Core Reversal Research Articles

Core Reversal in Vertically Coupled Vortices: Simulation and Experimental Study

Core Reversal in Vertically Coupled Vortices: Simulation and Experimental Study

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.

Influence of Dzyaloshinskii–Moriya interaction on the magnetic vortex reversal in an off-centered nanocontact geometry

The influence of Dzyaloshinskii–Moriya interaction (DMI) on the vortex reversal driven by an out-of-plane spin-polarized current in an off-centered nanocontact structure is investigated. The simulation results show that DMI plays a vital role in vortex core reversal, including reversal current density, reversal velocity and reversal time. Under the influence of DMI, magnetic vortices still reverse polarity through the nucleation and annihilation of vortex and anti-vortex, with some peculiar characteristics. These results open up new possibilities for the application of magnetic vortex-based spin-transfer encryption nano-storage.

Observation of Defect-Assisted Magnetic Vortex Core Reversal at Ultralow Critical Velocity

The stability of structures with nontrivial topology is a subject of great interest, both from fundamental and technological perspectives. The topology of a magnetic vortex, for example, results in high stability of the vortex core polarity, which is attractive for applications. This high stability, however, greatly impedes the required deterministic polarity switching, which is typically only achieved with large magnetic fields or strong dynamic driving. Here, we show that the interaction between the vortex core and manufactured nanoscale defects in the magnetic material lowers the required driving strength for core polarity reversal by more than an order of magnitude. We excite vortex dynamics in thin permalloy disks, and map the two-dimensional (2D) vortex core trajectory using 3D time-resolved Kerr microscopy. In pristine samples, we observe normal gyrotropic motion of the vortex core. After laser-induced generation of defects, however, we observe repeated vortex core reversal at much-reduced driving strength. Micromagnetic simulations reveal how local reduction of exchange coupling and saturation magnetization can create vortex core reversal sites for deterministic vortex core switching.

Vortex core reversal by elastic waves in ferromagnetic materials

Magnetic vortex has attracted increasing attention due to its potential application in magnetic logic and memory devices. With the development of straintronics, acoustic waves could remotely control the topologically nontrivial magnetic bits such as magnetic vortices. Understanding the dynamic response of magnetic vortex subjected to elastic waves is crucial for the future logic and memory devices. In the present work, a dynamic phase field model is developed to investigate the dynamic magnetoelastic coupling behavior between elastic waves and magnetic vortices. Phase field simulations show that the magnetic vortex core can flip over under the elastic waves with gigahertz frequency. Compared with the results of conventional phase field model without the elastodynamic term, it is found that the strain gradient caused by elastic wave plays an important role in the reversal of vortex core. When the strain gradient in the ferromagnetic material reaches a certain value, the vortex core is likely to flip over. The elastic waves with larger magnitudes and frequencies have the larger strain gradient, which are more easily to cause the reversal of vortex core. In addition to the strain gradient, magnetic damping constant also has great influence on the vortex core reversal by changing the precession process of magnetization. Larger magnetic damping constant makes the smaller precession of magnetization, which retards the reversal of vortex core. The results of this study suggest a mechanical way for the remote control of magnetic vortices by elastic waves.

Strain-driven radial vortex core reversal in geometric confined multiferroic heterostructures

The magnetic radial vortex is a nanoscale magnetization configuration that is typically stabilized by the interfacial Dzyaloshinskii–Moriya interaction (i-DMI). The existing control methods for the radial vortex core polarity rely on the use of current flow or magnetic fields, which may cause long consumption times or limit device miniaturization. Here, we investigate a repeated reversal of a radial vortex that can be driven by strain from a piezoelectric substrate using micromagnetic simulations. A phase diagram for the representative regions against perpendicular anisotropy, i-DMI, and the applied strain was obtained. The derived phase diagram was used to associate the mechanism of the core reversal with edge magnetization rotation during core magnetization switching, which exhibits a relationship by transforming a quasi-Bloch wall into a Néel wall. The existence of the i-DMI effect causes the core polarity and radial chirality of the radial vortex to be reversed simultaneously without resulting in larger core movements. These results offer an alternative and efficient way to achieve core reversal, which is expected to stimulate the radial vortex application in magnetoresistive memory devices.

