Magnetization Reversal Mechanism Research Articles

Temperature dependent structural and magnetic properties of permalloy (Ni80Fe20) nanotubes

One dimensional Permalloy (Ni80Fe20) nanotubes (NTs) were successfully fabricated using Anodic Aluminum Oxide (AAO) templates with 100 nm diameter by employing a low-cost electrodeposition route at room temperature. As prepared Ni80Fe20 NTs morphology, elemental analysis and structural details have been investigated by using field-emission scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDS) and X-Ray diffraction (XRD) respectively. The structure of prepared Permalloy nanostructures has face centered cubic (FCC) phase with polycrystalline in nature. The Ni80Fe20 NTs were annealed at 200 °C, 400 °C and 600 °C. The vibrating sample magnetometer (VSM) measurements showed that all the samples exhibit ferromagnetic nature. Furthermore, VSM has been employed to study the saturation magnetization (MS), Squareness (SQ) and Coercivity (HC) as the function of annealing temperature. A comparative study of magnetization reversal mechanism has also been conducted. The easy axis lies in the direction perpendicular to the NTS axis due to the dominance in shape anisotropy. Annealing temperature improved the magnetic properties of Permalloy (PA) NTs. This work will provide significant applications in the field of spintronics and high-density data storage devices.

Temperature dependent interfacial-coupling and magnetization reversal of La2/3Sr1/3MnO3 film with inverted hysteresis features

Temperature-dependent magnetic anisotropy and interfacial coupling of La2/3Sr1/3MnO3 (LSMO) epitaxial films with inverted hysteresis (IHLs) features are investigated using a broadband ferromagnetic resonance (FMR) spectrometer. Through the dispersion relation obtained along [100], [110] and [010] axes, a simplified method is proposed to investigation in-plane magnetic anisotropy by broadband FMR in a cryostat. According to this method, the perpendicular uniaxial anisotropy, magneto-crystalline anisotropy, and interfacial coupling between the LSMO film and hard-magnetic interface layer are investigated as a whole. The measured value of interfacial coupling is −17.6 Oe at 300 K, close to the bias field found in minor loop, i.e., −18.4 Oe. The interfacial coupling is enhanced with the dropping of temperatures, and the value of interfacial coupling changed to −112 Oe at 10 K. A Stoner-Wohlfarth-like model is employed to calculate the hysteresis loops of the films based on the measured values of in-plane magnetic anisotropy and interfacial-coupling. The calculation reveals the mechanism of magnetization reversal and the origin of IHLs found in LSMO films.

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

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

Analyzing the mechanism of reducing B content on the coercivity of sintered (Nd, Ce)-Fe-B magnets

In recent years, the research focus has shifted towards the preparation of non-heavy rare earth (RE)-Fe-B magnets with high coercivity (Hcj) by reducing the content of B element. In this work, we achieved resource-saving sintered (Nd, Ce)-Fe-B magnets by adjusting the B element content, optimizing microstructure, and enhancement of Nd/Ce element distribution. We conducted an analysis on the magnetic characteristics, phase composition, microstructure, magnetization reversal mechanism, and economic viability of these sintered magnets. With a decrease in the B element content from 0.96 wt% to 0.90 wt%, there is a increase of approximately 1 kOe in Hcj.At the same time, the temperature coefficient of Hcj has been improved, resulting in a absolute value decrease from 0.67%/°C to 0.63%/°C. The findings suggested that reducing the mass fraction of B element could facilitate the formation of a thicker grain boundary phase between the grains. It could significantly weaken the ferromagnetic coupling effect, and enhance the magnetic isolation effect between grains, resulting in a higher Hcj. From an economic perspective, compared to the cost of magnet with similar coercivity prepared by increasing the RE content, the cost of magnet with low B element content was reduced by 554.4 US$/t, approximately saving 3.08%. This study provides a new theoretical foundation and experimental basis for resource-saving sintered (Nd,Ce)-Fe-B magnet preparation.

