Welcome to the Homepage of the Group «Research in Theory of Magnetism» (RITM, Kyiv, Ukraine). We are focused on physics of the nonlinear phenomena in the Nanomagnetism, including dynamics of 2D magnetic solitons, vortices, topological singularities etc.

Publications Highlights

  • Curvature-induced domain wall pinning

A local bend of a nanowire is a source of pinning potential for a transversal head-to-head (tail-to-tail) domain wall. Eigenfrequency of the domain wall free oscillations at the pinning potential and the effective friction are determined as functions of the curvature and domain wall width. The pinning potential originates from the effective curvature-induced Dzyaloshinsky-like term in the exchange energy. The theoretical results are verified by means of micromagnetic simulations for the case of parabolic shape of the wire bend.Phys. Rev. B. 92, 104412 (2015), PDF.
  • Torsion-induced effects in magnetic nanowires
A magnetic helix wire is one of the simplest magnetic systems which manifests properties of both curvature and torsion. Possible equilibrium magnetization states in the helix wire with different anisotropy directions are studied theoretically. There exist two equilibrium states in the helix wire with easy-tangential anisotropy: a quasitangential magnetization distribution in the case of relatively small curvatures and torsions, and an onion state in the opposite case. The curvature and torsion also essentially influence the spin-wave dynamics in the helix wire, acting as an effective magnetic field. Originated from a geometry-induced effective Dzyaloshinskii interaction, this magnetic field leads to a coupling between the helix chirality and the magnetochirality and breaks mirror symmetry in the spin-wave spectrum: the modification of magnon dispersion relation is linear with respect to the torsion and quadratic with respect to the curvature. All analytical predictions on magnetization statics and dynamics are well confirmed by direct spin-lattice simulations.

Phys. Rev. B. 92, 054417 (2015), PDF.

  • Effects of a spin-polarized current assisted Ørsted field in magnetization patterning
A spin-polarized electrical current leads to a variety of periodical magnetic structures in nanostripes. In the presence of the Ørsted field, which always assists an electrical current, the basic types of magnetic structures, i.e., a vortex-antivortex crystal and cross-tie domain walls, survive. The Ørsted field prevents saturation of the nanostripe and a longitudinal domain wall appears instead. Possible magnetization structures in stripes with different geometrical and material properties are studied numerically and analytically.

J. Appl. Phys. 117, 213910 (2015), PDF.

  • Coupling of Chiralities in Spin and Physical Spaces: The Möbius Ring as a Case Study
Chirality symmetry breaking is a common phenomenon in the Nature. The most intriguing examples come from life sciences (chirality symmetry breaking of biological macromolecules, such as DNA) and from particle physics (e.g. neutrino helicity symmetry breaking). In magnetism, chirality is a characteristic of a number of topologically nontrivial structures, such as domain walls and magnetic vortices. Therefore, chirality symmetry breaking is a subject of very recent studies in various magnetic phenomena. In our study we consider a ferromagnetic Möbius ring with anisotropy of easy-normal type. In the case of strong  anisotropy a topologically protected Bloch domain wall forms the ground state of the ring. It is shown that magnetic chirality of the wall is determined by the geometric chirality of the Möbius ring, i.e. the chirality symmetry breaking takes place.

Phys. Rev. Lett. 114, 197204 (2015), PDF.

  • Curvature effects in statics and dynamics of low dimensional magnets

Nowadays there is a growing interest in studying of topological structures in curved magnetic
objects such as magnetic nanowires and nanoshells. We develop an approach to treat magnetic energy of a ferromagnet for arbitrary curved wires and shells on the assumption that the anisotropy contribution greatly exceeds the dipolar and other weak interactions. We show that the curvature induces two effective magnetic interactions: effective magnetic anisotropy and an effective Dzyaloshinskii-like interaction. We derive an equation of magnetization dynamics and propose a general static solution for the limit case of strong anisotropy. To illustrate our approach, we consider the magnetization structure in a ring wire and a cone surface: ground states in both systems essentially depend on the curvature, excluding strictly tangential solutions for a cone surface even in the case of strong anisotropy. We derive also the spectrum of spin waves in such systems.

