Recent aspects of hydrogenation disproportionation desorption and recombination (HDDR) phenomena in NdFeB-, SmFe- and SmCo-type alloys are reviewed in this paper. The effects of additives on hydrogen sorption and magnetic properties are discussed. The possibilities of producing magnetically anisotropic NdFeB HDDR magnets by hot deformation or alternatively pre-aligning and compacting anisotropic powders are described. Current models for the inducement of magnetic anisotropy in NdFeB HDDR powder are summarized and it is shown that anisotropic powder can be produced from a ternary compound when using a modified processing route. The application of ‘extreme’ hydrogenation conditions, namely high hydrogen pressure or reactive milling in a hydrogen atmosphere at enhanced temperatures, allows the disproportionation of thermodynamically very stable compounds such as Sm2Fe17−xGax (x≥l), SmCo5 or Sm2Co17. Reactive milling and subsequent recombination in vacuum lead to structures, of both the disproportionated and the recombined type, which are on a remarkably finer scale than those commonly observed for standard HDDR procedures. Exchange coupling between the nano-scaled grains can result in magnetically single phase behavior despite a multiphase microstructure and in particular for the Sm2Co17 alloy, a remanence enhancement was observed after recombination at temperatures ≤700°C.

This paper deals with the magnetic and technological properties of isotropic Nd-Fe-B magnets over a wide range of Nd and B contents. They can be used to determine the optimum composition based on requirements. In anisotropic magnets, Nd and B ranges for production of high energy product rings was determined in Nd-Fe-Co-Ga-B system. This is also used in production.

The magnetic properties of nanophase exchange enhanced RE-Fe-B based alloys ribbons prepared by melt spinning are presented and discussed. The alloy systems investigated include NdyFe94−yB6 and PryFe94−yB6, for y within the range 6-20 at%, and the data are compared in the context of the larger anisotropy field for the Pr2Fe14B phase in comparison with Nd2Fe14B. Experimental studies are also reported for nanophase (Nd-Pr)- Fe-B alloys with various proportions of Nd:Pr, to investigate the increase in coercivity of NdFeB by partial substitution of Nd by Pr, and for (Nd, Pr)-(Fe-Co)-B-based systems, to study the degree to which the remanence enhancement can be maintained for alloys in which Co substitutes for Fe to increase the thermal stability.

Chemical surface modification can be used as a method for corrosion protection of sensitive powders based on intermetallic alloys between rare earth and transition metals. Surface coating is used for preventing fine powders, based on Nd-Fe-B, or Sm2Fe17−xTaxN3−δ prepared by HDDR processing and mechanically alloying, from hydrolysis. Powders coated by chemisorbed organic substance, after exposing to a humid atmosphere, do not show any chemical or physical change.

Different coating agents were used and the sufficient amount of various materials was optimised with the emphasis on minimising their quantity. Simple experiment shows that the surfactant is successfully chemisorbed on the powder surface and that the coated powders are hydrophobic indefinitely. Magnetic properties were measured on samples after they were exposed to the same corrosion tests. Measurements on coated and bonded samples were compared with the measurements of non-coated and bonded samples. By using Auger electron spectroscopy the thickness of the coating was controlled. In order to distinguish the nature of the bonding between the powder surface and the surface-active substance FT-IR spectroscopy in absorbency and diffuse reflection modes was used. The protection of the fine particles is based on the formation of a covalent bond between the hydroxyl groups at the particle surface and the surface-active substance. The monomolecular layer of organic substance does not damage the magnetic properties of the powder, but successfully protect the powder against humidity.

Some boron-rich (with respect to the Nd 2Fe,4B composition) NdFeB alloys (with and without small additions of zirconium) have been investigated in an attempt to understand the influence of zirconium in the subsequent magnetic properties of HDDR hot pressed magnets. It was found that zirconium additions had a marked influence on the as-cast microstructures with a general refinement of the grain size and evidence of a zirconium-rich phase (probably ZrB2) at the grain boundaries. The magnetic property measurements of fully dense, isotropic HDDR hot pressed magnets indicated that those containing zirconium maintained their coercivity to significantly higher hot pressing temperatures and this was attributed to the pinning of grain boundaries by the zirconium rich phase.