Magnetization reversal and coercivity mechanism in ferrimagnetic M-type hexaferrite by controlling phase evolution

The correlation of phase evolution (annealing at different temperatures) with magnetic properties like magnetization reversal, coercivity mechanism, and energy product in M-type hexaferrite by substituting Sr in place of Ba has been reported in the present article. The Ba1-xSrxFe12O19 (x = 0.0, 0.5, & 1.0) have been prepared by the sol–gel auto-combustion method followed by annealing at different temperatures from 800 °C to 1200 °C. The structural and magnetic properties have been characterized by X-ray diffraction, Field emission scanning electron microscopy, Raman spectroscopy, and Vibrating sample magnetometer. The Rietveld refinement confirms contraction in unit cell parameters by replacing Sr in place of Ba and lattice expansion with increasing annealing temperature. There is no significant effect of substitution on magnetic properties found rather heat treatment affects greatly. The transition from a single domain to multidomain particles and their contribution to initial magnetization curves are analyzed. The differential susceptibility (dM/dH) calculated from room temperature magnetization confirms the existence of a pinning effect during magnetization reversal for samples heat treated below 1200 °C. Secondary phases or grain boundaries are believed to act like pinning centers. This work opens insight into the domain wall motion by microstructure engineering. The coercivity is tuned to 6.09 kOe by controlling the annealing temperature for the Ba1-xSrxFe12O19 (x = 1.0) sample. The maximum energy product (BH)max in the range of 0.94–1.27 MGOe has been achieved.

Understanding the Effect of Curvature on the Magnetization Reversal of Three-Dimensional Nanohelices.

Comprehending the interaction between geometry and magnetism in three-dimensional (3D) nanostructures is important to understand the fundamental physics of domain wall (DW) formation and pinning. Here, we use focused-electron-beam-induced deposition to fabricate magnetic nanohelices with increasing helical curvature with height. Using electron tomography and Lorentz transmission electron microscopy, we reconstruct the 3D structure and magnetization of the nanohelices. The surface curvature, helical curvature, and torsion of the nanohelices are then quantified from the tomographic reconstructions. Furthermore, by using the experimental 3D reconstructions as inputs for micromagnetic simulations, we can reveal the influence of surface and helical curvature on the magnetic reversal mechanism. Hence, we can directly correlate the magnetic behavior of a 3D nanohelix to its experimental structure. These results demonstrate how the control of geometry in nanohelices can be utilized in the stabilization of DWs and control of the response of the nanostructure to applied magnetic fields.

Computational modelling of a triaxial vibrating sample magnetometer

Magnetic Field Assisted Additive Manufacturing (MFAAM) enables 3D printing of magnetic materials of various shapes which exhibit a complex anisotropy energy surface containing contributions generated from different origins such as sample, particle, and agglomerate shape anisotropy, flow and field induced anisotropy, and particle crystal anisotropy. These novel magnet shapes require the need to measure the x, y, and z components of the magnetic dipole moment simultaneously to fully understand the magnetic reversal mechanism and unravel the complex magnetic anisotropy energy surface of 3D printed magnetic composites. This work aims to develop a triaxial vibrating sample magnetometer (VSM) by adding a z-coil set to a pre-existing biaxial VSM employing a modified Mallison coil set. The optimum size and location of the sensing coils were determined by modeling the sensitivity matrix of the z-coil set. The designed coil set was implemented using 3D printed spools, a manual coil winder, and gauge 38 copper wire. A 3D printed strontium ferrite nylon composite sample was used to estimate the sensitivity of the z-coils (50 mV/emu). The results herein are applicable for any VSM using a modified Mallison biaxial coil configuration allowing for a quick implementation on pre-existing systems.

