Ferromagnetic Disks Research Articles

Remagnetization processes of uniaxial ferromagnetic films with spatially modified parameters

The study examines the behavior of vortex-like magnetic inhomogeneities that arise in a ferromagnetic disk with spatially modulated uniaxial anisotropy under magnetic fields of varying orientations. The research identifies the characteristic remagnetization stages of the vortex-like inhomogeneities formed in the region of the defect. critical fields of their rearrangement are found and an explanation is given for the difference in the behavior of these inhomogeneities in perpendicular and planar magnetic fields. The effect of the helicity of the magnetic skyrmion localized on the defect on its remagnetization process in the planar field is revealed.

Chirality Control of Magnetic Vortices in Ferromagnetic Disk–Nanowire System

The results of experimental studies and micromagnetic modeling of magnetic states in a one-dimensional array are presented. The array has the form of a chain of ferromagnetic disks coupled with a ferromagnetic nanowire made of the same material. The disks are located on opposite sides of the nanowire, which makes it possible to obtain distributions when the chiralities of the magnetic vortex shells in neighboring disks alternate, which can find application in vortex spin nanooscillators. Using the application in situ of a magnetic field to an excited objective lens using Lorentz transmission electron microscopy, it is shown that in this system it is possible to control the chiralities of the shells of magnetic vortices, which is realized by magnetization in the sample plane along various azimuthal directions. When wire magnetized along a nanowire, vortex states with opposite chiralities are realized in disks located on opposite sides of it. An antivortex is formed in the nanowire itself at the boundary with the disk, since the local direction of magnetization in the wire and in the disk are anticollinear. When magnetized perpendicular to the nanowire, states with the same chirality are realized in all disks. In this case, two perpendicular domain walls are formed between the disks in the nanowire, and the vortex in the disk is shifted to one of the edges along the nanowire.

A spin–rotation mechanism of Einstein–de Haas effect based on a ferromagnetic disk

A spin–rotation mechanism of Einstein–de Haas effect based on a ferromagnetic disk

Parallel pumping of magnons in inhomogeneous spin textures probed through NV spin relaxometry

We combine micromagnetic simulations and nitrogen-vacancy (NV) defect center spin relaxometry measurements to study magnon modes in inhomogeneous spin textures. A thin, micrometer-scale ferromagnetic disk is magnetized in a vortex state in which the magnetization curls around a central core. Micromagnetic simulations show that at zero applied field, the magnetization dynamics of the disk consist of a low frequency gyrotropic mode and higher frequency azimuthal magnon modes, all far detuned from the NV spin transition frequencies. An in-plane static magnetic field breaks the azimuthal symmetry of the vortex state, resulting in the magnon modes transforming in frequency and spatial profile as the field increases. Experimentally, we probe the dynamics of vortex magnetization as a function of applied in-plane static field and ac driving frequency by optically monitoring a nearby NV defect center spin. At certain values of the applied magnetic field, we observe enhanced spin relaxation when driving at twice the frequency of the NV ground state spin transition in optically detected magnetic resonance measurements. We attribute this effect to parallel pumping of a magnon mode in the disk producing magnons at half the excitation frequency. Micromagnetic simulations support this finding, showing spatial and spectral overlap of a confined magnon mode and an NV spin transition, with sufficient interaction strength to explain the observed signal.

Scanning Spin Probe Based on Magnonic Vortex Quantum Cavities.

Performing nanoscale scanning electron paramagnetic resonance (EPR) requires three essential ingredients: First, a static magnetic field together with field gradients to Zeeman split the electronic energy levels with spatial resolution; second, a radio frequency (rf) magnetic field capable of inducing spin transitions; finally, a sensitive detection method to quantify the energy absorbed by spins. This is usually achieved by combining externally applied magnetic fields with inductive coils or cavities, fluorescent defects, or scanning probes. Here, we theoretically propose the realization of an EPR scanning sensor merging all three characteristics into a single device: the vortex core stabilized in ferromagnetic thin-film discs. On one hand, the vortex ground state generates a significant static magnetic field and field gradients. On the other hand, the precessional motion of the vortex core around its equilibrium position produces a circularly polarized oscillating magnetic field, which is enough to produce spin transitions. Finally, the spin-magnon coupling broadens the vortex gyrotropic frequency, suggesting a direct measure of the presence of unpaired electrons. Moreover, the vortex core can be displaced by simply using external magnetic fields of a few mT, enabling EPR scanning microscopy with large spatial resolution. Our numerical simulations show that, by using low damping magnets, it is theoretically possible to detect single spins located on the disc's surface. Vortex nanocavities could also attain strong coupling to individual spin molecular qubits with potential applications to mediate qubit-qubit interactions or to implement qubit readout protocols.