Melt-spun Nd2Fe14B (2–14–1) ribbons were produced under active vacuum and different partial pressures of inert gases of Ar and He. Microstructure and thermal analyses were performed to understand the microstructural evolution and glass formability (GF) of the ribbons. He atmosphere enhances the quenchability of the ribbons over Ar and vacuum. Ribbons made under 250 Torr He have more uniform microstructure and smoother surfaces than those under 760 Torr He. The higher quenchability induced by He, which increases the interfacial heat transfer coefficient between the melt and rotating wheel during melt spinning, is due to its higher thermal conductivity compared to Ar. The lower pressure stabilizes the turbulence between the melt-pool and Cu wheel, enhancing the heat transfer resulting in a more uniform quench. As a result, a more uniform ribbon microstructure can be obtained at relatively low wheel speeds.

SmFe based alloys interstitially modified with nitrogen are potential candidates for high energy permanent magnets. In order to obtain the optimum properties a thorough understanding of the starting material and processing parameters is required. The microstructures of two cast alloys of composition Sm13.8Fe82.2 Ta4.0 and Sm13.7 Fe86.3 were carefully examined with a SEM equipped with EDX and the exact stoichiometries of the phases were determined. The SmFeTa material was found to contain significant amounts of TaFe2as well as the Sm2Fe17, SmFe2, SmFe3 phases observed in the SmFe material but without the a-iron dendrites which are characteristic of the latter material. The optimum conditions necessary to provide the highest coercivities using the conventional HDDR process, and for the HDDR process combined with pre-milling were investigated. The coercivities obtained after using the HDDR process and subsequent nitriding were 680 kA/m for the SmFeTaN and 360 kA/m for the SmFeN samples. Significantly higher coercivites of 1000 kA/m for SmFeN and 1275 kA/m for SmFeTaN were achieved by reducing the particle size with milling prior to the HDDR process.The better coercivities obtained with the Ta containing sample were found to be due to the presence of a much smaller amount of a. The milling prior to the HDDR treatment improves the magnetic properties because of the small particle size which prevents the grains growing too large, with their consequent very negative effect on the coercivity.

Alloys of Pr-Fe-B-Cu were cast into Cu and stainless steel tubes. It could be seen that a preferred crystallographic orientation was obtained after cooling. Fine platelets of Pr2Fe1 4B matrix phase were formed, surrounded by various grain boundary phases, and there was a significant reduction in the amount of free Fe present, in comparison to conventional slab cast alloys. In order to improve the magnetic properties in the as-cast state, two alternative routes were used. The first involved a series of two step heat treatments to develop an improved microstructure. After an optimum heat treatment of 1000 'C for 24 hours + 500°C for 3 hours, significantly improved magnetic properties were obtained for a 17.5 % Pr alloy; Br = 752 mT, Hci = 613 kAm−1 and BHmax = 96 kJm−3. The second route involved a rapid upset forging [RUF] process, with a strain rate of 11.5 s−1, to hot deform the alloys. Following post-forging heat treatments, properties of Br = 966 mT, Hci = 780 kAm−3 and BHmax = 160 kJm−3 were obtained for a 15% Pr alloy. That nature of the improvements in properties as a result of heat treatments and RUF were investigated and are discussed in the present work.

We report progress on our exploratory research on surface modification of magnetic NdFeB fillers, characterization of suitable magnetic rare earth alloy powders and high-performance polymer matrices, processability, and properties of novel thermoplastic/NdFeB magnets. The results suggest that blending liquid crystal polymer (LCP) with a high-thermoplastic polymer such as polyphenylene sulfide (PPS) provides the required balance of properties. These properties include superior magneto-mechanical performance, minimal melt viscosity at optimal NdFeB volume loading, enhanced thermal stability, high stiffness, high strength, improved dimensional stability, and excellent chemical resistance; making the thermoplastics magnets suitable for use in high temperature and aggressive environments where commercial polymer-bonded magnets are not useable.

Neutron diffraction has been invaluable in the characterization of intermetallic magnetic phases due to the unique properties of the neutron and the availability of high performance powder diffractometers that allow large numbers of samples to be investigated. The structural data derived from these studies serves as fundamental input for theory, and constraints on analyses of data from other methods, such as Mossbauer spectroscopy. The empirical structure-property relations derived from these studies serves to guide synthesis of new compounds and may help to define the next generation of high performance permanent magnets.

