Magnetic Nanoelements Research Articles

Curvature-induced enhancement of thermal stability of skyrmions

Geometry plays an important role in the nucleation, stabilization, and manipulation of magnetization patterns within magnetic nanoelements. This work analyzes the impact of curvature on the thermal stability of skyrmions hosted on Gaussian-shaped nanoshells. Based on annihilation processes observed in flat nanoparticles, three distinct annihilation processes—skyrmion contraction, expansion, and displacement toward the nanodot border—are analyzed. We show that curvature-induced effective interactions significantly alter the energy barriers associated with these annihilation processes. The changes in energy are related to the relative alignment between the skyrmion core and the direction normal to the surface, highlighting the presence of favorable and unfavorable chiralities for skyrmion stabilization in curved nanodots. We also show that, unlike the one obtained for flat nanodots, where the skyrmion lifetimes typically span seconds, the lowest energy barrier values in curved nanodots reach values that ensure skyrmion lifetimes at room temperature to months before thermal fluctuations annihilate them. Curvature parameters can control the annihilation mechanism. This enhancement in skyrmion stability holds even without external additional stimuli. This underscores the profound impact of curvature on the dynamic behavior and thermal stability of skyrmions within magnetic nanoelements.

Topological Memory with Multiply-Connected Planar Magnetic Nanoelements

A coding scheme is introduced to store a set of linked bit strings in planar magnetic nanoelements with holes. Analytical expressions for the corresponding magnetization distributions are developed up to a homotopy and the specific examples are given for doubly- and triply-connected cases. The energy barriers, protecting the information-bearing states, are discussed. Compared to a set of disparate simply-connected nanoelements of the same total connectivity, the nanoelements with holes can hold much more information due to the possibility of linking the individual bits.

Switching field distribution of ultradense arrays of single-crystalline magnetic nanowires

Ultradense arrays of magnetic nanoelements present considerable interest for extending areal densities in magnetic recording media, provided that they display high switching fields and corresponding low standard deviations. Here, we report the switching field distribution of bottom–up synthesized single-crystalline vertical Co nanowires self-organized in 2D hexagonal superlattices. The combined shape and Co hexagonal compact magnetocrystalline anisotropies in individual nanowires of diameter as small as 6 nm define a robust perpendicular magnetic anisotropy despite important interactions in superlattices of 10 × 1012 NWs/in2. Using quantitative analysis of temperature-dependent first-order reversal curves, we capture the switching field distribution in this dipolar-coupled perpendicularly magnetized nanomagnets. First, the interwire dipolar interactions are treated separately and show a dominant mean field character with temperature independent amplitudes that scale with the nanowire packing fraction. Then, the intrinsic switching field distribution, namely, independent of interwire interactions, is determined as a function of temperature in the 5–300 K range. The mean value and deviation are both found to be driven by the intrawire dipolar interaction and the temperature-dependent uniaxial magnetocrystalline anisotropy, but of smaller amplitudes than those expected from bulk behavior. With coercive fields ranging between 0.3 and 0.8 T, the switching field deviations relative to coercivity reach 20%, which is a moderate value regarding pitch arrays as small as 8 nm.

Creation of smart compression garment using magnetic nanotextiles

PurposeThis paper aims to prove the expediency and effectiveness of magnetic textiles use obtained by adding nanopowder synthesized on the basis of oxides of divalent and trivalent iron oxides, taking into account bacteriostatic, magnetotherapeutic and compressive properties.Design/methodology/approachThe research includes methods of synthesis of nanoelements of bivalent and trivalent iron, methods of the theory of elasticity for determining the pressure between compression clothing and a limb, methods of creating an annular magnetic field with determination of its voltage, methods of determining the growth dynamics of mold bacteria and methods of approximation of experimental data.FindingsOn the base of the determination of the forces arising from the interaction of magnetic nanotextiles with a magnetic field, the expediency of using these materials in the creation of compression clothing has been proven. An additional medical value of magnetic textiles is the bacteriostatic effect. The content of magnetic nanoelements in the textile composition of 0.2% almost completely suppresses mold infectionsResearch limitations/implicationsCotton samples with the addition of nanocomponents based on ferric and ferric oxides were studied.Practical implicationsMagnetotextile materials can be used in magnetotherapy, compression clothing, in textile products that provide bacteriostatic properties.Originality/valueThe use of magnetic textile materials is a perspective direction for the creation of medical textile products with complex properties.

