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We achieve strong magnon-magnon coupling experimentally between edge and center modes in patterned hexagonal CoFeB films. The coupling strength is adjustable via the external magnetic field's direction and the nanomagnet's aspect ratio. Micromagnetic simulation indicates that the stable existence of the center mode determines the occurrence of coupling, which correlates with the direction of the edge demagnetization field. This work offers a novel perspective on the study of magnon-magnon coupling in a single material, which benefits magnon-based quantum information technology. [ABSTRACT FROM AUTHOR]

Nowadays, the concept of non-trivial topological protection and the nanoscale size of nanomagnetic particles constitute a major area of research. Due to topological protection stability, nanoscale size, and the requirement of low spin current density for motion, skyrmions have attracted great attention in next-generation spintronic devices as robust information carriers. We study the motion of an isolated magnetic skyrmion with induced interfacial Dzyaloshinskii-Moriya interaction (iDMI) instigated by spin waves and driven by spin current with variation in different parameters in a nanotrack of finite length using micromagnetic simulations. It is found that the magnetic skyrmion moves in the same direction as the direction of propagation of the spin waves. The skyrmion initially experiences an acceleration in its motion; thereafter, the velocity decreases exponentially. The motion of the magnetic skyrmion initiates as the momentum of the spin wave gets transferred to it. The motion of the magnetic skyrmion is found to be significantly dependent on the variation of parameters like frequency and amplitude of the incident spin waves, as well as the damping parameter and the strength of the applied spin-polarized current. The results obtained in this work could become useful to design skyrmion-based spintronic information-carrying and storage devices. [ABSTRACT FROM AUTHOR]

In this study, we investigate the thermal pinning and depinning behaviors of vortex domain walls (VDWs) in constricted magnetic nanowires, with a focus on potential applications in storage memory nanodevices. Using micromagnetic simulations and spin transfer torque, we examine the impacts of device temperature on VDW transformation into a transverse domain wall (TDW), mobility, and thermal strength pinning at the constricted area. We explore how thermal fluctuations influence the stability and mobility of domain walls within stepped nanowires. The thermal structural stability of VDWs and their pinning were investigated considering the effects of the stepped area depth (d) and its length (λ). Our findings indicate that the thermal stability of VDWs in magnetic stepped nanowires increases with decreasing the depth of the stepped area (d) and increasing nanowire thickness (th). For th ≥ 50 nm, the stability is maintained at temperatures ≥ 1200 K. In the stepped area, VDW thermal pinning strength increases with increasing d and decreasing λ. For values of d ≥ 100 nm, VDWs depin from the stepped area at temperatures ≥ 1000 K. Our results reveal that thermal effects significantly influence the pinning strength at constricted sites, impacting the overall performance and reliability of magnetic memory devices. These insights are crucial for optimizing the design and functionality of next-generation nanodevices. The stepped design offers numerous advantages, including simple fabrication using a single electron beam lithography exposure step on the resist. Additionally, adjusting λ and d allows for precise control over the pinning strength by modifying the dimensions of the stepped areas. [ABSTRACT FROM AUTHOR]

In this work, we investigate the influence of curvature on the dynamic susceptibility in FeGe nanowires, both curved and straight, hosting a skyrmionic tube texture under the action of an external bias field, using micromagnetic simulations. Our results demonstrate that both the resonance frequencies and the number of resonant peaks are highly dependent on the curvature of the system. To further understand the nature of the spin wave modes, we analyze the spatial distributions of the resonant mode amplitudes and phases, describing the differences among resonance modes observed. The ability to control the dynamic properties and frequencies of these nanostructures underscores their potential application in frequency-selective magnetic devices. [ABSTRACT FROM AUTHOR]

The evolution process of magnetic domains in response to external fields is crucial for the modern understanding and application of spintronics. In this study, we investigated the domain rotation in stripe domain films of varying thicknesses by examining their response to microwave excitation in four different orientations. The resonance spectra indicate that the rotation field of stripe domain film under an applied magnetic field approaches the field where the resonance mode of sample changes. The saturation field of the stripe domain film corresponds to the field where the resonance mode disappears when measured in the stripe direction parallel to the microwave magnetic field. The results are reproducible and consistent with micromagnetic simulations, providing additional approaches and techniques for comprehending the microscopic mechanisms of magnetic domains and characterizing their rotation. [ABSTRACT FROM AUTHOR]

We investigated the temperature dependence of agglomeration systems consisting of interacting single-domain magnetic nanoparticles (MNPs). In our study, we utilized electron spin resonance (ESR) experiments to measure how temperature-induced change in agglomerate state affects the peak-to-peak width of ESR spectra (Hpp) and validated these observations through simulations. Micromagnetic simulations were conducted by combining the classical magnetic dipolar Metropolis Monte Carlo method with the semi-classical macro-spin Landau-Lifshitz-Gilbert finite element method, and the results were compared with experimental findings. Our research elucidates the behavior of thermally-induced disruption of MNP agglomerates. We discovered that the temperature at which MNPs are completely dispersed might correspond to one at which the temperature dependence of Hpp disappears.