Analytical model of the deformation-induced inertial dynamics of a magnetic vortex

We present an analytical model to account for the deformation-induced inertial dynamics of a magnetic vortex. The model is based on a deformation of the vortex core profile based on the Döring kinetic field, whereby the deformation amplitudes are promoted to dynamical variables in a collective-coordinate approach that provides a natural extension to the Thiele model. This extended model describes complex transients due to inertial effects and the variation of the effective mass with velocity. The model also provides a quantitative description of the inertial dynamics leading up to vortex core reversal, which is analogous to the Walker transition in domain wall dynamics. Our work paves the way for a standard prescription for describing the inertial effects of topological magnetic solitons.

Modulation and phase-locking in nanocontact vortex oscillators

We have conducted experiments to probe how the dynamics of nanocontact vortex oscillators can be modulated by an external signal. We explore the phase-locking properties in both the commensurate and chaotic regimes, where chaos appears to impede phase-locking while a more standard behavior is seen in the commensurate phase. These different regimes correspond to how the periodicity of the vortex core reversal relates to the frequency of core gyration around the nanocontact; a commensurate phase appears when the reversal rate is an integer fraction of the gyration frequency, while a chaotic state appears when this ratio is irrational. External modulation where the power spectral density exhibits rich features, appears due to the modulation between the external source frequency, gyration frequency, and core reversal frequency. We explain these features with first- or second-order modulation between the three frequencies. Phase-locking is also visible between the external source frequency and internal vortex modes (gyration and core reversal modes).

Rapid Apparent Diffusion Coefficient Evolution After Early Revascularization.

Background and Purpose- In this era of endovascular therapy (EVT) with early, complete recanalization and reperfusion, we have observed an even more rapid apparent diffusion coefficient (ADC) normalization within the acute ischemic lesion compared with the natural history or IV-tPA-treated patient. In this study, we aimed to evaluate the effect of revascularization on ADC evolution within the core lesion in the first 24 hours in acute ischemic stroke patients. Methods- This retrospective study included anterior circulation acute ischemic stroke patients treated with EVT with or without intravenous tPA (IVT) from 2015 to 2017 compared with a consecutive cohort of IVT-only patients treated before 2015. Diffusion-weighted imaging and ADC maps were used to quantify baseline core lesions. Median ADC value change and core reversal were determined at 24 hours. Diffusion-weighted imaging lesion growth was measured at 24 hours and 5 days. Good clinical outcome was defined as modified Rankin Scale score of 0 to 2 at 90 days. Results- Twenty-five patients (50%) received IVT while the other 25 patients received EVT (50%) with or without IVT. Between these patient groups, there were no differences in age, sex, baseline National Institutes of Health Stroke Scale, interhospital transfer, or IVT rates. Thirty-two patients (64%) revascularized with 69% receiving EVT. There was a significant increase in median ADC value of the core lesion at 24 hours in patients who revascularized compared with further ADC reduction in nonrevascularization patients. Revascularization patients had a significantly higher rate of good clinical outcome at 90 days, 63% versus 9% (P=0.003). Core reversal at 24 hours was significantly higher in revascularization patients, 69% versus 22% (P=0.002). Conclusions- ADC evolution in acute ischemic stroke patients with early, complete revascularization, now more commonly seen with EVT, is strikingly different from our historical understanding. The early ADC normalization we have observed in this setting may include a component of secondary injury and serve as a potential imaging biomarker for the development of future adjunctive therapies. Clinical Trial Registration- URL: https://www.clinicaltrials.gov. Unique identifier: NCT00009243.

Radial vortex core reversal driven by a spin-polarized current in a nanocontact structure

In this paper, we propose a reversal mechanism of magnetic radial vortex core in a Permalloy nanodisk driven by a spin-polarized current injected through a nanocontact. The micromagnetic simulation results indicate that the reversal process of radial vortex core is through the rotation and fusion of an edge soliton pair. Due to the effect of interfacial Dzyaloshinskii–Moriya interaction, the radial chirality and polarity of radial vortex are reversed simultaneously. The transformation is uncovered by detailed studies of magnetization configuration, skyrmion number, energy and effective field. This reversal mechanism of radial vortex differs entirely from the previous mechanisms reported in the literatures. This work not only provides deeper insights into the dynamic properties of magnetic radial vortices, but also has great potential for developing new kinds of spintronic devices using the radial vortices.

