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Europium intermetallic compounds show a variety of different ground states and anomalous physical properties due to the interactions between the localized 4f electrons and the delocalized electronic states. Europium is also the most reactive of the rare earth metals which might be the reason why very few works are concerned with the properties of Eu-based thin films. Here we address the low-temperature magnetic properties of ferromagnetic EuPd _2 thin films prepared by molecular beam epitaxy. The epitaxial (111)-oriented thin films grow on MgO (100) with eight different domain orientations. We analyze the low-temperature magnetic hysteresis behavior by means of micromagnetic simulations taking the multi-domain morphology explicitly into account and quantify the magnetic crystal anisotropy contribution. By ab initio calculations we trace back the microscopic origin of the magnetic anisotropy to thin film-induced uniform biaxial strain.

Field-induced domain wall motion on thin magnetic films with perpendicular anisotropy has long been attributed with the universal creep theory, in which the domain wall (DW) is able to experience a slow-moving motion with driving fields below the depinning threshold. Despite the numerous research that has been conducted in regards to this phenomenon, creep motion thus far has yet to be observed on a typical single-layered magnetic film. The effects of the film’s thickness towards the creep motion are also scarcely explored. In this study, we conduct micromagnetic simulations of CoFeB nanowire strips with perpendicular anisotropy and varying thicknesses to investigate the dynamics of the creep motion being exhibited. We then analyze the obtained DW velocities and its’ agreement with the universal creep law equation. The velocities obtained with low driving fields is found to be in an agreement with the creep law equation. The varying thicknesses also seem to affect the overall DW velocity and DW width.

We explored a new helical spring magnet (HSM) concept using micromagnetic simulation along with analytical modeling. The present HSMs can be made of a single magnetic material or micro/nano-scale magnetic particles in a polymer matrix, which provides for active control over the motion of the HSMs by additional forces generated by the magnetic interactions inside the HSMs. A finite-element micromagnetic simulation was performed to calculate the different magnetic forces employed in the HSMs under applied magnetic fields. The effects of the HSMs' geometrical parameters, such as pitch length, wire diameter, and coil (spring) diameter, on their magnetic and mechanical forces were examined. The numerical calculations of an analytical model of a simplified three-rod magnet were in good quantitative agreement with the micromagnetic simulation results for the whole HSM geometry. The magnetic forces acting on the HSMs vary remarkably with the dimensions of the spring-geometrical parameters as well as the constituent material parameters. Thus, if the dimensions of HSMs are appropriately tuned in consideration of both the mechanical and magnetic parameters of the constituent materials, the resultant geometrical and material optimization will allow for an exter nal field control over the HSMs’ motion and, consequently, performance improvement. This could make the proposed HSMs highly suitable for active-control vibration dampers in mechanical engineering applications as well as soft robots in biomedical applications.

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The finite element micromagnetic simulation is used to study the role of complex composition of 2:17R-cell boundaries in the realization of magnetization reversal processes of (Sm, Zr)(Co, Cu, Fe)z alloys intended for high-energy permanent magnets. A modified sandwich model is considered for the combinations of 2:7R/1:5H phase and 5:19R/1:5H phase layers as the 2:17R-cell boundaries in the alloy structure. The results of the simulation represented in the form of coercive force vs. total width of cell boundary showed the possibility of reaching the increased coercivity at the expense of 180°-domain wall pinning at the additional barriers within cell boundaries. The phase and structural states of the as-cast Sm1-xZrx(Co0.702Cu0.088Fe0.210)z alloy sample with x = 0.13 and z = 6.4 are studied, and the presence of the above phases in the vicinity of the 1:5H phase was demonstrated. [ABSTRACT FROM AUTHOR]

Magnetic nanofibers belong to the geometries which are intensively investigated in simulations and experiments due to their unique magnetic properties, varying in their lengths, cross-sections, and bending radii. Besides basic research of different magnetization reversal processes and magnetization dynamics in bent nanofibers, these structures are of potential interest for data storage applications, data transport, or other tasks in spintronics devices. While previous simulations concentrated on the domain wall transport through coupled bent nanofibers, creating networks with many in- and outputs to establish nanofiber-based domain wall logics, here we show the influence of the constricted area, in which a rotating magnetic field is applied in the middle of bent or straight magnetic nanofibers, on the magnetization dynamics. Our micromagnetic simulations, performed by Magpar, reveal a strong impact not only of this area, but also of the curvature of the nanofiber as well as of an additional Dzyaloshinskii–Moriya interaction (DMI). [ABSTRACT FROM AUTHOR]

Magnetic nanowires (MNWs) are potential candidates for heating in biomedical applications that require rapid and uniform heating rates, such as warming cryopreserved organs and hyperthermia treatment of cancer cells. Therefore, it is essential to determine which materials and geometries will provide the optimal heating using available alternating magnetic fields (AMF). Micromagnetic simulations are used to investigate the heating ability of MNWs by predicting their hysteretic behavior. MNWs composed of iron (Fe), nickel (Ni), cobalt (Co) or permalloy (FeNi alloy, Py) with different diameters (10-200 nm) are simulated using object oriented micromagnetic framework (OOMMF). Hysteresis loops are obtained for each simulated MNW, and the 2D/3D magnetic moment map is simulated to show the reversal mechanism. The heating ability, in terms of specific loss power (SLP), is calculated from the area of the hysteresis loop times frequency for each MNW for comparison with others. It is estimated that a theoretical optimal heating ability of 2730 W/g can be provided by isolated Co MNWs with 50 nm diameters using a typical AMF system that can supply 72 kA/m field amplitude and 50 kHz in frequency. Generalized correlation between coercivity and size/material of MNWs is provided as a guidance for researchers to choose the most appropriate MNW as a heater for their AMF system and vice versa. [ABSTRACT FROM AUTHOR]

