High Coercive Force Research Articles

Hydrogen as a working atmosphere for manufacturing permanent magnets based on rare-earth metals

The phase compositions and magnetic properties of permanents magnets of the systems Sm – Co and Nd – Fe – B are analyzed. Features of the hydrogenation–disproportionation–desorption– recombination (HDDR) process in the Nd 2 Fe 14 B intermetallic are considered. Using the Dd – Fe – B system as an example, we assess stages of manufacture of commercial permanent magnets and show the prospect of using hydrogen as a working atmosphere for the manufacture of magnetic powder. It is established that the HDDR of Dd – Fe – B alloys leads to their homogenization, grain refinement to a grain size of 0.2 to 0.5 μm, and an increase in the volume content of the main ferromagnetic phase Dd 2 Fe 14 B. By optimizing such a treatment, we managed to increase the magnetic energy (by 10%) and the lift force (by 25 – 27%) of Dd – Fe – B commercial permanent magnets. In modern materials science, a new trend in chemicothermal treatment of metals that consists of using hydrogen as a working atmosphere is being intensively developed. Developed hydrogen technologies are based on the regularities of its effect on the phase transformations, specifically, atomic ordering and hydride formation, in metals [1 – 3]. Earlier, magnetic materials were treated in a hydrogen atmosphere to clear them from impurities by annealing at high temperatures (1100 – 1300°C) [4]. However, in the manufacture of permanent magnets based on rare-earth metals (REMs), the possibility of using hydrogen both during their milling (hydride embrittlement) and for the change of the phase–structural state arose. Optimizing conditions for such a treatment, one can increase the operating characteristics of magnets (the coercive force Hc and residual induction Br ) and decrease the consumption of energy in their manufacture. Rare-earth magnets based on samarium were first obtained in the early 1970s. Alloys of the samarium–cobalt (Sm – Co) system have high saturation magnetization, coercive force, heat resistance, and corrosion resistance. However, due to the high cost, their use in industry is limited to a range of elevated temperatures (150 – 500°C). At present, magnets based on the Nd – Fe – B system are most extensively used. Of all known ferromagnets they exhibit the highest value of the energy product ( BH ) max (to 50 MG ⋅ Oe), which is five times higher than those of the best Alnico type magnets (Al – Ni – Co) [4]. Moreover, the determining advantage of these magnets over other materials is the relatively low cost calculated per unit magnetic energy (Table 1).

Levitation performance of YBCO bulk in different applied magnetic fields

The maglev performance of bulk high-Tc superconductor (HTS) is investigated above three different types of permanent magnet guideways (PMGs). The main difference among these PMGs is the method used to concentrate the magnetic flux. The experimental results indicate that the levitation force depends only in part on the peak value of the magnetic field. The variation of the vertical component of the magnetic field (Bz), and the structure of the magnetic field are also responsible for the levitation force. These results imply that the permanent magnet with high coercive force is better at concentrating flux than iron. The conclusions contribute in a very helpful way to the design and optimization of PMGs for HTS maglev systems.

Acquisition of shock remanent magnetization for demagnetized samples in a weak magnetic field (7 μT) by shock pressures 5–20 GPa without plasma‐induced magnetization

Abstract— Demagnetized samples of cobalt precipitates in a copper matrix were shocked to 5, 10, and 20 GPa in a weak magnetic field of 7.7 μT to elucidate the origins of the natural remanent magnetization of meteorites and the magnetic anomalies of impact craters on the moon and Mars. The samples placed in the target acquired shock remanent magnetization (SRM) whose intensity increased up to 21.3 times compared with the demagnetized state, but SRM intensity and shock intensity were not correlated. The SRM direction was in most cases approximately perpendicular to the shock direction. The samples placed 4.8 mm from the impacted surface did not acquire significant magnetization, suggesting no plasma‐induced remanent magnetization (PIRM) up to 20 GPa. When the samples were divided into 8 sub‐samples, the SRM intensities of sub‐samples increased up to 40 times compared with bulk ones and their directions were scattered. Higher coercive force grains were magnetized perpendicular to the shock direction for shocks of 5 and 10 GPa, but at 20 GPa the directions were less systematically oriented. These results suggest that the proposed plasma‐induced magnetization of impactites should be reconsidered.