Monte Carlo study of the magnetization reversal times in a core/shell magnetic nanoparticle

In this work, we present an investigation of the magnetization reversal times for a ferromagnetic/antiferromagnetic core/shell nanoparticle, by means of Monte Carlo simulations. The magnetic behavior of the nanoparticle was evaluated with a classical Heisenberg Hamiltonian, which includes terms for the exchange interaction, external magnetic field interaction and uniaxial anisotropy. We determined the magnetization reversal times for the core and core interface, when the system is initially ordered and it is subjected to a negative magnetic field. We computed the influence of the core/shell exchange interaction, as well as the core anisotropy, core size and temperature dependence on the reversal times. It was found that there exists a strong influence of core/shell exchange interaction on the core sites reversal time. These parameters influence the reversal time in different ways. In general, the increment of the temperature and the core/shell exchange interaction produce a faster magnetization reversal. Moreover, the core interface reverses faster than the core bulk. On the other hand, the reversal time depends on the anisotropy and the uncompensated sites in the shell interface. In this way, it is possible to control the core reversal time in these kind of nanoparticles by means of the core/shell interface.

Unusual spin-wave dynamics in core-shell magnetic nanodisks

We investigated the spin dynamics of a vortex state in a core-shell magnetic nanodisk driven by an oscillating field applied perpendicular to the disk plane by means of micromagnetic simulations. The nanodisk comprises a Py (Fe0.2Ni0.8) core of 100 nm in radius, surrounded by a 50 nm thick Fe shell. Fourier transform analyses show that the Py core and the Fe shell dominate spin-wave oscillation at the fundamental and higher order radial modes, respectively. For oscillating driving field tuned to the fundamental eigenfrequency, the Py/Fe interface effectively confines spin-wave excitation in the Py core region. This effect leads to significantly more rapid vortex core (VC) reversal in comparison to homogeneous disks. Our work demonstrates that the higher order modes can drive much faster VC reversal than the fundamental mode, in sharp contrast to the results obtained in homogeneous disks. With excitation levels up to 30 mT, we find strong nonlinear spin-wave dynamics in the system, which results in mode frequency redshifting, therefore the observation of the most rapid VC reversals below eigenfrequencies and VC switching in wide ranges of frequencies.

Magnetization dynamics induced by Rashba effect in a Permalloy nanodisk

Magnetic vortex dynamics mediated by spin-polarized ac current of different amplitudes and frequencies are investigated by micromagnetic simulations in a system lacking structure inversion symmetry. Micromagnetic calculations reveal that the critical current density required to induce vortex core reversal may be decreased to below 1010Am−2 due to strong transverse magnetic field by Rashba effect. We also find the spin torque of ac current plays a trivial role in magnetic vortex dynamics in a broken inversion symmetry system when the current density is on the order of 1010Am−2 and the current with frequency close to the vortex eigenfrequency is the most efficient for reversal.

Voltage controlled core reversal of fixed magnetic skyrmions without a magnetic field.

Using micromagnetic simulations we demonstrate core reversal of a fixed magnetic skyrmion by modulating the perpendicular magnetic anisotropy of a nanomagnet with an electric field. We can switch reversibly between two skyrmion states and two ferromagnetic states, i.e. skyrmion states with the magnetization of the core pointing down/up and periphery pointing up/down, and ferromagnetic states with magnetization pointing up/down, by sequential increase and decrease of the perpendicular magnetic anisotropy. The switching between these states is explained by the fact that the spin texture corresponding to each of these stable states minimizes the sum of the magnetic anisotropy, demagnetization, Dzyaloshinskii-Moriya interaction (DMI) and exchange energies. This could lead to the possibility of energy efficient nanomagnetic memory and logic devices implemented with fixed skyrmions without using a magnetic field and without moving skyrmions with a current.

Topological textures and their bifurcation processes in 2D ferromagnetic thin films

In this paper, by the use of the topological current theory, the topological structures and the dynamic processes in thin-film ferromagnetic systems are investigated directly from the viewpoint of topology. It is found that the topological charge of a thin-film ferromagnetic system can be changed by annihilation or creation processes of opposite polarized vortex–antivortex pairs taking place at space–time singularities of the normalized magnetization vector field of the system, the variation of the topological charge is integer and can further be expressed in terms of the Hopf indices and Brouwer degrees of the magnetization vector field around the singularities. Moreover, the change of the topological charge of the system is crucial to vortex core reversal processes in ferromagnetic thin films. With the help of the topological current theory and implicit function theorem, the processes of vortex merging, splitting as well as vortex core reversal are discussed in detail.