• This paper proposes a simulation-experiment closed-loop analysis framework to guide the design of nanocomposite permanent magnets. • Transmission electron microscopy data from experimentally prepared magnets refine polycrystalline models, followed by micromagnetic simulation analysis. • The iterative process minimizes discrepancies between simulation predictions and actual performance, accelerating magnet development and optimization. This study presents a framework for simulation-experiment closed-loop analysis of nanocomposite magnets. Firstly, hysteresis loops of nanocomposite permanent magnets were calculated using micromagnetic simulations. Then, the experimental preparation was carried out by rapid quenching of the melt and spark plasma sintering (SPS). We analyzed the effect of different sintering temperatures on the physical phase composition and magnetic properties of the samples. Characterization results show that the samples sintered at 600 °C have the highest magnetic energy product. Using transmission electron microscopy data, the polycrystalline model was refined, and further micromagnetic simulations were performed. The close agreement between simulated and experimental data confirms the improved accuracy of our simulation model. This iterative process of experimental characterization and simulation improvement reduces the discrepancy between simulation predictions and actual sample performance. It could facilitate rapid iteration in magnet development and provide valuable insights for optimizing the preparation of nanocomposite permanent magnets. [ABSTRACT FROM AUTHOR]

• SmCo/Fe and SmCo/Co magnetic bilayer models are proposed. • The influence of the thickness of the magnetic layers is explored. • Our models are shown to describe well of experimental data. The magnetic SmCo/Fe and SmCo/Co heterostructures were numerically investigated in an external magnetic field using the micromagnetic simulation package MuMax3. The magnetization reversal curves and magnetic phase diagrams for these structures were plotted for different thicknesses of the magnetic layers and directions of the external magnetic field. The SmCo(20 nm)/Fe(20 nm) heterostructure reached saturation along the hard magnetization axis at an external magnetic field of 18 T, while a single SmCo(20 nm) layer achieved saturation at 18.2 T. The highest values of (BH) max were found to be 29.2 MG∙Oe for the SmCo(20 nm)/Fe(10 nm) and 35.5 MG∙Oe for the SmCo(20 nm)/Co(20 nm) heterostructures. [ABSTRACT FROM AUTHOR]

• The impact of the RE and B on magnetic properties and microstructure is clarified. • Micromagnetic simulations were used to study the effect of REFe 2 on coercivity. • A new recipe to develop the magnetic performance of MM-based magnets is proposed. To develop high-performance MM-based sintered magnets (MM=Misch Metal), the effects of varying the rare earth (RE) and boron (B) content on the microstructure and magnetic properties of (MM, Nd)-Fe-B sintered magnets were performed. By increasing the RE content from 30 wt.% to 33 wt.% and decreasing the B content from 1 wt.% to 0.9 wt.%, the coercivity (H cj) increases by 31.8 %. This is mainly attributed to an increase in the volume of the grain boundary (GB) and the formation of additional REFe 2 phase. Micromagnetic simulations confirmed that the REFe 2 phase, acting as a paramagnetic GB, has a positive effect on enhancing coercivity. During the demagnetization process, the propagation of reversed domains is hindered when the paramagnetic GB becomes thicker. This study provides a new recipe to develop MM-based sintered magnets with high comprehensive properties through varying RE and B contents. [ABSTRACT FROM AUTHOR]

In this work, we developed a series of anisotropic hot-deformed Nd-Fe-B magnets with ultrafine grain sizes varying from 125 nm to 234 nm in the lateral direction. Although grain refinement is known to increase the coercivity of magnets, all samples exhibited identical record high coercivity of 2 T. Microstructural analysis showed similar intergranular phase (IGP) composition and crystallographic texture, but an increased number of coarse and isotropic equiaxed grains in the flake boundary regions of the magnets with larger grain size. Although this could explain significant differences in the initial magnetization curves and magnetic domain patterns, the experimental data alone were not enough to identify the main factors limiting the coercivity. To elucidate these, a micromagnetic simulation was performed using a realistic 1:1 scale model. We showed that grain refinement below 200 nm has a limited potential for coercivity enhancement of hot-deformed Nd-Fe-B magnets unless the exchange interaction is weakened by reducing the magnetization of the IGP. Negative contributions to the coercivity from texture deterioration, coarse grains, and magnetostatic interaction were quantified for different values of IGP magnetization. A clear direction on how to further improve the coercivity of hot-deformed magnets was provided. [Display omitted] [ABSTRACT FROM AUTHOR]

This work presents the analysis of the stability of magnetic bimerons in a cylindrical nanotube. Through micromagnetic simulations, we study the influence of magnetic and geometrical parameters on the bimeron existence and size. The obtained results allow us to present diagram states showing the stability region of a bimeron as a function of the nanotube’s height and radius for different anisotropy and Dzyaloshinskii–Moriya interaction strengths. We also obtain two other magnetic states in the range of parameters where the bimeron is not stable: helicoidal and saturated states.

Heavy rare-earth (HRE) elements are important for the preparation of high-coercivity permanent magnets. A further understanding of the thermodynamic properties of HRE phases, and the magnetization reversal mechanism of magnets are still critical issues to obtain magnets that can achieve better performance. In this work, the Nd–Dy–Fe–B multicomponent system is investigated via the calculation of the phase diagram (CALPHAD) method and micromagnetic simulation. The phase composition of magnets with various ratios of Nd and Dy is assessed using critically optimized thermodynamic data. γ-Fe and Nd2Fe17 phases are suppressed when partial Nd is substituted with Dy (<9.3%), which, in turn, renders the formation of Nd2Fe14B phase favorable. The influence of the magnetic properties of grain boundaries (GBs) on magnetization reversal is detected by the micromagnetic simulations with the 3D polyhedral grains model. Coercivity was enhanced with both 3 nm nonmagnetic and the hard-magnetic GBs for the pinning effect besides the GBs. Moreover, the nucleation and propagation of reversed domains in core-shell grains are investigated, which suggests that the magnetic structure of grains can also influence the magnetization reversal of magnets. This study provides a theoretical route for a high-efficiency application of the Dy element, realizing a deterministic enhancement of the coercivity in Nd–Fe–B-based magnets. [ABSTRACT FROM AUTHOR]

Spin wave propagation in a magnonic crystal excited at a specific wavenumber was investigated by micromagnetic simulation modelling with meander-shaped antennae. The dispersion relation when excited at wave number k which corresponds to the boundary of the Brillouin zone, shows a pair of strong spots that originate from the magnonic upper and lower bands. When excited at wavenumber slightly offset from k above, additional strong spots appear and it shows intensity nonreciprocity reversal. It can be interpreted in terms of the repeated or reduced zone scheme of the Brillouin zone. These characteristics have the potential for novel control of nonreciprocity. [ABSTRACT FROM AUTHOR]

