Domain Wall Nucleation Research Articles

The effect of diameter on hysteresis loop squareness in iron nanowires array by micromagnetic simulation

Utilization of micromagnetic simulation is a very attractive subject to understand the magnetic behavior of nanowire arrays that tune their geometrical features and magnetic properties. Here, the micromagnetic simulations utilize to investigation of the single, and a 3 × 3 array of iron nanowires with diameters from 30 to 60 nm. The diameter size and the distance between the wires are selected according to the fabrication of iron nanowires grown in alumina templates. The simulation indicated that an increase in diameter led to a decrease in the coercive field and squareness ratio in both single nanowires and nanowire arrays. Besides, the simulation discloses that the magnetization reversal process of the nanowires with diameters more than 42 nm occurs through the nucleation and propagation of vortex domain walls. In the nanowires array, an increase in wire diameter increases the number of jumps in the hysteresis loop, indicating a more magnetic interaction between the nanowires.

Geometry-induced Bloch point domain wall in Permalloy conical frustum nanowires for advanced spintronics applications

In this study, we investigate the pseudo-static magnetic properties of Permalloy conical frustum nanowires using micromagnetic simulations. We thoroughly examine how both the major and minor radii influence the magnetic reversal mechanism when an external magnetic field is applied parallel to the nanowire axis. The obtained results show that under specific geometrical conditions, magnetization reverts though a Bloch point-type domain wall. In these cases, hysteresis curves exhibit two Barkhausen jumps during magnetization reversal, forming a plateau field range in which a Bloch point domain wall nucleates and propagates until its annihilation after the second Barkhausen jump. The nucleation of a Bloch point domain wall in a frustum conical nanowire geometry is reported. These findings highlight the significance of this geometry in nucleating these attractive topological defects for promising applications.

Granular Magnetization Switching in Pt/Co/Ti Structure with HfOx Insertion for In-Memory Computing Applications.

Exploring multiple states based on the domain wall (DW) position has garnered increased attention for in-memory computing applications, particularly focusing on the utilization of spin-orbit torque (SOT) to drive DW motion. However, devices relying on the DW position require efficient DW pinning. Here, we achieve granular magnetization switching by incorporating an HfOx insertion layer between the Co/Ti interface. This corresponds to a transition in the switching model from the DW motion to DW nucleation. Compared to the conventional Pt/Co/Ti structure, incorporation of the HfOx layer results in an enhanced SOT efficiency and a lower switching current density. We also realized stable multistate storage and synaptic plasticity by applying pulse current in the Pt/Co/HfOx/Ti device. The simulation of artificial neural networks (ANN) based on the device can perform digital recognition tasks with an accuracy rate of 91%. These results identify that DW nucleation with a Pt/Co/HfOx/Ti based device has potential applications in multistate storage and ANN.

Influence of the magnetic field on the nucleation and properties of 0-degree domain walls in uniaxial films with inhomogeneous magnetoelectric interaction

This study investigates the behavior of 0°-domain walls arising in uniaxial magnetic films with a flexomagnetoelectric (FME) effect in a magnetic field. It is shown that at certain magnetic field orientations, it is possible to significantly enhance (or weaken) the degree of manifestation of the FME effect in the studied films. In addition, by varying the magnitude and direction of the magnetic field, it is possible to significantly lower (up to zero) the value of the critical electric field at the origin of this inhomogeneity. It is also established that the 0°-domain wall of the non-Neel type, in which induced bound charges do not create a resultant field (field shielding); when a magnetic field is directed perpendicular to the plane of rotation of magnetic moments, a FME effect occurs, which increases with increasing magnitude of the magnetic field.