Magnetic vortex states manipulation in overlapping ferromagnetic disks

We present the study of vortex magnetic states in a system of two overlapping ferromagnetic disks by Lorenz microscopy (L-TEM) and micromagnetic modeling. It is shown that in the case of small overlaps in this system there are two magnetic configurations Vortex–Antivortex–Vortex (VAV) state and Vortex–Vortex (VV) state. They are promising for phase locking in vortex spin-transfer nanooscillators. However, our experiments show that the nucleation of VV and VAV states is random and, as a result, the arrays consisting of overlapping disks are inhomogeneous. To control the nucleation of vortices with a certain vorticity, we changed the shape of overlapping disks by cutting them off from different edges. Disks of one type have been cut at the same edge (SD), while disks of another type have been cut at different edges (AD). In situ L-TEM experiments showed that after the sample magnetizing in the magnetic field applied in the plane of disks, the VV states have been realized in both SD and AD arrays. On the other hand, when samples were magnetized in a perpendicular magnetic field, the VAV state was realized in SD array, while the VV state was still realized in the AD array. The peculiarities of vortex nucleation in SD and AD in an external magnetic field are analyzed by means of micromagnetic simulations. The proposed method for controlling the configuration of magnetic vortices can be applied to create large arrays of overlapping disks that are in the same magnetic state.

Acoustic Waves Induced by Einstein-de Haas Effect in the Ultrafast Core Reversal of Magnetic Vortex.

The interaction between acoustic wave and magnetization in ferromagnetic thin films has attracted great attention due to its interesting physics and potential applications. However, up to now, the magneto-acoustic interaction has mainly been studied on the basis of magnetostriction. In this Letter, we develop a phase field model of magneto-acoustic interaction based on the Einstein-de Haas effect, and predict the acoustic wave during the ultrafast core reversal of magnetic vortex in a ferromagnetic disk. Because of the Einstein-de Haas effect, the ultrafast change of magnetization at the vortex core leads to a large mechanical angular momentum, which induces a body couple at the vortex core and excites a high-frequency acoustic wave. Moreover, the displacement amplitude of the acoustic wave is highly dependent on the gyromagnetic ratio. The smaller the gyromagnetic ratio is, the larger the displacement amplitude is. The present work not only provides a new mechanism for dynamic magnetoelastic coupling but also sheds new insights on the magneto-acoustic interaction.

Synchronization of the Self-Oscillations of the Magnetic Vortices in Exchange-Coupled Ferromagnetic Disks

The low-frequency (gyrotropic) self-oscillations of the magnetic vortices in interacting ferromagnetic disks, which are caused by a spin-polarized current, are studied by numerical simulation. Various magnetization oscillation modes depending on the configuration of the magnetic state of the system are considered. The influence of the pumping current nonuniformity on the phase difference of the vortex gyration in neighboring disks is investigated. The overlap of the disks is shown to increase the interaction between the vortices and, hence, to decrease the dephasing of the vortex core oscillations. The prospects of using overlapping disks to ensure phase synchronization of arrays of spin-transfer vortex oscillators are discussed.

Synchronization of the Self-Oscillations of the Magnetic Vortices in Exchange-Coupled Ferromagnetic Disks

Synchronization of the Self-Oscillations of the Magnetic Vortices in Exchange-Coupled Ferromagnetic Disks

Size driven barrier to chirality reversal in electric control of magnetic vortices in ferromagnetic nanodiscs.

