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

New permanent magnet materials

Trends in structural and magnetic parameters are presented and discussed for Nd2Fe14−xTxB with T=Cr, Mn, Co, Ni, Al. Some data are also included on the Y analog systems. Partial Al substitution strongly increases c/a. We show that this has to do with preferential occupation of the larger atom (Al) in the σFe layer. Co and Ni substitutions increase Curie temperature (TC) while all others decrease it. Anisotropy constants K for 300 K decrease in all cases investigated compared to Nd2Fe14B. Some discussion is given to the general mode of preferential substitution and the concomitant effects on magnetic anisotropy.

Structural and magnetic properties of R2Fe14−xTxB (R=Nd, Y; T=Cr, Mn, Co, Ni, Al)

We have probed the spin reorientation transition in Er2Fe14B by neutron powder diffraction Our data and Rietveld analyses of them indicate that the transition is centered near 330 K, as magnetization measurements have shown, but extends over an interval of approximately 50 K It occurs by coherent rotation of the moments from the basal plane at low temperatures to the c axis above ∼350 K We find no evidence for any noncollinear arrangement of the Er and Fe moments

Neutron scattering studies of the spin reorientation in Er2Fe14B

We have measured the thermal expansion in ternary compounds of the type R2Fe14B (R  La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er and Lu) in the temperature range 4.2–1000 K. Data were also obtained above room temperature for Th2Fe14B. At temperatures well above the corresponding Curie temperatures we found a linear high temperature slope of Δ l l equal to α = 16 × 10−6 K−1, excepting Ce2Fe14B for which the slope was slightly higher (α = 20 × 10−6 K−1). The spontaneous volume magnetostriction derived from our data extends to temperatures considerably above the corresponding Curie temperatures. The spontaneous volume magnetostriction at 4.2 K ranges from 2.5% to 3.5% in the various compounds investigated and consists of contributions due to the iron sublattice and the rare earth sublattice.

F. Rothwarf

Papers

The anisotropy of NdFeB magnets