arXiv subjects

Denis D. Sheka

Publications and source records attributed to Denis D. Sheka.

At least 19 recordsLinked to original sources

arXiv2025

Chiral breakdown engineered by mesoscale Dzyaloshinskii-Moriya interaction in biaxial magnetic nanotubes

Curvilinear geometries in magnetic nanostructures provide a unique platform for exploring the interplay of symmetry, topology, and curvature in magnetization dynamics. In this work, we analytically study the static and dynamic properties of domain walls in biaxial magnetic nanotubes with intrinsic Dzyaloshinskii-Moriya interaction of different symmetries. We show that geometry-driven local and nonlocal interactions govern domain profiles and dynamics, enabling precise control over the wall propagation and Walker breakdown field. Furthermore, the combination of bulk-type Dzyaloshinskii-Moriya interaction and curvature leads to chirality symmetry breaking and chiral breakdown in domain wall motion. These findings offer a framework for tailoring domain wall textures in cylindrical nanotubes, unlocking new functionalities for advanced applications in curvilinear magnonics and data storage technologies.

cond-mat.mes-hall↗

arXiv2024

Magnetic Vortex Dynamics on a Spherical Cap

By tailoring geometrical properties of magnetic nanocap structures, new possibilities appear to control its magnetic properties, such as the dynamics of magnetic vortices. Here, we develop an approach to describe dynamics of magnetic vortices on a spherical cap. Analytic results for the gyrofrequency of the vortex state are in a good agreement with micromagnetic simulations.

cond-mat.mes-hall↗

arXiv2024

Antiferromagnetic nanoscale bit arrays of magnetoelectric Cr$_2$O$_3$ thin films

Magnetism of oxide antiferromagnets (AFMs) has been studied in single crystals and extended thin films. The properties of AFM nanostructures still remain underexplored. Here, we report on the fabrication and magnetic imaging of granular 100-nm-thick magnetoelectric Cr$_2$O$_3$ films patterned in circular bits with diameters ranging from 500 down to 100 nm. With the change of the lateral size, the domain structure evolves from a multidomain state for larger bits to a single domain state for the smallest bits. Based on spin-lattice simulations, we show that the physics of the domain pattern formation in granular AFM bits is primarily determined by the energy dissipation upon cooling, which results in motion and expelling of AFM domain walls of the bit. Our results provide a way towards the fabrication of single domain AFM-bit-patterned memory devices and the exploration of the interplay between AFM nanostructures and their geometric shape.

cond-mat.mtrl-sci↗

arXiv2023

Control of magnetic response in curved stripes by tailoring cross-section

Curved magnetic architectures are key enablers of the prospective magnetic devices with respect to size, functionality and speed. By exploring geometry-governed magnetic interactions, curvilinear magnetism offers a number of intriguing effects in curved magnetic wires and curved magnetic films. The applicability of the current micromagnetic theory requires that the sample has constant width and thickness, which does not correspond in many cases to specificity of experimental sample preparation. Here, we put forth a self-consistent micromagnetic framework of curvilinear magnetism of nanowires and narrow stripes with spatially inhomogeneous cross-section. The influence of the varying cross-section is exploited and illustrated by an example of the simplest topological texture, which is a transversal head-to-head (tail-to-tail) domain wall. The cross-section gradient becomes a source of domain wall pinning which competes the curvature gradient. Eigenfrequencies of the domain wall free oscillations at the pinning potential are determined by both curvature and cross-section gradients. Prospects for curvilinear magnonics and spintronics are discussed.

cond-mat.str-el↗

arXiv2022

Field-induced spin reorientation transitions in antiferromagnetic ring-shaped spin chains

Easy axis antiferromagnets are robust against external magnetic fields of moderate strength. Spin reorientations in strong fields can provide an insight into more subtle properties of antiferromagnetic materials, which are often hidden by their high ground state symmetry. Here, we investigate theoretically effects of curvature in ring-shaped antiferromagnetic achiral anisotropic spin chains in strong magnetic fields. We identify the geometry-governed helimagnetic phase transition above the spin-flop field between vortex and onion states. The curvature-induced Dzyaloshinskii-Moriya interaction results in the spin-flop transition being of first- or second-order depending on the ring curvature. Spatial inhomogeneity of the Néel vector in the spin-flop phase generates weak ferromagnetic response in the plane perpendicular to the applied magnetic field. Our work contributes to the understanding of the physics of curvilinear antiferromagnets in magnetic fields and guides prospective experimental studies of geometrical effects relying on spin-chain-based nanomagnets.