Studies on the magnetism of cobalt ferrite nanocrystals synthesized by hydrothermal method

Fe–Co/CoFe 2O 4 nanocomposite and CoFe 2O 4 nanopowders were prepared by the hydrothermal method. The structure of magnetic powders were characterized by X-ray diffraction diffractometer (XRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), thermal gravity analysis (TGA) and differential thermal analysis (DTA) analysis, X-ray photoelectron spectrometry (XPS), and Fourier transform infrared spectra (FTIR) techniques, while magnetic properties were determined by using a vibrating sample magnetometer (VSM) at room temperature. The effects of hydrothermal reaction conditions on magnetic properties were also discussed in details. The values of saturation magnetization ( M s) and coercive fore ( H c) for Fe–Co/CoFe 2O 4 nanocomposite are 113 emu/g and 1.4 kOe, respectively. Furthermore, CoFe 2O 4 ferrite with a single-domain critical size of 70 nm was fabricated by controlling the hydrothermal reaction conditions carefully, which presents high coercive force (ca. 4.6 kOe) and high squareness ratio (ca. 0.65). One interesting thing is M s value of CoFe 2O 4 ferrite with a diameter of 40 nm is 86 emu/g which is comparable to that of the bulk counterpart.

Synthesis of Carbon Encapsulated Magnetic Nanoparticles with Giant Coercivity by a Spray Pyrolysis Approach

Carbon encapsulated magnetic (metal) nanoparticles (CEMNPs) have wide applications in biomedicine and the magnetic recording industry. However, synthesis of such particles with a high coercive force and good ferromagnetism is still a great challenge. The present study reports a new method for the continuous production of CEMNPs of high purity. This involves the spray pyrolysis of a mixture of iron pentacarbonyl and ethanol at 500-900 degrees C. Results show that the Fe (or Fe3C) particles synthesized at 700 and 900 degrees C were well encapsulated by graphitic layers with rare byproducts such as carbon nanotubes, nanofibers, or bulk amorphous carbon. Those synthesized at 700 degrees C had a particle size of 30-50 nm, a giant coercive force of 867 Oe, and a good magnetic remanence of 33% at room temperature. The present approach based on spray pyrolysis is advantageous over previous ones in suitability for large-scale production, and the synthesized material has wide applications in many fields.

Microstructure and magnetic properties of R-Fe-B-Cu (R = Pr, Nd) alloys deformed by equal-channel angular pressing and subsequent hot upsetting

Effects of equal-channel angular pressing with controlled back pressure (ECAP-BP) and subsequent upsetting on the formation of the texture, magnetic properties, and changes in the microstructure of R-Fe-B-Cu (17–20 at. % R, R = Pr, Nd) alloys have been investigated. It has been shown that, under the action of ECAP-BP at 470°C, a high coercive force H c arises and a diametral texture is formed in the Pr-Fe-B-Cu alloys. Application of hot compression to alloys processed by ECAP-BP increases the degree of texture if the compression is applied in the direction of the initial texture axis or reorientes the texture axis in the direction of compression if the compression is applied in the direction perpendicular to the initial texture. It has been established that to obtain a high level of magnetic hysteresis properties in the Nd-Fe-B-Cu alloys, the combined hot deformation of these alloys should be carried out at temperatures above 500°C.

TEM observation of sintered permanent magnetic strontium ferrite

Sintered permanent magnetic strontium ferrites were studied using transmission electron microscopy to investigate the microstructure morphology and its correlation with the magnetic properties. The present study shows that the microstructure of sintered permanent magnetic strontium ferrites is an important parameter in determining their magnetic properties. The microstructure morphology in low-performance ferrite magnet is obviously different from high-performance one. The magnetic properties of sintered permanent strontium ferrite depend strongly on the orientation degree of strong magnetic crystals. The presence of ferric oxide phase in ferrite magnet can deteriorate the magnetic properties. Moreover, proper quantities of crystal defects are beneficial to high coercive force due to the fixing of magnetic domain.

The spin and orbital moment contributions of each element to macroscopic magnetization in Co0.9Fe0.1 films

Co-Fe alloy films have such properties as high saturation magnetization, high Curie temperature and low coercive force. It is of importance to study the origin of high saturation magnetization in these alloys. In this paper, we study the spin (ms) and orbital moments (mo) of Fe and Co in the Co0.9Fe0.1 film by using X-ray magnetic circular dichroism (XMCD) and SQUID magnetometry. The ms and mo for Co are 1.58 and 0.31 μB, and for Fe are 1.63 and 0.36 μB respectively. The average magnetic moment (1.90 μB) determined by XMCD is in agreement with that obtained from SQUID measurements. The total magnetic moment ratio of Fe to Co is 10.5:89.5, while the ratio of the spin to orbital moment is 83.4:16.6. Considering the separation of the spin and orbital moment, an outcome of mFe-spin: mFe-orbit: mCo-spin: mCo-orbit = 8.6:1.9:74.8:14.7 is obtained.