New high density storage media and spintronic devices come about with a progressing demand for the miniaturization of ferromagnetic structures. Vortex ordering of magnetic dipoles in such structures has been repeatedly observed as a stable state, offering the possibility of chirality in these states as a means to store information at high density. Electric pulses and magnetoelectric coupling are attractive options to control the chirality of such states in a deterministic manner. Here, we demonstrate the chirality reversal of vortex states in ferromagnetic nanodiscs via pulsed electric fields using a micromagnetic approach and focus on the analysis of the energetics of the reversal process. A strong thickness dependence of the chirality reversal in the nanodiscs is found that emanates from the anisotropy of the demagnetizing fields. Our results indicate that chiral switching of the magnetic moments in thin discs can give rise to a transient vortex-antivortex lattice not observed in thicker discs. This difference in the chirality reversal mechanism emanates from profoundly different energy barriers to overcome in thin and thicker discs. We also report the polarity-chirality correlation of a vortex that appears to depend on the aspect ratio of the nanodiscs.

Gyrotropic Oscillations of Magnetic Vortices in Two Interacting Ferromagnetic Disks

Gyrotropic Oscillations of Magnetic Vortices in Two Interacting Ferromagnetic Disks

Improved durable vortex core MFM tip

• Vortex-Core probe offers long life time and stable magnetic moment. • Stability of vortex core in the disk increased for smaller diameter and larger disk thickness. • The stability of vortex core is improved by VC pinning at the disk center. • Asymmetry pinning center better pins VC than symmetrical one. • Probe with pinned VC gives 15 nm spatial resolution. Vortex-Core magnetic force microscopy (VC MFM) promises magnetic tip durability and long-term calibration of its magnetic moment that is crucial for quantitative scanning. It uses the vortex core of a ferromagnetic disk located at the tip apex as a magnetic probe. However, our experience showed that MFM images might be distorted when scanning magnetic objects larger than 100 nm due to a VC shift generated by the stray field of the scanned sample. We addressed the problem by changing the tip geometry and introducing artificial VC pinning at the disk center. These steps are essential for improving the VC tip for quantitative scanning. The scanning experiments with the optimized VC tip showed improved characteristics of the magnetic images – spatial resolution of the tip was on the level of commercial MFM tip with certain details depicted even better.

The emergence of k[formula omitted] skyrmions and their spin wave modes in a ferromagnetic disk

Using micromagnetic simulations, we show that varying the perpendicular anisotropy constant Kz and the Dzyaloshinskii–Moriya exchange constant Dint one can stabilize kπ skyrmions (k =1, 2, … 5) in a ferromagnetic nanodisk. We have studied their spin wave modes, and obtained the spatial profile of the radial and azimuthal modes, for different values of Dint. Hysteresis curves and the variation of the skyrmion topological charges were obtained as a function of the value of a planar applied magnetic field. The curves show how the application of the field induces the conversion between different kπ skyrmions.

Low-Frequency Spectrum of the Gyrotropic Modes of a Finite Chain of Interacting Ferromagnetic Disks

Low-Frequency Spectrum of the Gyrotropic Modes of a Finite Chain of Interacting Ferromagnetic Disks

Mutual Interaction between Superconductors and Ferromagnetic Skyrmionic Structures in Confined Geometries

The mutual interaction between a chiral ferromagnetic disk with confined skyrmionic structures and a superconducting disk is studied. The interaction is accounted for by solving simultaneously the Brown and London equations. We find that for a superconductor under an applied uniform field, the stray field of the shielding currents can trigger switching between different skyrmionic states in the ferromagnet that is not possible without the superconductor. We discuss the effect of the different parameters on the results and some possible applications such as multivalued logic, storage, and skyrmionic metamaterials.

Superconducting Triplet Rim Currents in a Spin-Textured Ferromagnetic Disk.