Merging of spin-wave modes in obliquely magnetized circular nanodots

Magnetic nanoelements attract great interest due to their prospects for data storage and signal processing. Spin-wave confinement in these elements implies wave-number quantization, discrete frequency spectra and thus complex resonance patterns, strongly dependent on the elements' geometry and static magnetic configuration. Here we report experimental observation of unconventional single-frequency resonance response of flat circular Permalloy nanodots, which is achieved via the application of a magnetic field at a certain critical angle ${\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\theta}}}_{B}$ with respect to the dot normal. This observation is explained as the merging of spin-wave eigenmodes under the transition of the spin-wave dispersion from the forward-volume to the backward-volume type, as elucidated by micromagnetic simulations in conjunction with an analytical theory. Our results offer a way for the creation of spin-wave systems with spectrally narrow magnetic noise.

Engineering spin wave spectra in thick Ni80Fe20 rings by using competition between exchange and dipolar fields

Control of the spin wave dynamics in nanomagnetic elements is very important for the realization of a broad range of novel magnonic devices. Here we study experimentally the spin wave resonance in thick ferromagnetic rings (100 nm) using perpendicular ferromagnetic resonance spectroscopy. Different from what was observed for the continuous film of the same thickness, or from rings with similar lateral dimensions but with lower thicknesses, the spectra of thick patterned rings show a nonmonotonic dependence of the mode intensity on the resonance field for a fixed frequency. To explain this effect, the theoretical approach by considering the dependence of the mode profiles on both the radial and axial coordinates was developed. It was demonstrated that such unusual behavior is a result of the competition between exchange and dipolar fields acting at the spin excitations in the structure under study. The calculations are in a good agreement with the experimental results.

Magnonic bands in periodic arrays of vertically-stacked cylindrical magnetic nanoelements

The spin-wave spectra are studied theoretically in periodic magnonic crystals (MCs) of vertically-stacked cylindrical nickel nanodisks or nanorings separated by nonmagnetic spacers, resulting in infinitely long segmented nanowire or nanotube MC arrays, respectively. A microscopic or Hamiltonian-based approach is employed for the magnetization dynamics, in which an external magnetic field is applied along the cylinder axis and the periodic length is varied by changing the length of the spacers. The results show a broadening and shift of the spin-wave frequency bands at smaller spacer length due to the strong dipolar inter-element coupling, which also modifies the spatially-inhomogeneous equilibrium magnetization in the finite-length cylindrical elements. The effects are found to be more pronounced in a nanotube MC than in a nanowire MC, mainly because the absence of the core in a nanotube favours vortex states.

Influence of the Dzyaloshinskii – Moriya interaction on the properties of magnetic states in nanostructures

Dzyaloshinskii – Moriya interaction (DMI) plays an important role in the formation of chiral spin textures useful in spintronics. Interfacial DMI is of great interest, as was recently shown, it leads to the stabilization of chiral spin textures not only in ferromagnetic metals, but also in insulator systems such as rare-earth iron garnets. In this paper, we explore transformations of uniform magnetic states induced by the DMI in ferrimagnetic system of restricted geometry. We show that change of the DMI constant induces a series of phase transitions in the square magnetic nanoelement: homogeneous magnetic state – vortex – skyrmion/meron – multidomains – domain structure (K>/<0) and construct phase diagrams in terms of “DMI constant – nanoelement size”. Our findings show that manifestation of DMI in a system of restricted geometry depends on the initial magnetic configuration stabilized by competition between exchange and relativistic interactions. The transformations of the “out – of - plane” and “in-plane” single-domain magnetic states induced by the DMI are considered, and their distinctive features are revealed. The influence of an electric field acting through inhomogeneous magnetoelectric interaction on localized magnetic states is discussed.