The influence of Dzyaloshinskiiâ€"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. [ABSTRACT FROM AUTHOR]

Well-ordered Co x Dy1âˆ' x nanotubes are electrodeposited into self-made anodic aluminum oxidation templates under different potentials. The composition of Co x Dy1âˆ' x alloy nanotubes can be tuned by deposition potentials. The deposited Co x Dy1âˆ' x alloy nanotubes are amorphous, however, there appears a diffraction peak corresponding to Co7Dy2 in the sample annealed at 600 °C. The easy magnetization direction is always along the longitudinal axis of arrays for all samples. Dysprosium alloying significantly increases the coercivity of Co x Dy1âˆ' x nanotubes in comparison with Co nanotubes. By using the hybrid Monte Carlo micromagnetic method, the exchange stiffness constant of Co7Dy2 nanotubes can be conveniently estimated to be 8.0 Ă— 10âˆ'11 J mâˆ'1. [ABSTRACT FROM AUTHOR]

Ferromagnetic nanostripes have gained massive attention due to their intriguing magnetic properties associated with dimensional confinements and shape anisotropy leading toward potential applications in magnetic storage, memory, and spin‐wave‐based devices. Consequently, reconfiguration of their static and dynamic magnetic properties by the geometric parameters and external field is imperative. Here, a combined experimental and numerical study of the reconfigurable spin‐wave dynamics in arrays of ferromagnetic nanostripes by the stripe thickness and external magnetic field strength and orientation is presented. Different uniform, localized, and standing spin waves in the nanostripes and their monotonic and nonmonotonic variation, including mode merging with these parameters, are observed. The observed variations are interpreted with the aid of simulated spin configurations, magnetostatic field maps, and spin‐wave mode profiles. Further numerical study reveals anisotropic spin‐wave propagation in nanostripes for different thicknesses and in different bias‐field geometry opening potential applications in magnonic circuit components such as reconfigurable magnonic waveguides and omnidirectional spin‐wave emitters. [ABSTRACT FROM AUTHOR]

Topological spin textures are of great significance in magnetic information storage and spintronics due to their high storage density and low drive current. In this work, the transformation of magnetic configuration from chaotic labyrinth domains to uniform stripe domains was observed in MnFe 1- x Co x Ge magnets. This change occurs due to the noncollinear magnetic structure switching to a uniaxial ferromagnetic structure with increasing Co content, as identified by neutron diffraction results and Lorentz transmission electron microscopy (L-TEM). Of utmost importance, a hexagonal lattice of high-density robust type-II magnetic bubble lattice was established for x = 0.8 through out-of-plane magnetic field stimulation and field-cooling. The dimensions of the type-II magnetic bubbles were found to be tuned by the sample thickness. Therefore, the stabilization of complex magnetic spin textures, associated with enhanced uniaxial ferromagnetic interaction and magnetic dipole-dipole interaction in MnFe 1- x Co x Ge through magnetic structure manipulation, as further confirmed by the micromagnetic simulations, will provide a convenient and efficient strategy for designing topological spin textures with potential applications in spintronic devices.

The potential to produce nanostructures with intricate shapes in large quantities holds promise for a range of applications in the fields of nanoelectronics and biomedicine. Here a method for fabricating jellyfish-like Ni nanowires (JFNWs) using bilayered nanoporous anodic alumina templates with through pores of varying diameters in each layer is presented. To assess the capabilities of this method, samples are created with different voltages during the second step of anodization, resulting in distinct geometrical characteristics of the second layer of the template, and subsequently synthesize Ni JFNWs. By employing magnetometry and first-order reversal curve (FORC) method, the magnetic properties are examined and a significant alteration in their magnetic behavior, attributed to the differing shapes of the JFNWs and the magnetostatic interactions within the array, is observed. The study utilizes magnetic force microscopy to evaluate the stray magnetic fields generated by the individual JFNWs and unveils their unusual and asymmetric distribution. Through simulations based on the experimental data, the study analyzes the field- and current-induced domain wall movement in a single JFNW and their array. The findings reveal non-trivial micromagnetic configurations in these structures, including a remarkable 'corkscrew' state, and allow for an examination of the process of magnetization switching., (© 2024 Wiley‐VCH GmbH.)

The magnetic properties of magnetic nanocomposites consisting of hard and soft magnetic phases are dependent not only on the intrinsic properties but also on the grain structure and volume ratio of the two phases. In this study, we performed a systematic micromagnetic simulation on the magnetic properties of Ce2Fe14B/α-Fe and Nd2Fe14B/α-Fe nanocomposites. The volume fractions of the hard magnetic Nd2Fe14B or Ce2Fe14B phase were varied from 80% to 40%, and the grain sizes of the hard magnetic phase and the soft magnetic α-Fe phase were changed independently from 10 nm to 40 nm. The results show that when the grain size of both hard and soft phases is 10 nm and the volume fraction of the hard phase is 70%, the highest maximum magnetic energy product can be obtained in both Ce2Fe14B/α-Fe and Nd2Fe14B/α-Fe nanocomposites. The hard magnetic properties of Ce2Fe14B/α-Fe nanocomposite decrease significantly when the volume fraction of the α-Fe phase exceeds 30%. However, for the Nd2Fe14B/α-Fe system, this situation only occurs when the α-Fe volume fraction exceeds 40%. The reason for this is not only because of the low anisotropic field and the smaller exchange coupling length between the soft and hard magnetic phases, but also because of the lower saturation magnetization of the hard phase. The grain size has greater effects on the magnetic properties compared to the volume fraction of the hard magnetic phase. The main reason is that as the grain size increases, the remanence of the nanocomposite decreases sharply, which also leads to a rapid decrease in the maximum magnetic energy product. The simulation results on the effects of phase ratio and grain size have been verified by experiments on melt-spun Ce2Fe14B/α-Fe alloys with various compositions prepared by melt-spinning followed by annealing for various lengths of time. Due to the influence of demagnetization energy, the hard magnetic phase with high saturation magnetization is preferred for the preparation of high-performance nanocomposite magnets.