Since the discovery of the long-range superconducting proximity effect, the interaction between spin-triplet Cooper pairs and magnetic structures such as domain walls and vortices has been the subject of intense theoretical discussions, while the relevant experiments remain scarce. We have developed nanostructured Josephson junctions with highly controllable spin texture, based on a disk-shaped Nb/Co bilayer. Here, the vortex magnetization of Co and the Cooper pairs of Nb conspire to induce long-range triplet (LRT) superconductivity in the ferromagnet. Surprisingly, the LRT correlations emerge in highly localized (sub-80 nm) channels at the rim of the ferromagnet, despite its trivial band structure. We show that these robust rim currents arise from the magnetization texture acting as an effective spin–orbit coupling, which results in spin accumulation at the bilayer–vacuum boundary. Lastly, we demonstrate that by altering the spin texture of a single ferromagnet, both 0 and π channels can be realized in the same device.

Electromagnetic proximity effect in superconductor/ferromagnet bilayers with in-plane magnetic texture

The so-called electromagnetic proximity effect manifests itself as the long-range leakage of the magnetic field from a ferromagnet (F) to a superconductor (S), which is in electrical contact with the ferromagnet. In the present paper, we study the proximity-induced supercurrent and magnetic field in S/F bilayer systems with an in-plane inhomogeneous ferromagnet. We find that at the S/F interface a spontaneous surface current appears which is perpendicular to the local in-plane magnetization. In this sense, S/F bilayers with a metallic ferromagnet mimic similar systems with an insulating ferromagnet, where a surface current appears due to the interplay of the exchange field and Rashba spin-orbit coupling. We consider three geometrical configurations of the ferromagnet: a uniformly magnetized strip, an infinite slab with two domains and a Néel wall between them, and a ferromagnetic disk with a magnetization vortex. It is found that the proximity effect in the magnetic strip geometry induces a magnetic field outside the ferromagnet (which vanishes in the absence of proximity coupling between S and F), and in the other two geometries it enhances the magnetic field far from the domain wall or the core of the magnetic vortex. Possible experimental methods to probe the described features are discussed.

Stabilizations and field-driven core reversals of magnetic vortices in Fe3Sn2 disks

A magnetic vortex is a structure with a polarized core surrounded by an in-plane spin swirl and was first reported in easy-plane disks. Here, we explore the effects of the disk thickness, temperature, and out-of-plane field on the magnetic vortex stabilization and core reversals in Kagome uniaxial ferromagnetic Fe3Sn2 disks. Magnetic ground domains in Fe3Sn2 disks turn from stripes to vortices after decreasing the disk thickness or temperature as the easy-plane anisotropy wins the competition with the uniaxial magnetocrystalline anisotropy. Topological transformations from charge Q=−1/2 vortices at a zero field to Q=−1 skyrmion-like configurations at a high out-of-plane field were identified in cooperation with micromagnetic simulations. Thus, our results promote future vortex-based spintronic applications.

Power transmission between two rotating bodies using magnetorheological fluid magnetized by permanent magnets

Magnetorheological fluid (MRF) is functional fluid whose viscosity changes under magnetic field. It is used in applications such as dampers and brakes. In this research, permanent magnets are used to hold MRF between two rotating bodies. Traction force generated by MRF under magnetic field used to transmit power between the rotating bodies without vibration and noise. In this paper, we research the arrangement of permanent magnets and the configuration of magnetic circuits that hold MRF in the gap between rotating bodies and transmit rotational power without supplying additional MRF, and report on the improvement of power transmission efficiency. In this research, three models are assembled. Nonmagnetic rollers are used as the rotating body in Model 1, nonmagnetic disks are used in Model 2, and ferromagnetic disks are used in Model 3. The results showed that Model 3 was superior in both power transmission efficiency (74.6 [%]) and maximum transmission torque (0.55 [N∙m]). This is due to the fact that the use of a disk as the rotating body enables power transmission through the MRF over a wider area than with a roller type, and that a ferromagnetic disk can hold the MRF strongly.

Phase Synchronization of Auto-Oscillations of Magnetic Vortices Induced by Spin-Polarized Current in Exchange-Coupled Ferromagnetic Disks

Phase Synchronization of Auto-Oscillations of Magnetic Vortices Induced by Spin-Polarized Current in Exchange-Coupled Ferromagnetic Disks