Effect of random anisotropy in stabilization of skyrmions and antiskyrmions

Ever increasing demand of skyrmion manipulation in nanodevices has brought up interesting research to understand the stabilization of these topologically protected chiral structures. To understand the actual shape and size of skyrmions and antiskyrmions observed experimentally, we have performed micromagnetic simulations to investigate their stabilization in presence of random anisotropy in magnetic thin film system. Previous experimental reports of skyrmion imaging in thin films depicts that the skyrmion shape is not perfectly circular. Here we show via simulations that the shape of a skyrmion and an antiskyrmion can get distorted due to the presence of different local anisotropy energy. The values of uniaxial anisotropy constant (Ku) and random aniostropy constant (Kr) are varied to understand the change in shape and size of a skyrmion and an antiskyrmion stabilized in a square magnetic nanoelement. The skyrmion or antiskyrmion shape gets distorted with the presence of random anisotropy energy in the system.

Skyrmion racetrack memory with an antidot

Skyrmion racetrack memory has a lot of potential in future non-volatile solid state devices. By application of current in such devices, both spin-orbit torque and spin-transfer torques are proven to be useful to nucleate and propagate skyrmions. However, the current applied during nucleation of successive skyrmions may have unwanted perturbation viz. Joule heating and the skyrmion Hall effect, on the propagation of previously generated skyrmions. Therefore, new methodology is desired to decouple the generation and propagation of skyrmions. Here, we present a novel route via micromagnetic simulations for generation of skyrmions from triangular antidot structure in a ferromagnetic nanotrack using local Oersted field. Antidots are holes in a magnetic nanoelement. Multiple skyrmions can be simultaneously generated by incorporating a greater number of antidots. Controlled skyrmion injection can be achieved by tuning the separation between the antidots that are placed at either end of the nanotrack. Here, we propose a novel design to realise skyrmionic racetrcak memory, where one can individually generate and manipulate the skyrmions within the nanotrack.

Spin–Orbit Torque and Dipole Coupling for Nanomagnetic Array Programmability

Computational architectures that rely on an array of dipole-coupled nanomagnetic elements require an energy-efficient method of programming individual elements within the array. As a low-energy, selective method of controlling magnetization, spin-orbit torque (SOT) represents a promising solution. Here, a finite-difference micromagnetic model is used to characterize the dipole coupling between adjacent CoFeB nanodisks and to determine the critical SOT current required to switch these disks. Additionally, a phase plot showing disk dimensions at which both vortex and single-domain in-plane magnetic states are stable is produced. A dipole-coupled array's response to dynamic application of SOT current is also simulated. The results show that the rate of applying SOT current to one element in the array strongly influences the stable states of adjacent elements and that the SOT current amplitude required for this influence is an order of magnitude lower than the previously determined critical switching current. This indicates that SOT current dynamics play a significant role in the behavior of a dipole-coupled array. Finally, an architecture to achieve programmability in nanomagnetic computational platforms with SOT is presented.

Micro-magnetic simulations of correlated switching in touching circular nano-magnetic elements

Past studies have demonstrated that logic states can be represented using the rotation of the magnetization at flux-closed remanence in individual ferromagnetic disks. In this work, we present the results of micro-magnetic simulations of touching circular elements that can be used for room operable magnetic quantum cellular automata. Like gears in a mechanical system, the chirality of the magnetization alternates from element to element, as determined through interaction with neighbors. The switching of touching symmetric elements occurred when the applied field was removed, meaning minimal energy loss during the process. Maintaining coherence of opposite chirality in chains of elements could be achieved with the introduction of a biasing element to eliminate the bidirectionality of interaction.