A brief review is presented of recent work dealing with the structure and magnetic properties of RCo7−x Zrx alloys (R=Sm, Pr, Er, Gd, and Y). The experimental results obtained are consistent with a model in which Zr atoms partly replace dumbbell Co atoms and play an important role in stabilizing the TbCu7 structure while significantly increasing the anisotropy field (HA). For example, when R=Sm, HA increases from 90 kOe for x=O to 130 kOe for x=0.2 at 300 K, and from 140 kOe for x=0 to 220 kOe for x=0.2 at 10 K. In the case of R=Y and Gd, HA is mainly contributed by the Co sublattice. For R=Y alloys, HA increases from 18 kOe for x=0 to 74 kOe for x=0.2 at 300 K and from 20 kOe for x=0 to 82 kOe for x=0.2 at 10 K. For R=Gd alloys, HA shows the largest enhancement. It increases from 35 kOe for x=0 to 140 kOe for x=0.2 at 300 K and from planar for x=0 to uniaxial with 182 kOe for x=0.2 at 10 K. In general, experimental results are in accord with the theory of Greedan and Rao for anisotropies of R-Co alloys. The magnetic moments for cobalt and rare earth in RCo7−xrx compounds (x=0∼A).2) have been estimated from the experimental values. The results show that they are nearly the same as those in RCo5 or R2Co17. The Co moment is 1.5∼;1.6 μB. Some phase transition phenomenon between RCo5, RCo7, R2Co17and R2Co7at different heat treatment conditions will also be discussed.

First-principle electronic structure studies complement experimental research on hard-magnet materials. Since the discovery of Nd 2Fel4B in 1984, the research in this area has been concentrated on T(Fe,Co)-rich rare-earth compounds such as RT12 and R2T17 and exchange coupled hard/soft phases. Self-consistent spin-polarized electronic structure calculations are carried out for the sequence YFc2→ YFe3→Y2Fe17→YFe12 to study the variation of the magnetization and Curie temperature as a function of the Fe concentration. Calculations are performed for R2T17 systems which show significant improvements in their Curie temperatures with interstitial and substitutional modifications. The calculated results are compared with the available experimental data. Computer simulations are carried out for FePt/Fe and SmCo5/Co1−x -Fex, hard/soft multilayers.

The Nd5Fel compound has attracted some attention lately as a possible permanent-magnet material. This compound has a very complicated crystal structure (264 atoms per unit cell with 14 and 7 different Fe and Nd sites respectively!) Using linear-muffin-tin-orbital method, self-consistent spin-polarized studies of NdFe1 - have been carried out to determine the electronic structure, magnetic moments, and the Curie temperature. The results show strong effect of the local environment on the magnetic properties of individual Fe sites. The calculated moments are compared with the magnetization data. The Curie temperature estimated for the iron sublattice using the mean-field approximation is in a reasonable agreement with the experimental result.

In this work the rare-earth anisotropy in the rare-earth - transition metal intermetallic compounds is investigated by studying the temperature-induced spin reorientation phenomena on aligned powder samples and by analyzing the temperature dependence of the magnetization measured along different principal axes of single crystals. From such investigations the crystalline-electric-field parameters of the rare-earth ions in these compounds have been derived. The results on different hard magnetic materials of the compositions R2Fe14B, RFe11 and their substituted compounds as well as R2Co17 are summarized.

Recent progresses of materials development and their utilization of Cr-and-Co-doped Fe3B/Nd2Fe14B-type nanocomposite permanent magnets are reported. A practical balance between enhanced intrinsic coercivity and temperature stability of magnetic properties has been realized by simultaneous additions of Cr and Co. This type of magnets are less susceptible to oxidation because of the less rare-earth content. Consequently, even fine powders (e. g., under 38 micrometer sieve) are physically and chemically stable in contrast to the conventional Nd2Fe14B-type melt-spun materials. The stability in terms of the structural losses is also superior to that of the Nd2Fe14B-type melt-spun materials. Utilization of the Cr-and-Co-doped Fe3B/Nd2Fe14B nanocomposite permanent magnets as a hard magnetic component of injection-molded resin-bonded magnets seems promising because of the excellent stability of the magnetic properties of fine powders. Direct quenching of a melt into the nanocomposite structure has become possible recently in addition to the conventional processing route of crystallization of an amorphous precursor, opening up the possibility of less-expensive production of the material.

Emerge is an European project based on the collaboration between three industrial partners (Rhodia, Philips and Magnetfabrik Bonn) and three research laboratories (Centre National de la Recherche Scientifique CNRS (Laboratory Louis NEEL), Trinity College of Dublin TCD, Vienna University IATP). The aim of this project is to improve the coercivity in enhanced remanence material. This three years project used statistical set-ups to reduce the number of samples to be realised and tested. The complete description of the project is developed in this paper and the main conclusions on sample composition and process issues are also discussed.