Magnetization reversal and domain wall pinning in nanowires with nonlinear modulation of diameter

Micromagnetic simulation of magnetization reversal is studied on cylindrical tapered nanowire with radii 50nm and 10nm at wider and narrower end respectively. The radius decreases from wider end to the narrower end non-linearly as R-βzδ where δ is the degree of non-linearity. Value of δ is either an integer (n) or reciprocal of integer (1/n). At remanence, a vortex and flower state at the wider and narrower extremes respectively are exhibited combined with ferromagnetic ordering in the majority of volume. When δ is below 1/2, pinning and depinning of domain wall drives the hysteresis following a Modified Kondorsky model. Otherwise, the magnetization reversal proceeds with nucleation and propagation of vortex domain wall according to Kondorsky model. The propagation of domain wall is dependent upon the value of δ which suggests tunability of magnetic characteristics with modulation of diameter. For instance, large values of coercivity is observed for wires that have strong domain wall pinning which is considerably reduced when depinning and propagation of domain wall takes over. A large range (20kA/m to 130kA/m) of coercivity can be obtained by changing the modulation.

Controlled Growth of Submillimeter-Scale Cr5Te8 Nanosheets and the Domain Wall Nucleation Governed Magnetization Reversal Process.

Two-dimensional (2D) ferromagnets have attracted widespread attention for promising applications in compact spintronic devices. However, the controlled synthesis of high-quality, large-sized, and ultrathin 2D magnets via facile, economical method remains challenging. Herein, we develop a hydrogen-tailored chemical vapor deposition approach to fabricating 2D Cr5Te8 ferromagnetic nanosheets. Interestingly, the time period of introducing hydrogen was found to be crucial for controlling the lateral size, and a Cr5Te8 single-crystalline nanosheet of lateral size up to ∼360 μm with single-unit-cell thickness has been obtained. These samples exhibit a leading role of domain wall nucleation in governing the magnetization reversal process, providing important references for optimizing the performances of associated devices. The nanosheets also show notable magnetotransport response, including nonmonotonous magnetic-field-dependent magnetoresistance and sizable anomalous Hall resistivity, demonstrating Cr5Te8 as a promising material for constructing high-performance magnetoelectronic devices. This study presents a breakthrough of large-sized CVD-grown 2D magnetic materials, which is indispensable for constructing 2D spintronic devices.

Propagation dynamics of the Bloch-type domain wall on CoFeB nanostripes driven by a nanosecond magnetic pulse

Despite the abundance of research on the dynamics of magnetic domain walls (DW) on thin-film media, in-depth discussions regarding the behavior around the so-called Walker breakdown phenomenon are still scarce. In this work, propagation dynamics of the Bloch-type DW on CoFeB nanostripes with perpendicular anisotropy driven by a nanosecond magnetic pulse are investigated using micromagnetic simulation. It is found that on smaller nanostripes, the Walker breakdown corresponds with the oscillatory motion of the DW. On larger nanostripes, the formation of a Bloch-line with previously unreported behavior can be observed for magnetic pulses above HWB. Using a nanosecond field pulse instead of a continuous external field may allow for more direct control over the oscillatory DW motion and nucleation of the Bloch-line. A deeper understanding of the dynamics of the Bloch-type DW and its correlation to the applied nanosecond pulse may lead to better control of the DW for future spintronic devices.

Investigation into the role of Co doping in the microstructure and magnetic characteristics of nanocrystalline Nd-Ce-La-Fe-B alloys

In this study, [Nd0.5(Ce0.5La0.5)0.5]17Fe78-xCoxB6 (x = 0 ∼ 1.5 at. %) ribbons are prepared via a direct melt-spun (wheel speed of 28 m/s). The impacts of Co doping on the crystal structure, Curie temperature Tc, magnetic parameters (Br, Hci and (BH)max) of [Nd0.5(Ce0.5La0.5)0.5]17Fe78-xCoxB6 alloys are systematically assessed. X-ray diffraction patterns and Rietveld refinement data demonstrate that the exclusive presence of a pure hard magnetic 2:14:1 phase without any secondary phases across all samples. Compared with Co-free sample (x = 0), the Br, Hci and (BH)max of Co-doped with x = 1 are increased by 15.0 %, 4.6 % and 42.9 %, respectively. The coercivity mechanism in the [Nd0.5(Ce0.5La0.5)0.5]17Fe78-xCoxB6 alloys arises from the integration of reversed domain nucleation and robust pinning of domain walls. The magnetic domain structure and quantitative analysis based on the microstructure and demagnetization behavior are conducted to investigate the exchange coupling interactions among the 2:14:1 grains. This exploration reveals that the enhanced magnetic properties stem from the refinement of the 2:14:1 phase's grains, leading to more uniform and smaller interaction domains.

