Values Of Coercive Force Research Articles

Rapid-annealing effect on the microstructure and magnetic properties of the Fe-rich nanocomposite magnets

The rapid-annealing effect on the microstructure and magnetic properties of the nanocomposite Fe93-x-yCoxNb2(Nd, Pr)yB5 (x=0–20, y=5–7 at. %) alloys produced by crystallization of an amorphous phase have been investigated. The melt-spun ribbons consist of an amorphous phase in the as-quenched state and the amorphous phase changes to a nanocomposite structure consisting mainly of bcc-(Fe, Co), (Nd, Pr)2(Fe, Co)14B and residual amorphous phases after annealing at temperatures of 973–1023 K. The nanocomposite alloys exhibit improved values of remanence (Jr), coercive force (HcJ), and maximum energy product [(BH)max] upon annealing at a higher heating rate (α) in the temperature range corresponding to the primary crystallization temperature of bcc-Fe phase. As α increases, the grain sizes of each phase decrease, especially for the Nd2Fe14B phase, and the ratio of total surface area of the Nd2Fe14B to bcc-Fe phases (Shard/Ssoft), which are evaluated assuming each grain is a sphere, becomes close to 1 for the Fe88Nb2Nd5B5 alloy. This result indicates that the homogeneity of soft and hard magnetic phases is improved by rapid annealing, and it causes the improvement of hard magnetic properties of the Fe-rich nanocomposite alloys.

Co-Fe合金の減衰能と磁気弾性的性質に及ぼす熱処理効果

The damping capacity and magnetoelastic properties were investigated for 64 mass%Co-Fe and 68 mass%Co-Fe alloys. The alloys were heat-treated for 7.2 ks at 1123 K, and then cooled with and without magnetic field of 16 kA/m. Measurements of the logarithmic decrement (δ) and ΔE effect (ΔE⁄E0) were carried out using a cantilever method, and the magnetic hysteresis loop was measured using a direct current BH tracer. The δ and ΔE⁄E0 of alloys heat-treated without a magnetic field showed low values; the magnetic hysteresis loop exihibited a “bulge” in the low magnetic field range and the coercive force (Hc) value was relatively large. The δ and ΔE⁄E0 values of alloys heat-treated in a magnetic field were considerably higher; the magnetic hysteresis loop had a “normal” shape and the Hc value was small. This indicates a close relationship between the damping capacity and magnetoelastic properties of the alloys.

Ferromagnetic resonance studies of video tapes

The magnetic anisotropy fields (Hk=2K/M)and damping factors (α) of three kinds tapes of metal particle, oxide particle and metal-evaporated tapes for various home-use video systems were studied using 34-GHz ferromagnetic resonance (FMR). The Hk is in the 3-8 kOe range and varies linearly with the value of coercive force (Hc). The α value of metal-evaporated tape is the largest (0.24), indicating the small crystallite size. The smallest α is oxide particle tape (0.10), indicating well-aligned particles. Although the switching parameter Sw/Hc tends to increase with 1/α in general way, the relation of Sw/Hc vs. 1/α is different in particulate tape and thin-film tape in precisely.

Highly homogeneous nanoparticulate Fe films prepared by laser ablation

We report on the preparation, by means of the pulsed laser ablation technique, of particulate Fe films. Our results for the morphologic and magnetic characterization of these films showed that they were formed by quasi-spherical particles having average sizes in the range from 13 up to 25 nm and that they had room temperature coercive force values in the range from 240 Oe down to typical bulk-like Fe values. Measurements of the field evolution of the isothermal and demagnetization remanences evidenced that the hysteretic behavior of the films was strongly influenced by the occurrence of interparticle interactions.

Magnetization reversal features of Fe15Co20Ni65 sandwiches with various anisotropy of layers

Magnetization reversal process in Fe 15 Co 20 Ni 65 /Cr or Cu/Fe 15 Co 20 Ni 65 sandwiches having magnetic layers with mutually perpendicular easy magnetization axes or different value of coercive force has been studied. Dependences of hysteresis properties on interlayer thickness (L int ) for the sandwiches were measured and coupling constants for different L int were calculated also. On the basic of these data it is found that in the sandwiches for L int <10 A the exchange interlayer coupling dominates and for 10<L int <50 A the magnetostatic coupling is prevalent. The magnetostatic coupling may be realised through interface roughness (orange rind) and magnetization ripple.

