Several ternary compounds of tetragonal structure and with the composition R2Fe14B have found an application as starting materials for permanent magnets. Several physical properties of R2Fe14B compounds will be discussed in relation to the modification in properties that can be obtained by substituting other elements for R, Fe or B. Also investigated is the extent to which it will be possible to find alternative materials differing in structure and composition from R2Fe14B. For this purpose the results obtained on 3d-rich ternaries based on well-known structure types will be reviewed. Included in this survey will be novel ternary intermetallics made by crystallization from the amorphous state. These compounds, though stable enough for applications, are actually metastable. They are not found in the corresponding equilibrium diagrams.

Certain R2Co17 and R2 T14 B systems (R = a rare earth, T = Fe or Co) are of significance for use in the fabrication of high energy magnets. Improvements in these systems as high energy magnet materials are effected by partially replacing Co or T by appropriate d transition metals. A description is given of the magnetic behavior of a number of systems based on R2 Co17 and R2T14 B. Also, an account is given of recent work dealing with the systems RCo4 B and PrCo4-xFex B. The latter materials have large magnetizations and high Curie temperatures. However, they are not useful permanent magnet materials because they exhibit planar anisotropy.

Results of some Auger spectroscopy studies of R2 Fe14 B systems (R = Pr,Nd or Er) are presented. This work shows the intergranular region to be rich in B, O and R. This region is undoubtedly non-magnetic, a feature which is most probably of significance in regard to the observed coercivity of R2 Fe14 B magnets.

Single crystals of R2 Fe14 B weighing 5 to 15 g were grown from a liquid melt by tri-arc and levitation Czochralski methods. The arc method was used for the growth of smaller size crystals from 5 to 7 g, and the levitation method was used for the growth of larger sizes, from 10 to 15 g. Crystals of (R1)2 –x(R2)xFe14 B could also be grown with isomorphous replacement of rare earth atoms. The starting alloy composition for the crystal growth process was chosen based on solidification microstructure. In this paper we discuss the solidification microstructure, isomorphous replacement, seeding and their interactions with the crystal growth processes and crystal quality.

Thin Films (≃4000 A) of R2Fe14B compounds have been synthesized by d.c. triode sputtering for R=Nd,Sm and Er. Deposition onto single-crystal substrates heated to 600”C and with rates of 1.3 A/s results in nearly epitaxial film growth such that the c-axis of the tetragonal structure is perpendicular to the film plane. As a consequence, intrinsic anisotropy effects are observed in the magnetic properties of the as-made films. Although sapphire and quartz substrates give the best results, similar directed growth is observed on recrystallized Nb foils. Magneto-optical measurements on the Nd-based compounds give a remnant polar Kerr angle θ=0.22° and coercive field Hc=5.5 k0e. The easy axis is in the film plane for R=Sm and Er and a remnant magnetization greater than 80% of saturation is observed for H applied parallel to the film plane. Electrical resistivity measurements on Nd2Fe14B films indicate that spin-disorder scattering from the rare earth site is important and features from both the Curie and spin reorientation temperatures are observed.

Initial magnetization and demagnetization data are reported for three forms of rapidly solidified Nd-Fe-B permanent magnet materials: melt-spun ribbons, hot pressed magnets, and die upset magnets. In all three materials the results are consistent with domain wall pinning at grain boundary phases as the coercivity mechanism. Optimally quenched ribbons are comprised of randomly oriented single domain Nd2Fe14B grains, and both initial magnetization and demagnetization are controlled by strong domain wall pinning at grain boundaries. Maximum coercivity is accompanied by a low initial permeability. Coercivity is reduced in overquenched ribbons by partial retention of a magnetically soft amorphous or very finely crystalline microstructure. Coercivity decreases in underquenched ribbons because wall pinning weakens as the grain size increases above optimum. Correlation of magnetization and demagnetization behaviors suggests that maximum coercivity in ribbons is largely determined by the resistance to domain wall formation in grains smaller than the single domain particle limit. Grain size is much less important in the aligned die upset magnets. Domain walls are initially free to move until they become strongly pinned at grain edges, and complete magnetization requires an applied field greater than the coercive field. Hot pressed magnets show a mixture of ribbon and die upset behavior.