Universal Conductance Fluctuations in a MnBi2Te4 Thin Film.

Quantum coherence of electrons can produce striking behaviors in mesoscopic conductors. Although magnetic order can also strongly affect transport, the combination of coherence and magnetic order has been largely unexplored. Here, we examine quantum coherence-driven universal conductance fluctuations in the antiferromagnetic, canted antiferromagnetic, and ferromagnetic phases of a thin film of the topological material MnBi2Te4. In each magnetic phase, we extract a charge carrier phase coherence length of about 100 nm. The conductance magnetofingerprint is repeatable when sweeping applied magnetic field within one magnetic phase. Surprisingly, in the antiferromagnetic and canted antiferromagnetic phases, but not in the ferromagnetic phase, the magnetofingerprint depends on the direction of the field sweep. To explain our observations, we suggest that conductance fluctuation measurements are sensitive to the motion and nucleation of magnetic domain walls in MnBi2Te4.

Epitaxial growth and electric control of magnetic behaviors in exchange biased IrMn/FeGa/MgO/PMN-PT heterostructures

We grew exchange biased (001)-textured IrMn/epitaxial FeGa bilayers on ferroelectric PMN-PT(001) substrate with assistance of an MgO buffer layer and investigated the electric control of magnetic behaviors. The as-grown sample exhibits square, asymmetrically shaped, and two-step loops at different magnetic field orientations due to the combined effect of the intrinsic magnetocrystalline anisotropy and the collinear uniaxial and unidirectional magnetic anisotropies, which can be interpreted by the magnetization reversal of domain wall nucleation. After polarized by an electric field, another kind of asymmetrically shape loops with three steps in the descending and two steps in the ascending branch is additionally observed because the uniaxial magnetic anisotropy is greatly enhanced and becomes perpendicular to the exchange bias. The angular dependent magnetic behavior suggests the exchange bias is slightly rotated by the electrically induced strain. The electric field dependence of coercive field and uniaxial magnetic anisotropy exhibit an asymmetrical butterfly-like shape, indicating a strain-mediated magnetoelectric coupling with a net 109° polarization switching of PMN-PT.

Enabling Logic Computation Between Ta/CoFeB/MgO Nanomagnets

Dipolar coupled magnets proved to have the potential to be capable of successfully performing digital computation in a highly parallel way. For that, nanomagnet-based computation requires precise control of the domain wall nucleation from a well-localized region of the magnet. Co/Pt and Co/Ni multilayer stacks were successfully used to demonstrate a variety of computing devices. However, Ta/CoFeB/MgO appears more promising thanks to the lower switching field required to achieve a full magnetization reversal, reduced thickness (less than 10 nm), and its compatibility with magnetic tunnel junctions. In this work, the switch of the information is achieved through the application of a magnetic field, which allows to scale more the nanomagnets with respect to current-driven magnetization reversal-based devices and to go towards 3D structures. We experimentally demonstrate that Ga <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> ions can be used to tune the energy landscape of the structured magnets to provide signal directionality and achieve a distinct logic computation. We prove that it is possible to define the artificial nucleation center in different structures with two irradiation steps and that this approach can enable logic computation in ultra-thin films by dipolar interaction. Moreover, different from previous studies, the results coming from the irradiation analysis are then used for real logic devices. We present the experimental demonstration of a set of fully working planar inverters, showing that it is possible to reach a coupling field between input and output, which is strong enough to reliably implement logic operations. Micromagnetic simulations are used to study the nucleation center’s effectiveness with respect to its position in the magnet and to support the experiments. Our results open the path to the development of more efficient nanomagnet-based logic circuits.