High-performance Nd-Fe-B-based rare-earth permanent magnets play a crucial role in the application of traction motors equipped in new energy automobiles. In particular, the anisotropic hot-deformed (HD) Nd-Fe-B magnets prepared by the hot-press and hot-deformation process show great potential in achieving high coercivity due to their fine grain sizes of 200–400 nm, which are smaller by more than an order of magnitude compared to the traditional sintered Nd-Fe-B magnets. However, the current available coercivity of HD magnets is not as high as expected according to an empirical correlation between coercivity and grain size, only occupying about 25% of its full potential of the anisotropy field of the Nd2Fe14B phase. For the sake of achieving high-coercivity HD magnets, two major routes have been developed, namely the grain boundary diffusion process (GBDP) and the dual alloy diffusion process (DADP). In this review, the fundamentals and development of the HD Nd-Fe-B magnets are comprehensively summarized and discussed based on worldwide scientific research. The advances in the GBDP and DADP are investigated and summarized based on the latest progress and results. Additionally, the mechanisms of coercivity enhancement are discussed based on the numerous results of micromagnetic simulations to understand the structure–property relationships of the HD Nd-Fe-B magnets. Lastly, the magnetization reversal behaviors, based on the observation of magneto-optic Kerr effect microscopy, are analyzed to pinpoint the weak regions in the microstructure of the HD Nd-Fe-B magnets.

The interest in magnetic nanostructures exhibiting perpendicular magnetic anisotropy and exchange bias (EB) effect has increased in recent years owing to their applications in a new generation of spintronic devices that combine several functionalities. We present a nanofabrication process used to induce a significant out-of-plane component of the magnetic easy axis and EB. In this study, 30 nm thick CoO/Co multilayers were deposited on nanostructured alumina templates with a broad range of pore diameters, 34 nm ≤ Dp ≤ 96 nm, maintaining the hexagonal lattice parameter at 107 nm. Increase of the exchange bias field (HEB) and the coercivity (HC) (12 times and 27 times, respectively) was observed in the nanostructured films compared to the non-patterned film. The marked dependence of HEB and HC with antidot hole diameters pinpoints an in-plane to out-of-plane changeover of the magnetic anisotropy at a nanohole diameter of ∼75 nm. Micromagnetic simulation shows the existence of antiferromagnetic layers that generate an exceptional magnetic configuration around the holes, named as antivortex-state. This configuration induces extra high-energy superdomain walls for edge-to-edge distance >27 nm and high-energy stripe magnetic domains below 27 nm, which could play an important role in the change of the magnetic easy axis towards the perpendicular direction. [ABSTRACT FROM AUTHOR]

Binary Fe–Co and ternary (FeCo)(1−x)Nix alloy thin films were electrodeposited on indium tin oxide (ITO)-coated glass substrate. The morphology of electrodeposited films was significantly influenced by nickel (Ni) concentration which varied from uniformly distributed needle-like structures in FeCo to spherical granules. With increasing Ni content, the coercivity as well as remanence decreases. The angular variation of coercivity predicts that the magnetization reversal mechanism in FeCo film occurs by domain wall depinning through a local defect, whereas the domain reversal is followed by a coherent rotation of spins in Fe38Co37Ni25 films having 25 (atomic)% Ni. MOKE microscopy images reveal flame-like domains in FeCo, which transforms to ripple-like domains in Fe38Co37Ni25. A deeper understanding of the magnetization reversal and observed high coercivity behavior in the out-of-plane hysteresis is presented using micromagnetic simulation. [ABSTRACT FROM AUTHOR]

A conventional energy product calculated by the product of the B-field and the H-field is not sufficient for representing the performance of a magnet because it considers the homogeneous and only the uniaxial magnetic properties of the magnet. The conventional energy product has been compared with another energy product obtained by integrating the scalar product of the B-field and the H-field of each cell composed of the three-dimensional components. We investigated a model system by micromagnetic simulation using finite differential method (FDM) and calculated the full hysteresis of the magnet. The model system of a Nd2Fe14B magnet composed of grains with a diameter of about 100 nm was assumed. In the case of the isotropic multi-grain magnet, the energy product calculated by the integration method was 28% larger than the energy product obtained by the conventional way, although a discrepancy between the distribution of the magnetizations and the demagnetizing fields at the reversal process resulted in the decrease of the energy product. [ABSTRACT FROM AUTHOR]

Two-dimensional ferromagnetic nanodot structures exhibit intriguing magnetization dynamics and hold promise for future magnonic devices. In this study, we present a comparative experimental investigation into the reconfigurable magnetization dynamics of non-ellipsoidal diamond and triangular-shaped nanodot structures, employing broadband ferromagnetic resonance spectroscopy. Our findings reveal substantial variations in the spin wave (SW) spectra of these structures under different bias field strengths (H) and angles (φ). Notably, the diamond nanodot structure exhibits a variation from nearly symmetric W-shaped dispersion to a skewed dispersion and subsequent transition to a discontinuous dispersion with subtle variation in bias field angle. On the other hand, in the triangular nanodot array a SW mode anti-crossing appears at φ = 15° which is starkly modified with the increase in φ to 30°. By analyzing the static magnetic configurations, we unveil the nature of the SW spectra in these two shapes. We reinforce our observations with simulated spatial power and phase maps. This study underscores the critical impact of dot shape and inversion symmetry on SW dynamical response, highlighting the significance of selecting appropriate structures and bias field strength and orientation for required functionalities. The remarkable tunability demonstrated by the magnonic crystals underscores their potential suitability for future magnonic devices. • Bias-field engineering of spin wave dynamics in diamond and triangular-shaped nanodot arrays arranged in square lattice. • Reconfigurability of spin wave spectra and mode softening phenomena in diamond-shaped nanodot array. • Mode hybridization due to magnon-magnon coupling in triangular-shaped nanodot array and its reconfigurability. [ABSTRACT FROM AUTHOR]

