Demagnetization Curves Research Articles

Influence of heat treatment, near the temperature region of Fe α-γ transformation, on the interphase exchange coupling of Nd2Fe14B + Fe nanocomposites

Hard-Soft Exchange coupled nanocomposite magnets rely on a precise control of both crystal structure and microstructure for the constituent phases. In Nd2Fe14B + 10 wt%α-Fe magnetic nanocomposites obtained through mechanical milling, the nanostructure refinement is achieved by annealing for a short duration. This work builds upon previous results in two ways: (i) increasing rapid annealing temperature and rate, (ii) taking advantage of the Fe α/γ transformation in order to limit the growth of the soft phase crystallites. Annealing was performed at temperatures between 800 °C and 950 °C under purified inert He gas for durations between 38 s and 65 s. The structure and microstructure of the nanocomposites were studied through XRD and SEM, while their magnetic properties and the effectiveness of the interphase exchange coupling were studied using demagnetization curves (±10 T) and dM/dH plots.

Observation of unexpected uniaxial magnetic anisotropy in La2/3Sr1/3MnO3 films by a BaTiO3 overlayer in an artificial multiferroic bilayer.

We studied in detail the in-plane magnetic properties of heterostructures based on a ferroelectric BaTiO3 overlayer deposited on a ferromagnetic La2/3Sr1/3MnO3 film grown epitaxially on pseudocubic (001)-oriented SrTiO3, (LaAlO3)0.3(Sr2TaAlO6)0.7 and LaAlO3 substrates. In this configuration, the combination of both functional perovskites constitutes an artificial multiferroic system with potential applications in spintronic devices based on the magnetoelectric effect. La2/3Sr1/3MnO3 single layers and BaTiO3/La2/3Sr1/3MnO3 bilayers using the pulsed-laser deposition technique. We analyzed the films structurally through X-ray reciprocal space maps and high-angle annular dark field microscopy, and magnetically via thermal demagnetization curves and in-plane magnetization versus applied magnetic field loops at room temperature. Our results indicate that the BaTiO3 layer induces an additional strain in the La2/3Sr1/3MnO3 layers close to their common interface. The presence of BaTiO3 on the surface of tensile-strained La2/3Sr1/3MnO3 films transforms the in-plane biaxial magnetic anisotropy present in the single layer into an in-plane uniaxial magnetic anisotropy. Our experimental evidence suggests that this change in the magnetic anisotropy only occurs in tensile-strained La2/3Sr1/3MnO3 film and is favored by an additional strain on the La2/3Sr1/3MnO3 layer promoted by the BaTiO3 film. These findings reveal an additional mechanism that alters the magnetic behavior of the ferromagnetic layer, and consequently, deserves further in-depth research to determine how it can modify the magnetoelectric coupling of this hybrid multiferroic system.

Irreversible magnetic behavior with temperature variation of Ni0.5Co0.5Fe2O4 nanoparticles

In this paper, the irreversible magnetic behavior of Ni0.5Co0.5Fe2O4 nano-ferrite has been discussed with varying the temperature. The nano-ferrite discussed in present work synthesized using sol gel technique. The formation of the phase of Ni0.5Co0.5Fe2O4 was confirmed using X-ray diffraction (XRD), and their Rietveld refinement shows it crystallized into cubic structure with lattice constant 8.3541 Å under space groupFd3-m. The homogeneity and the morphology of the sample was analysed using FESEM along with EDAX measurement. The irreversibility of Ni0.5Co0.5Fe2O4 was studied in three modes ZFCW, FCC and FCW using field dependent magnetic measurements in temperature range of 2 K–300 K under the presence of 50 Oe. However, the magnetic hysteresis of sample was studied at different temperature T = 5 K, 50 K, 100 K, 120 K, 200 K, 300 K from varying the field upto ±5 Tesla. Further, the asymmetry observed in the magnetization and demagnetization curve gives the value of Hm, which increased with lowering the temperature due to magnetic crystalline phase as well as the micro-strain of the sample. Finally, retentivity, coercivity and saturation magnetization of the present sample makes it a potential candidate for magnetic memory devices, magnetic resonance imaging, medical diagnosis applications.