Measurement of the activation volume in magnetic random access memory

Measuring thermal stability in magnetic random access memory devices is non-trivial. Recently, there has been much discussion on the appropriate model to use: single domain or domain wall nucleation. Of particular challenge is assessing the maximum size at which the single domain model can be assumed. Typically, this is estimated to be in the range of 20–30 nm based on a value of the exchange stiffness (Aex) that is assumed, estimated using indirect measurements or derived from significantly thicker films. In this work, it is proposed that this maximum size can be measured directly via the “activation volume” (Vact) or the “activation diameter” (Dact), which originates from the concept of magnetic viscosity. This is conducted by measuring, using the time dependence of magnetization at different applied fields, Dact in perpendicular magnetic tunnel junction pillars of varying effective anisotropy constant (Keff) and diameter. It is shown that the trend in Dact follows 1/Keff dependence, in good agreement with the analytic model for the critical diameter of coherent switching. Critically, it is also found that the smallest size for which a single domain, with coherent reversal, occurs is 20 nm. Thus, in devices with technologically relevant values of Keff, the macrospin model may only be used in 20 nm, or smaller, devices.

Impact of Reference-Layer Stray Field on the Write-Error Rate of Perpendicular Spin-Transfer-Torque Random-Access Memory

A finite-temperature micromagnetic study of magnetization switching and write-error rates in a perpendicular magnetic tunnel junction with and without synthetic antiferromagnetic layer (SAF) is presented. In the absence of SAF, magnetization switching is induced by domain-wall nucleation and propagation. Although the various modes of domain-wall propagation are observed to delay switching, it does not show an appreciable impact on the overall write-error-rate slopes. In the presence of the nonuniform stray field from the SAF assembly, the domain-wall-based switching modes turn on more complex magnetization dynamics that impedes the switching process. In cases where the SAF layers fail to balance each other contributing to a stronger stray field, incoherent switching modes give rise to metastable states with significantly longer lifetimes, and a dramatic change in the write-error slopes is observed. Simulation results are compared to recent experimental findings from time-domain measurements of spin-transfer-torque switching and measurements of anomalous write-error rates. These results directly prove the long-predicted relation of the SAF stray field to write-error anomaly in perpendicular spin-transfer-torque magnetic random-access memory and could be useful to solve the anomalous write-error problems.

Fourfold magnetic anisotropy induced in CoFeB/IrMn bilayers by interfacial exchange coupling

Exchange bias (EB) occurring in ferromagnetic (FM)/antiferromagnetic (AFM) bilayers conventionally can lead to a unidirectional magnetic anisotropy () as well as an accompanied uniaxial magnetic anisotropy (). We observed an additional fourfold magnetic anisotropy () induced by interfacial exchange coupling in amorphous CoFeB/epitaxial IrMn bilayers with an EB. Because of the combined effect of the three kinds of magnetic anisotropies, one- and two-step magnetic switching processes were observed at different magnetic field orientations, which usually appear in single-crystal FM layer with an intrinsic magnetocrystalline anisotropy but not in amorphous FM layer. The angular dependent magnetic switching fields can be nicely fitted by a phenomenological model based on domain wall nucleation and propagation with the in-plane along <100>. The ferromagnetic resonance measurements indicate that the specific strength of for EB along [100] is larger than that for EB along [110]. The induced can be understood by considering two types of AFM domains caused by both monatomic steps and defects and their induced net uncompensated spins along the in-plane <100> axes. The different dependence of on the EB direction are because of the different effects of growth magnetic field on the presence of AFM domains.