Transmission electron microscopy has been used to characterize the microstructure of Nd-Fe-B magnets produced by melt-spinning and subsequent hot-pressing/die-upsetting. For a material of starting composition Nd.135Fe.815B.05, the basic microstructure od melt-spun, hot-pressed and dieupset magnets consists of two phases. In the optimally processed melt-spun ribbons and hot-pressed samples, small and randomly oriented Nd2Fe14B grains are surrounded by a thin noncrystalline Nd-rlch phase. The die-upset material consists of closely stacked flat Nd2Fe14B grains surrounded by a second phase of approximate composition Nd7Fe3. No Nd11Fe4B4 phase is observed in these materials, but it can form if the chemical composition and/or processing parameters are varied. In all these materials, Lorentz microscopy reveals that magnetic domain walls are pinned by the second phase. The differences in the hard magnetic properties of the three kinds of MAGNEQUENCH magnets closely correlate with the differences in the distribution of Nd2Fe14B crystallites and the pinning sites.

Permanent magnets from the Fe77Nd15B8 alloy were produced using liquid dynamic compaction (LDC). Process parameters such as gas and metal mass flow rates were carefully controlled to refine the microstructure. Accordingly, grain sizes between 0.1 and 5 microns were achieved in the LDC compact. In contrast to our earlier LDC results high intrinsic coercivity in excess of 7500 Oe was obtained for thick (> 7 nm) as-LDC'd samples with lower neodymium content.

Nd2-xPrxCo14B intermetallics crystallize in tetragonal form isostructural to Nd2Fe14B. Structural and magnetic properties of the polycrystalline materials have been determined. Room temperature magnetic measurements indicate that these intermetallics exhibit uniaxial anisotropy throughout the entire composition range. Substitution of praseodymium in Nd2-xPrxCo14B significantly enhances the anisotropy field both at 78 K and at room temperature. Magnetization versus temperature plots show spin reorientation TSR2 at 546 K for Nd2Co14B and at 668 K for Pr2Co14B. TSR2 increases linearly with composition x. The 3d sublattice anisotropy which seeks the tetragonal plane appears to dominate above these temperatures. In addition to the above, low temperature magnetic measurements demonstrate the spin reorientation in Nd2Co14B and Nd1.9Pr0.1Co14B. Further additions of praseodymium strengthen the axial anisotropy at temperatures down to 4.2 K. Effect of competing crystal field interactions and the consequences of praseodymium substitution that result in considerable increase in anisotropy field in these quaternaries are discussed.

The magnetocrystalline anisotropy energy has been determined for single crystal Y1.8 Er0.2Fe14B in the (001) and (100) planes between 5 K and 300 K using torque magnetometry techniques. The results are compared with a model based on crystal field theory. Excellent agreement was obtained between the model and experiment. Both experiment and model showed a first order spin reorientation between the [001] and an angle 70 degrees from the [001] in the (100) plane.

The bulk magnetic properties of (ErxY1-x)2Fe14B and (ErxPr1-x)2Fe14B systems were studied over the temperature range 4.2-1100 K. Lattice parameters, saturation magnetizations, Curie temperatures and spin reorientation temperatures were determined. Theoretical description of the detailed magnetic behavior is presented, based on a crystal field model. The (ErxY1-x)2Fe14B compounds were all found to exhibit plane-to-axis spin reorientations similar to that observed for Er2Fe14B, with the transition temperature decreasing with increasing Y content. In contrast, the spin reorientations in the (ErxPr1-x)2Fe14B systems appear to be of the cone-to-axis type. Since higher order crystal field terms appear to be significant only in the cases of Nd3+ and Ho3+, the results are discussed in terms of a crystal field Hamiltonian involving only 2nd order terms. Using known values of the exchange field, Fe anisotropy and the ratios of the crystal field coefficients, the multi-ion cr 6.tal field problem was formulated in terms of a single adjustable parameter (B0 2(f). It is shown that 2nd order crystal field terms are capable, not only of explaining the conical anisotropy of the (ErxPr1-x)2Fe14B systems, but also the decrease in the Er moment upon passing through the spin reorientation (as has been observed for Er2Fe14B). The magnetic structure of Er1.5Pr0.5Fe14B is also predicted.