Interparticle interference and elastic interphase deformations upon X-ray scattering by precipitates statistically distributed in a crystal: III. Analysis of results of applied investigations

Based on theoretical data [1, 2], results of X-ray diffraction investigations of the initial stages of decomposition of Fe-25Ni-25Al and NaCl-AgCl solid solutions have been analyzed critically. It has been established that the interpretation of diffuse effects in diffraction patterns of the Fe-Ni-Al alloy in terms of the three-phase model is wrong; numerous errors and inaccuracies have been revealed in the representation and explanation of diffuse scattering in works devoted to the NaCl-AgCl alloys. A new interpretation of effects of diffuse scattering is given and a quantitative estimation is given to the structural parameters of alloys and elastic interphase stresses. It is shown that diffraction triplets consisting of relatively wide maxima caused by scattering by the matrix and precipitates and of a peaked maximum caused by scattering by both phases can be formed in the fundamental reflections of alloys with precipitates isomorphic to the matrix. Kinetic and structural factors that determine the formation of high magnetic properties, including the high coercive force and magnetic saturation, in the Fe-Ni-Al alloy are discussed. The necessity of a further reconsideration of the results of investigations that rest upon the three-phase model of solid-solution decomposition when interpreting diffuse scattering is noted.

Progress of Sm-Fe-N Anisotropic Magnets

A reduction and diffusion method (R/D) is used to make a mother alloy of Sm-Fe-N anisotropic magnets. Reduction of 0.5wt% of samarium content compared to the conventional powder increases magnetization. Milling condition and surface treatment improve the squareness of demagnetization curve, the aging property and the heat resistance. The maximum energy product of 292 kJ/m 3 is obtained with the powder. High coercive force is maintainable even if the powder is exposed for 300h in 80 °C 90%RH. The maximum energy product of 141 kJ/m 3 is obtained with an injection molded anisotropic magnet. The aging property estimated by irreversible flux loss is comparable to the conventional MQP-B magnets. The heat resistance temperature ( T −5%) at which die initial irreversible flux loss becomes −5% is 125-more than 150 °C for Sm-Fe-N magnets and 150–170 °C for hybrid magnets. The magnetic properties of bonded HDDR Nd-Fe-B magnet were improved by substituting for Nd-Fe-B powder with Sm-Fe-N powder. A new technology to make anisotropic bonded Sm-Fe-N thin cylinder magnets by an injection molding using unsaturated polyester (UP) resin was developed.

Transient Analysis for Magnetic Circuit of a Novel Synchronous Motor Basing on Controllable Flux

The paper refers to a novel permanent magnet motor with composite-rotor, to solve the difficulties of extend speed by “flux weakening”. The rotor is composed of two rotor segments: one is made of AINiCo magnet and the other of NdFeB. Relying on high coercive force and high remnant magnetic flux density quality of NdFeB, permanent magnet motor could get enough air gap magnetic fields. On the other way, by making using of low coercive force characteristics of AINiCo magnet, die permanent motor could be changed the orientation and strength of magnetization when applying a pulse of magnetizing (demagnetizing) current, thus magnetic flux in air gap could be varied over wide range. To achieve the aim, the paper analyzes magnetization process of permanent magnet and transient magnetic circuit in the motor, and then computes the distribution of air gap magnetic field in different magnetization by using finite element method. The numeral results show the validity of the scheme.

Approach to enhance the coercivity in perpendicular FePt∕Ag nanoparticle film

[ FePt 2 nm ∕ Ag d nm ] 10 multilayers were prepared on 100 °C MgO (100) single-crystal substrates with magnetron sputtering. The ordering and perpendicular texture of L10-FePt∕Ag particle films with high coercive force were realized at a lower postannealing temperature and by controlling accurately the atomic ratio of Fe to Pt of L10-FePt films. The results of the x-ray-diffraction pattern show that the FePt particles have excellent (001) texture when the multilayers were sputtered on 100 °C substrates with the thickness of the Ag layer d=3–9nm, and the mean size of L10-FePt particles reduces with increasing thickness of the Ag layer. After the [FePt2nm∕Ag5nm]10 was vacuum annealed at 600 °C for 15 min, the mean size of L10-FePt particles is roughly about 8 nm and its perpendicular coercive force reaches 979kA∕m(12.3kOe). This particle film has little magnetic exchange coupling and is a candidate for future perpendicular magnetic recording media with ultrahigh density.

