High Intrinsic Coercivity Research Articles

Permanent Magnets Beyond Nd-Dy-Fe-B

Permanent magnets are one of the key parts in energy conversion devices to realize the utmost efficiency. DC motors up to the 100-kW class are used in some ‘‘strong’’ hybrid electric vehicles (HEV), in which the top-grade Nd-Dy-Fe-B-type permanent magnets are required. Such motors have inner permanent magnet (IPM) rotors with which the combination of magnet torque and reluctance torque is optimized. For such an optimized design, permanent magnets are shaped in thin plate-like blocks with a thickness of about 5 mm, inserted into slots in a laminated rotor core, and magnetized perpendicularly to the largest surfaces of the block. The weight of the magnets may amount to about 2 kg or less per vehicle. The reason why Nd-Dy-FeB permanent magnets are used is simply because the highest performance of this class of material in terms of residual flux density, Br, can be generated with them. These magnets have high intrinsic coercivity, HcJ, typically about 2.0–2.4 MA/m. Such high HcJ values are certainly not necessary at room temperature, but the magnet temperature can rise to about 200C at which HcJ is only about 0.6–0.8 MA/m, being marginally enough to withstand the armature magnetic fields generated by driving current. Dysprosium (Dy) or terbium (Tb) is used in the magnet in order just to obtain such high HcJ but have been indispensable in the permanent magnets. Since Dy and Tb can be mined industrially only in a southern province of China in a limited quantity, Dy and Tb are now typical critical elements, overreliance on these elements should be avoided. For this very reason, permanent magnets beyond Nd-Dy-Fe-B are now extensively sought. Development of permanent magnets beyond the Nd-Dy-Fe-B is a challenge because Nd2Fe14B, the main hard magnetic phase of the magnet, is the compound that has the largest magnetization at room temperature among all existing hard magnetic compounds, and that, in terms of elemental abundance, already consists mainly of Fe and Nd, both of which are the most abundant elements carrying magnetic moments in the 3d and 4f transition metal series, respectively. One of the approaches is to reduce the overall average Dy content over the entire commercial grades down to a sustainable level, which may be about 1–1.5 mass percent in the magnets, even if the crustal abundance ratio with Nd is considered. Another approach is to seek for an entirely new material with or without rare earth elements. In advanced economic blocks that have benefitted from functional critical elements, governments have taken strategic actions to promote developments in technologies that will lead to reduced society’s dependence upon those elements. In Japan, well

Mesoporous urchin-like α-Fe2O3 superstructures with high coercivity and excellent gas sensing properties

This paper reports the synthesis, morphology, magnetic properties and gas sensing characteristics of mesoporous urchin-like α-Fe2O3 superstructures prepared by one-step thermal decomposition of iron nitrate aqueous solution. The advantages of this fabrication technique include simplicity, convenience, efficiency, capability of scale-up fabrication, and good control of product structure and morphology. The urchin-like α-Fe2O3 superstructures are made of oriented aggregations of nanorods (diameter ∼ 10–130 nm; length ∼ 150–700 nm) with an overall size in the range of 0.5–5 μm. The nanobuilding blocks of the urchin-like superstructures are either single-crystal nanorods or a mixture of single-crystal knotted or branched sub-nanorods and elongated particles. Unusually high intrinsic coercivity, iHc, of up to 5.3 kOe and reduced Morin transition temperatures were observed for the urchin-like α-Fe2O3 superstructures. The urchin-like α-Fe2O3 samples showed excellent gas sensing performance for acetone, ammonia, and ethanol.

Optimization of magnetic properties of sintered (SmGdDy)(Co, Fe, Cu, Zr)z magnets by Dy–Co addition

Liquid phase sintering is used to optimize the magnetic properties and temperature stability of (SmGdDy)(Co, Fe, Cu, Zr)z sintered magnet. DyCo0.51 and Sm0.67Dy0.11Gd0.22(Co0.693Fe0.201Cu0.081Zr0.0252)7.94 alloys are chosen as liquid phase and main phase, respectively. (SmGdDy)(Co, Fe, Cu, Zr)z permanent magnets are prepared with different content of liquid phase. XRD analysis shows that all magnets consist of Th2Zn17-type phase and CaCu5-type phase. Cellular structure studies indicate that the H1-5 cell boundary phase shifts gradually from thin and part of discontinuous to thick and continuous. The cell size decreases with the increase of the content of liquid phase. In addition, the Cu content in cell boundary phase is getting lower with the increase of LP adding ratio. The homogeneous and continuous cellular structure is got when the adding ratio of liquid phase is 3 wt% and the highest intrinsic coercivity is achieved at room temperature (23.88 kOe) and 200 °C (12.06 kOe). It’s interesting to find that temperature dependence of remanence (α) and the temperature dependence of intrinsic coercivity (β) can be optimized with the increase of the content of liquid phase, in which the α(20–100 °C) and β(20–200 °C) are −0.0075%/°C and −0.140%/°C for the magnet with 5 wt% liquid phase, and −0.016%/°C and −0.322%/°C for the magnet with 1 wt% liquid phase.