Understanding of magnetization reversal mechanism of precipitation-hardening samarium-cobalt magnets from micromagnetic simulation

The precipitation-hardening Sm-Co magnets not only have irreplaceable applications under extreme conditions such as high temperatures, but are also the most typical magnets with pinning-controlled magnetization reversal mechanism. However, there have always been two views on this type of pinning, attractive or repulsive. In this article, the modification of the parallel interface to the titled interface, led to occurrence of quasi-repulsive and quasi-attractive pinning modes, by employing micromagnetic simulation methods. The corresponding domain wall energy relationship between the pinning phase and main phase was analyzed for different pinning modes, according to the morphology of domain wall from the simulation results. The analysis results indicated that relatively small difference in domain wall energy between the two phases led to quasi-repetitive and quasi-attractive pinning modes. Our findings can provide a reference for further understanding of the magnetization reversal mechanism of precipitation-hardening Sm-Co magnets. The emphasis on the influence of 1:5/2:17 interface orientation on pinning also provided a perspective on the magnetization reversal mechanism of other nanostructured permanent magnet materials.

“Tailoring thermal stability and in-plane magnetic anisotropy of CoFeB alloy thin films via growth conditions engineering”

The present work investigates the influence of growth conditions on the magneto-structural properties of magnetron-sputtered CoFeB alloy thin films. Films grown at room temperature (RT) are soft magnetic (coercivity <9 Oe), show amorphous microstructure, and possess a distinct uniaxial magnetic anisotropy (UMA) in the film plane. The amorphous microstructure and UMA persist up to a growth temperature (Ts) of 300 °C. CoFeB film grown at Si (100) substrate shows amorphous to crystalline phase transformation at Ts = 500 °C. In contrast, under the same growth conditions, the CoFeB film prepared on the SiO2 substrate maintains an amorphous structure up to Ts = 500 °C. The difference in the microstructures was explained in terms of the out-diffusion of B to the interfacial layer and segregation near the substrate for the films having crystalline and amorphous structure, respectively. On both the substrates Addition of W buffer layer facilitated the growth of smooth and nanocrystalline CoFeB films at Ts ∼300 °C. Upon crystallization, UMA evolution with a simultaneous increase in the coercive field is interpreted in terms of joint effect of random anisotropy model and surface/interface roughness-induced magnetization reversal mechanism. Unlike post-annealing, the CoFeB films prepared at high temperature shows superior thermal stability against crystallization. The relationship between the soft magnetic performance, UMA evolution, and thermal stability of CoFeB films with the growth conditions is established. The study reveals the role of growth condition engineering, to obtain CoFeB thin films with controlled microstructure and magnetic performance, which is crucial for designing the spintronics devices.

Coercivity, Activation Volumes, and Interactions in BaFe12O19

Coercivity and magnetic viscosity studies are presented in coprecipitated and ceramic BaFe12O19 hexaferrites. According to magnetic viscosity formalism, the activation volumes are obtained and for ceramic sample the temperature dependence of the activation volume is reported. Using remanence curves, interactions are evaluated by means of the ΔM technique. In both samples, interactions are evidenced, particularly demagnetizing‐like interactions. The activation volume values seem to indicate that coherent rotation is not the mechanism of magnetization reversal even in the sample with particle size below the critical single‐domain size. It is confirmed that the activation volume increases with temperature in 75–300 K temperature range, for both coprecipitated and ceramic samples.

Granular micromagnetic HAMR model: investigation of damping dependence and parametric optimization for high performance

Heat assisted magnetic recording (HAMR) is a novel high-density magnetic recording technology that relies on thermal assist from a laser during the writing process. To achieve high writing performance, it is important to study and optimize the crucial factors affecting the magnetization reversal mechanism at elevated temperature. In this work, we use a multiscale approach that combines atomistic and micromagnetic models to study the magnetization reversal behavior in the recording medium. The atomistic model allows to parameterize accurately the macroscopic approach, which is utilized to model the system and its dynamics. We perform a parametric investigation of the switching properties as a function of the HAMR setup characteristics as well as the material properties, such as magnetic damping. The results show that high damping and moderate external fields can achieve high-performance HAMR media characterized by high switching probability, short switching time and low peak temperature. We demonstrate that switching occurs via the linear reversal mechanism. By systematic variation of the longitudinal susceptibility we force a transition to coherent reversal and demonstrate that this reduces the switching probability, showing linear reversal to be an important component within the HAMR process.

Current-Induced Magnetization Switching in Light-Metal-Oxide/Ferromagnetic-Metal Bilayers via Orbital Rashba Effect.

