnetic medium with weakly inhomogeneous uniaxial magnetic anisotropy, saturation magnetization, and exchange stiffness, and derive, to second order in the amplitudes of the inhomogeneities, the micromagnetic solutions for the equilibrium magnetization textures. Further, we compute the corresponding magnetic SANS cross section up to the third order. For the special case of scattering geometry where the incident neutron beam is perpendicular to the applied magnetic field, twice the cross section along the direction orthogonal to both the field and the neutron beam cancels the cross section along the field direction in the second order. This cancellation does not depend on the defect shape and amplitudes of the exchange inhomogeneities. Hence, such a cross-section difference has only a third-order contribution in the amplitudes of the inhomogeneities. It provides a separate gateway for a deeper analysis of the sample's magnetic structure. We derive and analyze analytical expressions for the dependence of this difference on the scattering-vector magnitude for the case of spherical Gaussian inhomogeneities. © 2015 American Physical Society.

Metlov K.L.,Donetsk Institute for Physics and Technology NAS
Journal of Applied Physics | Year: 2013

Frequency of free magnetic vortex precession in circular soft ferromagnetic nano-cylinders (magnetic dots) of various sizes is an important parameter, used in design of spintronic devices (such as spin-torque microwave nano-oscillators) and characterization of magnetic nanostructures. Here, using a recently developed collective-variable approach to non-linear dynamics of magnetic textures in planar nano-magnets, this frequency and its amplitude-dependent shift are computed analytically and plotted for the full range of cylinder geometries. The frequency shift is positive in large planar dots, but becomes negative in smaller and more elongated ones. At certain dot dimensions, a zero frequency shift is realized, which can be important for enhancing frequency stability of magnetic nano-oscillators. © 2013 AIP Publishing LLC.

Metlov K.L.,Donetsk Institute for Physics and Technology NAS
Physical Review B - Condensed Matter and Materials Physics | Year: 2013

A collective-variable approach for the study of nonlinear dynamics of magnetic textures in planar nanomagnets is proposed. The variables are just arbitrary parameters (complex or real) in the specified analytical function of a complex variable describing the texture in motion. Starting with such a function, a formal procedure is outlined, allowing a (nonlinear) system of differential equations of motion to be obtained for the variables. The resulting equations are equivalent to Landau-Lifshitz-Gilbert dynamics as far as the definition of collective variables allows it. Apart from the collective-variable specification, the procedure does not involve any additional assumptions (such as translational invariance or steady-state motion). As an example, the equations of weakly nonlinear motion of a magnetic vortex are derived and solved analytically. A simple formula for the dependence of the vortex precession frequency on its amplitude is derived. The results are verified against special cases from the literature and agree quantitatively with experiments and simulations. © 2013 American Physical Society.

Metlov K.L.,Donetsk Institute for Physics and Technology NAS
Journal of Magnetism and Magnetic Materials | Year: 2013

The energy of magnetic vortex core and its equilibrium radius in thin circular cylinder were first presented by Usov and Peschany in 1994. Yet, the magnetostatic function, entering the energy expression, is hard to evaluate and approximate. Here, precise and explicit analytical approximations to this function (as well as equilibrium vortex core radius and energy) are derived in terms of elementary functions. Also, several simplifying approximations to the magnetic Hamiltonian and their impact on theoretical stability of magnetic vortex state are discussed. © 2013 Elsevier B.V. All rights reserved.

Metlov K.L.,Donetsk Institute for Physics and Technology NAS
Physical Review Letters | Year: 2010

The assumption of a certain hierarchy of soft ferromagnet energy terms, realized in small enough flat nanoelements, allows us to obtain explicit expressions for their magnetization distributions. By minimizing the energy terms sequentially, from the most to the least important, magnetization distributions are expressed as solutions of the Riemann-Hilbert boundary value problem for a function of complex variable. A number of free parameters, corresponding to positions of vortices and antivortices, still remain in the expression. Thus, the presented approach is a factory of realistic Ritz functions for analytical (or numerical) micromagnetic calculations. Examples are given for multivortex magnetization distributions in a circular cylinder, and for two-dimensional domain walls in thin magnetic strips. © 2010 The American Physical Society.

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