Effect of composition and heat treatment on MnBi magnetic materials

The metallic compound MnBi is a promising rare-earth-free permanent magnet material, unique among all candidates for its high intrinsic coercivity (Hci) and its large positive temperature coefficient. The Hci of MnBi in thin-film or powder form can exceed 12 and 26kOe at 300 and 523K, respectively. Such a steep rise in Hci with increasing temperature is unique to MnBi. Consequently, MnBi is a highly sought-after hard phase for exchange coupling nanocomposite magnets. However, the reaction between Mn and Bi is peritectic, and hence Mn tends to precipitate out of the MnBi liquid during the solidification process. As result, when the alloy is prepared using conventional induction or arc-melting casting methods, additional Mn is required to compensate the precipitation of Mn. In addition to composition, post-casting annealing plays an important role in obtaining a high content of MnBi low-temperature phase (LTP) because the annealing encourages the Mn precipitates and the unreacted Bi to react, forming the desired LTP phase. Here we report a systematic study of the effect of composition and heat treatments on the phase content and magnetic properties of Mn–Bi alloys. In this study, 14 compositions were prepared using conventional metallurgical methods, and the compositions, crystal structures, phase content and magnetic properties of the resulting alloys were analyzed. The results show that the composition with 55at.% Mn exhibits both the highest LTP content (93wt.%) and magnetization (74emug−1 with 9T applied field at 300K).

Structural and Magnetic Properties of Nanostructured Pr0.75 Y 0.25Co5 Powders Obtained by Mechanical Milling and Subsequent Annealing

Pr0.75 Y 0.25Co5-based as-cast alloys were processed by high-energy ball milling to obtain nanostructured powders with high coercivity. The powders obtained after 4 h of milling exhibited nearly amorphous behavior in X-ray diffraction patterns. DSC scans of the as-milled powders indicated a process of crystallization by broad, exothermic transition peak at 503 °C. Annealing of the milled powders at 850 °C for 2.5 min in high vacuum produced fine grains of size ranging 15–30 nm with optimal microstructure and hard magnetic properties. Magnetic measurements of the annealed powders evaluated a high intrinsic coercivity, i H c of 9.3 kOe, and a remanence ratio, M r/ M max of 0.72. The magnetic hardening was attributed to higher anisotropy field of the powders and microstructural uniformity achieved by the processing methodologies.

Microstructure and Magnetic Properties of Rapidly Solidified Sm(Co<sub>bal</sub>Fe<sub>0.11</sub>Cu<sub>0.10</sub>Zr<sub>x</sub>)<sub>7 </sub>Alloys

We reported a systematic study of the effect of Zr content on the structural and magnetic properties of rapidly quenched Sm (CobalFe0.11Cu0.10Zrx)7 alloys with x varying from 0 to 0.04. The results show that the addition of Zr greatly affected the microstructure and magnetic properties of the heat-treated ribbons. The Zr-free ribbon had crystallized as the 1:7H structure as the main phase while the ribbon with Zr addition adopted the 2:17R structure type. A Sm2Co7 phase was observed in the heat-treated ribbons when the Zr content x increased to 0.04. It is found that while the Zr content was increased from 0 to 0.04, saturation magnetization of the heat-treated ribbons was decreased from 11.9kGs to 3.3 kGs, and the intrinsic coercivity was increased from 0.07 kOe to 11.7 kOe. The highest intrinsic coercivity, Hci=11.7 kOe, was obtained in the ribbon with Zr content of 0.03.