Micromagnetic finite element calculations provide the theoretical limits for the remanence, the coercive field, and the coercive squareness of nanocomposite Nd2Fe14B/α-Fe,Fe3B,Fe23B6magnets. The influence of the intrinsic magnetic properties and the microstructure were investigated using an energy minimization technique. The coercive field reaches a maximum as a function of the average grain size at about 15 nm - 20 nm. The replacement of α-Fe with Fe3B improves the coercive field but deteriorates the loop shape. The reduction of the magnetization and the exchange constant in the soft magnetic Fe3B phase by 20% improves the coercive field without a significant loss in the remanence. An increase of the hard phase anisotropy by 8% enhances the coercive field by more the 100 kA/m in two-phase α-Fe/Nd2Fe14B and by 60 kA/m in two-phase Fe3B/Nd2Fe14B magnets. Dynamic simulations for magnetostatically interacting particles of a bonded magnet show a slight influence of the particle arrangement on magnetization reversal. The interaction field in the range of 100 kA/m to 200 kA/m rapidly decreases the distance from the particle.

A series of RExTMbalBy (3≤x≤5.5, 15≤y≤25 at%) nanocomposite magnets were prepared using the classical method of alloy rapid quenching and subsequent devitrification. Appropriate elemental additions and substitutions allowed the technical magnetic properties to be optimized. The analysis of the both hysteresis cycles and static susceptibility measurements in the range 20K-300K reveals that magnetization reversal in these materials can be interpreted schematically as a two-step process : the initial magnetization reversal is dominated by reversible processes while irreversible processes, which characterize the coercivity mechanism, occur at larger field values. To complement this study, X-Ray Magnetic Circular Dichroism (XMCD) spectroscopy was applied as an element-selective probe of magnetization reversal in nanocomposite samples. This allowed the hysteresis cycles of both hard and soft phases to be separated from the total cycle. Magnetization processes in these bulk heterogeneous systems are discussed in the light of information brought by the complementary applied experimental techniques.

A residual amorphous phase can be observed when Nd content is less than 8 at% in a a Fe/NdFeB nanocomposite with various additional elements which was prepared by melt-spinning and heat treatment. The surface velocity of the single-roller melt-spinner for optimal coercive force (Hcj) was 10-15 m/s. By adding Nb, Hcj increases monotonically. Nearly homogeneous crystal size of around 5 nm is observed in a sample composition of NdxFex Cox Nb2,B2 With Nd=8 at%, the maximum Hcj is as large as 575kA/m (7.22kOe) at Nb=2.5 at%, and the best (BH)max is 154kJ/m3 (19.4MGOe) at Nb=0.5 at% in optimally heat-treated and pulse-current magnetized samples. By high-resolution transmission electron microscopy, a residual amorphous phase is observed in all alloy ribbons with Nd=8 at%, which shows soft magnetic properties and is thought to act as a crystal growth inhibitor. To estimate the existing ratio of the amorphous phase and crystallized phases, the Mössbauer effect has been studied. It was found that the ratio of the crystallized Nd2Fe14B1 phase increases with increasing Nb content and that several wt% of the residual amorphous phase is certified. Epoxy-resin bonded magnets were prepared by compression molding, and revealed that, comparing to the MQP bonded magnet using conventional MQP-B powder, their (BH)max are in the same order of 72kJ/m3 (9MGOe), and magnetization at lower magnetic field than 800kA/m (10kOe) is superior. Initial flux loss at 100°C for 1.8ks, however, is in the range of 3-8% depending on Nb contents.

Many high performance permanent magnets are nanostructured materials. The magnetic properties of nanoparticles are discussed in terms of characteristic length scales, including the maximum monodomain size and the exchange length. Experimental results for ball milled SmCo5 nanoparticles are presented, showing deviations from idealized behavior. Because of the short exchange length, this can be understood in terms of independent nucleation of reverse domains in grains within larger particles. With a much longer exchange length, FeCo alloy nanoparticles show reduced coercivity in a high density compact, in accordance with the random anisotropy model. The SmCo5 and FeCo nanoparticles were mixed and compacted in an attempt to make an exchange spring nanocomposite. However, significant exchange between the hard and soft phases was not observed because the sample density was too low. Processing considerations for improved co-compaction of these nanoparticles are discussed.