A review is given of the formation, the crystal structure and the magnetic properties of several classes of rare earth based intermetallic compounds that lend themselves as starting materials of permanent magnets. These compounds include R2Fe14B, R2Fe14C and R2Co14B and the large class of ternary rare earth compounds having the tetragonal ThMn12 structure. Special emphasis is given to the changes in magnetic properties of R2Fe17 compounds observed after interstitial solution of C or N atoms. The magnetic properties of all these compounds are discussed in terms of current models based on intersublattice and intrasublattice exchange and the interplay between the rare earth sublattice anisotropy and 3D sublattice anisotropy. A substantial portion of the review is devoted to manufacturing routes of permanent magnets and a description of the coercivity mechanisms operative in the magnets. A comparison is made of the performance and economic costs of various types of magnets and novel applications are briefly discussed.

https://iopscience.iop.org/article/10.1088/0034-4885/54/9/001/pdf

New developments in hard magnetic materials

A comprehensive review of the Nd–Fe–B permanent magnet material is given. A historical survey, the basic magnetic properties of Nd2Fe14B and isomorphous compounds, a phase diagram for the ternary Nd–Fe–B system, processing techniques and magnetic properties of sintered Nd–Fe–B magnets, and the future directions for improvements of this type of permanent magnet are discussed.

https://iopscience.iop.org/article/10.1143/JJAP.26.785/pdf

Nd–Fe–B Permanent Magnet Materials

New permanent magnet materials

The coercive field of sintered and melt-spun NdFeB magnets

A review is given on the basic properties of 2:14:1 hard magnetic phases as well as technological procedures used for manufacturing R-Fe-B and R-Fe-C permanent magnets. First, we analyse the phase diagrams, crystal structure of hard magnetic phases and composition ranges in which these phases form solid solutions, as a result of various types of substitution. Then, the magnetic properties of and -based compounds are described. The routes used for manufacturing R-Fe-B and R-Fe-C permanent magnets are detailed. The magnets' microstructure as well as their coercivities are described in correlation with composition and manufacturing routes. The properties of nanostructure magnets are then presented. Finally, the stability of Nd-Fe-B and Nd-Fe-C magnets in various working conditions is analysed.

Permanent magnets based on R-Fe-B and R-Fe-C alloys

Among the first crystallized melt quenched alloys to exhibit interesting hard magnetic properties was (Fe0.82B0.18)0.9Tb0.05La0.05. The high coercive force of this alloy results from a mixture of R6Fe23 and Fe3B phases with a very fine grained microstructure (∼300 A). The hard magnetic state only exists over a narrow range of annealing temperatures due to phase decomposition and grain growth at higher temperatures. Recent results on alloys with compositions corresponding to varying ratios of R6Fe23 and Fe3B have suggested a new magnetically hard ternary phase near the composition R0.15Fe0.783B0.067. In contrast to earlier alloys, the hard magnetic state of this material remains essentially unchanged over a rather wide range of annealing temperatures up to at least 800 °C. When R is mostly Nd or Pr, isotropic magnets with energy products as high as 13 MGOe can be produced.

N. C. Koon

Papers

Crystallization of FeB alloys with rare earths to produce hard magnetic materials (invited)