The precipitation-hardened Sm-Co permanent magnets relied on the pinning of nanocellular structures to achieve high coercivity, and have an irreplaceable position in high-temperature applications. In this article, by employing micromagnetic simulations and magnetic domain observations, the pinning behavior of lamellar phases (Z-phase) were investigated, which has not been well understood before. The results showed that the Z-phase can serve as a strong pinning position, but due to the parallel lamellar distribution, the magnetic domain walls can move around. Additionally, the effect of Z-phase on coercivity was achieved by changing the morphology of magnetic domain walls during magnetization reversal. For attractive pinning, the coercivity was reduced by the Z-phase, while for repulsive pinning, the Z-phase increased the coercivity when γ Z ＜0.54γ H. Our findings can provide a novel understanding of the coercivity mechanism of precipitation-hardened Sm-Co permanent magnets. [Display omitted] • Lamellar phases can serve as a strong pinning position in Sm-Co magnets. • The coercivity was reduced by the Z-phase for attractive pinning. • For repulsive pinning, the Z-phase increased the coercivity when γ Z ＜0.54γ H. [ABSTRACT FROM AUTHOR]

Recent years have seen greater interest in manipulating vortex states in magnetic nanostructures for non-volatile memory and logic networks. We show how reducing saturation magnetization locally in ferromagnetic thick nanodot vortex-based spin-torque nano-oscillators modulates their frequency using micromagnetic simulations. When a spin-polarized current and a static in-plane magnetic field are applied to the vortex core of an isolated thick nanodot, the uniform gyrotropic modes and the first higher-order gyrotropic mode resonate at different frequencies in various saturation magnetization areas. The intensity of the first higher-order mode gets almost suppressed in areas with modified saturation magnetization. With linewidths ranging from 25 to 70 MHz with the considered dimensions, these small spin-torque vortex-oscillator devices made of thick Permalloy nanodots seem promising for use in gigahertz signal processing. Our study suggests that locally modifying saturation magnetization may be a cost-effective technique to build dense oscillator and array networks for neuromorphic computing without lithographical fabrication stages. [ABSTRACT FROM AUTHOR]

Three-dimensional antisphere arrays (3DAAs) of Ni have been fabricated using electrochemical deposition into self-assembled polystyrene sphere templates, which offers the advantage of straightforward scalability. Using the first-order reversal curve (FORC) method, the magnetic reversal mechanism is identified from the characteristic features in the FORC distribution. A left-bending boomerang-like feature is observed in the thinnest sample, which transforms to a ridge oriented along the local coercivity Hc axis with increasing sample thickness. This transformation identifies a change in the reversal process from an exchange dominated domain-growth reversal to a localized weakly interacting particle-like reversal. Micromagnetic simulations confirm the decrease in domain growth and increase of pinning behaviors as the thickness of the Ni 3DAAs structure increases, providing strong support to the FORC analysis and interpretation.

Three-dimensional antisphere arrays (3DAAs) of Ni have been fabricated using electrochemical deposition into self-assembled polystyrene sphere templates, which offers the advantage of straightforward scalability. Using the first-order reversal curve (FORC) method, the magnetic reversal mechanism is identified from the characteristic features in the FORC distribution. A left-bending boomerang-like feature is observed in the thinnest sample, which transforms to a ridge oriented along the local coercivity Hc axis with increasing sample thickness. This transformation identifies a change in the reversal process from an exchange dominated domain-growth reversal to a localized weakly interacting particle-like reversal. Micromagnetic simulations confirm the decrease in domain growth and increase of pinning behaviors as the thickness of the Ni 3DAAs structure increases, providing strong support to the FORC analysis and interpretation.

As an ideal high-density storage unit, magnetic nanorings have become a research hotspot in recent years. We can both study the evolution of microscopic state of magnetization and acquire macroscopic magnetic properties by micromagnetic simulation, which has thus been widely used. However, traditional micromagnetism cannot simulate complex stress state. Due to the introduction of microelasticity theory, the phase field method for magnetic materials can be used to calculate the coupling effect of stress and magnetic field. However, the computing model usually needs to satisfy periodic boundary condition. In this paper, the phase field simulation combined with the finite element method is employed. By using user defined element, the evolution of magnetic domain structures of the double-hole nanorings has been studied. In different diameter of the holes and external magnetic field direction, we have found seven kinds of magnetic domain evolution mechanism. Among them, the twin-vortex evolution mechanism with high stability and low demagnetization interference characteristics of advantages, has good application prospect in magnetic random-access memory (MRAM) unit.

Abstract. Topic. Micromagnetic modeling of the propagation of backward volume magnetostatic waves (MSBVW) beams, excited by an antenna, placed in the center of yttrium iron garnet (YIG) film, has been carried out. Aim. To explore MSBVWbeam focusing with an increase in the amplitude of the exciting field at the antenna under conditions when only four-magnon (4M) processes are allowed for the MSBVW. Methods. The problem was solved using micromagnetic modeling by the finite-difference method solving the Landau–Lifshitz equation using the OOMMF software package. Results. It is shown that, depending on the position of the signal frequency in the MSBVW spectrum, an increase in the amplitude of the input signal above a certain threshold can lead to both the effect of wave beam focusing due to the development of modulation instability and the spatiotemporal chaotization of the amplitude distribution in the beam due to 4M decay processes. Changing of MSBVW-beam instability character at the frequency variation is associated with a change in the angular spectrum width of the beam and interaction between MSBVW and so-called «width» modes of the film. The obtained results can be used to analyze the effects of the propagation of nonlinear spin waves in YIG film waveguides.

• Skyrmions generatation from the periodic domain is realized by introducing a nucleation region. • The orthogonal currents are applied to balance the transversal skyrmion motion due to skyrmion Hall effect. • The ferromagnetic interlayer interactions are demonstrated feasible to stabilize skyrmion without the external magnetic fields. • A multi-terminals logic output is achieved by adjusting the strength of currents along x and y directions. Magnetic skyrmions are topologically nontrivial whirling spin textures with great potential for next-generation magnetic logic and memory applications as spintronic information carriers. The abilities to precisely manipulate skyrmion generation and motion using electric current are considered to be important issues for developing the high-efficiency skyrmion-based devices. In this paper, a skyrmionic logic device structure is proposed to realize the generation of Néel-type skyrmions from the topologically trivial domains. Micromagnetic simulations are employed to investigate the controllable skyrmion motions manipulated by the orthogonal current excitations in a perpendicular ferromagnetic multilayer nanotrack. The transversal skyrmion motion induced by the skyrmion Hall effect is balanced through introducing a y-axial current. A three-terminals logic output is achieved by adjusting the strength of currents along x and y directions. The interlayer coupling effect for stabilizing skyrmion nucleation is finely tuned via a ferromagnetic pinning layer in absent of the external magnetic field. It is found that both the perpendicular magnetic anisotropy and the Dzyaloshinskii-Moriya interaction are critical for the formation of stabilized skyrmions. The controllable skyrmion motion and multi-terminal information transmission offer a novel path for designing skyrmion-based logic and memory devices. [ABSTRACT FROM AUTHOR]