Dynamics and Modeling of Multidomains in Ferroelectric Tunnel Junction—Part-II: Electrostatics and Transport

We have derived the 2-D analytical multidomain electrostatics model of the ferroelectric tunnel junction (FTJ) in part-I of this work. Here, we have used the 2-D potential functions from part-I to determine the depolarizing energy density of the ferroelectric layer. The coupled solution of the Landau Ginzburg–Devonshire (LGD) equation with Poisson’s equation is obtained to capture the domain texture in the ferroelectric layer. The nucleation and movement of the domain wall are incorporated into the model by minimizing net ferroelectric energy (depolarization energy density + free energy density + gradient energy density). Due to the mathematical complexity, spontaneous polarization <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$({P}_{s})$ </tex-math></inline-formula> and coercive electric field <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$({E}_{c})$ </tex-math></inline-formula> are assumed to be fixed, and the net system energy is minimized only by domain wall nucleation and movement in the ferroelectric layer. The analysis of this article is twofold; first, we study the impact of domain dynamics on electrostatics in an FTJ. Subsequently, the obtained electrostatics is used to study the variations in tunneling current, and ON/OFF ratio [tunnel electroresistance (TER)] originated from multidomain dynamics. We show that ON/OFF tunneling current density and TER varies locally in the ferroelectric region, and approximately one-decade local variations in current density are observed. The optimization techniques to achieve a uniform and maximum TER are also discussed. Furthermore, the impact of the bottom insulator layer on ferroelectric’s gradient energy is also studied.

All optical writing and current-driven shifting of bits in ferrimagnetic strips: A micromagnetic study

Nucleation of domains and domain walls by means of ultrashort laser pulses, and their current-driven shifting along a ferrimagnetic strip with high perpendicular magnetic anisotropy on top of a heavy metal, are both explored here by means of advanced micromagnetic modeling. Our results indicate that these systems are ideal candidates to develop high-density and high-efficient domain wall-based memory devices where the information is coded in series of bits in the form of perpendicular up and down domains flanked by chiral domain walls.

Deterministic domain wall rotation in a strain mediated FeGaB/PMN-PT asymmetrical ring structure for manipulating trapped magnetic nanoparticles in a fluidic environment.

The manipulation of domain walls (DWs) in strain-mediated magnetoelectric (ME) heterostructures has attracted much attention recently, with potential applications in precise and location-specific manipulation of magnetic nanoparticles (MNPs). However, the manipulation ability in these structures is restricted to magnetostrictive circular ring structures only, where the required onion state is metastable, less thermally stable, and cannot be obtained easily. This work investigates the highly shape anisotropic FeGaB magnetostrictive elliptical ring structures of different aspect ratios and trackwidths on the PMN-PT piezoelectric substrate to manipulate fluid-borne MNPs using active control of DWs. The proposed model utilizes the attribute that the required onion state in a magnetostrictive elliptical ring is thermally stable and easily obtained compared to magnetostrictive circular ring structures. By varying the trackwidth of elliptical rings, nucleated DWs are rotated at different angles to capture and transport fluid-borne MNPs. Up to a critical trackwidth, DW rotation is predicted by dominant stress anisotropy energy that leads the rotation of DWs and attached MNPs toward the dominant tensile strain direction of PMN-PT with reversibility. Increasing the trackwidth beyond the critical trackwidth caused a complete 90° rotation of DWs and attached MNPs without reversibility and is given by dominant shape anisotropy energy. The fundamental relationship of capture probability with the size and velocity of injected MNPs is also demonstrated. The nucleation and rotation of DWs are predicated using the coupled elastodynamic and electrostatic Finite Difference Method (FDM) micromagnetic model. Dynamics of MNP capture and rotation are envisaged using an analytical model.

Bulk generalized Dzyaloshinskii-Moriya interaction in PT -symmetric antiferromagnets