Magnetic Tunability of Permalloy Artificial Spin Ice Structures

This paper reports tunable ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ artificial spin ice structures of various geometrical lattice arrangements as a function of film thickness. We achieve the magnetic tunability by three distinct methods namely, geometrical arrangements of nanomagnetic elements in the form of square, kagome, and triangular lattices with the variation in film thickness (20, 30, and 50 nm) for each geometry and the applied field orientations. Magnetic force microscopy reveals that the nanoelements are in single-domain states, obeying the spin ice rules for the 20-nm-thick spin ice structures. A combination of nanoelements in single-domain and vortex states is observed with the increase in thickness up to 30 nm. For the 50-nm-thick elements, vortex and flux closure states are in evidence. Broadband ferromagnetic resonance spectroscopy establishes the presence of distinct resonant modes that are spatially localized in the nanomagnets of different orientations and, hence, can be controlled by the applied field orientations. The role of shape anisotropy on the static and dynamic properties is investigated systematically and complemented by extensive micromagnetic simulations. The results show great potential towards designing reconfigurable magnonic crystals for microwave filter applications.

Pushing down the lateral dimension of single and coupled magnetic dots to the nanometric scale: Characteristics and evolution of the spin-wave eigenmodes

Planar magnetic nanoelements, either single- or multi-layered, are exploited in a variety of current or forthcoming spintronic and/or ICT devices, such as read heads, magnetic memory cells, spin-torque nano-oscillators, nanomagnetic logic circuits, magnonic crystals and artificial spin-ices. The lateral dimensions of the elemental magnetic components have been squeezed down during the last decade to a few tens of nanometers, but they are still an order of magnitude larger that the exchange correlation length of the constituent materials. This means that the spectrum of spin-wave eigenmodes, occurring in the GHz range, is relatively complex and cannot be described within a simple macrospin approximation. On the other hand, a detailed knowledge of the dynamical spectrum is needed to understand or to predict crucial characteristics of the devices. With this focused review we aim at the analysis and the rationalization of the characteristics of the eigenmodes spectrum of magnetic nanodots, paying special attention to the following key points: (i) Consider and compare the case of in-plane and out of-plane orientation of the magnetization, as well as of single- and multi-layered dots, putting in evidence similarities and diversities, and proposing a unifying nomenclature and labelling scheme; (ii) Underline the evolution of the spectrum when the lateral size of magnetic dots is squeezed down from hundreds to tens of nanometers, as in current devices, with emphasis given to the occurrence of soft modes and to the change of spatial localization of the fundamental mode for in-plane magnetized dots; (iii) Extend the analysis from isolated elements to twins of dots, as well as to dense arrays of dipolarly interacting dots, showing how the discretized eigenmodes distinctive of the single element transform in finite-width frequency bands of spin waves propagating through the array.

Micromagnetic simulations study of skyrmions in magnetic FePt nanoelements

The magnetization reversal in 330 nm triangular prismatic magnetic nanoelements with variable magnetocrystalline anisotropy similar to that of partially chemically ordered FePt is studied using micromagnetic simulations employing Finite Element discretizations. Several magnetic properties including the evaluation of the magnetic skyrmion number $S$ are computed in order to characterize magnetic configurations exhibiting vortex-like formations. Magnetic vortices and skyrmions are revealed in different systems generated by the variation of the magnitude and relative orientation of the magnetocrystalline anisotropy direction, with respect to the normal to the triangular prism base. Micromagnetic configurations with skyrmion number greater than one have been detected for the case where magnetocrystalline anisotropy was normal to nanoelement's base. For particular magnetocrystalline anisotropy values three distinct skyrmions are formed and persist for a range of external fields. The simulation-based calculations of the skyrmion number S revealed that skyrmions can be created for magnetic nanoparticle systems lacking of chiral interactions such as Dzyaloshinsky-Moriya, but by only varying the magnetocrystalline anisotropy.

Tunable ground state in heterostructured artificial spin ice with exchange bias

We describe an artificial spin ice (ASI) composed of exchange biased heterostructured nanomagnetic elements with unidirectional anisotropy and compare it with a conventional ASI constituted by ferromagnets with uniaxial anisotropy (Ising spins). The introduction of a local exchange bias field, aligned along one of the sublattices of the square ASI, lifts the spin-reversal symmetry of the vertices. By varying the lattice constant of the square array, we control the ratio of exchange bias (EB) to dipolar field (${H}_{\mathrm{EB}}/{H}_{\mathrm{dip}}$) and tune the ground state from an antiferromagnetic to a ferromagnetic configuration with an effective magnetic moment. The geometric frustration of dipolar interactions is moderated by a nonfrustrated local field, leading to a mesoscopic system with specific metastable states observed during the demagnetization process.