A Study on the Design of IPMSM for Reliability of Demagnetization Characteristics-Based Rotor

This paper describes a study on an design of IPMSM (Interior Permanent Magnet Synchronous Motor) for reliability of demagnetization characteristics-based rotor. The IPMSM in this paper is located at a lower part of a railway vehicle and has a totally enclosed type structure so heat does not circulate within it and its temperature rises high. In order to increase power density of the motor, a neodymium permanent magnet with high energy density is used. However, this neodymium permanent magnet has properties that demagnetization occurs at a high temperature or by a reverse magnetic field and that as a new demagnetization curve called a recoil line is drawn in the B-H characteristic curve, magnet becomes permanently demagnetized. This permanent demagnetization property decreases the torque characteristics of or power output of the motor, eventually making it impossible to drive the railway vehicle and also reducing its reliability. In order to prepare for these problems, it is definitely necessary to consider analysis of the demagnetization property in the analysis stage. Hence, this study aims to propose an analysis method of demagnetization characteristics by considering the recoil line and to establish a base capable of securing reliability of IPMSM for traction of and highlights the proposed analysis method through a performance test and temperature saturation test of the prototype.

Correlation between microstructural heterogeneity and energy product in hot deformed Nd-Fe-B magnets

We systematically investigated the local crystallographic texture, magnetic domain structure, and the associated magnetic reversal process of different regions, namely fine-grain (FG), coarse-grain (CG), and large-grain (LG) regions in hot-deformed Nd-Fe-B magnets. The FG region possesses the strongest (00 l) texture and plays a dominant role on the magnetic properties. In contrast, the CG region with partially aligned sub-micron grains and the LG region with randomly aligned micro-sized grains exhibit much weaker (00 l) texture, where the reversal magnetic domains primarily nucleate, deteriorating the remanence and the squareness of the demagnetization curve seriously. This finding highlights a new strategy to achieve high energy density in hot-deformed Nd-Fe-B magnets, i.e. maximizing the volume fraction of highly (00 l) textured FG regions and reducing the undesirable CG and LG regions. Our further experiment enhancing the maximum energy product of the magnet from 42.6 to 51.1 MGOe through optimizing the microstructure homogeneity unambiguously demonstrates the general applicability of the strategy.

Optimal design of a flux reversal permanent magnet machine as a wind turbinegenerator

Flux reversal permanent magnet generators are well suited for use as wind turbine generators owing to their high torque generation ability and magnetic gear. However, they suffer from poor voltage regulation due to their high winding inductance. In this paper, a design optimization method is proposed for flux reversal generators in wind turbine applications. The proposed method includes a new multiobjective function. Cost, volume of the generator, and mass of the permanent magnet are considered in it independently and simultaneously. Besides the new objective function, the main superiority of this paper compared with published papers is considering winding inductance in optimization procedures as a constraint and analyzing the optimization results for different values of it. Also, for the first time, the equations for permanent magnet sizing are considered based on a demagnetization curve for designing a flux reversal generator. For this purpose, a step-by-step design procedure is proposed and sensitivity analysis is performed to determine the sensitivity of output parameters to specific electrical loading, the height of the permanent magnets, and the machine length-to-diameter ratio. Then a multiobjective optimization based on a genetic algorithm is carried out and the best combination of pole number and number of slots/pole/phase is obtained. Then, for this combination, the optimum value of the constraint is obtained, too. Then specifications and dimensions of the optimum flux reversal machine as a wind turbine generator is presented. Finally, a time-stepping finite element method is used to validate the design and optimization results.

Effects of Sm content on the phase structure, microstructure and magnetic properties of the SmxZr0.2(Fe0·8Co0.2)11.5Ti0.5 (x=0.8–1.4) alloys