The grain boundary diffusion process (GBDP) is one of the most efficient treatments for increasing the coercivity (H c) of Nd-Fe-B magnets. However, this enhancement typically occurs at the expense of remanence (M r). In this study, micromagnetic simulations were performed to quantify this tradeoff in hot-deformed Nd-Fe-B magnets subjected to the GBDP using a Nd-based eutectic alloy. The GBDP was imitated in a series of models with a gradually increasing volume fraction of the infiltrated Nd-rich nonmagnetic phase. This infiltration reduced the remanence and grain connectivity via the remaining thin magnetic intergranular phase (IGP), which in turn increased the coercivity. We distinguished between the roles of exchange and magnetostatic interactions in this coercivity enhancement. Furthermore, the simulated M r vs. H c curves defined realistic limits for coercivity that depended on the IGP magnetization, which was estimated to be 0.9 ± 0.1 T by reproducing experimental M r vs. H c data from the literature. [Display omitted] [ABSTRACT FROM AUTHOR]

The grain boundary diffusion process (GBDP) has been widely applied to increase the coercivity of Nd-Fe-B magnets. After GBDP with Dy/Tb-rich diffusion sources the thickness of the Dy/Tb-rich shell formed on the epitaxial layer of the 2:14:1 main phase grain decreases from the magnet surface to the center. However, the influence of the Dy/Tb-rich shell gradient distribution on magnetic properties has not been thoroughly studied. In this work, a sintered Nd-Fe-B magnet was subjected to GBDP with Pr 60 Tb 10 Cu 30 alloy at 860°C for various diffusion times (3 h, 6 h and 9 h). The coercivity improves rapidly from 884 kA/m (without GBDP) to 1533 kA/m after GBDP of 3 h. The coercivity further increases to 1741 kA/m with increased diffusion time to 6 h. But only marginal coercivity enhancement (rising to 1803 kA/m) can be obtained by further prolonging the diffusion time to 9 h. Microstructure analysis indicates that the long diffusion time leads to the surface grain coarsening, which degrades the diffusion efficiency. Meanwhile, micromagnetic simulation indicates that if the thickness of the Tb-rich shell in magnet center is less than 4 nm, the coercivity increases significantly with the enhanced thickness uniformity of the Tb-rich shell. But if the thickness of the Tb-rich shell in magnet center is higher than 4 nm, the coercivity cannot be improved effectively by further increasing the thickness uniformity of the Tb-rich shell. The results in this work clarify the mechanism of the magnetic property dependence on the diffusion time and help to optimize the GBDP parameters in the future. • Nd-Fe-B magnets were subjected to grain boundary diffusion with Pr 60 Tb 10 Cu 30 alloy. • The coercivity increasing rate decreases with the prolonged diffusion time. • Long diffusion time leads to the surface grain coarsening. • The diffusion depth contributes more to the coercivity than Tb-rich shell thickness. [ABSTRACT FROM AUTHOR]

As a potential candidate for critical chemical element free permanent magnet, (Fe 0.7 Co 0.3) 2 B, shows promising intrinsic magnetic properties, including large magnetization, high Curie temperature and moderate magnetocrystalline anisotropy (MCA). Still, the coercivity that has been achieved is relatively low. To understand and improve coercivity, we study the robustness of MCA related to the fluctuation of chemical composition and the structural distortion and its effect on coercivity in (Fe 0.7 Co 0.3) 2 B from first principles DFT (density functional theory) calculation and micromagnetic simulation. A fluctuation of cobalt content of 10%, or 1% isotropic lattice strain, or 2% tetragonal distortion reduces MCA energy up to 30%. By considering randomly distributed local MCA energy reduction up to 30%, the calculated coercivity of anisotropic bulk magnets reduced from 3.6 kOe to 2.2 kOe. Rational microstructure design, such as grain size refinement and development of strong crystal texture of MCA easy-axis (i.e., degree of alignment for (Fe 0.7 Co 0.3) 2 B grains), can enhance coercivity and mitigate the effect of local MCA reduction. In addition to acceptable intrinsic magnetic properties, the promising hard magnetic phase needs good MCA robustness to facilitate the development of practical magnets. [ABSTRACT FROM AUTHOR]

With the widespread applications of sintered NdFeB, the impact-prone scenarios are often encountered. Such impacts will cause magnetic disorder inside the magnet and further demagnetization. At present, research on impact-induced demagnetization of sintered NdFeB mostly focuses on experimental investigations, lacking clear theoretical expressions and visual reproduction of the microscopic demagnetization process. To end this, this paper uses the impact-induced demagnetization experiment, theoretical expression, and micromagnetic simulation to reveal and quantify the demagnetization behavior of NdFeB. An impact-induced demagnetization experimental platform capable of producing corresponding load amplitudes is first established. The experimental results demonstrate the existence of reversible and irreversible demagnetization of NdFeB under impact. A magneto-mechanical constitutive model for sintered NdFeB considering stress history is further developed using the principle of thermodynamic, pinning effect and proximity effect. The residual magnetization intensity between the numerical simulation and the experimental results have good agreement. Finally, the demagnetization behavior of the magnet is visually displayed through the micromagnetic simulation. Employing the LLG equation and kinematic equations for elastic waves, a set of magnetization dynamics equations is established to analyze impact-induced demagnetization characteristics in sintered NdFeB. These results will provide the reference for the application of NdFeB in impact environment. • Carry out impact-induced demagnetization experiment of sintered NdFeB. • Obtain the magneto-mechanical constitutive demagnetization model of sintered NdFeB. • Reveal the demagnetization mechanism of sintered NdFeB by micromagnetic simulation. • Verify the constitutive model through the impact-induced demagnetization experiment and micromagnetic simulation results. [ABSTRACT FROM AUTHOR]

