Variation Of Coercive Force Research Articles

Study on Coercivity-Stress Relationship of X80 Steel under Biaxial Stress

Buried pipelines are prone to deformation under surface displacement, pressure occupation, and/or other actions, resulting in stress concentration. Conducting regular stress detection is an important approach to reducing pipeline accidents. This study performed experiments on measuring the coercivity stress of X80 steel, the most commonly used material for high-pressure large-diameter gas transmission pipelines. The coercivity of the specimen under different stress combinations was measured using the DC-driven coercive force measurement system, and the variation of coercive force with biaxial stress was also analyzed. The modified coercive force-stress expression of the generalized Hooke’s law was proposed; the model for the coercive force-stress relationship of X80 steel under the biaxial stress state was established based on this expression. The study indicates that the coercive force method is reliable for accurately measuring the stress of pipeline steel and evaluating the safety margin of in-service pipelines.

A Novel NDT Method for Coercive Force of Grain-Oriented Electrical Steel Applied in Cigarette Machines

Coercive force of grain-oriented electrical steel applied in cigarette machines is tested by a novel NDT method in which Barkhausen noise was applied. Linear relationship between coercive force and Barkhausen noise of conventional and high-permeability electrical steel was built and the precision of the model was verified. Experimental results show that the coercive force is linearly related to Barkhausen noise. The variation of coercive force with magnetic induction intensity increase was also interpreted in view of magnetic domain changing process, and the correlation formula derivation of MBN and coercive force domain wall was illustrated as well.

ＷＣ‐Ｃｏ系超硬合金の圧縮疲労特性

As WC-Co based alloys have high hardness, compressive strength and Young's modulus and excellent deformation resistance, they have been widely applied for wear and impact resistance tools. Therefore, it becomes very important to acquire knowledge on compressive fatigue characteristics of these alloys. In this study, the effects of Co content, WC grain size, HIP and coating treatment on compressive fatigue life of medium or coarse-grain WC-7-20mass% Co alloys and micrograin WC-6-l5mass%Co-0.4-1.2mass%Cr3C2 alloys were mainly investigated and discussed with the variation of coercive force during fatigue process.The results obtained were as follows: (1) The fatigue life increased with decreasing Co content in the medium and coarse-grain alloys, but it was almost same with no relation to Co content in the micrograin alloys. The fatigue life showed an upward tendency with decreasing WC grain size. (2) The effect of HIP treatment on fatigue life was not observed at all. The fatigue life of PVD-coated alloy did not change and that of CVD-coated alloy remarkably decreased, comparing with that of non-coated alloy. (3) The coercive force of medium-grain WC-12mass%Co alloy increased with increasing applied maximum stress and repeated cycle, and it became much larger in the repeated compressive fatigue test than static compressive test. This increase of the coercive force was considered to be equivalent to the degree of toughness deterioration due to fatigue hardening in Co phase during fatigue process.

Local Variation of Magnetic Parameters of the Free Layer in TMR Junctions

Local M-H loops have been measured on the free layer of a tunneling magnetoresistance (TMR) junction using the magneto-optical Kerr effect (MOKE) system, with an optical beam size of about 2 <TEX>$\mu$</TEX>m diameter. Tunnel junctions were deposited using the DC magnetron sputtering method in a chamber with a base pressure of 3<TEX>$\times$</TEX>10<TEX>$^{-9}$</TEX> Torr. The relatively irregular variations of coercive force H<TEX>$_c$</TEX>(∼17.5 Oe) and unidirectional anisotropy field H<TEX>$_{ua}$</TEX>(∼7.5 Oe) in the as-deposited sample are revealed. After <TEX>$200{^{\circ}C}$</TEX> annealing, He decreases to 15 Oe but H<TEX>$_{ua}$</TEX> increases to 20 Oe with smooth local variations. Two-dimensional plots of H<TEX>$_c$</TEX> and H<TEX>$_{ua}$</TEX> show the symmetric saddle shapes with their axes aligned with the pinned layer, irrespective of the annealing field angle. This is thought to be caused by geometric effects during deposition, together with a minor annealing effect. In addition, the variation of root mean square (RMS) surface roughness reveals it to be symmetric with respect to the center of the pinned-layer axis, with the roughness of 2.5 <TEX>$\AA$</TEX> near the edge and 5.8 <TEX>$\AA$</TEX> at the junction center. Comparison of surface roughness with the variation of H<TEX>$_{ua}$</TEX> suggests that the H<TEX>$_{ua}$</TEX> variation of the free layer is well described by dipole interactions related to surface roughness. As a whole, the reversal magnetization is not uniform over the entire junction area and the macroscopic properties are governed by the average sum of local distributions.

