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

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

Water purification using magnetic assistance: a review.

Nanophase hard magnets

The tetragonal L10 family of ferromagnets including the Co-Pt, Fe-Pt, Fe-Pd and Mn-Al alloy systems exhibits high, uniaxial magnetocrystalline anisotropies with first anisotropy constants K1 ~ 107–108 ergs/cm3. The domain parameters of these materials are similar to the rare earth permanent magnets and the defect structure is dominated by planar faults such as APB's, stacking faults, and twins. In this study the micromagnetic analysis of Kronmuller has been applied to elucidate the mechanism of coercivity controlling the hysteresis of the polytwinned L10 FePd equiatomic alloy and the results are consistent with pinning control involving atomically thick, planar defects. Also, Lorentz microscopy has been used to observe the interaction of magnetic domain walls with the planar faults characteristic of these materials. Finally, a thermomechanical treatment involving concomitant ordering and recrystallization is shown to eliminate the well-known polytwinned structures which generally evolve during the formation of the L10 phase leading to enhanced coercivities. The fundamental structure-property relationships involved are discussed.

Magnetic hardening and coercivity mechanisms in L10 ordered FePd ferromagnets

Neodymium-iron-boron permanent magnets

Crystallization and magnetic properties of melt-spun neodymium-iron alloys

Temperature dependence of the magnetostriction of polycrystalline Y6Fe23, Ho6Fe23 and Er6Fe23

Permanent magnet research and technology have been propelled into a new era by the rare earth-iron-boron materials, R 2 Fe 14 B. Energy products surpassing all previous values have been attained in magnets based on Nd 2 Fe 14 B, the prototypical compound. In this review we place Nd-Fe-B in the historical context of permanent magnet evolution, summarize the intrinsic properties of the R 2 Fe 14 B phases, and discuss the properties of practical Nd-Fe-B magnets produced by the two methods in present commercial use.

J.J. Croat

Papers

Neodymium-iron-boron permanent magnets

Crystallization and magnetic properties of melt-spun neodymium-iron alloys

Temperature dependence of the magnetostriction of polycrystalline Y6Fe23, Ho6Fe23 and Er6Fe23

Neodymium-Iron-Boron Permanent Magnets