Initial Magnetization Curve Research Articles

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.

Magnetic studies of ultrafine CoFe2O4 nanoparticles with different molecular surface coatings.

Surface functionalized ultrafine CoFe2O4 nanoparticles (NPs), with mean diameter ∼5 nm, were investigated by means of DC magnetization and AC susceptibility over the temperature range of 4-400 K. All NPs present the same CoFe2O4 core, with different molecular surface coatings, increasing gradually the number of carbon atoms in the coating layer: glycine (C2H5NO2), alanine (C3H7NO2), aminobutanoic acid (C4H9NO2), aminohexanoic acid (C6H13NO2), and aminododecanoic acid (C12H25NO2). Samples were intentionally fabricated in order to modulate the core-core magnetic dipolar interaction, as the thickness of the coating layer increases with the number of carbon atoms in the coating molecule. The magnetic data of the uncoated CoFe2O4 NPs were also collected for comparison. All investigated CoFe2O4 NPs (coated and uncoated) are in a magnetically blocked state at room temperature as evidenced by ZFC/FC measurements and the presence of hysteresis with ∼700 Oe coercivity. Low temperature magnetization scans show slightly constricted hysteresis loops with coercivity decreasing systematically with a decreasing number of carbon atoms in the coating molecule, possibly resulting from differences in magnetic dipole coupling between NPs. Large thermomagnetic irreversibility, slow monotonic increase in the FC magnetization and non-saturation of the magnetization give evidence for the cluster glass (CG) nature in the CoFe2O4 NPs. The out of phase part (χ'') of AC susceptibility for all samples shows a clear frequency dependent hump which was analyzed to distinguish superparamagnetic (SPM), cluster glass (CG) and spin glass (SG) behavior by using Néel-Arrhenius, Vogel-Fulcher, and power law fittings. These analyses rule out the SPM state and suggest the presence of significant inter-cluster dipolar interaction, giving rise to CG cooperative freezing in the high-temperature region. In the low-temperature range, however, the disordered spins on the nanoparticle's surface play an important role in the formation of the SG-like state, as evidenced by Arrott plots and temperature dependency of dM/dH in the initial magnetization curves. In summary, the magnetic measurements showed that undercooling the system evolves from a SPM state of weakly interacting spin clusters, through the CG state induced by strong dipolar interaction, to the SG state resulting from the frustration of the disordered surface spins.

Ti‐Doped Nanocomposite Pr2Fe14B/α‐Fe Ribbons with High Coercivity

Nanocomposite permanent magnetic alloys are prepared by the melt‐spinning technique with wheel speed ranging from 21 to 33 ms−1. X‐ray diffraction results show that the as‐spun ribbon is rich of soft α‐Fe, hard Pr2Fe14B magnetic phases, and no other metastable phases are found in our alloy ribbon. Initial magnetization curves reveal that the coercivity mechanism of the ribbons vary with the content of Ti. For x = 2, coercivity is mainly determined by pure pinning; while for x = 1, 3, and 4, it is mainly controlled by the mixture of nucleation of domain and localized pinning at grain boundaries. The coercivity value of 1026 kA m−1, the highest value reported so far, is mainly generated from a pure pinning mechanism in the ribbon with x = 2. The recoil permeability shows a minimum value range from to for x = 2 alloy, suggesting strong exchange coupling between the magnetic grains and then leading to the concurrent reversal of soft and hard phase grains during the demagnetization process. The Henkel plots demonstrate only exchange interaction for x = 1 and exchange interaction with slightly magnetostatic interaction for x = 2, 3, and 4. The exchange interaction exhibits maximum strength for x = 2, which is the reason for high coercivity.

Analytical Inverse Preisach Model and Its Comparison With Inverse Jiles–Atherton Model in Terms of Accuracy and Computational Speed

Inverse hysteresis model with magnetic flux density and magnetic field intensity as input and output respectively is more preferred, compared with the forward one, for resolving the electromagnetic issues of electrical equipment in terms of magnetic vector potential or with voltage sources. And an inverse hysteresis model with high accuracy and high computation speed both has broader range of applications. However, the well known classical Preisach hysteresis model is formulated with the forward form, and has the double integration of its distribution function, making it inconvenient and unpractical in the electrical engineering. In this paper, an analytical inverse Preisach model is proposed for the first time. Firstly, the analytical expressions of permeability of initial magnetization curve, descending and ascending branches of the hysteresis loop are derived using one closed form of Everett integral function. Subsequently the analytical inverse Preisach model is derived by the difference method, considering the reversible magnetization component that classical Preisach model cannot simulate with an odd function analytically. The experiment of one grain oriented silicon steel sample and one non-oriented silicon steel sample, under different magnetic excitation levels is conducted, and the accuracy and efficiency of the proposed analytical inverse Preisach model are confirmed by comparing its simulated hysteresis loops with the experimental ones and that computed with the widely used inverse Jiles-Atherton hysteresis model.