The Dzyaloshinskii-Moriya interaction (DMI) in magnetic materials plays an important role in spintronics, giving rise to chiral spin textures such as the nucleation and propagation of domain walls and skyrmions. The necessary ingredient for the emergence of the DMI is the lack of inversion symmetry combined with elements with strong spin-orbit coupling. We report on a first-principles investigation of generalized DMIs in bulk centrosymmetric crystals with noncentrosymmetric local sublattices, where we consider a prototype family of $\mathcal{PT}$-symmetric antiferromagnets, including, ${\mathrm{Mn}}_{2}\mathrm{Au}$, ${\mathrm{MnPd}}_{2}$ and MnCuAs in the tetragonal and orthorhombic phases. We employ a Green's function approach to calculate the interatomic relativistic exchange coupling which can, in turn, determine the sublattice elements of the generalized fifth-order DMI tensor ${D}_{\ensuremath{\alpha}\ensuremath{\beta},\ensuremath{\gamma}}^{s{s}^{\ensuremath{'}}}$. We demonstrate that the breaking of the local mirror symmetry and the resulting local Rashba-type spin-momentum locking yield local DMIs with opposite signs on the two antiparallel sublattices. We present numerical results and provide analytical expressions for the magnon dispersion in the presence of the sublattice DMI and show that the intra- (inter-) sublattice DMIs result in identical (opposite) contributions to the nonreciprocal components of the two low-frequency antiferromagnetic magnon modes.

Electrical detection of domain evolution in magnetic Weyl semimetal Co3Sn2S2 submicrometer-wide wire devices

Microscopic understanding of magnetization switching via domain nucleation and/or domain-wall propagation is fundamental knowledge for developing magnetic and spintronic devices. Here, we explore the underlying mechanism of the large coercivity of the magnetic Weyl semimetal ${\mathrm{Co}}_{3}{\mathrm{Sn}}_{2}{\mathrm{S}}_{2}$ thin films, which is roughly ten times larger than that of ${\mathrm{Co}}_{3}{\mathrm{Sn}}_{2}{\mathrm{S}}_{2}$ bulk single crystal, by measuring Hall resistance in constricted wire devices. The discretized steplike variations appear in the hysteresis loops of the Hall resistance in $0.6\phantom{\rule{4pt}{0ex}}\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{m}$ wide and narrower devices, indicating that the size of the reversed magnetic domain is comparable to the active area of the Hall devices. By counting the number of discrete features, the average diameter of the reversed magnetic domain is estimated to be 80 nm. Individually, the diameter of the reversed domain nucleus is evaluated to be roughly 2 nm. Considering the difference in the diameters of the reversed magnetic domain and the reversed domain nucleus, we ascribed the large coercivity of the ${\mathrm{Co}}_{3}{\mathrm{Sn}}_{2}{\mathrm{S}}_{2}$ thin films to a large nucleation field owing to the uniform crystallinity within grains and strong domain-wall pinning at grain boundaries specific to the thin films. With the large nucleation field in the films, an engineering of the domain-wall pinning sites is a promising approach to control the nucleation, manipulation, and detection of the single domain wall in ${\mathrm{Co}}_{3}{\mathrm{Sn}}_{2}{\mathrm{S}}_{2}$ thin-film devices.

Highly responsive photodetection based on bismuth ferrite ceramics: The roles of depolarization field and domain network

This study presents the highly-sensitive, millisecond-response, and self-powered photosensors based on the indium-tin-oxide (ITO)/Bi0.93R0.07FeO3 ceramic/Au heterostructures (R = Nd, Gd) at 360-nm irradiation. Photocurrent density (Jph), photoconductive gain (G), photoresponsivity (R) and specific detectivity (D*) were significantly boosted by an electric-field poling, and can reach 1.4 mA/cm2, 25%, 71 mA/W and 2.4 × 1011 Jones in the ITO/Bi0.93Gd0.07FeO3/Au; 0.17 mA/cm2, 14.5%, 42 mA/W and 2.6 × 1012 Jones in the ITO/Bi0.93Nd0.07FeO3/Au. Fast response times of 0.6 ms and 1.0 ms are respectively detected in the ITO/Bi0.93Gd0.07FeO3/Au and ITO/Bi0.93Nd0.07FeO3/Au. The enhanced photodetection is driven collectively by the p-n heterojunction, the depolarization field, and the electric-field-induced domain-wall nucleation. The complex network consisting of grain boundaries and nanoscale domain walls provides electrically conductive passages in the grain matrix. This work delivers an energy self-sufficient route using the polycrystalline rare-earth-doped BiFeO3 ceramics for the high-performance photodetection in the ultraviolet-A spectrum.