cond-mat.mes-hall↗

arXiv2022

Circular stripe domains and cone state vortices in disk-shaped exchange coupled magnetic heterostructures

Vertically stacked exchange coupled magnetic heterostructures of cylindrical geometry can host complex noncolinear magnetization patterns. By tuning the interlayer exchange coupling between a layer accommodating magnetic vortex state and an out-of-plane magnetized layer, one can efficiently realize new topological chiral textures such as cone state vortices and circular stripe domains. We study how the number of circular stripes can be controlled by both the interlayer exchange coupling and the sample geometrical parameters. By varying geometrical parameters, a continuous phase transition between the homogeneous state, cone state vortex, circular stripe domains, and the imprinted vortex takes place, which is analysed by full scale micromagnetic simulations. The analytical description provides an intuitive pictures of the magnetization textures in each of these phases. The possibility to realize switching between different states allows for engineering magnetic textures with possible applications in spintronic devices.

cond-mat.mes-hall↗

arXiv2022

Impact of curvature-induced Dzyaloshinskii-Moriya interaction on magnetic vortex texture in spherical caps

Geometric curvature of nanoscale magnetic shells brings about curvature-induced anisotropy and Dzyaloshinskii-Moriya interaction (DMI). Here, we derive equations to describe the profile of the magnetic vortex state in a spherical cap. We demonstrate that the azimuthal component of magnetization acquires a finite tilt at the edge of the cap, which results in the increase of the magnetic surface energy. This is different compared to the case of a closed spherical shell, where symmetry of the texture does not allow any tilt of magnetization at the equator of the sphere. Furthermore, we analyze the size of the vortex core in a spherical cap and show that the presence of the curvature-induced DMI leads to the increase of the core size independent of the product of the circulation and polarity of the vortex. This is in contrast to the case of planar disks with intrinsic DMI, where the preferred direction of circulation as well as the decrease or increase of the size of vortex core is determined by the sign of the product of the circulation and polarity with respect to the sign of the constant of the intrinsic DMI.

cond-mat.mes-hall↗

arXiv2021

Nematic versus ferromagnetic shells: new insights in curvature-induced effects

Within the framework of continuum theory, we draw a parallel between ferromagnetic materials and nematic liquid crystals confined on curved surfaces, which are both characterized by local interaction and anchoring potentials. We show that the extrinsic curvature of the shell combined with the out-of-plane component of the director field gives rise to chirality effects. This interplay produces an effective energy term reminiscent of the chiral term in cholesteric liquid crystals, with the curvature tensor acting as a sort of anisotropic helicity. We discuss also how the different nature of the order parameter, a vector in ferromagnets and a tensor in nematics, yields different textures on surfaces with the same topology as the sphere. In particular, we show that the extrinsic curvature governs the ground state configuration on a nematic spherical shell, favouring two antipodal disclinations of charge +1 on small particles and four $+1/2$ disclinations of charge located at the vertices of a square inscribed in a great circle on larger particles.

cond-mat.soft↗

arXiv2021

Curvature-driven homogeneous Dzyaloshinskii-Moriya interaction and emergent weak ferromagnetism in anisotropic antiferromagnetic spin chains

Chiral antiferromagnets are currently considered for broad range of applications in spintronics, spin-orbitronics and magnonics. In contrast to the established approach relying on materials screening, the anisotropic and chiral responses of low-dimensional antifferromagnets can be tailored relying on the geometrical curvature. Here, we consider an achiral, anisotropic antiferromagnetic spin chain and demonstrate that these systems possess geometry-driven effects stemming not only from the exchange interaction but also from the anisotropy. Peculiarly, the anisotropy-driven effects are complementary to the curvature effects stemming from the exchange interaction and rather strong as they are linear in curvature. These effects are responsible for the tilt of the equilibrium direction of vector order parameters and the appearance of the homogeneous Dzyaloshinskii-Moriya interaction. The latter is a source of the geometry-driven weak ferromagnetism emerging in curvilinear antiferromagnetic spin chains. Our findings provide a deeper fundamental insight into the physics of curvilinear antiferromagnets beyond the $σ$-model and offer an additional degree of freedom in the design of spintronic and magnonic devices.