Journal of Physics A: Mathematical and Theoretical 48 , 125202 (2015), PDF.

  • Vortex polarity switching in magnets with surface anisotropy
Schematic of a wired core modelVortex core reversal in magnetic particle is essentially influenced by a surface anisotropy. Under the action of a perpendicular static magnetic field the vortex core undergoes a shape deformation of pillow- or barrel-shaped type, depending on the type of the surface anisotropy. This deformation plays a key point in the switching mechanism: We predict that the vortex polarity switching is accompanied (i) by a linear singularity in case of Heisenberg magnet with bulk anisotropy only and (ii) by a point singularities in case of surface anisotropy or exchange anisotropy. We study in details the switching process using spin-lattice simulations and propose a simple analytical description using a wired core model, which provides an adequate description of the Bloch point statics, its dynamics and the Bloch point mediated switching process. Our analytical predictions are confirmed by spin-lattice simulations for Heisenberg magnet and micromagnetic simulations for nanomagnet with account of a dipolar interaction.

Low Temp. Phys. 41, 466–481 (2015), PDF.

  • Curvature Effects in Thin Magnetic Shells
Cone

The submbicron-scaled magnetism of solid state is a rapidly developing research area with a promising applied potential. Nanomagnets are often considered as elements of fast nonvolatile memory devices of high density. Up to now the theoretical study in this direction was restricted mostly by planar structures. In the current research we propose an universal approach for describing the magnetization dynamics of an arbitrary curved magnetic film. We believe that the proposed method significantly expands facilities of the nanomagnetism area by introducing into the consideration a wide range of new objects: magnetic nanotubes, curvilinear nanowires, nanocaps of spherical and other geometries.

Phys. Rev. Lett. 112, 257203 (2014)PDF.

  • Periodic magnetic structures generated by spin-polarized currents in nanostripes
Diagram

We investigated the influence of a spin-polarized current on long ferromagnetic nanostripes. The magnetization behavior is analyzed for all range of the applied currents, up to the saturation. It is shown that the saturation current is a nonmonotonic function of the stripe width. For a stripe width increasing it approaches the saturation value for an infinite film. A number of stable periodic magnetization structures are observed below the saturation. Type of the periodical structure depends on the stripe width.

Appl. Phys. Lett. 103, 222401 (2013)PDF.

  • Equilibrium states of soft magnetic hemispherical shell
Diagram

We predict existence of two magnetic phases in hemispherical permalloy magnetic shells: the onion state and the vortex one.

SPIN , 1340003 (2013), 10.1142/S2010324713400031PDF.

  • Regular and chaotic vortex core reversal by a resonant perpendicular magnetic field
Poincare evolution of the vortex polarity in the core model. Background: part of the phase diagram with insets of dynamical regimes. We predict the regular and chaotic dynamics of the vortex polarity under the action of perpendicular ac magnetic field and propose a simple analytical description in terms of a reduced vortex core model.

Phys. Rev. B, 88, 014432 (2013), 10.1103/PhysRevB.88.014432PDF.

  • Magnetic vortex-antivortex crystals generated by spin-polarized current
Vortex-antivortex superlattice (theory) We study vortex pattern formation in thin ferromagnetic films under the action of strong spin-polarized currents: The stable square vortex-antivortex superlattices (vortex crystals) appear slightly below the critical current.

Phys. Rev. B 86, 144401 (2012), 10.1103/PhysRevB.86.144401PDF.

  • Bloch Point Structure in a Magnetic Nanosphere
Bloch point pinned at the hole We classify possible types of Bloch points and analytically derive the shape of the magnetization distribution for different Bloch points.

Phys. Rev. B 85, 224401 (2012), 10.1103/PhysRevB.85.224401PDFSupplementary materials.

  • Out-of-surface vortices in spherical shells
Phi_new_small We show that the curvature of the underlying surface leads to a coupling between the localized out-of-surface component of the vortex with its delocalized in-surface structure, i.e., polarity-chirality coupling.

Phys. Rev. B 85, 144433 (2012), 10.1103/PhysRevB.85.144433PDF.

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