In the future, DW memory will replace conventional storage memories with high storage capacity and fast read/write speeds. The only failure in DW memory arises from DW thermal fluctuations at pinning sites. This work examines, through calculations, the parameters that might help control DW thermal stability at the pinning sites. It is proposed to design a new scheme using a stepped area of a certain depth (d) and length (λ). The study reveals that DW thermal stability is highly dependent on the geometry of the pinning area (d and λ), magnetic properties such as saturation magnetization (Ms) and magnetic anisotropy energy (Ku), and the dimensions of the nanowires. For certain values of d and λ, DWs remain stable at temperatures over 500 K, which is beneficial for memory applications. Higher DW thermal stability is also achieved by decreasing nanowire thickness to less than 10 nm, making DW memories stable below 800 K. Finally, our results help to construct DW memory nanodevices with nanodimensions less than a 40 nm width and less than a 10 nm thickness with high DW thermal stability. [ABSTRACT FROM AUTHOR]

(Fe0.7Co0.3)2B are potential permanent magnets material due to its large saturation magnetization and high Curie temperature. However, it has moderate magnetocrystalline anisotropy (MCA) and low coercivity. One way to improve its coercivity is to combine the contributions from magnetocrystalline- and magnetic-shape anisotropy by preparing (Fe0.7Co0.3)2B nanowires. We study the effects of size, morphology, and surface defects on the hard magnetic properties of nanowires using micromagnetic simulation. The hard magnetic properties of (Fe0.7Co0.3)2B nanowire-bonded magnets are estimated, including the role of inter-wire magnetostatic interaction. By considering the existence of local reductions in MCA energy of up to 30% on the surface layer of nanowires, the anisotropic bonded magnet with a 65% vol. of (Fe0.7Co0.3)2B nanowires would have typical remanence, Br= 7.6–8.4 kG, coercivity, Hci= 9.6–9.9 kOe, and maximum energy product, (BH)m = 14–17.8 MGOe. Developing effective technology for synthesizing nanowires and fabricating corresponding bonded magnets is promising for manufacturing practical magnets based on the magnetic phase with a relatively low or moderate MCA, such as (Fe0.7Co0.3)2B. [ABSTRACT FROM AUTHOR]

Swirling spin configurations with a non-trivial topological state are popularly known as magnetic skyrmions. They are often observed in magnetic thin films with perpendicular magnetic anisotropy and Dzyaloshinskii–Moriya interaction (DMI). Due to their small size and non-collinear spin texture, they are being considered as a potential candidate for spintronic devices such as data storage devices, and logic-based devices. These skyrmions have distinct properties and interactions, that can be utilized to control their movement by applying an external electric current. In this work, we have successfully demonstrated the operation of skyrmion-based universal NAND and NOR logic gates. We have further performed arithmetic addition of two and three single-bit by using a half and full adder, fabricated based on the XOR and AND gates. We have observed that the presence of only uniform DMI is not enough to achieve all logic operations. The presence of a negative DMI at the edges is crucial in achieving proper logic operations. The findings are very important for the realization of skyrmion-based logic and computational operations. [ABSTRACT FROM AUTHOR]

We investigated the temperature dependence of agglomeration systems consisting of interacting single-domain magnetic nanoparticles (MNPs). In our study, we utilized electron spin resonance (ESR) experiments to measure how temperature-induced change in agglomerate state affects the peak-to-peak width of ESR spectra (H pp) and validated these observations through simulations. Micromagnetic simulations were conducted by combining the classical magnetic dipolar Metropolis Monte Carlo method with the semi-classical macro-spin Landau-Lifshitz-Gilbert finite element method, and the results were compared with experimental findings. Our research elucidates the behavior of thermally-induced disruption of MNP agglomerates. We discovered that the temperature at which MNPs are completely dispersed might correspond to one at which the temperature dependence of H pp disappears. [ABSTRACT FROM AUTHOR]

Nanoscaled magnetic particle ensembles are promising building blocks for realizing magnon based binary logic. Element-specific real-space monitoring of magnetic resonance modes with sampling rates in the GHz regime is imperative for the experimental verification of future complex magnonic devices. Here we present the observation of different phasic magnetic resonance modes using the element-specific technique of time-resolved scanning transmission x-ray microscopy within a chain of dipolarly coupled Fe3O4 nanoparticles (40-50 nm particle size) inside a single cell of a magnetotactic bacterium Magnetospirillum magnetotacticum. The particles are probed with 25 nm resolution at the Fe L3 x-ray absorption edge in response to a microwave excitation of 4.07 GHz. A plethora of resonance modes is observed within multiple particle segments oscillating in- and out-of-phase, well resembled by micromagnetic simulations.