cond-mat.str-el↗

arXiv2021

Boundary conditions for the Néel order parameter in a chiral antiferromagnetic slab

Understanding of the interaction of antiferromagnetic solitons including domain walls and skyrmions with boundaries of chiral antiferromagnetic slabs is important for the design of prospective antiferromagnetic spintronic devices. Here, we derive the transition from spin lattice to micromagnetic nonlinear $σ$-model with the corresponding boundary conditions for a chiral cubic G-type antiferromagnet and analyze the impact of the slab boundaries and antisymmetric exchange (Dzyaloshinskii--Moriya interaction) on the vector order parameter. We apply this model to evaluate modifications of antiferromagnetic domain walls and skyrmions upon interaction with boundaries for different strengths of the antisymmetric exchange. Due to the presence of the antisymmetric exchange, both types of antiferromagnetic solitons become broader when approaching the boundary and transform to a mixed Bloch--Néel structure. Both textures feel the boundary at the distance of about 5 magnetic lengths. In this respect, our model provides design rules for antiferromagnetic racetracks, which can support bulk-like properties of solitons.

cond-mat.mes-hall↗

arXiv2020

Nanoscale mechanics of antiferromagnetic domain walls

Antiferromagnets offer remarkable promise for future spintronics devices, where antiferromagnetic order is exploited to encode information. The control and understanding of antiferromagnetic domain walls (DWs) - the interfaces between domains with differing order parameter orientations - is a key ingredient for advancing such antiferromagnetic spintronics technologies. However, studies of the intrinsic mechanics of individual antiferromagnetic DWs remain elusive since they require sufficiently pure materials and suitable experimental approaches to address DWs on the nanoscale. Here we nucleate isolated, 180° DWs in a single-crystal of Cr$_2$O$_3$, a prototypical collinear magnetoelectric antiferromagnet, and study their interaction with topographic features fabricated on the sample. We demonstrate DW manipulation through the resulting, engineered energy landscape and show that the observed interaction is governed by the DW's elastic properties. Our results advance the understanding of DW mechanics in antiferromagnets and suggest a novel, topographically defined memory architecture based on antiferromagnetic DWs.

cond-mat.mes-hall↗

arXiv2020

Curvature effects on phase transitions in chiral magnets

Periodical equilibrium states of magnetization exist in chiral ferromagnetic films, if the constant of antisymmetric exchange (Dzyaloshinskii-Moriya interaction) exceeds some critical value. Here, we demonstrate that this critical value can be significantly modified in curved film. The competition between symmetric and antisymmetric exchange interactions in a curved film can lead to a new type of domain wall which is inclined with respect to the cylinder axis. The wall structure is intermediate between Bloch and Néel ones. The exact analytical solutions for phase boundary curves and the new domain wall are obtained.

cond-mat.str-el↗

arXiv2020

Curvilinear one-dimensional antiferromagnets

Antiferromagnets host exotic quasiparticles, support high frequency excitations and are key enablers of the prospective spintronic and spin-orbitronic technologies. Here, we propose a concept of a curvilinear antiferromagnetism where material responses can be tailored by a geometrical curvature without the need to adjust material parameters. We show that an intrinsically achiral one-dimensional (1D) curvilinear antiferromagnet behaves as a chiral helimagnet with geometrically tunable Dzyaloshinskii--Moriya interaction (DMI) and orientation of the Néel vector. The curvature-induced DMI results in the hybridization of spin wave modes and enables a geometrically-driven local minimum of the low frequency branch. This positions curvilinear 1D antiferromagnets as a novel platform for the realization of geometrically tunable chiral antiferromagnets for antiferromagnetic spin-orbitronics and fundamental discoveries in the formation of coherent magnon condensates in the momentum space.