Multi-level magnetic recording is a new concept for increasing the data storage capacity of hard disk drives. However, its implementation has been limited by a lack of suitable media capable of storing information at multiple levels. Herein, we overcome this problem by developing dual FePt-C nanogranular films separated by a Ru-C breaking layer with a cubic crystal structure. The FePt grains in the bottom and top layers of the developed media exhibited different effective magnetocrystalline anisotropies and Curie temperatures. The former is realized by different degrees of ordering in the L1 0 -FePt grains, whereas the latter was attributed to the diffusion of Ru, thereby enabling separate magnetic recording on each layer under different magnetic fields and temperatures. Furthermore, magnetic measurements and heat-assisted magnetic recording simulations showed that these media enabled 3-level recording and could potentially be extended to 4-level recording, as the ↑↓ and ↓↑ states exhibited non-zero magnetization. [Display omitted] [ABSTRACT FROM AUTHOR]

Greigite magnetosomes produced by magnetotactic bacteria (MTB) are widely distributed in natural environments, but large uncertainties remain regarding their magnetic biosignatures. Here, we have constructed micromagnetic models with realistic biogenic greigite particles to quantify the magnetic properties and magnetotaxis efficiency of greigite‐producing MTB cells. Our calculations suggest coercivity (Bc) of ∼15–21 mT for intact greigite‐producing rod‐shaped MTB and many‐celled magnetotactic prokaryotes, with Bc decreasing to ∼11 mT for greigite magnetofossils with clumped particles. These magnetic signatures make biogenic greigite distinguishable from typical biogenic magnetite and inorganic greigite, providing reliable magnetic criteria to detect biogenic greigite in a wide range of environmental and geological settings. Our numerical calculations suggest that rod‐shaped greigite‐producing MTB have a similar magnetotaxis efficiency to magnetite MTB, likely by biomineralizing more greigite crystals to compensate for the lower saturation magnetization of greigite and less ordered chains in greigite MTB cells, demonstrating biological‐controlled optimization of their magnetic nanostructure. Plain Language Summary: Magnetic bacteria can produce greigite (Fe3S4) or magnetite (Fe3O4) nanoparticles arranged in chains and use them as nano‐compass for navigating along the geomagnetic field lines. Dead magnetic bacteria can be fossilized in the geological records that retain important signals of past geomagnetic field, environmental conditions, and biochemical processes in the Earth surface. Until now magnetic biosignatures of bacterial greigite are unclear because it is difficult to obtain pure greigite magnetotactic bacteria samples to measure their magnetic properties. Here, we constructed computer models that mimic those found in living greigite‐producing magnetic bacteria. Model calculations determined robust magnetic fingerprints of biogenic greigite that can be used to search for fossilized biogenic greigite nanoparticles in natural environments. Moreover, according to our calculations, greigite‐producing magnetic bacteria have similar navigational ability to bacterial magnetite counterparts by producing more biogenic greigite crystals in the bacterial cells, suggesting that magnetic nanostructures are optimized by those magnetic microorganisms. Key Points: Micromagnetic calculations reveal robust magnetic biosignatures of biogenic greigite that have been difficult to determine experimentallyModeled hysteresis properties of biogenic greigite provide magnetic criteria for their identification in a wide range of environmentsGreigite‐producing magnetotactic microorganisms likely have optimized their magnetic nanostructure for navigational and other purposes [ABSTRACT FROM AUTHOR]

Exchange-coupled magnets are promising candidates for a new generation of permanent magnets. Here, we investigated the effect of soft magnetic shell thickness and the aspect ratio of the hard magnetic core on the magnetic properties for isolated core/shell cylinder exchange-coupled magnets, as well as the packing effect of the cylindrical array via a micromagnetic simulation method. It was found that the shape anisotropy contributions to the magnetic properties in the cylindrical core/shell exchange-coupled magnets are closely related to the thickness of the soft magnetic shell. When the soft magnetic shell is thin, the magnetic properties are dominated by the hard–soft exchange coupling effects, and the contributions of shape anisotropy are quite limited. When the soft magnetic shell is relatively thick, utilizing shape anisotropy would be an effective method to improve the magnetic performance of hard–soft exchange-coupled magnets. The present work provides an in-depth fundamental understanding of the underlying magnetization reversal mechanism. This work could be useful for designing high-performance permanent magnets and avoiding pitfalls. [ABSTRACT FROM AUTHOR]

To avoid electron leakage and energy consumption as in present‐day bipolar junction transistor, the magnetic analog of the bipolar junction transistor using magnetic vortices proves to be an excellent alternative. This alternative exploration is implemented using micromagnetic simulations, which use spin‐polarized current to study the dynamics of magnetic vortices in physically separated but magnetostatically coupled nanodisks. Here, a magnetic vortex transistor (MVT), when the nanodisks are arranged in a triangular layout, is designed. Significant gain is observed for the scalene triangle configuration for a three‐vortex system when the polarities of the vortices are +1, −1, +1 and for the equilateral triangular configuration when the polarities are −1, +1, +1, respectively. The mechanism of energy transfer in such magnetostatically coupled vortices has been explained in terms of the dynamics of antivortex solitons' movement through the stray field. The variation of damping parameters has no significant effect on signal transfer and amplification. Through this extensive micromagnetic calculation, the possibility of tuning the MVT by varying the configuration, polarity combination, and changing the position of the input signal is demonstrated. This study offers a prospect for nonvolatile, all magnetic information‐carrying nanoscale devices and magnetic logic gates and switches. [ABSTRACT FROM AUTHOR]

In the current work, a core-shell type exchange coupled composite structure was constructed by micromagnetic simulation with a phase FePt core and an iron shell. Four types of switching loops with magnetic domain structure evolution were demonstrated. Based on the simulation results, a switching type diagram was constructed, which displays various hysteresis loops as a function of core radius and shell thickness. Furthermore, the effects of switching type and composite structure on the coercivity and remanent magnetization were predicted and discussed. This finding indicates that core-shell type FePt/Fe composite structure film has a large advantage in designing exchange-coupled bit patterned media to realize high-density storage devices at the nanoscale. [ABSTRACT FROM AUTHOR]