Transformer modelling considering power losses using an inverse Jiles-Atherton approach

Power transformers are devices with non-linear behavior due to the saturation of the ferromagnetic core. When modelling such devices, the saturation effect must be taken into account because it greatly affects their performance and efficiency. In this paper, an electromagnetic model for power transformers is proposed and experimentally validated using an electrical T-model coupled with a reluctance network to model the magnetic part. The electrical circuit and the reluctance network are linked by two B–H approaches. The B–H relationships are modelled by the full hysteresis cycle based on the inverse Jiles-Atherton theory and by the initial magnetization curve. The results obtained with the inverse Jiles-Atherton theory model reproduce the magnetic core behavior with more accuracy than the one based on the initial magnetization curve, especially at low load conditions where saturation plays a more prominent role on the no-load current. The proposed model can be applied to other magnetic devices such as inductors for power electronic applications or electromechanical relays, among others.

High-performance anisotropic SmCo-based magnet with novel microstructure

A novel microstructure of magnetically anisotropic SmCo-based magnet with high-performance is reported. The magnet consists of SmCo7-H (TbCu7-hexagonal type), Sm2Co17–H (Th2Ni17-hexagonal type) and SmCo3-R (SmCo3-rhombohedral type) phases. The maximum magnetic energy product of the magnet is 231.69 kJ/m3, and the intrinsic coercivity is 1005.47 kA/m. An outstanding intrinsic coercivity temperature coefficient (β) of −0.125%/K between 298 and 773 K is obtained, which is very close to the β of commercial high-temperature Sm2Co17-based sintered magnets. The initial magnetization curve indicates that the coercivity mechanism is controlled by a domain wall pinning mechanism. The SmCo3-R lamellar phase may be a potential pinning center or self-pinning center. The microstructure of the magnet is different from that of any previous SmCo-based magnets. These findings provide a new idea for preparing high-performance SmCo-based permanent magnets.

Demagnetization Parameters Evaluation of Magnetic Shields Based on Anhysteretic Magnetization Curve.

To achieve the nearly zero-field environment, demagnetization is an indispensable step for magnetic shields composed of high-permeability material, which adjusts the magnetization of the material to establish magnetic equilibrium with the environmental field and improve the shielding performance. The ideal demagnetization can make the high-permeability material on the anhysteretic magnetization curve to have a higher permeability than on the initial magnetization curve. However, inappropriate parameters of degaussing field cause the magnetization state to deviate from the anhysteretic magnetization curve. Therefore, this article proposes a new assessment criterion to analyze and evaluate the parameters of degaussing field based on the difference between the final magnetization state after demagnetization and theoretical anhysteretic state of the shielding material. By this way, the magnetization states after demagnetizations with different initial amplitude, frequency, period number and envelope attenuation function are calculated based on the dynamic Jiles-Atherton (J-A) model, and their magnetization curves under these demagnetization conditions are also measured and compared, respectively. The lower frequency, appropriate amplitude, sufficient period number and logarithmic envelope attenuation function can make the magnetization state after demagnetization closer to the ideal value, which is also consistent with the static magnetic-shielding performance of a booth-type magnetically shielded room (MSR) under different demagnetization condition.

Magnetic phase recognition of artificial kagome spin ice through initial magnetization curve

Magnetic phase recognition of artificial kagome spin ice through initial magnetization curve

Examination of Thickness Evaluation of Steel Plate Using DC Bias Magnetic Field by 3-D FEM Analysis Considering Minor Loop Magnetic Characteristics

Thickness evaluation of steel plates and large-diameter steel pipes in various plant structures is considered important for plant safety. In this paper, an electromagnetic inspection method that can evaluate this thickness using a pulsed magnetic field is investigated. When a steady square-wave pulse magnetic field with DC bias is applied from the surface of the steel plate, the relationship between the leakage flux on the steel plate and the plate thickness is investigated. The inspection principle is evaluated by electromagnetic field analysis considering eddy current and minor loop magnetic property of the steel plate and verification experiments. In addition, it is also confirmed that the calculation results considering the hysteresis curve in the steel plate are more useful than those considering only the initial magnetization curve.