Spin wave mediated unidirectional vortex core reversal by two orthogonal monopolar field pulses: The essential role of three-dimensional magnetization dynamics

Scanning transmission x-ray microscopy is employed to investigate experimentally the reversal of the magnetic vortex core polarity in cylindrical Ni81Fe19 nanodisks triggered by two orthogonal monopolar magnetic field pulses with peak amplitude B0, pulse length τ=60 ps, and delay time Δt in the range from −400 ps to +400 ps between the two pulses. The two pulses are oriented in-plane in the x- and y-directions. We have experimentally studied vortex core reversal as a function of B0 and Δt. The resulting phase diagram shows large regions of unidirectional vortex core switching where the switching threshold is modulated due to resonant amplification of azimuthal spin waves. The switching behavior changes dramatically depending on whether the first pulse is applied in the x- or the y-direction. This asymmetry can be reproduced by three-dimensional micromagnetic simulations but not by two-dimensional simulations. This behavior demonstrates that in contrast to the previous experiments on vortex core reversal, the three-dimensionality in the dynamics is essential here.

Switching probabilities of magnetic vortex core reversal studied by table top magneto optic Kerr microscopy

We have studied vortex core reversal in a single submicron Permalloy disk by polar Kerr microscopy. A sophisticated lock-in-technique based on repetitive switching of the magnetic vortex core and a continuous calibration allows for a reliable determination of the switching probability. This highly sensitive method facilitates the detection of a change in the magnetic moment of the tiny magnetic vortex core which is about 1.5 × 10−17 A m2. We have investigated vortex core switching caused by excitation of the vortex core gyromode with varying frequencies and amplitudes. The frequency range in which switching occurs was found to broaden with increasing excitation amplitude, whereby the highest frequency in this range shifts stronger to higher frequencies than the lowest frequency to lower frequencies. The experimental results are in good agreement with micromagnetic simulations.

Local energy of magnetic vortex core reversal

The polarity of magnetic vortex core can be switched by current or magnetic field through a vortex-antivortex pair creation and annihilation process, in which the significant change of the exchange energy during the switching takes an important role. To further unveil the energetic origin of magnetic vortex switching, we investigate the evolution of the maximum exchange energy density of the sample by using micromagnetic finite-element simulations based on the Landau-Lifshitz-Gilbert equation including the adiabatic and the nonadiabatic spin torque terms. Our micromagnetic calculations indicate that maximum exchange energy density for the considered sample must exceed a critical value of ~3.0106 J/m3 in order to achieve the magnetic vortex switching. The threshold value corresponds to the maximum exchange energy density at the time of creation of new vortex-antivortex pair. Following the nucleation of antivortex, the maximum exchange energy density increases rapidly with the antivortex approaching the original vortex. The maximum exchange energy density can become large at the time of annihilation of two vortexes. To explain well the critical value of the local maximum exchange energy density, we use the rigid vortex model（in which the spin distribution is unchangeable while vortex is displaced) to develop an analytical model. For a magnetic vortex confined in a thin ferromagnetic nanodisk, the magnetization distribution is unchanged along the thickness and can be seen as a two-dimensional model when the thickness is less than or on the order of the exchange length. The components of vortex magnetization vector in a ferromagnetic dot can be expressed by using a complex function w(,). Corresponding to the trivortex state appearing in vortex core reversal process, the local exchange energy density Wex around the vortexes cores is obtained. Simultaneously, we obtain the maximum exchange energy density:Wex2.3106 J/m3. In a realistic system, the shape of vortexes will deform during the vortex core reversal, which leads to the analytical result lower than the simulation value. Based on this reason, the analytical result matches well with our simulation value.

Ground state search, hysteretic behaviour, and reversal mechanism of skyrmionic textures in confined helimagnetic nanostructures.

Magnetic skyrmions have the potential to provide solutions for low-power, high-density data storage and processing. One of the major challenges in developing skyrmion-based devices is the skyrmions’ magnetic stability in confined helimagnetic nanostructures. Through a systematic study of equilibrium states, using a full three-dimensional micromagnetic model including demagnetisation effects, we demonstrate that skyrmionic textures are the lowest energy states in helimagnetic thin film nanostructures at zero external magnetic field and in absence of magnetocrystalline anisotropy. We also report the regions of metastability for non-ground state equilibrium configurations. We show that bistable skyrmionic textures undergo hysteretic behaviour between two energetically equivalent skyrmionic states with different core orientation, even in absence of both magnetocrystalline and demagnetisation-based shape anisotropies, suggesting the existence of Dzyaloshinskii-Moriya-based shape anisotropy. Finally, we show that the skyrmionic texture core reversal dynamics is facilitated by the Bloch point occurrence and propagation.

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.