Permanent Magnets Design and Magnetic Field Analysis of Multi-disc Coreless Permanent Magnet Synchronous Motor

This paper presents a new multi-disc coreless permanent magnet synchronous motor whose permanent magnets are arranged in the “Halbach array”. Unit motor combined structure is adopted to simplify processing technology of the motor. Airgap magnetic flux density is increased taking advantage of high remanence and coercive force of NdFeB permanent magnet material and high gathering magnetism function of Halbach array. This also makes coreless motor possible. The structure and the characteristics of the motor using Halbach array are also introduced. The FEM analysis has been done to build the mathematical model and study the magnetic field characteristics of the motor. And finally the interrelated design parameters of permanent magnet and arrangement between unit motors are determined according to the results of die FEM analysis.

Deposition Feature of Ni Nanoparticles on Halloysite Template and Magnetic Properties of the Composite

A novel cermet composite with Ni nanoparticles deposited on a hollow cylindrical halloysite template is fabricated by electroless plating. Ni nanoparticles have a uniform distribution on the template, and their average diameter is mainly in the range of 20-30 nm. The halloysite template will be beneficial to make the Ni nanoparticles achieve high stability and well-dispersed state. Different heat treatment temperatures have a great effect on the crystal structure of Ni nanoparticles and the magnetic properties of the composite. With the heat-treated temperature increase, Ni nanoparticles gradually become crystallized, and the composite values of inherent coercive force (iHc), saturated magnetization (Ms), and residual magnetization (Mr) increase respectively. After complete crystallization of Ni nanoparticles at 673 K, the composite has the maximum values of iHc (253.6 Oe), Ms (57.37 emu/g), and Mr (21.64 emu/g). The sum contribution of the magneto-crystalline anisotropy, single magnetic domain effect, and the pinning effect of multiple-twinned planar defects in Ni nanoparticles would result in the high value of iHc. The new nanosized cermet composite will be at such an advantage for its practicable fabrication method, higher coercive force, high stability, and low cost that it would have great potential to be utilized to design and prepare magnetic materials.

Nanocrystalline Fe-Pt-B base hard magnets with high coercive force obtained from amorphous precursor

Nanocomposite alloys of fct γ1-Fe(Co)Pt, fcc γ-Fe(Co)Pt, and Fe(Co)2B phases with good permanent magnet properties have been formed by melt spinning followed by annealing. An amorphous phase was formed for Fe56.25−xCoxPt18.75B25 (x=0–20) alloys by melt spinning. The thermal stability of the amorphous alloys increases gradually with increasing Co content. The structure crystallized by annealing for 900s at 785K consisted of fct γ1-Fe(Co)Pt, fcc γ-Fe(Co)Pt, and Fe(Co)2B phases, and their average grain sizes were about 20nm. The nanocomposite alloys exhibit good hard magnetic properties. As the Co content increases, the coercivity Hci increases, the remanence Br and maximum energy product (BH)max decrease. The Br, reduced remanence Mr∕Ms, Hci, and (BH)max were 0.96T, 0.83, 340kA∕m, and 102.2kJ∕m3, respectively, for the 0at.% Co alloy. Higher coercivity (426kA∕m) was obtained for the 20at.% Co alloy albeit with lower Br(0.68T), Mr∕Ms(0.80), and energy product (66.7kJ∕m3). The good hard magnetic properties for the low Pt-containing alloys are interpreted as resulting from exchange magnetic coupling between nanoscale hard fct γ1-Fe(Co)Pt and soft fcc γ-Fe(Co)Pt or Fe(Co)2B magnetic phase.

High-field magnetisation of SmCo5−xCux (x≈2.5) determined in pulse fields up to 48 T

The strong influence of Cu substitution on the magnetic properties of SmCo5, especially on the coercive field, is well known. Induction-melted samples (x=2,2.5,3) show a phase separation within the 1:5 structure. An unusual jump from a low to a high magnetisation state in an increasing external field is observed. Using pulse field, high coercive forces up to about μ0 JHc≈30T and a high critical field μ0HU for the unusual jump reaching 45T at 4.2K for x=3 are observed for the first time. Hc and HU increase with decreasing temperature. An influence of the field changing rate on the measured HU is found.

