Nanocomposite Permanent Magnets Research Articles

Fe65B22Nd9Mo4 bulk nanocomposite permanent magnets produced by crystallizing amorphous precursors

The Fe65B22Nd9Mo4 nanocomposite permanent magnets in the form of a rectangular cross sectioned rod have been prepared by annealing the amorphous precursors. The thermal behavior, structure and magnetic properties of the magnets have been investigated by differential scanning calorimetry, X-ray diffractometry, electron microscopy and magnetometry techniques. The as-cast Fe65B22Nd9Mo4 alloy showed soft magnetic properties, which changed into magnetically hard after annealing. Results provoke that the magnetic properties of the alloy are sensitive to thermal processing conditions. The optimum hard magnetic properties with a remanence (Br) of 0.56T, coercivity (iHc) of 920.7kA/m and maximum energy product (BH)max of 50.15kJ/m3 were achieved after annealing the alloy at 983K for 10min. The good magnetic properties of Fe65B22Nd9Mo4 magnets are ascribed to the exchange coupling between the nano-scaled soft α-Fe, Fe3B and hard Nd2Fe14B magnetic grains.

Manipulation of the magnetic exchange interaction in SmCo films with high thermal stability by controlling phase transformation

High thermal stability and tunable magnetic exchange interaction (MEI) in SmCo materials have been the critical problem in applications to magnetic recording media and nanocomposite permanent magnets. We constructed SmCo films with a high thermal stability and tunable MEI by controlling the phase transformation through properly increasing the Sm concentration (20.5–37.7 at.%) and controlling the annealing process. Microstructure studies show that the SmCo5 phases ensure that the film has a high thermal stability. Moreover, we manipulated the MEI in the film with non-magnetic precipitated SmCo2 particles in the vicinity of SmCo5 particles. These results provide a novel way to tune the MEI in SmCo materials while maintaining a high thermal stability.

Atom probe study on the bulk nanocomposite SmCo/Fe permanent magnet produced by ball-milling and warm compaction

The microstructure and compositions of the bulk nanocomposite SmCo/Fe permanent magnet were studied using transmission electron microscopy and 3-dimensional atom probe techniques. The excellent magnetic properties were related to the uniform nanocomposite structure with nanometer α-Fe particles uniformly distributed in the SmCo phase matrix. The α-Fe phase contained ∼26 at% Co, and the SmCo phase contained ∼19 at% Fe, confirming that the interdiffusion of Fe and Co atoms between the two phases occurred. The formation of the α-Fe(Co) phase explained why the saturation magnetization of the nanocomposite permanent magnet was higher than that expected from the original pure α-Fe and SmCo 5 powders, which enhanced further the maximum energy product of the nanocomposite permanent magnet.

The magnetic, structure and mechanical properties of rapidly solidified (Nd 7Y 2.5)–(Fe 64.5Nb 3)–B 23 nanocomposite permanent magnet

The Nd 7Y 2.5Fe 64.5Nb 3B 23 nanocomposite permanent magnets in the form of rods with 2 mm in diameter have been developed by annealing the amorphous precursors produced by copper mold casting technique. The phase evolution, structure, magnetic and mechanical properties were investigated with X-ray diffractometry, differential scanning calorimetry, electron microscopy, magnetometry and universal uniaxial compression strength techniques. The heat treatment conditions under which the magnets attained maximum magnetic and mechanical properties have been established. The results indicate that magnet properties are sensitive to grain size and volume content of the magnetic phases present in the microstructure. The composite microstructure was mainly composed of soft α-Fe (20–30 nm) and hard Nd 2Fe 14B (45–65 nm) magnetic phase grains. The maximum coercivity of 959.18 kA/m was achieved with the magnets annealed at 760 °C whereas the highest remanence of 0.57 T was obtained with the magnets treated at 710 °C. The optimally annealed magnets possessed promising magnetic properties such as j H c of 891.52 kA/m, B r of 0.57 T, M r/ M s = 0.68, (BH) max of 56.8 kJ/m 3 as well as the micro-Vickers hardness ( H v) of 1138 ± 20 and compressive stress ( σ f) of 239 ± 10 MPa.