The effects of misch-metal–and/or Al substitutions in the Nd-Fe-B alloy system have been investigated. Selected compositions were processed into magnets using the conventional powder metallurgy technique. As expected, misch-metal substitutions act to degrade the intrinsic magnetic characteristics and thus degrade the properties of the sintered magnet. It was observed that at a level of 25% of the total rare earth being comprised of misch-metal, energy products of the order of 25 MGOe were still able to be achieved. Processing of such material is difficult because higher sintering temperatures are required to fully densify the magnet, and the grain growth that occurs at these temperatures dramatically reduces the coercivity. The effects of misch-metal substitutions are quantified in this discussion.

Al was found to be beneficial for improving the coercivity in both the Nd-Fe-B and misch-metal-Fe-B alloy systems. Unfortunately Al substitutions reduce both the remanence and the Curie temperature. If the Al content is kept to 1 wt.% or less, magnets exhibiting reasonable properties can be made. Al doping as opposed to doping with expensive heavy rare earth elements such as Th or Dy is a less costly method of improving Hei, in the Nd-Fe-B magnets.

The temperature dependence of magnetic properties for NdFeCoB alloys were investigated. It is well known that dysprosium substitution for neodymium increases the intrinsic coercivity. Aluminum also improves the intrinsic coercivity, Hci, at room temperature but impairs the Curie temperature and the intrinsic coercivity at elevated temperatures. The rate of increase of the Hci with increasing dysprosium in the NdFeCoB system is less than when dysprosium is added to the NdFeB system. A combination of aluminum and dysprosium is more effective than aluminum alone in raising Hci. Molybdenum addition into the NdDyFeCoB alloy was found to reduce the temperature coefficients of Br and Hci. Temperature coefficients of Br and Hci obtained on a Nd10Dy5Fe67Co10Mo1Al1B6 alloy are -0.07 %/°C and -0.4 %/°C respectively.

The effect of small aluminum additions on the temperature dependence of remanence and intrinsic coercivity in Co- and Dy-containing Nd-Fe-B was studied. Sintered magnets were prepared and demagnetization curves measured at temperatures between −50 to +200°C. Curie temperatures and irreversible flux losses in open circuit were determined. Al increases the coercivity while decreasing remanence, energy product and Curie temperature. Other unfavorable side effects are the increase in temperature coefficients of Br and, especially, MHc. Substitution of Al is not beneficial for magnets used at elevated temperature.

The state-of-the-art description of the magnetic hardening mechanism in the sintered Nd-Fe-B permanent magnet is given. Recent experimental results concerning the coercivity-anisotropy (Hc -HA) correlation in B-rich Pr-Fe-B and Nd-Fe-B sintered magnets and the inhluence of the surface conditions of the sintered Nd-Fe-B magnets on the coercivity are reported. These results are interpreted in terms of the μ0Hc versus cμ0 HA -NIS plot, where I is the spontaneous magnetization of R2 Fe14 (R=Pr or Nd) and N the effective demagnetization field coefficient.