The orbital angular momentum (OAM) generation as well as its associated orbital torque is currently a matter of great interest in spin-orbitronics and is receiving increasing attention. In particular, recent theoretical work predicts that the oxidized light metal Cu can serve as an efficient OAM generator through its surface orbital Rashba effect. Here, for the first time, the crucial current-induced magnetic-field-free in-plane magnetization reversal is experimentally demonstrated in CoFeB/CuOx bilayers without any heavy elements. We show that the critical current density can be comparable to that of strong spin-orbit coupling systems with heavy metals (Pt and Ta) and that the magnetization reversal mechanism is governed by coherent rotation in the grains through the second-harmonic and magneto-optical Kerr effect measurements. Our results indicate that light metal oxides can play an equally important role as heavy metals in magnetization reversal, broadening the choice of materials for engineering spintronic devices.

Angular dependence of the magnetization relaxation in Co/Pt multilayers

We study the influence of defects in Co/Pt multilayers on the room-temperature magnetization reversal and relaxation mechanisms via angle-dependent magnetic viscosity and coercive field measurements. The data reveal a transition from pinning-dominated domain wall propagation to a sequence of pinning-dominated and uniform switching, with increasing tilt away from the normal direction. The leading role of the dendritic domain wall propagation in the nanogranular exchange-coupled films is corroborated by the scaling of relaxation times, the angular dependence of the coercive field, and Kerr microscopy.

Antiferromagnetic fourfold anisotropy induced exchange bias and magnetization reversal behaviors in CoFeB/IrMn bilayers

Bilayers of an amorphous CoFeB ferromagnet coupled to an epitaxially grown IrMn antiferromagnet, whose anisotropy is fourfold symmetric, are modeled to study the angular (φ) dependence of magnetization reversal mechanism and exchange bias behavior, based on a modified Monte Carlo method. It is found that positive and negative exchange-bias field (HE) maximum values appear in the antiferromagnetic easy-axis directions, and the HE zero-value transition/return points exist in the antiferromagnetic hard-axis directions. Furthermore, the value of HE(φ) is well fitted by considering cos3φ/sin3φ terms with larger or comparable coefficients as compared to the first-order terms in the Fourier expansion. Finally, the microscopic spin configuration results evidence these phenomena originating from the tailoring of antiferromagnetic spins by simply rotating the field-cooling direction between antiferromagnetic easy- and hard-axis directions. We open an exchange-bias mode governed by antiferromagnetic fourfold anisotropy and propose a method to precisely detect antiferromagnetic higher-order anisotropies on an exchange-bias support.

Magnetization reversal and coercivity mechanism in truncated conical nanowires of permalloy

Magnetization reversal and coercivity mechanism in truncated conical nanowires of permalloy

Static and Dynamic Magnetic Properties of Fe3O4 Nanotubes

In this paper, our objective was to investigate the static and dynamic magnetic properties of Fe3O4 nanotubes that are 1000 nm long, by varying the external radius and the thickness of the tube wall. We performed a detailed numerical analysis by simulating hysteresis curves with an external magnetic field applied parallel to the axis of the tubes (along the z-axis). Our findings indicate that nanotubes with an external radius of 30 nm exhibit non-monotonic behavior in their coercivity due to a change in the magnetization reversal mechanism, which was not observed in nanotubes with external radii of 80 nm. Additionally, we explored the dynamic susceptibility of these nanotubes and found that the position and number of resonance peaks can be controlled by manipulating the nanotube geometry. Overall, our study provides valuable insights into the behavior of Fe3O4 nanotubes, which can aid in the design and improvement in pseudo-one-dimensional technological devices.