Magnetic skyrmions in FePt nanoparticles having Reuleaux 3D geometry: a micromagnetic simulation study.

The magnetization reversal in magnetic FePt nanoelements having Reuleaux 3D geometry is studied using micromagnetic simulations employing Finite Element discretizations. Magnetic skyrmions are revealed in different systems generated by the variation of the magnitude of the magnetocrystalline anisotropy which was kept normal to the nanoelement's base and parallel to the applied external field. The topological quantity of skyrmion number is computed in order to characterize micromagnetic configurations exhibiting skyrmionic formations. Micromagnetic configurations with a wide range of skyrmion numbers between -3 and 3 are indicative for the existence of one or multiple skyrmions that have been detected and stabilized in a range of external fields. Internal magnetic structures are shown consisting of Bloch type skyrmionic entities in the bulk altered to Néel skyrmions on the nanoelement's bottom and top base surfaces. The actual sizes of the formed skyrmions and the internal magnetization structures were computed. In particular, the sizes of the generated and persistent skyrmions were calculated as functions of the magnetocrystalline anisotropy value and of the applied external magnetic field. It is shown that the size of skyrmions is linearly dependent on the external field value. The slope of the linear curve can be controlled by the magnetocrystalline anisotropy value. The magnetic skyrmions can be created for FePt magnetic systems lacking of chiral interactions by designing the geometry-shape of the nanoparticle and by controlling the value of magnetocrystalline anisotropy.

Subnanosecond magnetization dynamics driven by strain waves

Abstract

The microstructural evolution of chemical disorder and ferromagnetism in He+ irradiated FePt3 films

This paper investigates the role of ion-induced disorder on the morphology and magnetic properties of chemically ordered FePt3 films. The effects are studied for 15 keV He+ ions as a function of the ion fluence for 0, 2 × 1016 and 2 × 1017 ions cm−2. Substitutional mixing of the L12-type Fe-Pt sites takes place within the region of the chemically ordered FePt3 film affected by the irradiation. This accompanies a paramagnetic-to-ferromagnetic transition, as determined by room-temperature magnetometry. Dark-field transmission electron microscopy (TEM) measurements confirm that the 15 keV He+ ions induce a 120 nm-thick chemically disordered layer into the sub-surface region of the nominally 280 nm-thick ordered FePt3 film. The average domain size and the fractional density of the chemically ordered domains within the irradiated FePt3 microstructure are found to mutually decrease with increasing ion fluence. Selected-area electron diffraction results demonstrate that the film’s single crystallinity is preserved after irradiation, irrespective of the ion fluence. High-resolution TEM elucidates the coexistence of ordered domains and precipitate disordered domains in the near-surface, low-ion impacted regions of the FePt3 film. Collectively, this work provides detailed insights into the material-science relationship between ion-induced disorder and ferromagnetism in FePt3, as a step towards creating fully customisable, ion-beam-synthesised magnetic nano-elements.

Apparent ferromagnetism in the pinwheel artificial spin ice

Magnetic artificial spin ice provides examples of how competing interactions between magnetic nanoelements can lead to a range of fascinating and unusual phenomena. We examine theoretically a class of spin ice tilings, called the pinwheel, for which near degeneracy of the spin configuration energies can be achieved. Pinwheel tiling is a simple but crucial variant on the square ice geometry, in which each nanoelement of square ice is rotated some angle about its midpoint. Surprisingly, this rotation leads to an intriguing phase transition; and even though the spins are not parallel to one another, a ferromagnetic phase is found for rotation angles near ${45}^{\ensuremath{\circ}}$. Here, magnetic domains and domain walls are found when viewed in terms of net magnetization. Moreover, the ferromagnetic behavior of the system depends on its anisotropy which we can control by array shape and size.