Preparation and Magnetic Properties of Barium Hexaferrite Nanoparticles Produced by the Citrate Process

The sol‐gel synthesis technique has been used to produce barium ferrite particles. Several methods such as X‐ray diffractometry, transmission electron microscopy, and magnetism measurement (VSM) have been used to obtain detailed information on the crystallography and magnetic properties of the precursor and the calcined particles. The correlation of BaFe12O19 formation with the precursor properties and thermal treatment has been studied. The optimum conditions for preparing BaFe12O19 nanoparticles at a low calcining temperature are reported, and homogeneous ultrafine BaFe12O19 with almost‐ideal single‐domain behavior and coercive force (5950 Oe) and specific magnetization (70 emu/g) values that are similar to the theoretically predicted values is obtained.

Mössbauer analysis of the phase distribution of Fe64.5Co18B16SiC0.5 soft magnetic samples in powder form

We report on data, obtained from the analysis of Mössbauer spectra, on the phase distribution present in FeCoBSiC samples which were prepared by milling a melt spun amorphous precursor in a high energy ball mill. The milling process reduced the material to a powdered form, induced the precipitation of FeCo and Fe3C crystallites and resulted in relatively high coercive force values. In contrast with this, short time thermal treatments, carried out in the as-milled samples at temperatures in the range from 575 up to 650 K, produced a very remarkable softening which, according to our phase distribution results, should be associated to the partial dissolution of a certain percentage of the atoms involved in the grain boundary structures. This resulted in improved coupling between the crystalline and amorphous phases forming the sample and consequently, in an easier displacement of the domain walls.

Preparation of nanocrystalline FeSi alloys and their magnetic properties

Nanocrystalline FeSi alloys have been prepared using the mechanical alloying method. The structure of the obtained FeSi alloys is the same as that of bcc α-Fe. The final powders obtained mainly consist of particles with a grain size of from a few hundred nanometers to several microns, and the particles are composed of subgrains with a microcrystalline size of about 15–20 nm and an almost completely random orientation with respect to each other. The magnetic properties have also been investigated. The specific saturation magnetization ( σ s) values are slightly less than those of the corresponding single-crystalline and polycrystalline FeSi alloys, while the coercive force ( H c) values are much higher than those of the corresponding bulk FeSi alloys. The magnetic spectra are all relaxation-type except for Fe 75Si 25; the values of μ′ and μ″ are comparable to those of ferrite composite materials and domain wall displacements are predominant in the magnetization process.

Magnetic viscosity and microstructure: Particle size dependence of the activation volume

Ba hexaferrite samples having average particle sizes in the range from the tens up to the thousands of nm have been prepared by using conventional ceramic techniques and the exothermic reaction between Ba and Fe nitrates and oxalic dihydrized acid. The magnetic characterization of these samples included the measurement of the temperature dependence of the high field magnetization, the coercive force, and the evaluation, from thermally activated demagnetization measurements, of the activation volume at different temperatures. From the results of this characterization it was possible to reach conclusions about the occurrence of remarkable differences between the average particle size dependence of the coercive force and that corresponding to the activation volume: whereas the coercive force value increased monotonically with the decrease of the particle size, an activation volume minimum was observed for particle diameters of the order of hundreds of nm. We suggest that this minimum reflects a change of the mechanism ruling the magnetization reversal of the particles forming our samples.

Magnetic properties of iron nanoparticles grown in a glass matrix

Iron particles of diameters in the range 5.5–8.5 nm have been synthesized within a suitably chosen oxide glass by subjecting the latter to a 3Na+⇄Fe3+ exchange reaction followed by reduction treatment in hydrogen. Magnetic measurements have been carried out over the temperature range 20–300 K. The coercive force increases as the iron particle size is decreased. The maximum value of coercive force corresponding to 0 K (Hc0)∼548 Oe is obtained for a specimen having diameter of 6.5 nm. The magnetocrystalline anisotropy constant in the case of nanosized iron particles is estimated to be one order of magnitude higher than that of bulk iron.

焼結ＦｅＣｒ系磁性合金における磁気測定時間による保磁力値の変化について

焼結ＦｅＣｒ系磁性合金における磁気測定時間による保磁力値の変化について

Magnetic properties of mechanically alloyed amorphous Fe 50B 50

Samples with nominal composition Fe 50B 50 were prepared by high energy ball milling. For milling times in the range from 40 to 140 h the milling product consisted of FeB and Fe 2B (in addition to α-Fe) whereas a majority amorphous phase was formed in the samples milled for 200 h which displayed remarkably low coercive force values.