Precursor-directed synthesis of quasi-spherical barium ferrite particles with good dispersion and magnetic properties

The precursor-directed method has been firstly used for the preparation of M-type barium ferrite (BaFe12O19) particles in this article, where Fe3O4 microspheres were selected as the precursor. During the preparation process, it can be found that the volume ratio of water to ethanol plays an important role in determining the crystalline phase, and a high-purity magnetoplumbite structure can be achieved under the optimized conditions. Thanks to the morphology and dispersion of Fe3O4 microspheres, the obtained product presents a quasi-spherical shape, good dispersion, and relatively uniform particle size even after high-temperature heat treatment. Especially, the as-prepared barium ferrite exhibits excellent magnetic properties with both high intrinsic coercivity (5342 Oe) and large saturation magnetization (68.3 emu g−1), which are better than most results from some unconventional techniques. A series of Co–Ti substituted barium ferrite particles [Ba(CoTi)xFe12−2xO19, 0.25 ≤ x ≤ 1.0] are also prepared through the same technique by using Co–Ti substituted Fe3O4 microspheres as precursors, and they also display high-purity crystalline phase, quasi-spherical shape, and good dispersion, indicating that this route can be versatile for barium ferrite with different chemical compositions. However, with an increasing heteroatom content, the intrinsic coercivity and saturation magnetization drastically decrease because of the selective substitution of Fe3+ sites by Co2+ and Ti4+ in the magnetoplumbite structure.

Comparison on the magnetic properties, phase evolution and microstructure of directly quenched Pr–Fe–Ti–B-based ribbons and rods

Magnetic properties, phase evolution, and microstructure of directly quenched PrxFe82.5−x−yTi2.5ZryB15 (x=7–11; y=0 and 0.5) alloys, including ribbons with about 0.2mm in thickness and rod magnets with 0.9mm in diameter, have been systematically studied. Due to the uniformly finer microstructure, resulted from the higher cooling rate, PrxFe82.5−xTi2.5B15 (x=9–11) ribbons showed better magnetic properties as compared to PrxFe82.5−xTi2.5B15 (x=9–11) rods. Nevertheless, the improved maximum magnetic energy product (BH)max with higher intrinsic coercivity (iHc) of 12.5–13.8kOe and enlarged applicable composition range (x=8–10.5) could be obtained in Zr-substituted PrxFe82−xZr0.5Ti2.5B15 rods as a result of grain refinement by Zr substitution for Fe. The optimal magnetic properties of Br=7.1kG, iHc=15.0kOe, and (BH)max=8.6MGOe, and Br=5.9kG, iHc=14.5kOe and (BH)max=7.6MGOe were obtained for Pr9.5Fe73Ti2.5B15 ribbons and Pr9.5Fe72.5Ti2.5Zr0.5B15 rod magnet, respectively.

The preparation of sintered NdFeB magnet with high-coercivity and high temperature-stability

The NdFeB magnets with high intrinsic coercivity have been produced by using the conventional powder metallurgy method (including SC, HD and JM) of sintered NdFeB magnets. The effects of grain boundary phases on the microstructure and magnetic properties of as-sintered and annealed magnets have been tried to investigate. Also the Curie temperature of the magnets was studied. By adopting suitable component ratio of some heavy rare-earth atoms and some micro-quantity additives, we have prepared high-coercivity sintered NdFeB magnets with magnetic properties of jHc=36.3kOe, Br=11.7kGs and (BH)max=34.0MGOe. The temperature coefficient of residual magnetic flux of the magnets (between 20 and 200°C) is -0.113%/°C, while the temperature coefficient of intrinsic coercivity -0.355%/°C. The Curie temperature of the magnets is about 342°C.

Magnetic materials from co-precipitated ferrite nanoparticles

Some of recent technological advances in electronics need very compact magnetic materials. The paper presents the morphology and the magnetic properties of very dense polycrystalline magnetic materials obtained from co-precipitated manganese ferrite nanoparticles. The ferrite nanoparticles, with average diameter in the range of 13–25 nm, were obtained through an original low-cost co-precipitation route from aqueous solution of Mn 2+ and Fe 3+ ions generated by redox reactions between stoichiometric amounts of MnO 2 (piroluzite) and FeSO 4·7H 2O raw materials. Very dense homogeneous polycrystalline magnetic materials with high square hysteresis loop ( B r/ B s = 0.91) and low intrinsic coercivity were obtained using the co-precipitated un-doped manganese ferrite nanoparticles.

Study on the Nanostructure and Magnetic Properties of NdFeNbB Permanent Magnets

Nd2Fe14B/-Fe nanocomposite permanent magnet contains the hard and soft magnetic phases, Nd2Fe14B and -Fe respectively. An exchange coupling effect exists between the two magnetic phases. The effect of alloying element Nb on its nanostructure and properties have been studied. Adding Nb to the alloy is effective to refine grains, a relatively small grain size causes a high intrinsic coercivity, remanence and therefore a high maximum energy product, (BH)max. MFM (Magnetic Force Microscope) was used to observe the magnetic micro-domain structure in the nanophase alloys. The length of the magnetic contrast shows a significant dependence on the microstructure and phase constitution, and the longer length is correspond with the larger exchange coupling effect between the soft and hard magnetic phases.