Sintered, precipitation hardened SmCo 2:17 magnets contain a multiphase microstructure. Our electron microscopic investigations reveal that the size of the rhombic, cellular precipitation structure and the formation of cell interior and cell boundary phases is determined by the nominal composition of the alloy and the postsintering heat treatment conditions and primarily control the intrinsic coercivity of the magnet. Selected area electron diffraction together with high resolution electron microscopy showed a high density of basal stacking faults (microtwinning) of the cell interior phase of low coercivity (iHc < 700 kA/m) magnets with a (c/a)*- ratio of the basic structural unit of > 0.843. High coercivity magnets (iHc>) 1000 kA/m), containing a high density of the platelet phase perpendicular to the c-axis, exhibit cell diameters up to 200 nm with a (c/a)*-ratio of the basic structural unit of the cell interior phase of < 0.843.

Microstructures of Nd15−xDyxFe77B8 prepared by alloying with Dy, and by using Dy2O3 as a sinl'ken adidive, have been determined using electron microprobe and transmission electron microscopy. The results have shown a higher Dy concentration near the grain boundaries of the 2–14–1 phase for magnets doped with Dy2O 3, as compared to the Dy-alloyed magnets. A two-step post sintering heat treatment was also studied for the two systems. The resultant concentration gradient of Dy in the 2–14–1 phase of the oxide-doped magnets is explained by the reaction of Dy2O3 with the Nd-rich grain boundary phase and its slow diffusion into thg 4–14–1 phase. Increased Dy concentration near the grain boundary is more effective in improving the coercivity, as domain reversal nucleation originates at or near this region.

This paper addresses the effect of Dy addition upon the magnetic properties, microstructure and microcomposition of Fe-Nd-B magnets. It is shown that increasing additions of Dy causes the remanence, Br to decrease linearly. The intrinsic coercivity, iHc, increases sharply for small additions of Dy, but the increase is not proportional for higher Dy contents. The iHc increases almost linearly with the effective anisotropy field of the RE2Fe14B phase until the Dy content is about 10% of the total rare earth content. Above this concentration, there is a strong deviation from linearity. Various types of possible concentration profiles of the substituted rare earth are suggested. It is also argued that preferential segregation of Dy to the interfaces could be beneficial in increasing the nucleation field. Morphologically there is no apparent effect of Dy on the microstructure. However, in the 5 atomic % Dy sample, Dy rich oxides were observed. It is shown through Energy Dispersive Xray Spectroscopy (EDXS) line profiling that Dy partitions preferentially into the RE2Fe14B phase in all the cases. No segregation of Dy to the interphase interfaces has been detected.

A microstructural and microanalytical study of aluminum substituted Nd-Fe-B sintered permanent magnets were carried out to determine the effect, if any, of aluminum on structure, composition, and magnetic properties, particularly on the observed increase in coercivity. It was found that Al enrichment occurred in the Nd-rich phase at the grain boundaries. The possible role(s) of this enrichment on the observed coercivity increase are discussed.

The effect of absorbed hydrogen on the structural and magnetic behavior of Pr2Fe14−xCoxB has been investigated. Introduction of hydrogen in the host material reduces the average magnetic moment of the 3d element by ˜ 4 to 7%. Anisotropy fields for all systems studied are significantly reduced upon hydrogenation. Evidence of spin reorientation at temperatures ranging between ˜ 295 K and 496 K for some of the hydrides is observed. Pr2Co14B and its hydride exhibit uniaxial anisotropy at room temperature and at 77 K.

The role of hydrogen in expanding the lattices of these intermetallics and modifying their magnetic properties is discussed.

The ability of neutron diffraction in determining the nature and extent of magnetic ordering is illustrated for the intermetallic compounds, Y6(Fe,Mn)23, and ErFe3. Substitution with other 3d transition metals influences the Fe-Fe exchange forces such as to alter, sometimes considerably, the magnetic properties, e.g., local site magnetic anisotropies in Er(Fe,Ni)3 and thermal expansion anomalies in the R2 (Fe,Co)14B compounds. When the 3d atoms are near neighbors in the periodic chart, their nuclear scattering lengths for neutrons are sufficiently different to permit the detection of preferential occupation of the several nonequivalent crystallographic 3d metal sites, i.e., atomic ordering, present in the R6M23, and R2Fe14B structures.