It is proposed that permanent magnets can be made of composite materials consisting of two suitably dispersed ferromagnetic and mutually exchange-coupled phases, one of which is hard magnetic in order to provide a high coercive field, while the other may be soft magnetic, just providing a high saturation J/sub s/, and should envelop the hard phase regions in order to prevent their corrosion. A general theoretical treatment of such systems shows that one may expect, besides a high energy product (BH)/sub max/, a reversible demagnetization curve (exchange-spring) and, in certain cases, an unusually high isotropic remanence ratio B/sub r//J/sub s/, while the required volume fraction of the hard phase may be very low, on the order of 10%. The technological realization of such materials is shown to be based on the principle that all phases involved must emerge from a common metastable matrix phase in order to be crystallographically coherent and consequently magnetically exchange coupled. >

The exchange-spring magnet: a new material principle for permanent magnets

Recycling of rare earths: a critical review

Metallic glasses as structural materials

A comprehensive review is presented of the extractive metallurgy of rare earths. The topics covered are: world rare earth resources and production; ore processing and separation of individual rare earths; reduction, refining, and ultrapurification of rare earth elements; methods for rare earth materials analysis; and a selection of the numerous rare earth applications. World rare earth reserves are abundant and would last for well beyond the next century. However, all of the 16 naturally occurring rare earth elements are not equally distributed in the ore minerals. This, compounded with the problems specific to the isolation and recovery of each of the rare earths, sets the stage for an unequal rare earth availability. The close chemical similarity of rare earths looses its importance when divergent physical properties determine the processes for rare earth element reduction and refining. The rare earth metals, alloys, and compounds have been as pure as could be determined. Finally, the commercial...

Extractive Metallurgy of Rare Earths

Determination of the crystal structure of ${\mathrm{Nd}}_{2}$${\mathrm{Fe}}_{14}$B, a new ternary phase, is reported. It has recently been demonstrated that permanent magnets having large coercivities and energy products can be formed from this phase, underscoring its potential technological importance. We relate the crystal structure and intrinsic magnetic properties by considering analogies with previously known rare-earth---transition-metal materials.

Relationships between crystal structure and magnetic properties in Nd 2 Fe 14 B

We report the properties of a new class of high‐performance permanent magnets prepared from Nd‐Fe‐B and Pr‐Fe‐B alloys Magnetic hardening is achieved by rapid solidification Energy products of these isotropic materials can exceed 14 MGOe with intrinsic coercivities of ∼15 kOe X‐ray and microstructural analyses indicate that the alloys exhibiting optimum characteristics are comprised of roughly spherical crystallites, strongly suggesting that the coercivity mechanism is of the single‐domain particle type The crystallites are composed of an equilibrium R‐Fe‐B intermetallic phase having tetragonal symmetry, and the stability of this phase with respect to other rare earths and other metalloids has been investigated

Pr‐Fe and Nd‐Fe‐based materials: A new class of high‐performance permanent magnets (invited)

We have explored the hard magnetic properties of melt‐spun Nd‐Fe‐B alloys. A maximum energy product of 14.1 MG Oe has been observed, the highest value ever reported for a light rare earth‐iron material. X‐ray analyses indicate that the alloys exhibiting optimum characteristics are comprised of roughly spherical crystallites of an equilibrium Nd‐Fe‐B intermetallic phase. The observed grain sizes are in or near the estimated single‐domain range, suggesting that the coercivity arises principally from the formation of single‐domain particles.

High‐energy product Nd‐Fe‐B permanent magnets

Rietveld analyses of room‐temperature neutron powder diffraction data for seven Nd2(CoxFe1−x)17 alloys (x = 0, 0.1, 0.3, 0.5, 0.7, 0.9, 1) are reported. In the ternary systems we find that the c‐type transition metal sites are preferentially occupied by Fe ions; concomitantly, the d and h sites have Co occupations larger than those predicted by stoichiometry, while the f sites deviate only slightly from random occupation. The magnetic moments obtained from the data refinements indicate a transition from a low to a high spin state on the d,  f, and h sites as Fe is incorporated into the structure; a reverse trend is exhibited by the moments on the c site.

R. W. Lee

Papers

Pr‐Fe and Nd‐Fe‐based materials: A new class of high‐performance permanent magnets (invited)

High‐energy product Nd‐Fe‐B permanent magnets

Neutron diffraction studies of Nd2(CoxFe1−x)17 alloys: Preferential site occupation and magnetic structure