Demagnetization Testing for a Mixed-Grade Dovetail Permanent-Magnet Machine

A dovetail machine is a novel design developed to solve the strength problems of traditional buried magnet machines. A mixed-grade construction can be easily applied to a dovetail machine, because a dovetail machine has several magnets in a single pole in different positions. The basic idea of the mixed-grade construction is to use high intrinsic coercivity material in the positions of the high demagnetization risk and high remanence material in the positions of low demagnetization risk. We have developed a demagnetization model that takes into account the temperature dependence of the properties of the permanent-magnet materials to model a dovetail permanent-magnet motor with mixed-grade construction. We compared the model with a real motor. By comparing the testing and the calculations, we show that our demagnetization model can predict the demagnetization of the permanent magnets with reasonable accuracy. We discuss the benefits of the mixed-grade construction in a dovetail machine.

High-coercivity SmCo5 thin films deposited on glass substrates

Structural and magnetic properties of SmCo5 thin films grown on glass substrates with a Cr underlayer were investigated. The nanocrystalline SmCo5 film with a (112¯0) texture exhibited high in-plane intrinsic coercivity up to 26.5 kOe and a large in-plane magnetic anisotropy. The crystallinity of the SmCo5 layer plays an important role, which is strongly dependent on the texture and the morphology of the Cr underlayer. A smooth Cr underlayer with a (200) texture and a good crystallinity is desired for the SmCo5 film with a good magnetic performance. This work demonstrates a process to achieve high intrinsic coercivity and large in-plane magnetic anisotropy SmCo5 films on glass substrates deposited at a relatively low temperature (400 °C).

Study on the Magnetic Properties and Domain Structure of NdFeNbZrB Nanocomposite Permanent Magnets

A new kind of nanocomposite rare-earth magnets of Nd2Fe14B/ α-Fe were prepared by melt-spinning method. Effects of alloying element and processing parameter on the microstructure and magnetic properties of nanocomposite materials have been investigated. Zr is effective to enhance coercivity of alloys because of a refinement of grains, so that in alloy of Zr content with 1.0 at% (Zr1.0) has the smallest grain size of 17 nm and therefore causes the highest intrinsic coercivity . Addition of Zr can also enhance the ability of amorphous-forming. The combination of adding of Zr and using a smaller diameter of the nozzle in the melt-spinning method is effective for the forming of amorphous structure. According to the MFM study, the length of the magnetic contrast in the alloy is much larger than the mean grain size. The large length corresponds to that of interaction domains(ID), which is related to the exchange coupling effect.

Effect of boron addition on the magnetic properties of the Fe–Nd–Al alloys prepared by suction casting

The microstructure and magnetic properties of the Fe–Nd–Al alloys prepared by suction casting with boron addition have been investigated. The increasing boron content in the Fe–Nd–Al alloys significantly increases the intrinsic coercivity ( i H c) and decreases the proportion of the amorphous phase. The magnetization at the maximum applied field ( σ ′ s ) of the Fe–Nd–Al–B alloys decreases, while the coercivity increases markedly after annealing. The high intrinsic coercivity is due to the presence of the Nd 2Fe 14B phase.

Magnetic aftereffect and magnetic force microscopy studies of Fe–B∕FePt-type nanocomposite ribbons

Magnetic aftereffect and surface magnetic domain structure of (Fe0.675Pt0.325)100−xBx(x=12–20) nanocomposites have been investigated for correlating with their corresponding phases and magnetic properties. Exchange-coupling effect between grains is present for all the studied ribbons as evidenced by Henkel plot. The volume fraction of magnetically soft phases decreases with increasing B concentration from x=12 to x=18. It leads to the reduction in the activation volume of the reverse domain. Among all samples, the (Fe0.675Pt0.325)82B18 ribbon having the highest intrinsic coercivity (Hci) exhibits minimum value in activation volume V=1.1×10−18cm3, because it possesses the least volume fraction of the magnetic soft phases, i.e., Fe2B or Fe3B phases. Magnetic force microscopy studies reveal the magnetic domain structures of the ribbons, confirming the existence of strong exchange coupling between magnetic grains.