To introduce a paramagnetic intergranular phase into the ThMn12-type permanent materials, SmFe12-based alloys with the composition of SmxZr0.2(Fe0.8Co0.2)11.5Ti0.5 (x = 0.8–1.4, at.%) are designed and prepared by arc-melting and melt-spinning. Roles of extra Sm on the phase constitution and magnetic properties are systematically investigated. First of all, within the range of x = 0.8–1.2, the as-cast ingots mainly constitute of 1:12, 1:10, 1:2 and α-(Fe,Co) phase. With increasing Sm content, both 1:12 and α-(Fe,Co) phases decrease whereas 1:2 and 1:10 phases increase. Similarly, for the heat-treated ribbons with x = 0.8–1.2, α-(Fe,Co) phase also decreases with the increasing Sm content. Consequently, the Js and Jr of the alloys exhibit a decreasing trend while the Hc and (BH)max show an increasing trend. As a result, excellent magnetic properties of Jr = 1.04 T for x = 0.8 alloy, (BH)max = 61 kJ/m3 and Hc = 3.0 kOe for x = 1.2 alloys are acquired. Henkel curves of the annealed ribbons prove the existence of an exchange coupling effect in all alloys, which has contributed to the remanence enhancement. Interestingly, the observed kink in the demagnetization curve of x = 1.2 annealed alloy suggests the ineffective exchange coupling between soft and hard grains. An amorphous grain boundary phase is observed in the annealed x = 1.2 alloy while the refined 57Fe Mössbauer spectrum confirms its paramagnetic behavior. The present work may provide a dependable guideline for designing the composition and microstructure of 1:12-based permanent material.

Relationship between target materials and various properties of PLD-made isotropic Nd-Fe-B films

In order to deposit isotropic Nd-Fe-B film magnets on metal substrates, a laser beam with an energy density of approximately 3 J/cm2 was defocused on the surface of Nd-Fe-B sintered targets with three densities of 5.69, 6.14, and 6.64 g/cm3, respectively. The different Nd content (Nd/(Nd + Fe)) between each film and the corresponding targets decreased as the density of the targets increased. We have reported that the deposition using a Nd-Fe-B alloy target under the same laser energy density and T-S distance enabled us to obtain the good transfer of composition from an alloy target to a film. It was clarified that the similar result could be obtained in the use of a sintered target with the density of 6.64 g/cm3. Comparison of magnetic properties, normalized demagnetization curves, and microstructure between the films prepared using a sintered target (6.64 g/cm3) and a Nd-Fe-B alloy target, respectively, was carried out. The values of coercivity and (BH)max of the samples prepared by the sintered target were higher than those of the films prepared using the alloy one. Moreover, the use of a Nd-Fe-B sintered target enabled us to improve the reproducibility of squareness in demagnetization curves. It is considered that the results of comparison are attributed to the different microstructure of the films such as the precipitation due to uneven distribution of α-Fe and Nd elements. We also suppose that the formation of microstructure in each film originates from the microstructure of the corresponding targets.

Magnetic Granulometry and Mössbauer Spectroscopy of Synthetic FemOn–TiO2 Composites

FemOn–TiO2 particle aggregates have been obtained using the sol–gel method and hydrothermal treatment. It is shown that the synthesis conditions favor forming composites that contain titanomagnetite in very low concentrations. Hysteresis loops and demagnetization curves of anhysteretic remanent magnetization were measured with a vibration sample and a SQUID magnetometer, respectively. Mossbauer spectroscopy, bulk magnetic properties, scanning electron microscopy (SEM) observations, coupled with the theoretical analysis of magnetostatic interaction between finely dispersed particles lead to a conclusion that the studied composites can be viewed as groups of interacting clusters 2– $20~\mu \text{m}$ in size, consisting of chemically heterogeneous, mostly single-domain or close to single-domain particles with an average size of about 100 nm.

Isothermal Remanent Magnetization (IRM) and Direct Current Demagnetization (DCD) Curves for Nanocrystalline Sm-Y-Co Alloys

Magnetization curves were measured using the methods of isothermal remanent magnetization (IRM) and direct current demagnetization (DCD) for nanocrystalline Sm0.5Y0.5Co5 alloys. The study of magnetic properties for nanocrystalline Sm0.5Y0.5Co5 alloys was performed with the Physical Property Measurement System (PPMS). The alloys exhibited a high remanent coercivity of 2.1 MA/m. The ratio of magnetic susceptibilities for nanocrystalline Sm0.5Y0.5Co5 alloys was χd/χr = 4.6, indicating the presence of magnetic interactions in the alloys. Nanocrystalline Sm0.5Y0.5Co5 alloys with magnetic interactions were studied by measuring ΔM curves against the magnetic field, and the alloys showed dipolar and exchange interactions.

