A. B. Borisov

Three-dimensional skyrmion states in thin films of cubic helimagnets

A direct three-dimensional minimization of the standard energy functional shows that in thin films of cubic helimagnets chiral skyrmions are modulated along three spatial directions. The structure of such 3D skyrmions can be thought of as a superposition of conical modulations along the skyrmion axis and double-twist rotation in the perpendicular plane. Numerical solutions for chiral modulations demonstrate that 3D skyrmion lattices and helicoids are thermodynamically stable in a broad range of applied magnetic fields. Our results disclose a basic physical mechanism underlying the formation of skyrmion states recently observed in nanolayers of cubic helimagnets.

New type of stable particlelike states in chiral magnets.

We present a new type of thermodynamically stable magnetic state at interfaces and surfaces of chiral magnets. The state is a soliton solution of micromagnetic equations localized in all three dimensions near a boundary, and it contains a singularity but nevertheless has finite energy. Both features combine to form a quasiparticle state for which we expect unusual transport and dynamical properties. It exhibits high thermal stability and thereby can be considered as a promising object for fundamental research and practical applications in spintronic devices. We identified the range of existence of such particlelike states in the thickness dependent magnetic phase diagram for helimagnet films and analyzed its stability in comparison with the isolated skyrmion within the conical phase. We provide arguments that such a state can be found in different B20-type alloys, e.g., Mn_{1-x}Fe_{x}Ge, Mn_{1-x}Fe_{x}Si, Fe_{1-x}Co_{x}Si.

New spiral state and skyrmion lattice in 3D model of chiral magnets

We present the phase diagram of magnetic states for films of isotropic chiral magnets calculated as function of applied magnetic field and thickness of the film. We have found a novel magnetic state driven by the natural confinement of the crystal, localized at the surface and stacked on top of the conical bulk phase. This magnetic surface state has a three-dimensional (3D) chiral spin-texture described by the superposition of helical and cycloidal spin spirals. This surface state exists for a large range of applied magnetic fields and for any film thickness beyond a critical one. We also identified the whole thickness and field range for which the skyrmion lattice becomes the ground state of the system. Below a certain critical thickness the surface state and bulk conical phase are suppressed in favor of the skyrmion lattice. Unraveling of those phases and the construction of the phase diagram became possible using advanced computational techniques for direct energy minimization applied to a basic 3D model for chiral magnets. Presented results provide a comprehensive theoretical description for those effects already observed in experiments on thin films of chiral magnets, predict new effects important for applications and open perspectives for experimental studies of such systems.

Experimental observation of chiral magnetic bobbers in B20-type FeGe

Chiral magnetic skyrmions1,2 are nanoscale vortex-like spin textures that form in the presence of an applied magnetic field in ferromagnets that support the Dzyaloshinskii–Moriya interaction (DMI) because of strong spin–orbit coupling and broken inversion symmetry of the crystal3,4. In sharp contrast to other systems5,6 that allow for the formation of a variety of two-dimensional (2D) skyrmions, in chiral magnets the presence of the DMI commonly prevents the stability and coexistence of topological excitations of different types7. Recently, a new type of localized particle-like object—the chiral bobber (ChB)—was predicted theoretically in such materials8. However, its existence has not yet been verified experimentally. Here, we report the direct observation of ChBs in thin films of B20-type FeGe by means of quantitative off-axis electron holography (EH). We identify the part of the temperature–magnetic field phase diagram in which ChBs exist and distinguish two mechanisms for their nucleation. Furthermore, we show that ChBs are able to coexist with skyrmions over a wide range of parameters, which suggests their possible practical applications in novel magnetic solid-state memory devices, in which a stream of binary data bits can be encoded by a sequence of skyrmions and bobbers.Electron holography enables direct experimental verification of the existence of chiral bobbers in thin films of chiral magnets.The use of chiral skyrmions, which are nanoscale vortex-like spin textures, as movable data bit carriers forms the basis of a recently proposed concept for magnetic solid-state memory. In this concept, skyrmions are considered to be unique localized spin textures, which are used to encode data through the quantization of different distances between identical skyrmions on a guiding nanostripe. However, the conservation of distances between highly mobile and interacting skyrmions is difficult to implement in practice. Here, we report the direct observation of another type of theoretically-predicted localized magnetic state, which is referred to as a chiral bobber (ChB), using quantitative off-axis electron holography. We show that ChBs can coexist together with skyrmions. Our results suggest a novel approach for data encoding, whereby a stream of binary data representing a sequence of ones and zeros can be encoded via a sequence of skyrmions and bobbers. The need to maintain defined distances between data bit carriers is then not required. The proposed concept of data encoding promises to expedite the realization of a new generation of magnetic solid-state memory.

Spiral vortices in ferromagnets

New types of magnetic defects in two-dimensional ferromagnets are found in the exchange approximation, and the influence of magnetic anisotropy on their structure is analyzed.

Mechanical properties and the structure of porous titanium obtained by sintering compacted titanium sponge

Mechanical properties and characteristics of the porous material obtained by sintering of compacted granules of titanium sponge of grade TG-OP-01 have been investigated with the purpose to develop a technology of production of biocompatible orthoimplants. The content of pores θ was from 20 to 60%. As θ increased from 20 to 50%, the Brinell hardness HB, maximum bending strength σbend, and impact toughness Kc decreased, following the empirical dependences HBθ = const, σbend × θ2 = const, and Kc × θ1.5 = const, respectively. At θ > 50%, there was observed a stronger drop in these values. The experiments on the absorption of water by the pores and an analysis of the filtration properties of the material made it possible to establish that the basis of the system of pores connected with the surface is composed by the gaps between the pressed granules 0.1–0.2 cm sintered during annealing rather than by the small (∼ 100 μm) pores in the granules.

DYNAMICAL TOROIDAL HOPFIONS IN A FERROMAGNET WITH EASY-AXIS ANISOTROPY

Three-dimensional toroidal precession solitons with a nonzero Hopf index, which uniformly move along the anisotropy axis in a uniaxial ferromagnet, have been found. The structure and existence region of the solitons have been numerically determined by solving the Landau-Lifshitz equation.

Three-dimensional spiral structures in a ferromagnet

New types of three-dimensional spiral structures are found in an isotropic ferromagnet.

Local anisotropy induced by an external indenter in a magnetic film

The magnetic characteristics of a film under the effect of an external indenter have been studied. It has been shown that magnetoelastic interactions induce additional magnetic anisotropy, which can be either positive or negative, depending on the values of the magnetoelastic constants. The effect of the geometrical parameters of the problem on the magnetoelastic energy has been investigated.

Stationary precession topological solitons with nonzero Hopf invariant in a uniaxial ferromagnet

Three-dimensional stationary precession solitons with nonzero Hopf indices are found numerically by solving the Landau-Lifshitz equation. The structure and existence domain of the solitons are found.