Tuning magneto-dielectric properties of Co2Z ferrites via Gd doping for high-frequency applications

Magneto–dielectric properties of Co2Z ferrite materials are tuned via Gd doping for applications in high-frequency antennas and filters in the present work. Ba3Co2Fe24−x Gd x O41 (x = 0.00, 0.05, 0.10, 0.15, and 0.20) materials are successfully prepared by using solid-state method at 925 °C for 4 h with 2.5-wt% Bi2O3 sintering aids. The content of Gd3+ ion can affect micromorphology, grain size, bulk density, and magneto–dielectric properties of the ferrite. With Gd3+ ion content increasing, saturation magnetization (M s) first increases and then decreases. The maximum value of M s is 44.86 emu/g at x = 0.15. Additionally, sites occupied by Gd3+ ions can change magnetic anisotropy constant of the ferrite. Magnetocrystalline anisotropy constant (K 1) is derived from initial magnetization curve, and found to be related to spin–orbit coupling and intersublattice interactions between metal ions. The real part of magnetic permeability (μʹ) and real part of dielectric permittivity (εʹ) are measured in a frequency range of 10 MHz–1 GHz. When x = 0.15, material has excellent magneto–dielectric properties (μ′ ≈ 12.2 and ε′ ≈ 17.61), low magnetic loss (tan δμ ≈ 0.03 at 500 MHz), and dielectric loss (tan δε ≈ 0.04 at 500 MHz). The results show that Gd-doped Co2Z ferrite has broad application prospects in multilayer filters and high-frequency antennas.

Coercivity Mechanism and Magnetization Reversal Process in Nanocomposite Pr2Fe14B/α‐Fe Alloy with Phosphorous Addition

An investigation is conducted for the coercivity mechanism in as‐spun Pr2Fe14B/α‐Fe ribbons containing different phosphorus (P) prepared at a wheel speed of 30 ms−1. With the increase of P addition, the volume of the amorphous phase in the ribbons is increased, and the grain size of Pr2Fe14B is decreased. The hysteresis loops of the ribbons with 0.1 at% P exhibit a homogeneous magnetization reversal process similar to single‐phase magnet, demonstrating that soft magnetic grains are fully coupled with the hard ones. The highest coercivity of 12.0 kOe (300 K)–38.0 kOe (100 K) is achieved in the ribbon with 0.3 at% P‐addition, which shows weak exchange coupling interaction between grains, and the high coercivity comes mainly from the decoupled hard phase. The values of the microstructure parameter in all compositions are less than 0.3 and imply that the main coercivity mechanism is the mixture of the nucleation of reverse domain and domain wall pinning. The minor hysteresis loops and initial magnetization curves show the dominant domain wall pinning in the ribbon with 0–0.1 at% P, nucleation and pinning in the ribbon with 0.3 at% P, and dominant domain nucleation in the ribbon with 0.5 at% P.

Modification on grain boundary phase distribution in PrNd-Cu alloy diffused hot-deformed Nd-Fe-B magnets

Grain boundary diffusion using PrNd-Cu alloy followed pressure induction were applied to optimize the preferential distribution of the diffused eutectic phases in hot-deformed Nd-Fe-B magnets. A value of high coercivity 2.5 T was achieved while the coercivity of the sample with grain boundary modification was then decreased, and the remanence was significantly increased due to the improved grain orientation and reduced volume fraction of the nonferromagnetic phase. Magnetic domain structures of samples were observed and magnetic isolation by diffused nonmagnetic phases was also obtained. The corresponding magnetic domain structure analysis indicated that coercivity was close correlated with the magnetic isolation of grains by the diffused phases. The coercivity mechanisms of the samples were assumed to be domain wall pinning model by the fitted linear relationship of Hcj1/2 versus T2/3 and the initial magnetization curve. The Lorentz TEM investigation confirmed the domain wall pinning at grain boundaries parallel to c-axis.

