Switching phenomena in magnetic vortex dynamics

Amagnetic nanoparticle in a vortex state is a promising candidate for the information storage. One bit of information corresponds to the upward or downward magnetization of the vortex core (vortex polarity). Generic properties of the vortex polarity switching are insensitive of the way how the vortex dynamics was excited: by an ac magnetic field, or by an electrical current. We study theoretically the switching process and describe in detail its mechanism, which involves the creation and annihilation of an intermediate vortex- antivortex pair

Vortex polarity switching in magnets with surface anisotropy

Vortex 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

Mesoscale Dzyaloshinskii-Moriya interaction: Geometrical tailoring of the magnetochirality

Crystals with broken inversion symmetry can host fundamentally appealing and technologically relevant periodical or localized chiral magnetic textures. The type of the texture as well as its magnetochiral properties are determined by the intrinsic Dzyaloshinskii-Moriya interaction (DMI), which is a material property and can hardly be changed. Here we put forth a method to create new artificial chiral nanoscale objects with tunable magnetochiral properties from standard magnetic materials by using geometrical manipulations. We introduce a mesoscale Dzyaloshinskii-Moriya interaction that combines the intrinsic spin-orbit and extrinsic curvature-driven DMI terms and depends both on the material and geometrical parameters. The vector of the mesoscale DMI determines magnetochiral properties of any curved magnetic system with broken inversion symmetry. The strength and orientation of this vector can be changed by properly choosing the geometry. For a specific example of nanosized magnetic helix, the same material system with different geometrical parameters can acquire one of three zero-temperature magnetic phases, namely, phase with a quasitangential magnetization state, phase with a periodical state and one intermediate phase with a periodical domain wall state. Our approach paves the way towards the realization of a new class of nanoscale spintronic and spinorbitronic devices with the geometrically tunable magnetochirality

Ферромагнетизм в графеновых и фуллереновых наноструктурах. Теория, моделирование, эксперимент

This work is devoted to the construction of the quantum field model, allowing, in particular, to describe ferromagnetic properties in graphen structures adequately to the results of physical and numerical experiments. The offered model describes properties of monoatom graphen layers ( forming two-dimensional surfaces), which are connected with presence of nontrivial function of distribution of the spin density, formed as a result of spontaneous breakdown of the spin symmetry of valent electrons in atoms of carbon. Within the limits of the offered model possible exact solutions for field function of the spin density, explaining, in particular, experimentally observed ferromagnetic properties of graphen films are specified. Quantitative estimations of a thickness of the domain wall, dividing areas with counterdirected vectors of magnetization were suggested, which allows to check up offered theoretical model experimentally.Работа посвящена построению квантовополевой модели, позволяющей, в частности, описывать ферромагнитные свойства в графеновых структурах адекватно имеющимся физическим и численным результатам. Предлагается модель, описывающая такие свойства графеновых моноатомных слоёв, образующих двумерные поверхности, которые связаны с наличием нетривиальной функции распределения спиновой плотности, образованной в результате спонтанного нарушения спиновой симметрии валентных электронов атомов углерода на указанных поверхностях. В рамках предлагаемой модели указываются возможные точные решения для функции спиновой плотности, объясняющие, в частности, экспериментально наблюдаемые ферромагнитные свойства графеновых плёнок. Делаются количественные оценки толщины доменной стенки, разделяющей области с разнонаправленной намагниченностью, позволяющие экспериментально проверить предлагаемую теоретическую модель

Equilibrium magnetic states in individual hemispherical permalloy caps

The magnetization distributions in individual soft magnetic permalloy caps on non magnetic spherical particles with sizes ranging from 50 to 800 nm are investigated. We experimentally visualize the magnetic structures at the resolution limit of the x ray magnetic circular dichroism photoelectron emission microscopy XMCD PEEM . By analyzing the so called tail contrast in XMCD PEEM, the spatial resolution is significantly enhanced, which allowed us to explore magnetic vortices and their displacement on curved surfaces. Furthermore, cap nanostructures are modeled as extruded hemispheres to determine theoretically the phase diagram of equilibrium magnetic states. The calculated phase diagram agrees well with the experimental observation

Localization of magnon modes in a curved magnetic nanowire

Spin waves in magnetic nanowires can be bound by a local bending of the wire. The eigenfrequency of a truly local magnon mode is determined by the curvature: a general analytical expression is established for any infinitesimally weak localized curvature of the wire. The interaction of the local mode with spin waves, propagating through the bend, results in scattering features, which is well confirmed by spin-lattice simulations

Magnetism in curved geometries

Extending planar two dimensional structures into the three dimensional space has become a general trend in multiple disciplines, including electronics, photonics, plasmonics and magnetics. This approach provides means to modify conventional or to launch novel functionalities by tailoring the geometry of an object, e.g. its local curvature. In a generic electronic system, curvature results in the appearance of scalar and vector geometric potentials inducing anisotropic and chiral effects. In the specific case of magnetism, even in the simplest case of a curved anisotropic Heisenberg magnet, the curvilinear geometry manifests two exchange driven interactions, namely effective anisotropy and antisymmetric exchange, i.e. Dzyaloshinskii Moriya like interaction. As a consequence, a family of novel curvaturedriven effects emerges, which includes magnetochiral effects and topologically induced magnetization patterning, resulting in theoretically predicted unlimited domain wall velocities, chirality symmetry breaking and Cherenkov like effects for magnons. The broad range of altered physical properties makes these curved architectures appealing in view of fundamental research on e.g. skyrmionic systems, magnonic crystals or exotic spin configurations. In addition to these rich physics, the application potential of three dimensionally shaped objects is currently being explored as magnetic field sensorics for magnetofluidic applications, spin wave filters, advanced magneto encephalography devices for diagnosis of epilepsy or for energy efficient racetrack memory devices. These recent developments ranging from theoretical predictions over fabrication of three dimensionally curved magnetic thin films, hollow cylinders or wires, to their characterization using integral means as well as the development of advanced tomography approaches are in the focus of this revie

Magnetically Capped Rolled up Nanomembranes

Modifying the curvature in magnetic nanostructures is a novel and elegant way toward tailoring physical phenomena at the nanoscale, allowing one to overcome limitations apparent in planar counterparts. Here, we address curvature driven changes of static magnetic properties in cylindrically curved magnetic segments with different radii of curvature. The curved architectures are prepared by capping nonmagnetic micrometer and nanometer sized rolled up membranes with a soft magnetic 20 nm thick permalloy Ni80Fe20 film. A quantitative comparison between the magnetization reversal processes in caps with different diameters is given. The phase diagrams of magnetic equilibrium domain patterns diameter versus length are generated. For this, joint experimental, including X ray magnetic circular dichroism photoelectron emission microscopy XMCD PEEM , and theoretical studies are carried out. The anisotropic magnetostatic interaction in cylindrically curved architectures originating from the thickness gradient reduces substantially the magnetostatic interaction between closely packed curved nanowires. This feature is beneficial for racetrack memory devices, since a much higher areal density might be achieved than possible with planar counterpart