Magnetic flux density measurements from grain boundary phase in 0.1 at% Ga-doped Nd–Fe–B sintered magnet

The magnetism of a narrow (~1.6 nm) grain boundary phase produced in a 0.1 at% Ga-doped Nd–Fe–B sintered magnet was examined using electron holography. The magnetic flux density was determined to be 0.8 ± 0.1 T, which was smaller than that for a commercial magnet free from Ga doping (~1.0 T). The presence of a ferromagnetic grain boundary phase reasonably explained the functionality of the 0.1 at% Ga-doped system, such as the improvement in the squareness of the demagnetization curve. The observations provide useful information for deeper understanding of the coercivity mechanism in Ga-doped Nd–Fe–B sintered magnets.

Highly Efficient Diffusion TbF <sub>3</sub> to Enhance the Coercivity of Sintered Nd-Fe-B Magnet by Cu Addition

In this paper, Cu element was added to the grain boundary of sintered Nd-Fe-B magnet to form a low melting point alloy phase in the grain boundary to promote the diffusion depth of Tb element. The intrinsic coercivity of the magnet with 0.2 % Cu addition increased significantly from 16.07 kOe for standard alloy without Cu to 24.38 kOe after the diffusion of TbF3 . The squareness of the demagnetization curves is maintained at 94 %, ensuring a good diffusion efficiency and uniformity. EPMA analysis showed that the distribution depth of (Nd, Tb) 2 Fe 14 B core-shell layer of the magnet was increased from 50 μm to 145 μm after the diffusion. The addition of Cu increased the ratio and average area of the grain boundary phases effectively, and formed a good diffusion channel. A larger amount of (Nd, Tb) 2 Fe 14 B shell layers formed in the magnet as a result of TbF 3 diffusion in Cu added magnets and which increased the intrinsic coercivity due to enhanced magnetocryalline anisotropy and favorable microstructure.In this paper, Cu element was added to the grain boundary of sintered Nd-Fe-B magnet to form a low melting point alloy phase in the grain boundary to promote the diffusion depth of Tb element. The intrinsic coercivity of the magnet with 0.2 % Cu addition increased significantly from 16.07 kOe for standard alloy without Cu to 24.38 kOe after the diffusion of TbF3 . The squareness of the demagnetization curves is maintained at 94%, ensuring a good diffusion efficiency and uniformity. EPMA analysis showed that the distribution depth of (Nd, Tb) 2 Fe 14 B core-shell layer of the magnet was increased from 50 μm to 145 μm after the diffusion. The addition of Cu increased the ratio and average area of the grain boundary phases effectively, and formed a good diffusion channel. A larger amount of (Nd,Tb)2Fe14B shell layers formed in the magnet as a result of TbF3 diffusion in Cu added magnets which increased the intrinsic coercivity due to enhanced magnetocryalline anisotropy and favorable microstructure.

Influence of Melt-Pool Stability in 3D Printing of NdFeB Magnets on Density and Magnetic Properties.

The current work presents the results of an investigation focused on the influence of process parameters on the melt-track stability and its consequence to the sample density printed out of NdFeB powder. Commercially available powder of Nd7.5Pr0.7Fe75.4Co2.5B8.8Zr2.6Ti2.5 alloy was investigated at the angle of application in selective laser melting of permanent magnets. Using single track printing the stability of the melt pool was investigated under changing process parameters. The influence of changing laser power, scanning speed, and powder layer thickness on density, porosity structure, microstructure, phase composition, and magnetic properties were investigated. The results showed that energy density coupled with powder layer thickness plays a crucial role in melt-track stability. It was possible to manufacture magnets of both high relative density and high magnetic properties. Magnetization tests showed a significant correlation between the shape of the demagnetization curve and the layer height. While small layer heights are beneficial for sufficient magnetic properties, the remaining main parameters tend to affect the magnetic properties less. A quasi-linear correlation between the layer height and the magnetic properties remanence (Jr), coercivity (HcJ) and maximum energy product ((BH)max) was found.