Sintering behavior of spin-coated FePt and FePtAu nanoparticles

FePt and [FePt]95Au5 nanoparticles with an average size of about 4nm were chemically synthesized and spin coated onto silicon substrates. Samples were subsequently thermally annealed at temperatures ranging from 250to500°C for 30min. Three-dimensional structural characterization was carried out with small-angle neutron scattering (SANS) and small-angle x-ray diffraction (SAXRD) measurements. For both FePt and [FePt]95Au5 particles before annealing, SANS measurements gave an in-plane coherence length parameter a=7.3nm, while SAXRD measurements gave a perpendicular coherence length parameter c=12.0nm. The ratio of c∕a is about 1.64, indicating the as-made particle array has a hexagonal close-packed superstructure. For both FePt and FePtAu nanoparticles, the diffraction peaks shifted to higher angles and broadened with increasing annealing temperature. This effect corresponds to a shrinking of the nanoparticle array, followed by agglomeration and sintering of the nanoparticles, resulting in the eventual loss of positional order with increasing annealing temperature. The effect is more pronounced for FePtAu than for FePt. Dynamic coercivity measurements show that the FePtAu nanoparticles have both higher intrinsic coercivity and higher switching volume at the same annealing temperature. These results are consistent with previous studies that show that additive Au both lowers the chemical ordering temperature and promotes sintering.

Developments with melt spun RE–Fe–B powder for bonded magnets

Rapidly quenched isotropic rare earth iron boride (RE–Fe–B) powders have found many applications throughout the electronics, automotive and white goods industries. The magnetic performance, thermal stability, corrosion resistance and processability of a powder are important factors when selecting a RE–Fe–B powder for a particular application. For electronic devices that operate at ambient temperatures, high remanence ( B r) tends to be a priority and RE 2Fe 14B/α-Fe nanocomposite powder magnets are favoured. Alternatively, automotive applications tend to require greater thermal stability and corrosion resistance, which are satisfied by single-phase RE 2Fe 14B powder magnets with higher intrinsic coercivity ( H ci). This article reviews the performance of commercially available rapidly solidified RE–Fe–B powders and recent developments made to address the demands of applications.

Effect of Pr concentration on the magnetic properties and phase evolution of Ti-substituted Pr yFe 88− yTi 2B 10 ( y = 9.5–7) nanocomposite ribbons

Magnetic properties, phase evolution, and microstructure of melt spun Pr y Fe 88− y Ti 2B 10 nanocomposite ribbons with different Pr concentration ( y = 9.5–7) have been investigated. For Pr 9.5Fe 78.5Ti 2B 10, only the 2:14:1 phase and the α-Fe phase existed, while an additional Fe 3B phase appears for the ribbons with y = 9–7.5. On further decrease of the Pr content ( y = 7), one extra phase, 2:23:3, is found. The exchange coupling effect, as evidenced by a Henkel plot is found, to decrease with the decrease of the rare earth content, which is attributed to the increasing of grain size with decreasing of Pr content. The magnetic properties of the ribbons first improve with the decrease of y, reaching a maximum at y = 8.5 ( B r = 9.6 kG, i H c = 8.5 kOe, and (BH) max = 17.8 MGOe), then decrease with further decrease of the Pr content ( B r = 9.8 kG, i H c = 5.6 kOe, and (BH) max = 14.5 MGOe for y = 7). In comparison with the Ti-free Pr x Fe 90− x B 10 ( x = 9.5–7) ribbons, improved magnetic properties with higher intrinsic coercivity can be obtained in Pr y Fe 88− y Ti 2B 10 nanocomposites. The maximum magnetic properties of Pr y Fe 88− y Ti 2B 10 nanocomposites can be found in the ribbons with lower rare earth content, i.e. x = 9.5 for the former and y = 8.5 for the latter.

Coercivity mechanism of Sm(Co, Cu) 5

The mechanism of the high intrinsic coercivity of the Sm(Co 1− x Cu x ) 5 (0≦ x<1) system was studied by relating the coherency between the lattice constants of hexagonal Sm(Co, Cu) 5 and hcp Co to the coercive force. It was found analytically that the intrinsic coercive force reaches a maximum in the composition range from x=0.6 to 0.8, where the lattice mismatch approaches zero, so that there is a strong correlation between lattice matching and coercive force. When a Sm ion was located within a Sm(Co, Cu) 5 grain and in the outmost edge of the a and c planes of its grain surrounded or not surrounded by the coherent Co phase, the crystal field parameter at each Sm 3+ site was calculated using a point charge model under the assumption that the Co and Cu atoms located in a grain and the hcp Co atoms situated at the interface uniformly have a charge of 3/5−. The results indicated that the Co phase precipitated coherently along the grain boundaries effectively enhances the magnetocrystalline anisotropy of Sm ions located in the outmost edges of the a and c planes of a Sm(Co, Cu) 5 grain.