Bulk SmCo5/α-Fe nanocomposite permanent magnets fabricated by mould-free Joule-heating compaction

Bulk SmCo5/α-Fe nanocomposite magnets have been prepared using a Joule-heating compaction technique. Nearly fully dense bulk magnets are obtained by compacting the milled powders under a pressure of 2 GPa at temperatures above 400 °C. Structural analysis shows that the grain size of both the SmCo5 and the α-Fe phases is in the range of 10 to 15 nm when the compaction temperature is lower than 500 °C, which ensures effective interphase exchange coupling. A further increase in compaction temperature leads to significant grain growth and deterioration of magnetic properties. A maximum energy product of about 18.5 MGOe was obtained in the bulk SmCo5/α-Fe nanocomposite magnets, which is 90% higher than that of the single-phase counterpart prepared under the same conditions.

NdFeB/αFe Nanocomposite Permanent Magnets with Enhanced Properties Prepared by Spark Plasma Sintering

Nanocomposite NdFeB/αFe magnets were obtained by spark plasma sintering technique using high energy ball-milled Nd-Fe-B melt-spun ribbons mixed in different weight ratios with Fe commercial powders. The remanence of SPS nanocomposite magnets increases with the Fe powders content from 6.1 for 4 wt.% Fe to 6.4 kG for 5 wt.% Fe, while the estimated maximum energy product is also increased from 9.0 to 10.6 MGOe.

Direct Iron Coating onto Nd-Fe-B Powder by Thermal Decomposition of Iron Pentacarbonyl

Iron-coated Nd-Fe-B composite powder was prepared by thermal decomposition of iron pentacarbonyl in an inert organic solvent in the presence of alkylamine. Though this method is based on a modified solution-phase process to synthesize highly size-controlled iron nanoparticles, it is in turn featured by a suppressed formation of iron nanoparticles to achieve an efficient iron coating solely onto the surfaces of rare-earth magnet powder. The Nd-Fe-B magnetic powder was successfully coated by iron shells whose thicknesses were of the order of submicrometer to micrometer, being tuneable by the amount of initially loaded iron pentacarbonyl in a reaction flask. The amount of the coated iron reached to more than 10 wt.% of the initial Nd-Fe-B magnetic powder, which is practically sufficient to fabricate Nd-Fe-B/α-Fe nanocomposite permanent magnets.

Physical Properties of Bonded Nanocomposite Type Hard-Soft Magnets

The exchange spring magnetic powders of SmCo5/α-Fe were obtained by mechanical milling and annealing. SmCo5+20 wt% α-Fe powder milled for 8 h and annealed at about 550 °C was considered to be more appropriate for our purpose. The isotropic nanocomposite permanent magnets were obtained by bonding the magnet powder in a binder matrix. Several ratios from 0.5 to 1.5 wt % between binder and magnetic powder were used. The composite material was compacted in dies at pressure from 600 to 800 MPa. The heat treatments for polymerisation were performed at 180 °C for 1h. The density and the microstructure of the magnets are discussed in connection with the properties of the starting powder and processing conditions. Magnetic characteristics of Br, Hc and (BH)max were obtained from hysteresis curves in magnetic fields up to 10 T.

Novel Nd2Fe14B nanoflakes and nanoparticles for the development of high energy nanocompositemagnets

High energy ball milling has been shown to be a promising method for large-scalefabrication of rare earth–transition metal nanoparticles. In this work, magnetically hardNd–Fe–B nanopowders with a coercivity in the range of 1.2–4 kOe have been produced bysurfactant-assisted ball milling of nanocrystalline precursor alloys. The nanopowders consisted ofNd2Fe14B flakes with a thickness below 100 nm and an aspect ratio as high as102–103 and anisotropic square nanoparticles with a size of 11 nm. Both the nanoparticles andnanoflakes showed a strong [001] out-of-plane texture. The nanoparticles showed a spinreorientation temperature which is lower (117 K) than the bulk value (135 K). The successfulfabrication of Nd–Fe–B nano-thin flakes and anisotropic nanoparticles provides hope for thedevelopment of nanocomposite permanent magnets with an enhanced energyproduct.

The magnetization reversal behaviour for SmCo6.8Zr0.2 and SmCo6.8Zr0.2/α-(Fe,Co) nanocrystalline magnets at low temperature