Magnetic property improvement of melt spun LaCo5-based nanocomposites with Y, Fe and C substitutions

Magnetic properties, crystal structure, and microstructure of La1-xYxCo4.7-yFeyC0.3 alloys (x = 0–1; y = 0–0.5) prepared by melt spinning are reported. Very low coercivity (iHc) of 0.2 kOe is found for LaCo4.7C0.3 ribbon due to the existence of large amount of magnetically soft LaCo13 (1:13) and La2Co7 (2:7) phases. Interestingly, Y substitution for La was found effective in suppressing the formation of impurity phases and stabilizing 1:5 phase. Y entrance into 1:5 phase increases the Curie temperature and improves saturation magnetization (4πMs) and magnetocrystalline anisotropy field (Ha), simultaneously. Fine microstructure observed for all studied samples due to C-doping contributes to high iHc and good squareness of demagnetization curve. The stabilized 1:5 phase and increased 4πMs with Y substitution for La and fine microstructure due to C-doping give rise to significant improvement of the permanent magnetic properties. The magnetic properties of LaCo4.7C0.3 ribbons can be enhanced to remanence (Br) = 4.2–5.4 kG, iHc = 8.0–14.5 kOe, (BH)max = 3.6–6.6 MGOe for x = 0.25–1. Finally, for alloy with x = 0.5, Fe-doping could further improve magnetization and (BH)max of La0.5Y0.5Co4.7-yFeyC0.3 ribbons, from Br = 4.4 kG and (BH)max = 4.4 MGOe for y = 0 to Br = 4.7–4.8 kG and (BH)max = 5.0–5.1 MGOe for y = 0.3–0.4, respectively. The result of this study provides a significant way to largely improve magnetic performance of isotropic LaCo5-based nanocomposites.

Removing the surfactant of SmCo5 nanoflakes via ligand-exchange and vacuum heat-treatment

The SmCo5 nanoflakes covered with polyvinylpyrrolidone (PVP) surfactant are obtained via surfactant-assisted ball milling method and were treated by ligand-exchange and vacuum heat-treatment methods. No significant change in the phase structure and surface morphology of as-milled SmCo5 nanoflakes was found upon those as confirmed by X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis, respectively. Moreover, X-ray photoelectron spectroscopy (XPS) analysis revealed that the ligand-exchange and vacuum heat-treatment are effective techniques for removing the polyvinylpyrrolidone (PVP) surfactant from the surface of SmCo5 nanoflakes. Meanwhile, the mechanism of coercivity is studied by the temperature dependence of demagnetization curves, which indicates that the magnetization reversal is controlled by co-existed mechanisms of pinning and nucleation.

Tuning magnetic properties, thermal stability and microstructure of NdFeB magnets with diffusing Pr-Zn films

Grain boundary diffusion process (GBDP) serves as a promising approach in improving magnetic properties and thermal stability of NdFeB permanent magnets. Herein, non-heavy rare earth Pr-Zn films deposited on the magnet surface using DC-magnetron sputtering system are reported. The thermal stability and coercivity enhancement mechanism of Pr-Zn GBDP magnets were investigated. Results show that the coercivity of Pr-Zn GBDP magnet increases from 963.96 kA m−1 to 1317.14 kA m−1 without any remanence reduction. Notably, the demagnetization curve of Pr-Zn GBDP magnet still remains a high squareness ratio. The temperature coefficient of coercivity and anti-demagnetization ability of Pr-Zn GBDP magnet under high temperatures are improved after GBDP treatment. The well-optimized rare earth-rich (RE-rich) grain boundary phases and high effective anisotropy field of (Nd,RE)2Fe14B magnetic hardening layers surrounding main grains are the key factors to impact the magnetic properties and thermal stability of NdFeB permanent magnets via GBDP treatment.

A Comparative Investigation of Presaturated Core Fault Current Limiters Biased by DC Current and Permanent Magnet

In this paper, the dc biasing magnetomotive force (mmf) in presaturated core fault current limiter (PCFCL) is replaced with permanent magnet (PM) to overcome the well-known drawbacks of the dc biased PCFCLs. These drawbacks are excessively induced voltage across the terminals of the dc coil during fault condition and high total power losses during the steady-state condition. The novel contributions of this paper are the introduction of the selection criteria of PM material and its representation by actual nonlinear $B$ – $H$ hysteresis loop of Jiles-Atherton method, and the use of PM alone as an equivalent to the dc biasing coils for three-phase fault current limiter (FCL), contrary to work reported in earlier papers. Single- and three-phase PCFCLs are modified by replacement of dc saturation coil by the equivalent PM through time-domain finite element (FE) simulation of COMSOL Multiphysics package. Experimental verification shows that the implementation of accurate nonlinear representation of PM demagnetization curve with FE simulation gives a realistic performance of the PM biased PCFCL, especially in the fault condition. The performance of PCFCL can be evaluated through the voltage drop and total power losses during the steady-state condition and the fault current clipping ratio during fault condition of the grid. Furthermore, a comparative investigation of dynamic behavior is considered between dc and PM biased PCFCLs. It is found that the PM biased PCFCL has enriched capability of limiting any type of fault current with leading advantages of reduced voltage drop and power losses significantly together with risk elimination of the high induced voltage across dc coil terminals. Moreover, the governing parameters of PM biased PCFCLs are subjected to performance analysis to study their inherent effects.