Exploration and manipulation of topological protected magnetic swirls, such as skyrmion, antiskyrmion, meron, and vortex, holds significance for fundamental research and practical applications in high‐density magnetic information storage and spintronics because of their high storage density and low driven current. This study unveils the existence of field‐free spontaneous (anti)meron chains with a topological charge of ±1/2 in the centrosymmetric rare‐earth magnet Tb6Co2.17Si2.5 via in situ real‐space observation. The spin reorientation transition from in‐plane to uniaxial anisotropy contributes to the spontaneous transformation from straight domain walls to topological (anti)meron chains and to stripe domains along the [110] zone axis during in situ cooling. The study further confirms the critical role of the noncollinear magnetic structure of Tb atoms in the formation of topological (anti)meron chains via real‐space observations, first‐principles calculations, and micromagnetic simulations. The spontaneous topological magnetic texture is strongly correlated with the 4f electrons of rare‐earth atoms, enriching and stimulating alternative generation mechanisms of topological spin textures from emerging rare‐earth magnets, and further applications in spintronics. [ABSTRACT FROM AUTHOR]

Copyright of Rare Metals is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Magnetic skyrmion random switching and structural stability are critical limitations for storage data applications. Enhancing skyrmions’ magnetic properties could improve their thermal structural stability. Hence, micromagnetic calculation was carried out to explore the thermal nucleation and stability of skyrmions in magnetic nanodevices. Different magnetic properties such as uniaxial magnetic anisotropy energy (Ku), saturation magnetization (Ms) and Dzyaloshinskii—Moriya interaction (DMI) were used to assess the thermal stability of skyrmions in magnetic nanowires. For some values of Ms and Ku, the results verified that the skyrmion structure is stable at temperatures above 800 K, which is higher than room temperature. Additionally, manipulating the nanowire geometry was found to have a substantial effect on the thermal structural stability of the skyrmion in storage nanodevices. Increasing the nanowire dimensions, such as length or width, enhanced skyrmions’ structural stability against temperature fluctuations in nanodevices. Furthermore, the random nucleation of the skyrmions due to the device temperature was examined. It was shown that random skyrmion nucleation occurs at temperature values greater than 700 K. These findings make skyrmion devices suitable for storage applications.

In this work, we investigate the influence of curvature on the dynamic susceptibility in FeGe nanowires, both curved and straight, hosting a skyrmionic tube texture under the action of an external bias field, using micromagnetic simulations. Our results demonstrate that both the resonance frequencies and the number of resonant peaks are highly dependent on the curvature of the system. To further understand the nature of the spin wave modes, we analyze the spatial distributions of the resonant mode amplitudes and phases, describing the differences among resonance modes observed. The ability to control the dynamic properties and frequencies of these nanostructures underscores their potential application in frequency-selective magnetic devices.

In this study, we investigate the thermal pinning and depinning behaviors of vortex domain walls (VDWs) in constricted magnetic nanowires, with a focus on potential applications in storage memory nanodevices. Using micromagnetic simulations and spin transfer torque, we examine the impacts of device temperature on VDW transformation into a transverse domain wall (TDW), mobility, and thermal strength pinning at the constricted area. We explore how thermal fluctuations influence the stability and mobility of domain walls within stepped nanowires. The thermal structural stability of VDWs and their pinning were investigated considering the effects of the stepped area depth (d) and its length (λ). Our findings indicate that the thermal stability of VDWs in magnetic stepped nanowires increases with decreasing the depth of the stepped area (d) and increasing nanowire thickness (th). For th ≥ 50 nm, the stability is maintained at temperatures ≥ 1200 K. In the stepped area, VDW thermal pinning strength increases with increasing d and decreasing λ. For values of d ≥ 100 nm, VDWs depin from the stepped area at temperatures ≥ 1000 K. Our results reveal that thermal effects significantly influence the pinning strength at constricted sites, impacting the overall performance and reliability of magnetic memory devices. These insights are crucial for optimizing the design and functionality of next-generation nanodevices. The stepped design offers numerous advantages, including simple fabrication using a single electron beam lithography exposure step on the resist. Additionally, adjusting λ and d allows for precise control over the pinning strength by modifying the dimensions of the stepped areas.