鉄系軟質磁性材料における磁化の遅れ現象に及ぼす抵抗率の影響

There is a phenomenon at DC magnetic measurement of soft magnetic material that the larger relative permeability and quicker measuring speed for hysteresis loop cause an increase in the apparent coercive force. It was recognized that the increase in the coercive force was caused by the delay of magnetization, and depended on the permeability in the magnetic soft iron. Then the study was performed to investigate the coercive force as a function of the resistivity in iron-base soft magnetic material. The delayed magnetization depended on the resistivity as well as the permeability of the material. Furthermore it changed with the thickness of specimen. Such a phenomenon can be explained from a view point of the penetration depth of magnetic field. Finally a method to evaluate the intrinsic value of coercive force was proposed.

Mössbauer effect evidence for disordering induced by mechanical alloying in the Fe-Si system

Based on X-ray diffraction (XRD) patterns, differential scanning calorimetry (DSC) investigations, the phase transitions induced by mechanical alloying (MA) on the Fe rich side on the Fe-Si system has been studied. Starting from a mixture of Fe and Si powders, MA leads to an expansion (up to 27.5 at.% Si) of the A2 crystalline disordered solid solution phase domain. In this composition field, an amorphous phase is also detected. The expansion of the disordered A2 phase domain up to 27.5 at.% Si has been confirmed by Mossbauer spectroscopy investigations. An annealing of the mechanically alloyed powders at 800 o C for 48 hours leads exactly to the thermodynamically stable structures consistent with the sample stoechiometry. A high coercive force value of 17.2 kA/m at 1 kHz frequency and 0.15 T magnatic induction was reported for the 9.5 at. % Si

Palaeomagnetism of the Ferrar dolerite in the northern Prince Albert Mountains (Victoria Land, Antarctica)

SUMMARY During the sixth Italian expedition to Victoria Land (Antarctica), Jurassic Ferrar dolerite sills were sampled at 23 sites in the northern Prince Albert Mountains. The rock magnetic properties were similar in all sites. Saturation remanence, remanent coercive force and Curie-temperature values pointed to titanomagnetite as the main carrier of magnetization. Thermal and AF demagnetizations were both effective in removing weak, secondary magnetization components below 250 C and 20 mT peak field. Further demagnetization resulted in single-component, stable ChRM. Palaeomagnetic directions were derived after AF demagnetization at 20 mT, and site-mean VGPs computed for all sites but one (Thern A), where systematic changes of the ChRM inclination have resulted in single-sample VGPs dispersed astride the equator. The site-mean VGPs distribution is clearly bimodal: 15 VGPs cluster around a palaeopole (lat. 48S, long. 226E) in good agreement with the literature data, whereas seven VGPs are displaced toward a palaeopole at higher latitude (lat. 79S, long. 181E). All VGPs can be regarded as Jurassic, because no evidence of substantial secondary components was found, and experimental data showed that the PTVRM (partial thermal viscous remanent magnetization) acquisition rate is too low to account for complete remagnetization of the rocks giving the anomalous VGPs. The Thern A and the southward displaced VGPs plot along a great circle through the East Antarctica Jurassic palaeopole. As a high reversal rate in the Jurassic is well documented by magnetostratigraphy, and the reversal VGP path is often confined to a narrow longitudinal band, we suggest that the great circle is a palaeomeridian and the sills characterized by anomalous VGPs were intruded during transitional phases of the geomagnetic field. Similar anomalous VGPs previously reported from the Ferrar Supergroup rocks of Northern Victoria Land also plot along the same palaeomeridian, which is only 30 away from the palaeomeridian derived from the Jurassic intrusives of the Dufek Massif in the Transantarctic Mountains (Burmester & Sheriff 1980). The Jurassic palaeopole of East Antarctica has been recalculated from 103 VGPs available in the literature. The position is 57S, 221E when all VGPs are taken into account, and 51S, 223E when the high-latitude VGPs from Victoria Land are discarded as anomalous.

Question of the mechanism of formation of the damping condition of high-chromium ferritic steels

Damping in high-chromium ferritic steels is formed primarily as the result of magnetoelastic dispersion of energy, the level of which is determined by the character of interaction of the magnetic domain boundaries moving in the field of applied cyclic stresses with some subsystems of stoppers, defects of the crystalline structure. The maximum in damping capacity of Kh16, Kh16M4, and Kh25 steels is observed after annealing at 900–1000°C and is the result of superposition of individual oppositely acting on the magnetic constituent of damping contributions during evolution of structural defects under different temperature-time actions. An analysis of the contribution of different crystalline lattice defects makes it possible to predict the path of formation of the controlled level of vibration absorbing properties of high-chromium steels. The high-damping condition is characterized by a low level of internal stresses, moderate values of coercive force, and non-linearity of the elastic properties under the action of external stresses. The damping capacity after annealing in the temperature area of stratification of high-chromium ferrite and appearance of 475°-brittleness is determined by the action of two primary tendencies, the reduction of internal stresses as the result of ecomposition of the supersaturated solid solution of interstitial impurities and the subsequent increase in them as the result of stratification of high-chromium ferrite and formation of zones enriched with chromium.