Magnetic properties and microstructures of hydrogenation-disproportionation-desorption-recombination processed Nd-Fe-B powders by grain boundary diffusion of Nd-Cu-Al

The relatively low coercivity of the anisotropic bonded Nd-Fe-B powders prepared by hydrogenation-disproportionation-desorption-recombination (HDDR) method hinders their application in high-tech fields, such as new energy vehicles and robots. The low melting point Nd80Cu10Al10 (wt.%) ternary alloy is employed as the source for grain boundary diffusion to enhance the coercivity of the HDDR powders. The effect of the content and diffusion temperature on the magnetic properties is systematically investigated. It is found that the coercivity is enhanced after grain boundary diffusion. The coercivity increases monoclinically from 1026.0 to 1301.0 kA m−1 with the addition of 2% ∼ 6% (wt.%) diffusion source at 820 °C. Through the initial magnetization curve, the S-shape indicates that the coercivity mechanism is more inclined to pinning type. Meanwhile, the derivation curve shifts towards higher value gradually, indicating that the pinning field increases due to the grain boundary diffusion. Correspondingly, TEM micrographs show that the content of the grain boundary phase is increased, and the grain boundary is widened. In the range of 770-870℃, the coercivity increases at first and then decrease as the temperature exceeds 820 °C. The maximum of the coercivity of the HDDR powders of Hcj = 1301.0 kA m−1 is obtained with the addition of 6 wt% Nd-Cu-Al, diffusion treated at 820 °C.

Magnetic properties and coercivity mechanism of high Ce-content CeNdFeB film with Tb diffusion

Magnetic properties and coercivity mechanism of an anisotropic Ce–Nd–Fe–B film with a high Ce content (70 at. % Nd is replaced by Ce) have been investigated. After grain boundary diffusion with Tb layers, an enhancement of coercivity from 4150 to 9250 Oe is observed. Combining the initial magnetization curves, micromagnetic theory, and in-siut observation of magnetic domains in the demagnetization process, it is confirmed that the coercivity mechanism for the high-Ce-content magnets is the mixed type dominated by the pinning mechanism. Moreover, as the thickness of the Tb diffusion layer increases, the pinning center of domain walls changes from narrow planar inhomogeneities to wide planar inhomogeneities. A growing role of pinning plays in determining the coercivity of samples with increasing the thickness of Tb layer due to the increase in magnetocrystalline anisotropy after Tb substitution in the RE2Fe14B phase. In addition, the CeFe2 intergranular phase leads to the enhancement of coercivity due to decoupling the hard magnetic phase grains. Our results provide an insight into the coercivity mechanism of high-Ce-concentration Ce–Nd–Fe–B magnetic materials and promote the comprehension of the effect of Tb diffusion in the magnetization reversal process.

Investigating the structural, magnetic, magnetocaloric and critical behavior of Mg0.35Zn0.65Fe2O4 ferrite

In this study, we successfully prepared the Mg0.35Zn0.65Fe2O4 ferrite sample via standard solid-phase reaction and comprehensive studied its structure, magnetism, magnetocaloric effect and critical behavior. The XRD data refined by Rietveld technology confirms that the sample had a pure phase spinel structure. We conducted a magnetic study on the sample using SQUID and indicated that the sample’ ferromagnetic curie temperature Tc = 295 K, the paramagnetic curie temperature TP = 330 K, and the saturation magnetization can reach 125.41emu/g at an ultra-low temperature of 5 K. We tested the initial magnetization curve near the phase transition temperature (Tc) from ferromagnetic (FM) to paramagnetic states (PM) and produced an Arrott plot that confirmed the presence of a secondary phase transition. The sample’ maximum magnetic entropy change |ΔSMmax| was 1.642 J/kg K and relative cooling power RCP was 185.2 J/kg in the external magnetic field H = 50 kOe. The critical parameter β, γ, δ, and TP were calculated using a Modified Arrott plot (MAP), Kouvel-Fisher (KF) plot and critical isotherm (CI) methods to study the type of magnetic order in the ferromagnetic state that corresponded to the magnetic state equation and mean field model. By analyzing the critical parameters, we obtained the ferromagnetic exchange integral constant J(r) ~ r−4.8498, which was between the 3D Heisenberg model and mean field models and very close to the mean field model, confirming that the long-range and short-range ferromagnetic orderly interactions coexisted with interaction phases and the long-range ferromagnetic orderly interaction dominates.

Effect of P and Zr on the microstructure and magnetic properties of nanocomposite Pr2Fe14B/α-Fe ribbons

We have investigated the effect of P and Zr on the magnetic properties and microstructure of the as-spun ribbon Pr2Fe14B/α-Fe achieved at a wheel speed of 30 m/s. X-ray diffraction and TEM results reveal that P-addition can decrease the grain size of 2:14:1 hard phase whereas further Zr-addition decreases the soft α-Fe phase. The combination of doping P and Zr reduces overall grain size and refines the microstructure considerably, and results in enhanced coercivity significantly. The strong exchange coupling interaction in our ribbons were confirmed by Henkel plots, which leads to higher coercivity of 964 kA/m. The initial magnetization curves reveal the coercivity in P-added alloy is controlled by the nucleation of reversal domain and that in further Zr-added alloy is the domain wall pinning.