cond-mat.mes-hall↗

arXiv2020

The effect of curvature on the eigenexcitations of magnetic skyrmions

Spectrum of spin eigenmodes localized on a ferromagnetic skyrmion pinned by a geometrical defect (bump) of magnetic films is studied theoretically. By means of direct numerical solution of the corresponding eigenvalue problem and finite element micromagnetic simulations we demonstrate, that the curvature can induce localized modes with higher azimuthal and radial quantum numbers, which are absent for planar skyrmions (for the same parameters). The eigenfrequencies of all modes, except the breathing and gyromodes decreases with increasing curvature. Due to the translational symmetry break, the zero translational mode of the skyrmion gains a finite frequency and forms the gyromode, which describes the uniform rotation of skyrmions around the equilibrium position. In order to treat the gyromotion analytically we developed a Thiele-like collective variable approach. We show that Néel skyrmions in curvilinear films experience a driving force originating from the gradient of the mean curvature. The gyrofrequency of the pinned skyrmion is proportional to the second derivative of the mean curvature at the point of equilibrium.

cond-mat.str-el↗

arXiv2020

Domain wall diode based on functionally graded Dzyaloshinskii-Moriya interaction

We present a general approach for studying the dynamics of domain walls in biaxial ferromagnetic stripes with functionally graded Dzyaloshinskii-Moriya interaction (DMI). By engineering the spatial profile of the DMI parameter we propose the concept of a diode, which implements filtering of domain walls of certain topological charge and helicity. We base our study on phenomenological Landau-Lifshitz-Gilbert equations with additional Zhang-Li spin-transfer terms using a collective variable approach. In the effective equations of motion the gradients of DMI play the role of a driving force which competes with current driving. All analytical predictions are confirmed by numerical simulations.

cond-mat.str-el↗

arXiv2020

Unidirectional tilt of domain walls in equilibrium in biaxial stripes with Dzyaloshinskii-Moriya interaction

The orientation of a chiral magnetic domain wall in a racetrack determines its dynamical properties. In equilibrium, magnetic domain walls are expected to be oriented perpendicular to the stripe axis. We demonstrate the appearance of a unidirectional domain wall tilt in out-of-plane magnetized stripes with biaxial anisotropy and Dzyaloshinskii--Moriya interaction (DMI). The tilt is a result of the interplay between the in-plane easy-axis anisotropy and DMI. We show that the additional anisotropy and DMI prefer different domain wall structure: anisotropy links the magnetization azimuthal angle inside the domain wall with the anisotropy direction in contrast to DMI, which prefers the magnetization perpendicular to the domain wall plane. Their balance with the energy gain due to domain wall extension defines the equilibrium magnetization the domain wall tilting. We demonstrate that the Walker field and the corresponding Walker velocity of the domain wall can be enhanced in the system supporting tilted walls.

cond-mat.mes-hall↗

arXiv2019

Curvature induced magnonic crystal in nanowires

A new type of magnonic crystals, curvature induced ones, is realized in ferromagnetic nanowires with periodically deformed shape. A magnon band structure of such crystal is fully determined by its curvature: the developed theory is well confirmed by simulations. An application to nanoscale spintronic devises with the geometrically tunable parameters is proposed, namely, to filter elements.

cond-mat.mes-hall↗

arXiv2019

Spontaneous deformation of flexible ferromagnetic ribbons induced by Dzyaloshinskii-Moriya interaction

Here, we predict the effect of the spontaneous deformation of a flexible ferromagnetic ribbon induced by Dzyaloshinskii-Moriya interaction (DMI). The geometrical form of the deformation is determined both by the type of DMI and by the equilibrium magnetization of the stripe. We found three different geometrical phases, namely (i) the DNA-like deformation with the stripe central line in the form of a helix, (ii) the helicoid deformation with the straight central line and (iii) cylindrical deformation. In the main approximation the magnitude of the DMI-induced deformation is determined by the ratio of the DMI constant and the Young's modulus. It can be effectively controlled by the external magnetic field, what can be utilized for the nanorobotics applications. All analytical calculations are confirmed by numerical simulations.

cond-mat.str-el↗