Recently, it has been reported that the skyrmion Hall effect can be suppressed in an elliptical skyrmion-based device. Given that defects are unavoidable in materials, it is necessary and important to investigate the dynamics of an elliptical skyrmion in a defective racetrack device. In this work, the current-driven dynamics of an elliptical skyrmion in a defective racetrack device are systematically studied using micromagnetic simulations. The system energy analysis reveals that the magnetic parameters of the circular defect play critical roles in determining the type (repulsive or attractive) and the magnitude of the force on the elliptical skyrmion. The simulated trajectories show that the primary motion modes of the elliptical skyrmion in the defective racetrack can be divided into four types, which are dependent on the values of the Dzyaloshinskii–Moriya interaction (DMI) constant Dd, the perpendicular magnetic anisotropy constant Kd, the magnitude of the driving current density J, and the size d of the defect. Further investigation of the motion-mode phases of the skyrmion reveals the synthetic effects of Dd, Kd, J, and d. Finally, the minimum depinning current density J, which linearly depends on the parameters of Dd and Kd, is obtained for a skyrmion completely pinned in the defect. Our findings give insights into the dynamics of an elliptical skyrmion in the presence of a defect with different magnetic parameters in a racetrack device and may be useful for performance enhancement of skyrmion-based racetrack memory devices. [ABSTRACT FROM AUTHOR]

The interlayer coupling and magnetization reversal behaviors in NiFe/NiO/NiFe trilayers were investigated using polarized neutron reflectivity and Monte Carlo (MC) simulations. Our results reveal that the shape of NiFe loops transitions from square to tilted as the NiO thicknesses decrease, indicating changes in the direction of NiFe layer's easy axis. This phenomenon can be attributed to variations occurring at NiO/NiFe interfaces for different NiO layer thicknesses. With thin NiO layer, interdiffusion at the NiO/NiFe interfaces leads to frustrated coupling, resulting in a noncollinear interlayer coupling. This observation is supported by MC simulations. Conversely, hardly any coupling frustration is observed for the sample with a thick NiO layer. Our findings propose a novel way to tailor the interlayer coupling through interface engineering. [ABSTRACT FROM AUTHOR]

In this work, the influences of core–shell structure and cerium (Ce) content on magnetic properties of Nd-Ce-Fe-B permanent magnets were investigated by the finite difference micromagnetic (FDM) simulation method, and the process of magnetization reversals was systematically discussed. For the (Nd1-xCex)2Fe14B-(Dy2Fe14B) core–shell grains with different core sizes, the coercivity (Hcj) dramatically increases with the shell thickness from 0 to 2 nm and then increases slightly with shell thickness increase to 20 nm; for the grains with the same shell thickness, the Hcj decreases with the increase of core size, and it is due to the increase of demagnetization energy. Moreover, the Hcj remarkably deteriorates as the Ce content increases, which is caused by the lower magnetic crystal anisotropy field (HA). The hysteresis loops of core–shell models exhibit excellent squareness with a shell thickness of 2 nm at different Ce proportions. However, a significant two-step demagnetization feature for the model with the thicker shell sizes and higher Ce content results from the different demagnetization abilities of the shell and core. In addition, in the angular distributions of magnetization, the reversal of the core region is later than that of the outer shell, and the applied field required to inverse increases with the shell thickness. The abnormal two-step demagnetization was caused by the twice abrupt changes that occurred in the Zeeman energy (Eext) and total energy (Etot). [ABSTRACT FROM AUTHOR]

In the present work, a study on magnetic symmetry breaking is carried out in 3 × 3 arrangements of hollow Ni nanopillars. For the study, the results of ferromagnetic resonance were used by micromagnetic simulation. The external field is varied in intensity and in different orientations in order to determine the resonance field in each direction. We mainly use the configuration with a field parallel to the axis of the pillars (PL) and for a field perpendicular to the pillars. Then, a sweep is made at different angles in the plane perpendicular to the pillars. The array is composed of PL with a square base of 30 nm lateral dimension and 120 nm length. Each PL has an internal cavity that extends the length of the structure. Two sizes of this well were chosen, 10 and 20 nm. The distance between pillars is changed by four values so that the walls of the pillars are distanced in a-D = 5, 10, 20, and 50 nm. This variation was schooled in such a way that the dipole interactions were from very low values to relatively large values to the point of breaking the symmetry due to magnetostatic interactions between PL. The angular dependence of this frequency as a function of the perpendicular field determines the anisotropy field and with that, it is possible to understand from the point of view of symmetry associated with the independent PL, the effects of dipole interactions between pillars in the arrangement. [ABSTRACT FROM AUTHOR]