The skyrmion Hall effect (SkHE) is usually considered as a drawback in technological applications based on magnetic skyrmion and much effort has been made to suppress the SkHE. Instead, here we demonstrate that the SkHE can be used to design a useful skyrmion diode. Micromagnetic simulations show that, in a stripe-shaped device with a lateral asymmetry, diode-like forward breakover and reverse cutoff functions can be realized based on the current-induced motion of skyrmion. Our results may suggest a promising pathway toward the development of spintronic devices based on unidirectional skyrmion transport. [ABSTRACT FROM AUTHOR]

Skyrmionium is a magnetic configuration of zero-topological charge, which has promising applications in the field of Spintronics. In this paper, for a nanoring structure at different dimensions, we study the eigenmode frequency of a ground state skyrmionium. Furthermore, we demonstrated the nucleation of skyrmionium on a nanodisk by applying an oscillating perpendicular magnetic field. The applied magnetic field oscillates at a frequency equal to the eigenfrequency of skyrmionium in a nanoring. Additionally, we determined the threshold magnetic field intensity and pulse duration required for nucleation of skyrmionium on a nanodisk at each eigenfrequency. Furthermore, we demonstrate that a perpendicular oscillating magnetic field with an eigenmode frequency can switch skyrmion and 3π skyrmions to skyrmionium. Our results will facilitate the research of skyrmionium in different nanostructures and serve as a foundation for skyrmionium based applications. [ABSTRACT FROM AUTHOR]

Modulated FeCo nanowires were studied by ferromagnetic resonance (FMR) using micromagnetic simulation. The diameter-modulated nanowires were chosen with smaller and larger diameters of 100 and 130 nm, respectively, while 300 nm for the length of the shortest cross section and 450 nm for the longest cross section were used. The dimensions were chosen based on the experiments presented in the literature. For the simulation, the OOMMF package and the standard problem for FMR were combined to determine the magnetization dynamic. With the fast Fourier transform (FFT), three frequencies were associated to different locations along the nanowire: region A (at the ends of the nanowire) with 100 nm in diameter (f1); region B with 130 nm (f2); and region C with 100 nm in diameter and localized inside the nanowires (f3). These frequencies were analyzed in the FMR condition to determine the effective field experienced by these segments and the self-demagnetizing contribution. [ABSTRACT FROM AUTHOR]

Grain boundary diffusion (GBD) of multiple rare earths has proven effective in enhancing the performance of permanent magnets, whereas the diffusion behaviors are still unclear due to their intricate interactions. In this work, the synergistic effects of three rare earths (Tb, Ce, and La) on the grain boundaries diffusion process are investigated, achieving a high‐performance diffused Nd–Fe–B magnet with the high coercivity of 1822 kA m−1 and thermal stability of −0.447% K−1. This promotion originates from the improved diffusion depth of Tb at specific regions, induced by the coordination effects of Ce and La, as evidenced via electron probe microanalysis and the first‐principles calculations. It thus results in the uniform formation of Tb‐diffused magnetic harden grains, associated with the strongly pinned domain walls, which is further observed directly by in situ Lorentz transmission electron microscopy and reconstructed by micromagnetic simulations. The study provides insight into the role of element GBD mechanism and has promising potential to explore other element systems as diffusion sources for permanent magnets. [ABSTRACT FROM AUTHOR]

The magnetization reversal processes of hydrogenation–disproportionation–desorption–recombination (HDDR) anisotropic Pr–Fe–B magnetic powders were systematically investigated. It is found that with the increase of external magnetic field, the magnetization first increases rapidly, and then increases slowly, and finally increases rapidly again. The increasing trend of coercivity with the increase of external magnetic field is just opposite to that mentioned above. Further micromagnetic simulation studies suggest that the coercivity is mainly controlled by the nucleation of reversed domain, and this is also indicated by the subsequent investigation for the evolution of the magnetic domain under the magnetic field. [ABSTRACT FROM AUTHOR]

In this article, we experimentally demonstrate the propagation of magnetostatic surface spin wave (MSSW) in the orthogonally joined yttrium iron garnet stripes in the form of T-shaped connection. By the means of microwave spectroscopy technique, we show how the transmission is affected by the distance between orthogonal yttrium iron garnet stripes. Micromagnetic modeling demonstrates the presence of nonreciprocal propagation of spin waves (SWs) between all three ports of the T-junction. A mechanism for controlling the spatial-frequency selection of a propagating signal by variation of the air gap in the area of connected orthogonal sections is shown. The proposed structure can act as a magnonic splitter for 3-D topology of multilevel magnonic networks (MNs). The transition to a multilayer topology of MNs opens the possibility of increase of the packing density of computational units based on the principles of magnonics. [ABSTRACT FROM AUTHOR]

Transition curvature is a major contributor to transition broadening in current heat-assisted magnetic recording (HAMR) devices. In this article, in order to clarify HAMR performance versus bit length (BL), a detailed study of the dependence of HAMR transition curvature on BL is presented. With the help of micromagnetic simulations, HAMR bits with three different BLs (10.5, 15, and 21 nm) are written and transition curvatures are calculated. Both thermal exchange coupling composite (ECC) media and pure FePt media are tested. By tracking the change of transition curvature during the HAMR cooling process, an erase-after-write effect is observed. DC-erased media, introduction of intergranular exchange, and removal of dipole interactions are also examined for potential optimization of the HAMR system. [ABSTRACT FROM AUTHOR]

We report the results of a micromagnetic simulation of a vortex domain wall (VDW) dynamic pinning probability in 20 nm thick planar permalloy wires with triangular notches and widths of 100, 200, and 300 nm. The notches have a fixed depth ratio of 30%, which is depth divided by wire width. Both the wire width and the applied field influenced the dynamic VDW pinning probability and the pinning probability decreased as the wire width increased. We observed a pinned domain wall (DW) structure around the notches and defined two types of pinning states, transverse DW (TDW) pinning and VDW pinning. The 100 nm wide wire had more TDW pinning-type events than did the 200 and 300 nm wide wires. The 300 nm wide wire had the lowest number of TDW pinning-type events. These results were caused by different wire widths having different DW energy densities and different pinning potential energy landscapes. When the applied field was larger than the Walker field, the wide wires easily led to a complex VDW motion. [ABSTRACT FROM AUTHOR]