Characterizations analysis of magneto-structural transitions in Ce-Co doped SrM based nano Sr1−xCexFe12−xCoxO19 hexaferrite crystallites prepared by ceramic route

Herein, to analyze as a smooth microwave signal contributor in recording media, we have successfully synthesized Ce-Co co-doped SrM based nano hexaferrite crystallites through conventional ceramic route. The magneto-structural properties were studied analytically by employing different characterizations. Consequently, Rietveld refinement of X-ray diffraction (XRD) certified the phase singularity, with space group P63/mmc hexagonal. Field Emission Scanning Electron Microscope (FESEM) revealed the aggregation of grains into regular hexagonal platelets of sizes ~1 to 2 µm. Transmission electron microscope (TEM) estimated the grain size and high crystallinity. X-ray photoelectron spectroscopy (XPS) corroborated the host and dopant elements in diverse chemical states, affecting the magneto-structural parameters. Vibrating sample magnetometer (VSM) validated the ferromagnetic nature of samples at 300 K by measuring their saturation magnetization (MS). Accordingly; Stoner Wolhfarth (S-W) model confirmed the MS values, the domain singularity with squareness ratio (SQR) of ~0.51 and the high uniaxial magneto-crystalline anisotropy (MCA) of the prepared hexaferrites. Different magnetic parameters were analyzed using coupled intrinsic and normal demagnetization curves, results in Br > 1 kG & Hc > 1 kOe, validating our samples usage in permanent magnetic industry.

A Non-Linear Permanent Magnet Working Point Migration Model and its Application to Simulation of a Polarized Magnetic System

The paper addresses the issue of the working point migration in non-linear permanent magnets (PM). Starting from the considerations of energy, a novel working-point migration (WPM) model is proposed which can be incorporated into a magnetic equivalent circuit (MEC). The basic theory of working-point migration is discussed which focus on the second quadrant of the magnetic hysteresis loop. The calculation method of the WPM model is proposed based on the relationship between the recoil line model and the affine transformation hysteresis loop. The establishment method for the resultant working point is described by combining the demagnetization curve and the recoil line models in the minor hysteresis loop. The static characteristic of a bistable polarized magnetic system (BPMS), as used in actuators, is calculated using the magnetic circuit method based on the WPM, while a finite element model (FEM) is also derived. The calculation method for the WPM used in MEC is also discussed. The WPM based MEC model yields reasonable results, compared with FEM, of the latching force but with much faster calculation speeds. Furthermore, the working-point state of the PM is clearly illustrated. The test system of the BPMS prototype is established. The test data and WPM model calculation results are compared. It is shown that the WPM model provides accurate prediction of static characteristics of an electromagnetic system.

Enhanced magnetic properties of anisotropic Nd-Fe-B nanocomposites by Fe(C) coating

Nanostructured anisotropic Nd-Fe-B/Fe(C) composite powders were prepared by coating Fe(C) soft-magnetic nanoparticles on HDDR Nd-Fe-B hard magnetic powders using iron pentacarbonyl Fe(CO)5 as soft-phase precursor. The effect of Fe(CO)5-loading amount on soft-phase purity, coating morphology and magnetic properties of the composite powders was investigated. Dense and continuous Fe(C) soft-phase coatings with average particle sizes of 58–68 nm are obtained at Fe(CO)5 loading amounts of x ≤ 12 wt%, leading to enhanced remanence and improved energy product of the coated powders. Positive value in δM-plots and single-phase-like demagnetization curves are observed in the Nd-Fe-B/Fe(C) composite powders, indicating the exchange coupling effect between the coated Fe(C) soft phases and the Nd-Fe-B hard phase.