Analysis of the magnetization reversal mechanism of heterogenous grain boundary diffused Nd-Fe-B sintered magnets

The grain boundary diffusion (GBD) method could effectively improve the coercivity of Nd-Fe-B sintered magnets, but the uneven distribution of heave rare earth (HRE) elements would lead to the decrease of squareness factor of the demagnetization curve of the magnets, which limited the application of this method. In this paper, three typical-structure regions were summarized based on microstructure characterization, and the magnetic properties and magnetization reversal mechanism were analyzed. The differences in nucleation positions and magnetization reversal processes of the three regions were discussed, and it found that the remarkable differences among them resulted in the low squareness factor of GBD Nd-Fe-B sintered magnets. In addition, the efficiency of Tb elements could be further improved to enhance the coercivity of Nd-Fe-B sintered magnets by forming homogeneous and complete core-shell structure grains, which could strengthen the nucleation field and even transfer the nucleation position from the grain surface defect layer to the grain interior region. Our results could provide guidance and reference for further improving the permanent magnetic properties of Nd-Fe-B sintered magnets by optimized GBD method.

Magnetic Behavior of the Arrays of Iron Cylindrical Nanostructures: Atomistic Spin Model Simulations

Cylindrical ferromagnetic nanowires are of particular interest in nanomaterials science due to various manufacturing methods and a wide range of applications in nanotechnology, with special attention given to those with diameters less than the single domain limit. In the current study, the simulations of magnetic properties of isolated iron nanowires with a diameter of 5 nm and various aspect ratios, as well as two types of arrays of such nanowires (with hexagonal and square arrangement), were performed using atomistic spin model. In the case of a single nanowire, change of coercive field for different applied field directions with aspect ratio was discussed. It was shown that the evolution of the magnetization reversal mechanism from coherent rotation to domain wall propagation appears with increasing length of single nanowire. For the arrays of cylindrical nanostructures, it was revealed that different number of nearest neighbors for each nanostructure in square and hexagonal arrays have an influence on their magnetostatic interactions, which are the most significant for shortest interwire distances. The corresponding spin configurations during the remagnetization process showed the appearance of intermediate magnetization states (when a part of wires is magnetized parallel and part antiparallel to the field direction), connected with Barkhausen effect, which influence the observed hysteresis curves.

Micromagnetic Study of the Dependence of Output Voltages and Magnetization Behaviors on Damping Constant, Frequency, and Wire Length for a Gigahertz Spin Rotation Sensor.

In this report, we studied the dependence of output voltage on the damping constant, the frequency of the pulse current, and the wire length of zero-magnetostriction CoFeBSi wires using multiphysics simulation considering eddy currents in micromagnetic simulations. The magnetization reversal mechanism in the wires was also investigated. As a result, we found that a high output voltage can be achieved with a damping constant of ≥0.03. We also found that the output voltage increases up to a pulse current of 3 GHz. The longer the wire length, the lower the external magnetic field at which the output voltage peaks. This is because the demagnetization field from the axial ends of the wire is weaker as the wire length is longer.

Magnetization reversal mechanisms in highly corrugated thin films

Nanopatterned ferromagnetic (FM) thin films have specific characteristics that make them a workhorse for sensors based on magnonic, magnetoplasmonic, or anisotropic magneto-resistive effects. Undulated FM thin films have been studied because of their tunable uniaxial anisotropy. They have been traditionally understood by means of Schlömann's model taking account of shape-induced magnetic anisotropies in softly corrugated systems. Here, we show how it cannot describe accurately the magnetic behavior of highly corrugated FM systems within a thickness region of less than the ripple amplitude. We report on the magnetization reversal processes detected in Permalloy films deposited onto highly corrugated patterns (250 nm in periodicity, 180 nm in amplitude) in a wide thickness range (15–150 nm), finding both that the anisotropy of the system does not correspond to a uniaxial type for FM thicknesses larger than 40 nm and that the anisotropy of the system increases with the FM thickness. Based on the results, we hypothesize that whereas Schlömann's model is valid for softly corrugated thin films, it fails to explain magnetization reversal processes of highly corrugated thin films, especially when the ripple amplitude is much greater than the deposited FM layer thickness. By means of micromagnetic simulations, we find an increment of anisotropy with thickness, just as in the experimental, as well as determine the arise of magnetic domains at the ridges of high thickness corrugated FM thin films. This approach will help to get a better understanding of operating mechanisms in magnetic field sensors based on undulated ferromagnetic materials.