A numerical method for computing ferromagnetic resonance (FMR) is adapted for a confined bilayer exchange-coupled film. In this method, the system is unbalanced from the equilibrium by a short weak external magnetic field, and then the OOMMF micromagnetic simulation package is used to simulate the system's return to equilibrium and to record the resulting magnetization dynamics. After performing the Fourier transform of the time series of magnetization variation, the frequency dependences of the spectral density are calculated. The resonance frequencies are determined by correlating the maxima of these dependences with their corresponding frequencies. The study concerns the main modes and corresponding distributions of magnetization excitation in films of varying sizes when the constant magnetic field changes from saturating to zero. The FMR has been investigated, specifically, for the earlier studied vortex distribution of magnetization in the normal state. It is shown that the transition of the main mode to the low-frequency region occurs when the intensity of the external magnetic field decreases. This technique can be used to analyze the dynamic properties of multiferroic heterostructures in the future. [ABSTRACT FROM AUTHOR]

Copyright of Momento: Revista de Física is the property of Universidad Nacional de Colombia, Departamento de Fisica and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Magnetotactic bacteria (MTB) have the remarkable capability of producing intracellularly membrane-enveloped magnetic nanocrystals (i.e. magnetosomes) and swimming along geomagnetic field lines. Despite more than 50 years of research, bacterial diversity and magnetosome biomineralization within MTB are relatively less known in the Gammaproteobacteria class than other groups. This is incompatible with the status of Gammaproteobacteria as the most diverse class of gram-negative bacteria with a number of ecologically important bacteria. Here, we identify a novel MTB strain YYHR-1 affiliated with the Gammaproteobacteria class of the Pseudomonadota phylum from a freshwater lake. In YYHR-1, most magnetosome crystals are organized into a long chain aligned along the cell long axis; unusually, a few small superparamagnetic crystals are located at the side of the chain, off the main chain axis. Micromagnetic simulations indicate that magnetostatic interactions among adjacent crystals within a chain reduce the Gibbs energy to enhance chain stability. Genomic analysis suggests that duplication of magnetosome gene clusters may result in off-chain magnetosomes formation. By integrating available genomic data from Gammaproteobacteria , the phylogenetic position of MTB in this class is reassigned here. Our new findings expand knowledge about MTB diversity and magnetosome biomineralization, and deepen understanding of the phylogenetics of the Gammaproteobacteria. [ABSTRACT FROM AUTHOR]

Abstract Harnessing extra degrees of freedom at the heterostructure interface is of crucial importance to bring additional functionalities in modern spintronic devices. Here, a vertical hysteresis loop shift (vertical bias) is demonstrated in an exchange biased system of ferromagnetic thin film heterostructure of La0.67Sr0.33MnO3 (10 nm)‐SrRuO3 (SRO) (20 nm), after field cooling with ±1 T below 100 K close to the Curie temperature (TC) ≈125 K of SRO and loop sweeping under ±1 T field. Besides, a positive exchange bias (HEB) is also observed below TC ≈125 K showing a maximum ≈11 mT at 2 K. The vertical shift is modeled closely using micromagnetic simulations and the layers’ thickness dependency is demonstrated. The reason for the shift is attributed to the simultaneous role of the interfacial antiferromagnetic interaction and the hard anisotropy of SRO against the Zeeman energy. Finally, from the experimental and simulation results, a generalized model of controllable and tunable vertical shift is proposed applicable for other material systems possessing glassy phases, uncompensated/canted spins, absent interfacial exchange coupling, etc., and hence can be informative for the use of vertical shift in future spintronic devices.

In the future, DW memory will replace conventional storage memories with high storage capacity and fast read/write speeds. The only failure in DW memory arises from DW thermal fluctuations at pinning sites. This work examines, through calculations, the parameters that might help control DW thermal stability at the pinning sites. It is proposed to design a new scheme using a stepped area of a certain depth (d) and length (λ). The study reveals that DW thermal stability is highly dependent on the geometry of the pinning area (d and λ), magnetic properties such as saturation magnetization (Ms) and magnetic anisotropy energy (Ku), and the dimensions of the nanowires. For certain values of d and λ, DWs remain stable at temperatures over 500 K, which is beneficial for memory applications. Higher DW thermal stability is also achieved by decreasing nanowire thickness to less than 10 nm, making DW memories stable below 800 K. Finally, our results help to construct DW memory nanodevices with nanodimensions less than a 40 nm width and less than a 10 nm thickness with high DW thermal stability.