Nanocrystalline nickel-zinc ferrite prepared by the glass-ceramic route

Ni 0.5Zn 0.5Fe 2O 4 crystals having dimensions in the range 15–29 nm have been grown within borate glass by a two-stage heat treatment in the temperature range 873–953 K. Magnetic measurements have been carried out over the temperature range 20–300 K. The maximum coercive force corresponding to 0 K, ( H co) ∼ 334 Oe, is obtained for a sample containing ferrite crystals of median diameter 15 nm. Field-cooled and zero-field-cooled magnetization data for different samples are consistent with the blocking temperatures extracted from the variation of coercive force as a function of temperature. The magnetocrystalline anisotropy constant for nanocrystalline nickel-zinc ferrite is found to be an order of magnitude higher than that for the bulk material.

特集 磁性材料の化学 Ｓｍ２（Ｆｅ０．８Ｃｏ０．２）Ｎ３．０３圧縮成形ボンド磁石の磁気余効

The magnetic field and temperature dependences of the magnetic aftereffect constant Sv were examined for the Sm2(Fe0.8Co0.2)17 N3.03 compression bonded magnet at a temperature range from 77 to 397K. Sv increases gradually with increasing the magnetic field. The temperature dependence of coercive force HCJ, for Sm2(Fe0.8Co0.2)17N3.03, Sm-Co and Nd-Fe-B magnets decrease with the increasing temperature. However, the temperature dependence of Sv for the Sm2(Fe0.8Co0.2)17N3.03 compression bonded magnet was reverse to HCJ, in contrast to Sm-Co and Nd-Fe-B magnets. The temperature variation of coercive force ΔHCJ/ΔT in these rare-earth magnets linearly decreases as the activation volume v increases. Larger v for low ΔHCJ/ΔA T values and smaller v for higher HCJ, values are necessary, so that balanced performance of HCJ, should be obtained by choosing appropriate v values.

Magnetic properties of anisotropic Nd(Fe,Mo)12Nx powders

Anisotropic magnets based on Nd(Fe,Mo)12Nx nitrides were prepared by a milling process. Particle size dependence of the coercivity has been investigated. The optimizing size which accompanies the highest coercive force was examined by transmission electron microscopy. A coercive force iHc of up to 6.2 kOe, and a maximum energy product (BH)max of up to 12.0 MG Oe were obtained. The mechanism of the coercivities is discussed with a comparison of the powders prepared by mechanical alloying. The variation of coercive force as a function of temperature is reported.

Study of the coercivity of Pr(Mo, Fe)12Nx

A study of the permanent magnetic properties of Pr(Mo, Fe)12Nx powders has been carried out by using a mechanical alloying technique. The variations of coercive force and remanence as a function of composition and crystallizing temperature have been investigated. A coercive force of up to 6.0 kOe was obtained. The temperature dependence of the coercive force is presented in comparison with that of Nd2Fe14B-type magnets.

Magnetic hardening of mechanically alloyed Sm 2Fe 17N x

Samples of Sm 2Fe 17N x having the Th 2Zn 17 structure were prepared using mechanical alloying and subsequent heat treatment. The magnetic behaviour is found to depend significantly on heat treatment temperature. Coercive forces of up to 31.3 kOe at 300 K have been measured from hysteresis loops with maximum applied fields of 50 kOe. At 423 K the coercive force was 17 kOe. The variations of coercive force, magnetization and remanence as functions of maximum applied field are consistent with the view that the magnetization of this material is pinning controlled.

The Magnetization Process In Monoclinic Pyrrhotite (Fe7S8) Particles Containing Few Domains

SUMMARY Monoclinic pyrrhotite was prepared by the silica tube technique. All characteristic properties (X-ray, magnetic) point to the successful synthesis of Fe7S8. the material was separated into seven size fractions in the particle size range 1–30 μm, in which the particles are believed to contain only few domains. the magnetization process was studied at room temperature. the characteristics of the finest fraction (<0.8 μm) are consistent with coherent rotations of monodomain moments and a triaxial symmetry of anisotropy within the basal plane. the in-plane anisotropy constant is about 3 times 103 Jm−3. In the sized fractions (1–30 μm) the variation of ratio of isothermal remanence to saturation magnetization (Mrs/Ms) can be fitted by models of increasing complexity based on the statistically expected number of domains in particles of a particular size. the data can also be fitted by (Mrs/Ms) = 0.52 exp(-0.047L1/2) where L is the mean particle size in μm. the form of this expression is suggestive of the relevance of Poisson statistics to the magnetization process, but the numerical parameter within the exponential is physically implausible. Unlike titanomagnetite (Fe2.4 Ti0.6O4) particles of the same size range, the variation of coercive force (Hc) cannot be modelled on the basis of independent surface and volume pinning of domain walls. There is evidence for only one wall pinning mechanism in the 1–30 μm range in Fe7S8. A pleasing fit to the data is obtained by Hcr, = 75.5exp(-0.025L1/2)kA m-1. A similar dependence is found for Hcr, the coercive force of isothermal remanence. the susceptibility in the 1–30 μm range appears to be essentially the ‘intrinsic’ susceptibility and, if so, again provides evidence for only one wall pinning mechanism.