This paper reports that the SmCo6.8Zr0.2 nanocrystalline permanent magnets and SmCo6.8Zr0.2/α-(Fe,Co) nanocomposite permanent magnets are successfully produced by mechanical alloying and subsequently annealing at 700 °C for 10 minutes. The x-ray diffraction results show that the phase structure of SmCo6.8Zr0.2 nanocrystalline permanent magnets is composed of SmCo7 phase and SmCo6.8Zr0.2/α-(Fe,Co) nanocomposite permanent magnets is composed of SmCo7 and α-(Fe,Co) phases. The mechanism of magnetization reversal is mainly controlled by inhomogeneous domain wall pinning in SmCo6.8Zr0.2 and SmCo6.8Zr0.2/α-(Fe,Co) magnets. The inter-grain exchange interaction at low temperature is investigated, which shows that the inter-grain exchange interaction of SmCo6.8Zr0.2/α-(Fe,Co) magnets increases greatly by the decrease of the measured temperature. According to Δmirr–H/Hcj, Δmrev–H/Hcj and χirr–H/Hcj curves at room temperature and 100 K, the changes of irreversible and reversible magnetization behaviours of SmCo6.8Zr0.2 and SmCo6.8Zr0.2/α-(Fe,Co) magnets with the decreasing temperature are analysed in detail. The magnetic viscosity and the activation volume of SmCo6.8Zr0.2 and SmCo6.8Zr0.2/α-(Fe,Co) magnets at different temperatures are also studied.

Phase structure, magnetic properties, and magnetization behavior of Sm(Co,Zr)7/α-(Fe–Co) nanocomposite made by mechanical alloying

The Sm(Co,Zr)7/α-(Fe–Co) nanocomposite permanent magnets are produced by mechanical alloying and subsequent annealing. The soft magnetic phases were introduced by two different processes. In the first process, the crushed as-cast SmCo6.8Zr0.2 powders were blended and milled with different weight percent of fine iron powders (SCZ+Fe) and in the second process, the crushed as-cast (SmCo6.8Zr0.2)1−xFex (x=0,0.1,0.25,0.5,0.75) powders (SCZF) were milled directly. XRD analysis showed that the phase structure of annealed SCZ+Fe are consisted of SmCo7 phase and α-(Fe,Co) phase, while the composites of annealed SCZF are composed of α-(Fe,Co) and SmCo hard phases that varied with different Fe content. The best energy product of the annealed SCZF and SCZ+Fe are 11.3 MGOe (SCZF-3) and 10 MGOe (SCZ+Fe-1), respectively, resulting from the very strong exchange coupling between the hard phase and the soft phase. According to the Henkel-plots of SCZF-3 and SCZ+Fe-1, a stronger intergrain exchange coupling effect was observed in SCZF-3 sample. The irreversible nucleation fields of SCZF-3 and SCZ+Fe-1 are also studied in detail.

Rare earth–cobalt hard magnetic nanoparticles and nanoflakes by high-energymilling

High-energy ball milling has been shown to be a promising method for large-scale fabrication ofrare earth–transition metal nanoparticles. In this work, we report crystallographically anisotropicSmCo5,PrCo5 andSm2(Co, Fe)17 nanoparticles (particle size smaller than 10 nm) obtained by surfactant-assistedball milling and study their size and properties as a function of the millingconditions. By milling nanocrystalline precursor alloys, we obtainedSmCo5 platelets (flakes) approximately 100 nm thick with an aspect ratio as high as102–103. The unusual shape evolution of this brittle material is attributed to its increasedplasticity in the nanocrystalline state. The nanoflakes are susceptible to re-crystallizationannealing and exhibit a room-temperature coercivity of up to 19 kOe. The successfulfabrication of rare earth–cobalt nanoparticles and ultra-thin flakes provides hope for thedevelopment of nanocomposite permanent magnets with an enhanced energyproduct.

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.

Effect of Nd2Fe14B/α-Fe nanocomposite permanent magnets with a combined Co and Nb additions

Influence of Co+Nb on the Nd8Fe82Co3Nb1B6 nanocomposite magnets was investigated by adding Co element combined with Nb element. Results show that the high temperature stability of two phases is increased. Adding Co+Nb could improve the glass forming ability of the alloy, reduce the size of grains, increase the exchange coupling ability of two phases, and obviously increase the magnetic properties of the alloy. The optimal magnetic properties are Br=1.14 T, Hcj=320 kA/m, (BH)max-109.3 kJ/m3.

Rapidly solidified Sm–Co–V nanocomposite permanent magnets

Alloys around the Sm–Co eutectic composition provide an opportunity to form two-phase nanocomposite permanent magnets consisting of nanoscale Co fibers embedded in Sm2Co17.While ternary alloying elements may refine the scale of the structure, they may also disrupt the eutectic growth and lead to the formation of primary Co. Thus, microstructural selection maps were constructed for conventionally solidified Sm–Co–V alloys. It was found that V additions enlarged the primary Sm2Co17-forming region and, at (Sm0.09Co0.91)97V3, resulted in a eutectic structure. Upon rapid solidification, this alloy was determined to have a coercivity of 5 kOe with a high remanent ratio. However, the V addition reduced the magnetization, which limited the energy product to 4.3 MG Oe. The rapidly solidified structure consisted of primary SmCo7 dendrites along with an intergranular Co region, suggesting that eutectic structure formation is skewed by underlying metastable phase relationships.