Physical Properties of Polycrystalline Mn2O3-Substituted LiNiBi Ferrite

Lithium ferrites are a low-cost material which have been prominent in the high frequency core industry because of their excellent temperature performance and high squareness ratio. In order to develope the lithium ferrites with the high squareness and low coercive force, the ferrites of <TEX>$Li_{0.48}Bi_{0.02}Ni_{0.04}Fe_{2.46-x}O_4$</TEX> were investigated the by varying composition, temperature and frequency. Electric loss of the Li-ferrite was lowered with the substitution of <TEX>$Mn_{2}O_3$</TEX>. The addition of <TEX>$Mn_{2}O_3$</TEX> increased the magnetic induction (Bm&Br) but decreased the coercive force (Hc) and the squareness ratio (R=Br/Bm). Also, the Br value was stable at environmental temperature variation.

Determination of the Magnetic Compensation Composition in Al-Substituted Bi-DyCoIG Nanoparticles With Enhanced Coercive-Force

In garnet particles, higher coercive-force is required to develop a practical magnetooptical storage media. Therefore, in this paper, we develop high coercive-force particles reflecting the magnetic compensation induced by the aluminum substitution, and increasing the magnetic anisotropy under the influence of cobalt substitution for applying the garnet particles to magnetooptical storage media. Nanoparticles of aluminum substituted bismuth dysprosium cobalt iron garnet (Bi-DyCoAlIG) exhibit a magnetic compensation at room temperature. The Bi-DyCoAlIG (BiDy/sub 2/Co/sub 0.2/Al/sub x/Fe/sub 4.8-x/O/sub 12/: 0/spl les/x/spl les/0.9) nanoparticles were made using coprecipitation and heat-treatment processes. The X-ray diffraction patterns of nanoparticles are clear that all the peaks were assigned to the single phase of the garnet structure. The particles size of the Bi-DyCoAlIG nanoparticles was observed to be less than 30 nm by transmitted electron microscope images which is comparables to the estimated average particle size of Scherrer's equation. The magnetic profiles of the Bi-DyCoAlIG nanoparticles show shallow minima at the composition range of 0.6<x<0.8 reflecting the magnetic compensation induced by the aluminum substitution. Moreover, the coercive-force of the nanoparticles was also peaks at x=0.7, which can be consistently explained by the magnetic compensation. The BiDy/sub 2/Co/sub 0.2/Al/sub 0.7/Fe/sub 4.1/O/sub 12/ nanoparticles demonstrate the enhanced coercivity of 1000 Oe which is factor of 3 larger than the x=0.

Patterned Electroplating of Micrometer Scale Magnetic Structures on Glass Substrates

This paper has developed a new method of micro patterned electroplating that enables the fabrication of micrometer scale magnetic structures on glass substrates. In contrast to other methods, the process as developed here leaves the surrounding substrate area untouched: that is there is no seed layer present except underneath the magnetic structures. Patterned cobalt layers of thicknesses up to 8 /spl mu/m were fabricated exhibiting high saturation (1.5 T) and low coercive force. These layers have been used to create micrometer scale magnetic poles (magnetic tweezers) for biological and biophysical applications, where a clean untouched glass substrate in the sample area is most important. The technique, however, can be used in any situation where a residual seed layer on the surrounding area is unwanted, and afterwards removal of this seed layer is difficult or impossible.

Preparation of Co1?xNixFe2O4 nanoparticles by coprecipitation method

Co 1-x Ni x Fe 2 O 4 nanoparticles were prepared by a coprecipitation method. The effects of precipitants and Co/Ni ratio on particle shape and magnetic properties of Co 1-x Ni x Fe 2 O 4 nanoparticles were investigated. The physical properties such as particle size, phase, and magnetic properties of Co 1-x Ni x Fe 2 O 4 nanoparticles depended largely on the Co/Ni ratio and precipitant as well as heat-treatment. In case of using NaOH as a precipitant, the synthesized nanoparticles were magnetite phase including a small part of impurity which is needle shaped and non-magnetic material. On the other hand, the nanoparticles synthesized by NH 4 OH precipitant were amorphous phase. The coercive force of the nanoparticles was increased drastically with increasing heat-treatment temperature in the range of 400-600 °C. The high coercive force of magnetic nanoparticles with the size of 20-30 nm was obtained up to 1450-1800 Oe.