Induced anisotropy in fine particles for magnetic recording

An automatic apparatus allows fast studies of magnetic annealing and thermal variations of coercive force and remanent magnetization in cobalt substituted gamma -Fe/sub 2/O/sub 3/ particles for magnetic recording. Experimental results are interpreted with Neel's model (1954) of directional order. The model and results permit an understanding of the origin of surface anisotropy of surface cobalt doped particles.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Cation distribution in germanium-substituted magnesium ferrites

Magnetic properties of a series of tetravalent germanium substituted magnesium ferrites with the compositional formula Mg 1+ x Ge x Fe 2−2 x O 4. (x = 0, 0.1, 0.2, 0.3 and 0.4) are reported. The lattice constant was found to decrease with x. The observed variation of coercive force and remanence ratio with temperature for all the samples are consistent with the single domain behaviour of the material. Magnetisation measurements were carried out from 100°K to the Curie temperatures of the respective samples. O°K magnetisation values were obtained by extrapolating the magnetisation vs temperature plots. Taking into consideration the strong affinity of Ge 4+ ions for A-site occupancy and the 0°K magnetisation data, the cation distribution for this series of compositions has been worked out.

The effect of sputter-deposition conditions on the coercive force in amorphous rare-earth-transition-metal thin films

The origins of the coercive force in amorphous rare-earth-transition metal films have been investigated. The results are discussed in terms of how the growth conditions of the sputter-deposited films determine the pinning features that cause the coercive force. A model has been developed for the variation of coercive force with film thickness that enables a local pinning force per unit area to be deduced. This suggests that it should be possible to increase the coercive force by breaking up the microstructure with a multilayered structure. An increase in coercive force obtained by making such structures with tungsten is described. A reduction in coercive force obtained when the films are deposited in the presence of a perpendicular magnetic field is also reported and interpreted in terms of a growth annealing model. >

Shock induced magnetic effects in fine particle iron dispersions

In order to understand the role of shock waves in modifying existent remanent magnetization and in inducing magnetization via 1st and 2nd order shock transitions in iron and iron-nickel alloys a model system has been adopted. Dilute iron in copper (1.5 wt % Fe solutions can be annealed arbitrarily to produce a near ideal dispersion of spherical fcc precipitates having diameters ≲2000 Å. The size range is arbitrarily specified by the anneal. These spherical iron (antiferromagnetic-fcc) particles are then transformed to the ferromagnetic state (bcc) during a shock pulse in controlled external fields (≲1 Oe). The axial vector always takes on the sign of the external field indicating that on the microsecond time scale of the transformation the particles record the field sense. The amount of fcc iron transformed depends on both precipitate size and the peak input pressure. The observed deformation induced anistropy is dependent on particle size and shock level, but the 400 Å–600 Å particles show the maximum degree of anisotropy at all shock levels. Different sizes of precipitates support different degrees of magnetic hardening (i.e., response to alternating field demagnetization). Anisotropy in magnetic hysteresis, using hysteresis ratios - RI (ratio of saturation remanence to saturation magnetization) and RH (ratio of remanent coercive force to coercive force) indicate a shock induced weak field remanence anisotropy. Measurement of the angular variation of coercive force (HC) and remanent coercive force (HR) and the computed RH clearly identifies the direction of the shock, RH being a minimum and HC a maximum parallel to P. Unique properties observed in hysteresis behavior are related to the narrow iron particle size distribution, orientation, and mean particle spacing. These unique properties include discrete switching fields for magnetization reversal, and separation of region of hysteresis from region of shape response.

Coercive force in heat-treated R-Co alloys

The variation of coercive force as a function of annealing temperature is reported for some compacts of Co5Sm and Co5Pr alloys with the compositions of 63∼64 wt% Co-Sm and 60∼64 wt% Co-Pr, respectively. For the magnets made of these alloys, the curves of intrinsic coercive force against annealing temperature have two peaks; one at around 1080 °C and the other at 1190 °C in the Co-Sm sintered magnets. The curves of coercive force in sintered Pr-Co alloys have also two peaks; one near 1040 °C and the other near 1125 °C. This behavior can be correlated with the peritectic temperatures in the Co-Sm and Co-Pr phase diagrams. It is interpreted in terms of a model which assumes the existence of the pinning sites for the domain walls and their concentration at boundaries between the precipitates and grains of Co5R during the sintering. These precipitates are thought to consist of other R-Co compounds with lower melting points than RCo5.