(Fe0.7Co0.3)2B are potential permanent magnets material due to its large saturation magnetization and high Curie temperature. However, it has moderate magnetocrystalline anisotropy (MCA) and low coercivity. One way to improve its coercivity is to combine the contributions from magnetocrystalline- and magnetic-shape anisotropy by preparing (Fe0.7Co0.3)2B nanowires. We study the effects of size, morphology, and surface defects on the hard magnetic properties of nanowires using micromagnetic simulation. The hard magnetic properties of (Fe0.7Co0.3)2B nanowire-bonded magnets are estimated, including the role of inter-wire magnetostatic interaction. By considering the existence of local reductions in MCA energy of up to 30% on the surface layer of nanowires, the anisotropic bonded magnet with a 65% vol. of (Fe0.7Co0.3)2B nanowires would have typical remanence, Br= 7.6–8.4 kG, coercivity, Hci= 9.6–9.9 kOe, and maximum energy product, (BH)m = 14–17.8 MGOe. Developing effective technology for synthesizing nanowires and fabricating corresponding bonded magnets is promising for manufacturing practical magnets based on the magnetic phase with a relatively low or moderate MCA, such as (Fe0.7Co0.3)2B.

AbstractObjective 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).Method 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).Results 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.Conclusion 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.

There is a growing demand for magnetic materials straight forward in wind turbines and electric motors. Their functional properties depend critically on their microstructure and thus on the microstructure evolution during sintering or heat treatment. Field-assisted selective grain growth allows to optimize the microstructure. However, the simultaneous modeling of the structural and magnetic degrees of freedom on the micrometer scale is not possible with most simulation packages. Therefore, we extend the open-source software project OpenPhase and implement the micromagnetic equations needed to treat both degrees of freedom in the framework of the multi-phase field method. We apply our model to the field-assisted grain growth in Sm2Co17 polycrystal films. [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. [ABSTRACT FROM AUTHOR]

Grain boundary diffusion (GBD) process is an effective method to fabricate high-coercive Nd-Fe-B magnets with less consumption of heavy rare earth (HRE). By this approach, HRE-rich shell forms around the Nd2Fe14B grain, which can hinder the magnetic reversal starting at the edge of the grains and enhance the coercivity of whole magnet. Recently, an anti-core–shell structure was observed in the HRE diffused magnets after over-saturated diffusion, where the HRE concentration in the core is even higher than that in the shell. In this work, the effects of the anti-core–shell structure on the magnetization reversal and magnetic properties of diffused magnet have been clarified by micromagnetic simulations. Three-dimension models were established to analyze the demagnetization process. The results indicate that the anti-core–shell structure leads to a large stray field, which will accelerate the magnetization reversal of the whole magnet. As a result, the beneficial effect of HRE GBD on the coercivity has been reduced. Combined with the existing experimental results, the formation of anti-core–shell structure should be avoided during diffusion. Hence, appropriate diffusion time and diffusion source dosage should be selected for GBD process in order to obtain high-performance products and efficiently use the HRE resources. [ABSTRACT FROM AUTHOR]

Microwave-assisted switching (MAS) is simulated for different CoPt and CoPt/Co3Pt nanosrtuctures as a function of applied DC field and microwave frequency. In all the cases, the existence of microwave excitation can lower the switching field by more than 50%. However, this coercivity reduction comes at a cost in the required switching time. The optimal frequencies follow the trends of the ferromagnetic resonances predicted by the Kittel relations. This implies that: (a) when the DC field is applied along the easy axis, the coercivity reduction is proportional to the microwave frequency, whereas (b) when the coercivity is lowered by applying the DC field at an angle of 45° to the easy axis, extra MAS reduction requires the use of high frequencies.

Frequency mixing magnetic detection (FMMD) is a sensitive and selective technique to detect magnetic nanoparticles (MNPs) serving as probes for binding biological targets. Its principle relies on the nonlinear magnetic relaxation dynamics of a particle ensemble interacting with a dual frequency external magnetic field. In order to increase its sensitivity, lower its limit of detection and overall improve its applicability in biosensing, matching combinations of external field parameters and internal particle properties are being sought to advance FMMD. In this study, we systematically probe the aforementioned interaction with coupled Néel–Brownian dynamic relaxation simulations to examine how key MNP properties as well as applied field parameters affect the frequency mixing signal generation. It is found that the core size of MNPs dominates their nonlinear magnetic response, with the strongest contributions from the largest particles. The drive field amplitude dominates the shape of the field-dependent response, whereas effective anisotropy and hydrodynamic size of the particles only weakly influence the signal generation in FMMD. For tailoring the MNP properties and parameters of the setup towards optimal FMMD signal generation, our findings suggest choosing large particles of core sizes dC>25 nm with narrow size distributions (σ<0.1) to minimize the required drive field amplitude. This allows potential improvements of FMMD as a stand-alone application, as well as advances in magnetic particle imaging, hyperthermia and magnetic immunoassays.

