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

Nanophase hard magnets

A series of interstitial ternary nitrides R2Fe17Ny with 23<y<29 has been prepared for R=Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Lu and Y The nitrides have structures related to those of Th2Zn17 or Th2Ni17, but the unit cell volumes are 6-7% greater, and the magnetic ordering temperatures some 400 K higher than for the 2:17 parent compounds The Fe-Fe exchange interactions are more than doubled by nitrogenation, whereas the R-Fe exchange interactions are little changed All compounds exhibit easy-plane anisotropy at room temperature, except for Sm2Fe17Ny, which is easy-axis

https://iopscience.iop.org/article/10.1088/0953-8984/2/30/013/pdf

Magnetic properties of a new series of rare-earth iron nitrides: R2Fe17Ny(y approximately 2.6)

Perspective and Prospects for Rare Earth Permanent Magnets

Replacement of Dy and substitution of Nd in NdFeB-based permanent magnets by Ce, the most abundant and lowest cost rare earth element, is important because Dy and Nd are costly and critical rare earth elements. The Ce, Co co-doped alloys have excellent high-temperature magnetic properties with an intrinsic coercivity being the highest known for T ≥ 453 K.

Cerium: An Unlikely Replacement of Dysprosium in High Performance Nd–Fe–B Permanent Magnets

Magnetic properties of the series of ThMn12-structure intermetallic compounds R(Fe11Ti) have been determined for rare earths from Nd to Lu plus Y. The highest Curie temperature (607 K) is for R=Gd, and R-Fe exchange interactions are much stronger for light rare earths than for heavy ones. The temperature dependence of the iron sublattice magnetisation and anisotropy are determined for the Y and Lu compounds. Spin reorientation transitions are found as a function of temperature for the rare earths with a negative second-order Stevens coefficient alpha J(Nd, Tb, Dy), and a set of crystal-field parameters is derived to account for the transitions in a consistent way. A sharp increase in magnetisation observed for Sm(Fe11Ti) below 130 K in a field of about 10 T applied perpendicular to the easy direction indicates that J-mixing may be important for Sm3+. Compared with R2Fe14B, the iron anisotropy in R(Fe11Ti) is greater, and the rare-earth anisotropy is much weaker at low temperature, with the opposite sign for the rare-earth crystal-field coefficient A20. The average iron moment is 1.7 mu B in R(Fe11Ti) at 4.2 K; Mossbauer spectra are analysed to yield the average moments on each site. Limits set by the intrinsic magnetic properties on the performance of magnets made from these families of alloys are discussed.

https://iopscience.iop.org/article/10.1088/0953-8984/1/4/009/pdf

Intrinsic magnetic properties of the iron-rich ThMn12-structure alloys R(Fe11Ti); R=Y, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm and Lu

Magnetization curves have been measured in the temperature range from 4.2 to 300 K along the (100), (110), and (001) directions of a Dy(Fe{sub 11}Ti) single crystal in fields up to 7 T. The magnetic moment is along (100) below 58 K and parallel to the {bold c} axis above 200 K. Between the two spin-reorientation transitions (first order at 58 K, second order at 200 K) there is a canted spin structure where the net magnetization lies in a (010) plane and is inclined at an angle to the {bold c} axis. Three first-order magnetization processes are observed as a function of applied field below 150 K. All the data are used to derive a set of five crystal-field coefficients at the single rare-earth site of the ThMn{sub 12} structure: {ital A}{sub 20}={minus}32.3 K{ital a}{sub 0}{sup {minus}2}, {ital A}{sub 40}={minus}12.4 K{ital a}{sub 0}{sup {minus}4}, {ital A}{sub 44}=118 K{ital a}{sub 0}{sup {minus}4}, {ital A}{sub 60}=2.56 K{ital a}{sub 0}{sup {minus}6}, {ital A}{sub 64}=0.64 K{ital a}{sub 0}{sup {minus}6}. Spin reorientation observed in the other members of the {ital R}(Fe{sub 11}Ti) ({ital R}==rare earth) series, except those in Tb(Fe{sub 11}Ti), are explained by the same crystal-field coefficients.

Magnetization of a Dy(Fe11Ti) single crystal.