The field dependence of thermoremanent magnetization in igneous rocks

The thermoremanent magnetization (TRM) of ten igneous rocks with grain sizes varying from fine to coarse varies with applied field H as H p in fields 0.5Oe< H < 8–22Oe and as H p for 8–22Oe< H < 90–140Oe. Domain structure was investigated using alternating-field demagnetization (the Lowrie-Fuller test), the Koenigsberger ratio Q t , and saturation hysteresis parameters. Five of the rocks (three basalts and two diabases) appear to contain mostly single-domain (SD) or pseudo-single-domain (PSD) grains, two contain a mixture of coarse multidomain (MD) and ultra fine SD grains (the latter as impurities in silicates), and three contain largely MD grains above PSD size. However, there was no correlation of p and q values with either domain structure or degree of alteration upon heating. In spite of the wide range of grain sizes and domain structures, the high-field results were all consistent with a generalized version of Néel's two-domain model for TRM. The generalization presented here agrees with the predictions of independent approaches by Everitt and Schmidt. According to the theory, p = 1 and q = 1 − (1/n), where n is the index in the experimental power law variation of coercive force H c with saturation magnetization J s at high temperature, H c(T) ∝ J s n (T) . In practice, the former prediction is generally masked by the presence of PSD moments at low fields. The latter prediction was tested by determining n from high-temperature hysteresis measurements. Agreement between q determined from TRM data and 1 − (1/n) was poor for samples that altered upon heating. The agreement was somewhat better but not convincing for relatively unaltered samples. TRM and coercive force data for four PSD magnetite powders and two SD cobalt ferrite powders were also analysed. A definite grain-size trend was found with the magnetites: q increased with increasing grain size (0.04 μm to 0.22 μm) from 0.28 to 0.43 while 1 − (1/n) increased from 0.32 to 0.49. However, the fact that the TRM data for the SD cobalt ferrites also could be fit by the generalized theory casts doubt on the validity of the entire model, which is based on the motion of a domain wall in the presence of an internal demagnetizing field. Although the physical processes involved remain unclear, the data clearly indicate a previously unsuspected similarity in the TRM behaviour of SD, PSD and MD magnetite, spanning a size range 0.04 to > 100 μm.

ChemInform Abstract: EFFECT OF MILLING ON THE INTRINSIC COERCIVITY OF BARIUM FERRITE POWDERS

AbstractDer Einfluß des Mahlens auf die innere Koerzitivkraft von βaFelzOlg‐Pulver wird thermischen Fluktuationen, hervorgerufen durch superparamagnetische feine Teilchen, oder durch den Mahlvorgang erzeugten Gitterdefekten zugeschrieben.

Effect of Milling on the Intrinsic Coercivity of Barium Ferrite Powders

The effect of milling on the intrinsic coercive force of Ba ferrite powders has been ascribed to (1) thermal fluctuations caused by superparamagnetic fine particles and (2) lattice defects introduced by milling. To test these interpretations, the Moessbauer absorption spectra and angular variation of coercive force for ball‐milled powders were determined. With increasing milling time, the Moessbauer absorption spectrum becomes ambiguous, and the absorption lines are broadened; the quadrupole‐split center line corresponding to superparamagnetic BaFe12O19 cannot be detected. The angular variation of coercive force shows that milling considerably lowers either the nucleation field of the reverse domain or the effective anisotropy field or both. It is concluded that lattice defects, rather than superparamagnetic fine particles introduced by milling, greatly affect the intrinsic coercive force of Ba ferrite powders.

Magnetization reversal process in chemically precipitated and ordinary prepared BaFe12O19

BaFe12O19 particles were prepared by chemical precipitation and ordinary powder metallurgy methods. These particles were oriented in paraffin wax and the angular variation of coercive force was measured. By using the modified equation of Buessem et al., its variation was analyzed. It was found that reverse-domain nucleation greatly influences the intrinsic coercive force of BaFe12O19 particles; even stress-free chemically precipitated single-domain particles were influenced. Annealing the ball milled powder raised the coercive force drastically. This is due to the reduction of defect-nucleation centers. It should be noted that by the chemical precipitation method a coercive force of 6000 Oe was obtained, which is one of the highest reported up to this time for isotropic BaFe12O19.

Angular Variation of Coercive Force in Barium Ferrite

Magnetization reversals in high magnetocrystalline anisotropy materials like barium ferrite are believed to take place by coherent rotation or by reverse-domain nucleation. The former mechanism gives rise to a Stoner-Wohlfarth type of angular variation of coercive force and the latter mechanism gives rise to Kondorsky-type behavior. Angular variation of coercive force was measured in several oriented particle assemblies with the average particle size ranging from 0.4 to 200 μ. The results were interpreted by a simple model incorporating SW and Kondorsky behavior.