True random number generator (TRNG) is an important component for modern information security technologies. Among the candidates, TRNG with spin–orbit torque (SOT)-induced probabilistic magnetization switching is expected to be competitive for its advantages in anti-radiation, unlimited endurance, robust stability, and broad temperature range. However, specific mechanism of the randomness in SOT-induced magnetization switching are still not clear, which limits the demonstration of applicable SOT-TRNG. Here, we performed micromagnetic simulation of the SOT-induced probabilistic magnetization switching by using MuMax3. When various thermal noise seeds were introduced, not only stochastic precession trails within the current pulses, but also stochastic precession tendency after removing the current pulse were obtained, together resulting in random final magnetization states. Particularly, the consistency between mz orientations at the current withdraw moment and at the final stabilized magnetization state shows a positive correlation with the fluctuation range of mz during the current pulse duration, the latter of which narrows with the increasing of channel current density. Our work suggests the importance of thermal noise-related magnetization precession variations on the SOT-induced probabilistic magnetization switching, and suggesting the MuMax3 to be a practical tool for simulating the SOT-TRNG.

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In this article, the structural, magnetic, and microscopic properties of exchange-coupled TbFe/Fe/TbFe system are presented. Both TbFe layers exhibit out-of-plane magnetization, and Fe spacer layer ( S F ) has in-plane magnetization, whereas the entire exchange-coupled system shows considerable enhancement in magnetic properties. By increasing the thickness of S F from 2 to 20 nm, the out-of-plane coercive field and saturation magnetization get enhanced by more than thrice. Perpendicular anisotropy gets tailored as the thickness of S F increases, which also affects the microscopic magnetic domains. Maze-like magnetic domains have been observed at S F = 2 nm; however, domains get diffused and agglomerated upon further increment in thickness of S F . In addition, tilt in effective anisotropy has been achieved in the presence of S F , which is also verified using micromagnetic modeling. The results obtained in TbFe/Fe/TbFe exchange-coupled systems have the potential to be used in magnetic storage technology for hard disk drives. [ABSTRACT FROM AUTHOR]

The coercivity of permanent magnets is an important factor for the various applications including the electric vehicle motors. Improving the coercivity is highly demanded, however, it is quite sensitive to the microstructure of the magnet. For understanding the relation between the coercivity and the microstructure, the micromagnetic simulations of the permanent magnets have been studied extensively. Recently, as improving the computer performance and applying numerical techniques, such as the energy minimization, we can treat a multi-grain permanent magnet model in sub-micrometer scale. Hence, the effects of the magnetic grain size, the grain orientation and the magnetic properties of the grain boundary have been analyzed by the micromagnetic simulation. In addition, combining the micromagnetic simulation and the path analysis method make the thermal effect analysis enable. In this paper we summarize the foundation of the micromagnetic simulation for the coercivity analysis and review the studies on the coercivity analyses of the permanent magnet by using micromagnetic simulations. [ABSTRACT FROM AUTHOR]

Dual field generation layer (FGL) spin torque oscillators (STO) were investigated for use in microwave-assisted magnetic recording. The STOs were integrated into the gaps of write heads for stable oscillation. According to preliminary calculations, a FGL thickness of 5 nm was suitable, while the volume of 7.5 nm and 10 nm thick FGLs was too large and they did not oscillate stably. The most dominant factor influencing the FGL rotation was found to be the strength of the antiferromagnetic coupling between the FGLs. Other parameters were also varied, e.g., the exchange coupling between the negative spin injection layer (nSIL) and the trailing shield (TS), however, no significant effect was found. Unlike single FGL STOs, the oscillation frequency changed drastically, from 38 GHz to 73 GHz, on varying the injected current density to the STO. Recording simulations showed that the signal-to-noise ratio was maximized for an STO oscillation frequencies between 34 GHz and 50 GHz, depending on the recording media used in the calculations. Therefore, the dual FGL STO may be suitable for a wide range of recording media. Alternatively, the dual FGL STO may be used for dual-layer recording, where different resonance frequencies are needed for each of the layers, obviating the need for a second STO. [ABSTRACT FROM AUTHOR]

Skyrmions are promising for the next generation of spintronic devices, which involves the production and transfer of skyrmions. The creation of skyrmions can be realized by a magnetic field, electric field, or electric current while the controllable transfer of skyrmions is hindered by the skyrmion Hall effect. Here, we propose utilizing the interlayer exchange coupling induced by the Ruderman–Kittel–Kasuya–Yoshida interactions to create skyrmions through hybrid ferromagnet/synthetic antiferromagnet structures. An initial skyrmion in ferromagnetic regions could create a mirroring skyrmion with an opposite topological charge in antiferromagnetic regions driven by the current. Furthermore, the created skyrmions could be transferred in synthetic antiferromagnets without deviations away from the main trajectories due to the suppression of the skyrmion Hall effect in comparison to the transfer of the skyrmion in ferromagnets. The interlayer exchange coupling can be tuned, and the mirrored skyrmions can be separated when they reach the desired locations. Using this approach, the antiferromagnetic coupled skyrmions can be repeatedly created in hybrid ferromagnet/synthetic antiferromagnet structures. Our work not only supplies a highly efficient approach to create isolated skyrmions and correct the errors in the process of skyrmion transport, but also paves the way to a vital information writing technique based on the motion of skyrmions for skyrmion-based data storage and logic devices. [ABSTRACT FROM AUTHOR]