元素戦略プロジェクト「低希土類元素組成高性能異方性ナノコンポジット磁石の開発」の狙いと課題

Purpose and objectives of “Project for High Performance Anisotropic Nanocomposite Permanent Magnets with Low Rare-Earth Content”, a subproject of MEXT's “Element Science and Technology Project”, is briefly discussed and recent findings obtained from multi-scale analysis of a hydrogenation-disproportionaion-desorption-recombination (HDDR)-processed Nd-Fe-Co-Ga-B alloy is presented.

Spark plasma sintered Sm2Co17–FeCo nanocomposite permanent magnets synthesized by high energy ball milling

Nanocomposite Sm2Co17–5 wt% FeCo magnets were synthesized by high energy ball milling followed by consolidationinto bulk shape by the spark plasma sintering technique. The evolution of magneticproperties was systematically investigated in milled powders as well as in spark plasmasintered samples. A high energy product of 10.2 MGOe and the other magnetic properties ofMs = 107 emu g−1,Mr = 59 emu g−1,Mr/Ms = 0.55 and Hc = 6.4 kOe were achieved in a 5 h milled and spark plasma sinteredSm2Co17–5 wt% FeCo nanocomposite magnet. The spark plasma sintering was carried out at700 °C for 5 min with a pressure of 70 MPa. The nanocomposite showed a higher Curie temperature of955 °C forthe Sm2Co17 phase in comparison to its bulk Curie temperature for theSm2Co17 phase(920 °C). This higher Curie temperature can improve the performance of the magnet at highertemperatures.

Structure and magnetic properties of bulk isotropic and anisotropic Nd2Fe14B∕α-Fe nanocomposite permanent magnets with different α-Fe contents

Chemical coating, hot compaction, and hot deformation techniques have been applied to prepare bulk isotropic and anisotropic Nd2Fe14B∕α-Fe nanocomposite magnets. The effect of α-Fe content on the structure and magnetic properties of the magnets were studied. For the isotropic magnets, the remanence (Br) increases as the α-Fe content increases, while the coercive force (Hci) drops simultaneously. For the anisotropic magnets, the Br rises first, peaking at 2vol% of α-Fe content, then falls as the α-Fe content increases, and Hci drops significantly for all the α-Fe containing anisotropic magnets. Crystal structure analysis shows that only the magnets with no more than 2vol% α-Fe exhibit strong c-axis crystal texture of Nd2Fe14B phase after deformation. Microstructure observation also shows that there are many Nd2Fe14B equiaxial grains even after hot deformation in the magnets with α-Fe more than 2vol%.

Bulk anisotropy Nd‐Fe‐B/α‐Fe nanocomposite permanent magnets prepared by sonochemistry and spark plasma sintering

AbstractNdFeB/α‐Fe nanocomposite magnetic powders were prepared by sonochemical process. The powders were then submitted to a hot press and subsequent hot deformation process by spark plasma sintering (SPS) technique. Effect of α‐Fe content on structure and magnetic properties of both isotropic and anisotropic magnets was investigated. For hot pressed magnets, the remanence increases with the content of α‐Fe, while the coercive force drops simultaneously. After hot deformation, the magnets with no more than 2 vol% α‐Fe exhibit obvious anisotropic characteristic. For the magnets with more α‐Fe, however, the magnetic anisotropy disappears due to the absence of (00l) crystal texture after deformation. It is, therefore, expected that α‐Fe content plays an important role in the formation of C‐axis crystal texture during hot deformation process. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effects of magnetic field annealing on magnetic properties and microstructure of Nd‐Fe‐B‐Ti‐C based nanocomposite permanent magnet

AbstractRapidly solidified ribbons of Nd9Fe73B12.6C1.4Ti3Nb1 were annealed with or without a static magnetic field of 10 T. The remanence (Br) of the ribbon annealed with the magnetic field was higher than that of the ribbon annealed without the magnetic field. Improved magnetic properties of the ribbon annealed with the magnetic field may be attributed to a uniformly fine grain size. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)