The R-Fe-B (R, rare earth) sintered magnets prepared with different ratio of alloys of MM-Fe-B (MM, misch-metal) and Nd-Fe-B by dual alloy method were investigated. As expected, the high ratio of MM-Fe-B alloy degrades the hard magnetic properties heavily with intrinsic coercivity lower than 5 kOe. When the atomic ratio MM/R ≤ 21.5% the magnetic properties can reach a practical level of Br ≥ 12.1 kGs, Hcj ≥ 10.7 kOe, and (BH)max ≥ 34.0 MGOe. And the effect of Hcj enhancement by the grain boundary diffusion process is obvious when MM/R ≤ 21.5%. It is revealed that the decrement of intrinsic magnetic properties of R2Fe14B matrix phase is not the main reason of the degradation of the magnets with high MM ratio. The change of deteriorated microstructure together with phase component plays fundamental roles in low Hcj. In high MM ratio magnets, (a) after annealing, Ce atoms inside main phase are inclined to be segregated in the outer layer of the main phase grains; (b) there is no thin layer of Ce-rich phase as an analogue of Nd-rich phase to separate main phase grains; (c) excessive Ce tends to form CeFe2 grains.

Achievement of high coercivity in sintered R-Fe-B magnets based on misch-metal by dual alloy method

57Fe Mossbauer spectra have been obtained at room temperature and at 15 K for R2Fe17 and R2Fe17N3- delta ( delta approximately=0.4) with R identical to Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Lu and Y. The easy direction of magnetization of the R2Fe17 compounds can be identified by analysis of the hyperfine field and quadrupole interactions. For rare-earth elements with J>3, the sixth-order crystal-field parameter B66 determines the easy magnetization direction in the basal plane. In the nitrides, the average hyperfine field at 15 K increases by approximately 13% above the value in the parent compounds, from about 31 to 35 T, and it is anisotropic, being greatest when the magnetization lies along the c-axis. There is also an increase in the isomer shift of 0.12(2) mm s-1. Spin reorientation transitions occur in the nitrides of Er and Tm; only the samarium nitride has an easy c-axis at room temperature, but Sm, Er and Tm all have easy c-axis magnetic structures at 15 K.

A 5e Mossbauer study of a new series of rare-earth iron nitrides: R2Fe17N3- delta

A series of compounds with the ${\mathrm{ThMn}}_{12}$-type structure, R${\mathrm{Fe}}_{11.35}$${\mathrm{Nb}}_{0.65}$ (R=Y, Sm, Gd, Tb, Dy, Ho, Er, and Lu), were synthesized. The corresponding nitrides, obtained by gas-solid reactions, retained the same structure as their parent compounds, but with a relative volume expansion of 3%. The Nb atoms occupy the 8i sites in the ${\mathrm{ThMn}}_{12}$ structure. The highest Curie temperatures are 597 K and 773 K for ${\mathrm{GdFe}}_{11.35}$${\mathrm{Nb}}_{0.65}$ and its nitride, respectively. The average iron magnetic moment ${\mathrm{\ensuremath{\mu}}}_{\mathrm{Fe}}$ at T=1.5 K is 1.9${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$ for parent compounds and 2.1${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$ for the nitrides. By the introduction of nitrogen, the Fe-Fe exchange interaction is much strengthened while the R-Fe exchange interaction is slightly weakened. The enhancement of ${\mathrm{\ensuremath{\mu}}}_{\mathrm{Fe}}$ by the influence of interstitial nitrogen is about 11%. The Fe sublattices of R${\mathrm{Fe}}_{11.35}$${\mathrm{Nb}}_{0.65}$ compounds and their nitrides have an easy c-axis anisotropy. At T=1.5 K, the anisotropy constant ${\mathit{K}}_{1}$(Fe) is 25.7 K/f.u. for the parent compounds and 21.9 K/f.u. for the nitrides. The reduction of ${\mathit{K}}_{1}$(Fe) by the influence of interstitial nitrogen is about 17%. Spin-reorientation transitions were observed in ${\mathrm{DyFe}}_{11.35}$${\mathrm{Nb}}_{0.65}$ and ${\mathrm{ErFe}}_{11.35}$${\mathrm{Nb}}_{0.65}$ compounds. The complex magnetic behavior of R${\mathrm{Fe}}_{11.35}$${\mathrm{Nb}}_{0.65}$ at low temperatures can be explained by exchange and crystal-field-interaction models. The ${\mathrm{SmFe}}_{11.35}$${\mathrm{Nb}}_{0.65}$ compound has an anisotropy field of 10.0 T at room temperature. Its high Curie temperature and magnetization make it a potential candidate for applications as a permanent magnet.

Bo-Ping Hu

Papers

Intrinsic magnetic properties of the iron-rich ThMn12-structure alloys R(Fe11Ti); R=Y, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm and Lu

Magnetization of a Dy(Fe11Ti) single crystal.

Achievement of high coercivity in sintered R-Fe-B magnets based on misch-metal by dual alloy method

A 5e Mossbauer study of a new series of rare-earth iron nitrides: R2Fe17N3- delta

Structure and magnetic properties of RFe11.35Nb0.65 and RFe11.35Nb0.65Ny (R=Y, Sm, Gd, Tb, Dy, Ho, Er, and Lu).