Understanding the strain effect on coercivity enhancement in L10-Mn56Ga42 alloy via low-energy mechanical cryomilling

Varying degrees of strain were observed in L10-Mn56Ga42 powders via low-energy mechanical cryomilling, and the relationship between coercivity and strain was analyzed in detail. With increasing milling time, the powders do not show obvious variations in grain size and cell parameters, but a large strain is induced into the powders, leading to a remarkable enhancement in coercivity. A maximum coercivity of up to 6.7 kOe was accomplished for 20 h milled powders, with a high residual strain of about 0.96%. The initial magnetization curve indicates that the coercivity mechanism is mainly governed by the domain-wall pinning process where strain-induced defects act as pinning centers. Studies on heat treatments show that the strain-induced coercivity contributes to over 50% of the total coercivity, and a moderate annealing can eliminate the strain, changing the coercivity mechanism to a nucleation process.

GaN-Half-Bridge for Core Loss Measurements Under Rectangular AC Voltage and DC Bias of the Magnetic Flux Density

An accurate calculation of core losses is essential for the design of switched-mode power supplies. However, such predictions remain a challenge as core losses depend on various parameters, e.g., the waveform of the magnetic flux density, including its fundamental frequency and dc bias. To investigate the influence of dc bias, core loss measurements usually impose a dc current and, thus, dc bias of the magnetic field strength on ungapped toroid samples. However, hardware designers require the ac and dc excitation of the magnetic flux density. Therefore, this article proposes a half-bridge configuration to impose rectangular ac voltages on the core under test (CUT). By performing a pulsed operation in combination with an iteration method, the desired ac and dc excitation of the magnetic flux density can be obtained. GaN-HEMTs have been applied as they allow a printed circuit board design with low parasitic inductances and thus avoid ringing. The measurement setup is able to characterize magnetic materials at high frequencies for triangular magnetic flux density waveforms, including dc bias and different duty cycles. In addition, the influence of magnetic history on core losses can be studied, an issue usually not addressed in the literature. Moreover, the relationship between the dc bias of the magnetic field strength and the dc bias of the magnetic flux density is investigated. A simple functional approach, e.g., by using the initial magnetization curve, does not suffice to describe this relationship. Accordingly, core losses should be derived as a function of dc bias of the magnetic flux density for the results to be of practical use to hardware designers.

Magnetic Domain Analysis of Ferrite-Sintered Magnets in Thermally Demagnetized and AC Demagnetized States Using Multiple Distances Observation of MFM and Image Processing

The magnetic domain structure is an important factor in discussing the coercivity development mechanism. We have researched on the new observation method of magnetic domain structures using multiple distances observation of magnetic force microscope (MFM) and image processing. We observed magnetic domain structures in thermally demagnetized state and alternating current (ac) demagnetized state of ferrite-sintered magnets using this method. In addition, the initial magnetization curve was measured in each demagnetized state. There were differences in the domain structures and the initial magnetization curves in each demagnetized state. These differences suggest that irreversible multi-domain gains (IRMDGs) confirmed in rare-earth magnets are present even in sintered ferrite magnets.

Magnetic field-induced metamagnetic, magnetocaloric and pyrocurrent behaviors of Eu2CoMnO6

In this paper, the structural, transport, pyrocurrent and magnetic behaviors of Eu2CoMnO6 (ECMO) sample are reported. It was prepared by the solid-state reaction method and crystallized in monoclinic crystal structure with a space group of P21/n. A systematic study of the temperature and magnetic field-dependent susceptibility revealed a complex magnetic behavior with field-induced type-II metamagnetic phase transition of the first-order. The multiple sharp jumps in the initial magnetization curve for T < 10 K could be attributed to structural distortion related to the martensite-like strains from E*-type antiferromagnetic to ferromagnetic ordering. The metastable magnetic behavior for T ≥ 60 K is ascribed to a transformation of the low-field disordered spin glass phase to long-range ordered ferromagnetic phase, induced by critical fields. These critical fields are quite sensitive to the direction and magnitude of the field-cooling process. The temperature-dependent magnetocaloric effect with maximum value of ~3.3 J kg−1K−1 at TC ~ 123 K and the electrical transport behavior of ECMO were studied. Further, temperature dependent pyroelectric current and polarization data of ECMO are investigated.