Dynamic susceptibility of thin truncated conical permalloy nanodisk with height 20 nm is simulated using micromagnetic simulations. Starting from a relaxed magnetic vortex state, the effect of tapering on the resonant modes is analyzed by changing the bottom radius (R = 50 nmto 100 nm) and top radius (r = 10 nmtoR − 10 nm) of the conical nanodisk. When an in-plane excitation field pulse is applied, the lowest resonant peak frequency increases with increase in r. This mode is originated due to vortex core translation and the variation of the resonant peak with r is well explained using Thiele's equation. Other resonant modes appear at higher excitation frequencies which are identified as azimuthal modes. When an out-of-plane excitation field is applied, all the spin wave modes appear to be axially symmetric. Appearance of new resonant modes and shifting of the resonant frequency is recorded by changing either of the parameter r or R for both the excitation field directions. The spatial distribution of FFT of magnetization at each resonant mode links the area of nanodisk that has maximum contribution to each mode and combining this with average center of magnetization perturbation, shifting of resonance frequency is explained successfully. The findings from this study prove that tunable resonant frequency can be achieved by tapering of truncated conical nanodisks. [ABSTRACT FROM AUTHOR]

Microwave-assisted switching (MAS) is simulated for different CoPt and CoPt/Co3Pt nanosrtuctures as a function of applied DC field and microwave frequency. In all the cases, the existence of microwave excitation can lower the switching field by more than 50%. However, this coercivity reduction comes at a cost in the required switching time. The optimal frequencies follow the trends of the ferromagnetic resonances predicted by the Kittel relations. This implies that: (a) when the DC field is applied along the easy axis, the coercivity reduction is proportional to the microwave frequency, whereas (b) when the coercivity is lowered by applying the DC field at an angle of 45° to the easy axis, extra MAS reduction requires the use of high frequencies. [ABSTRACT FROM AUTHOR]

The effects of several factors, including alloy composition, cooling rate, and external magnetic field, on the microstructures and magnetic properties of Nd–Fe–B ternary alloy are explored in detail by using phase‐field and micromagnetic simulations during the copper mold casting process. It is found that the characteristic of regional distribution is formed due to the temperature gradient in the alloy, and that Nd19Fe71.5B9.5 (at%) exhibits the highest coercivity and pretty good comprehensive magnetic properties among all alloys studied, which can be interpreted by the refinement of Nd2Fe14B grain size and thickness of Nd‐rich grain boundary phase. The cooling rate has significant influence on the average grain size and the amount of amorphous phase. An increase of cooling rate results in a decrease of grain size and an increase of amorphous phase, which then affect the coercivity and remanence of the magnet. Applying an external magnetic field is demonstrated to be a good approach to enhance the coercivity and remanence by refining the grain size and restraining the formation of amorphous phase and T2 phase. [ABSTRACT FROM AUTHOR]

Recently, it has been reported that the skyrmion Hall effect can be suppressed in an elliptical skyrmion-based device. Given that defects are unavoidable in materials, it is necessary and important to investigate the dynamics of an elliptical skyrmion in a defective racetrack device. In this work, the current-driven dynamics of an elliptical skyrmion in a defective racetrack device are systematically studied using micromagnetic simulations. The system energy analysis reveals that the magnetic parameters of the circular defect play critical roles in determining the type (repulsive or attractive) and the magnitude of the force on the elliptical skyrmion. The simulated trajectories show that the primary motion modes of the elliptical skyrmion in the defective racetrack can be divided into four types, which are dependent on the values of the Dzyaloshinskii–Moriya interaction (DMI) constant Dd, the perpendicular magnetic anisotropy constant Kd, the magnitude of the driving current density J, and the size d of the defect. Further investigation of the motion-mode phases of the skyrmion reveals the synthetic effects of Dd, Kd, J, and d. Finally, the minimum depinning current density J, which linearly depends on the parameters of Dd and Kd, is obtained for a skyrmion completely pinned in the defect. Our findings give insights into the dynamics of an elliptical skyrmion in the presence of a defect with different magnetic parameters in a racetrack device and may be useful for performance enhancement of skyrmion-based racetrack memory devices.

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.