Mechanism of mechanical alloying phase formation and related magnetic and mechanical properties in the FeSi system

Based on X-ray diffraction (XRD) patterns, scanning electron microscopy (SEM) investigations, energy dispersive X-ray (EDX) chemical analysis, thermal analysis (DSC and DTA) and thermomagnetogravimetry (TMG) investigations, the far from equilibrium crystalline to amorphous phase transition induced by mechanical alloying (MA) in the Fe-rich side of the FeSi system was studied. Starting from a mixture of Fe and Si powders. MA leads to an expansion (up to 16 wt.% Si) of the A2 crystalline disordered solid solution phase domain. In this composition field an amorphous phase is also detected. For an Si content larger than 16 wt.% Si, a mixture of metastable phases (an amorphous and high temperature phases) and stable phases (low temperature phases) was detected. The crystalline to amorphous phase transition was a polymorphic phase transition which was attributed to instability of the crystalline lattice. The expansion of the A2 phase domain up to 15 wt.% Si was confirmed by Vicker's microhardness measurements, as well as the starting of the ordering reaction A2-B2 above 16 wt.% Si and the existence of the D03 phase at 17 wt.% Si. The influence of the structural state and the grain size on the magnetic properties such as the coercive force and the saturation magnetization was also studied. A high coercive force value of 17.2 × 10 3 A m −1 at 1000 Hz frequency and 0.15 T magnetic induction was reported for the 5 wt.% Si composition sample. An increase in ductility and low hysteresis loop were observed for the 10 wt.% Si composition.

Magnetization reversal mechanism evaluated by rotational hysteresis loss analysis for the thin film media

Rotational hysteresis loss analysis has been carried out for CoCrTa, CoNiPt, and CoCrPt thin-film media to evaluate the influence of microstructure on coercive force through the measurement of magnetic anisotropy. In each medium, the magnitudes of H/sub k//sup grain/ defined as a magnetic field where rotational hysteresis loss W/sub r/ diminishes remain almost a constant value independently of the values of coercive force. With increasing coercive force, the magnitude of the coercive force gradually becomes smaller than that of the switching field of magnetization H/sub p/ evaluated by torque analysis. Through the analysis of H/sub k//sup grain/ and H/sub p/, it is suggested that the coercive force in each examined medium is strongly dependent on the degree of intergranular exchange coupling and/or magnetostatic interactions. While on CoNiPt and CoCrPt media coercive force depends on the rotational hysteresis integral, no correlation between them was observed in CoCrTa films. >

Structure and Soft Magnetic Properties of Fe/CoZr Multilayer Films

Fe/CoZr multilayer films prepared by the RF magnetron sputtering method are reported. The films have a bcc structure with the (110) preferred orientation and consist of fine crystallites of about 10 nm in diameter. The growth of crystallites is very gradual below 350°C. Even after annealing at 450°C, the grain growth is unremarkable, and compositionally modulated structure still remains. The films show excellent soft magnetic properties after annealing above 400°C. Typical values of saturation magnetization, coercive force and saturation magnetostriction were about 21 kG, 1.8 Oe, and +2×10-5, respectively. The films also show a high permeability of about 1800 at frequencies up to 100 MHz.

Transport, magnetic and spectroscopic studies of nickel-gallium ferrites

X-ray, electrical conductivity, magnetic hysteresis and infrared spectroscopic studies for the system NiGa2−2xFe2xO4 were carried out. All the compounds, 0 x⩽1 showed cubic symmetry. The activation energy, thermoelectric coefficient values decreased with increasing Fe3+ concentrations but with increasing number of Fe3+ ions, the values of lattice constant and magnetic moment increased. Compounds with x⩽0.5 are p-type while those with x⩾0.75 are n-type semiconductors. Magnetic hysteresis loops indicated that the compounds with x⩾0.25 are ferrimagnetic while the compound NiGa2O4 (x=0.0) is anti-ferromagnetic. Constant and low values of coercive force indicated that the compounds with x⩾0.50 exhibit multidomain behaviour. X-ray intensity calculations, electrical conductivity, magnetic hysteresis and infrared studies indicate the presence of Ga3+ ions at the tetrahedral site, Ni2+ ions at the octahedral site and Fe3+ ions are present in both tetrahedral and octahedral sites. The probable ionic configuration for the system NiGa2−2xFe2xO4 is suggested to be Ga 1−x 3+ Fe 3+ [Ni2+ Fe3+ O 4 2− .