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

The impact of stress-driven structural transitions and of film strain on the magnetic properties of nm ferromagnetic films is discussed. The stress-induced bending of film-substrate composites is analysed to derive information on film stress due to lattice mismatch or due to surface-stress effects. The magneto-elastic coupling in epitaxial films is determined directly from the magnetostrictive bending of the substrate. The combination of stress measurements with magnetic investigations by the magneto-optical Kerr effect (MOKE) reveals the modification of the magnetic anisotropy by film stress. Stress-strain relations are derived for various epitaxial orientations to facilitate the analysis of the substrate curvature. Biaxial film stress and magneto-elastic coupling coefficients are measured in epitaxial Fe films in situ on W single-crystal substrates. Tremendous film stress of more than 10 GPa is measured in pseudomorphic Fe layers, and the important role of film stress as a driving force for the formation of misfit distortions and for inducing changes of the growth mode in monolayer thin films is presented. The direct measurement of the magneto-elastic coupling in epitaxial films proves that the magnitude and sign of the magneto-elastic coupling deviate from the respective bulk value. Even a small film strain of order 0.1% is found to induce a significant change of the effective magneto-elastic coupling coefficient. This peculiar behaviour is ascribed to a second-order strain dependence of the magneto-elastic energy density, in contrast to the linear strain dependence that is valid for bulk samples.

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The correlation between mechanical stress and magnetic anisotropy in ultrathin films

Multipolar interactions in f -electron systems: The paradigm of actinide dioxides

The secular determinants obtained in the previous paper are solved for the energy levels which are important in the absorption spectra of the normal complex ions, leaving the crystalline field strength as a parameter. The values of B and C (Racah's parameters) there needed are determined from the observed spectra of free ions or in some cases by extrapolation. The f -values of the transitions which connect the energy levels calculated are estimated and compared with the observed intensities. The difference of the spectral width among absorption bands and lines is also considered using the energy diagram obtained. Following the assignments determined by the above considerations, the calculated positions of lines and bands are rather in good agreement with the experimental data in divalent ions [MX 6 ] 2+ (M=Cr, Mn, Fe, Co, Ni), when we adjust the crystalline field parameter D q suitably. In trivalent ions [MX 6 ] 3+ (M=Ti, V, Cr, Mn, Fe), it is necessary besides the adjustment of D q to use smaller values ...

On the absorption spectra of complex ions. II

A theory of the antiferro-quadrupolar ordering in CeB 6 is developed in the framework of the Γ 8 -quartet basis. Possible ordered phases under the external magnetic field are classified in terms of their symmetry properties, and the relations between quadrupolar order parameters and field-induced moments are established. Then we formulate a mean-field theory of Ohkawa's RKKY model based on the group-theoretical consideration. The stability of the possible phases and the resulting phase diagrams are analyzed in connection with the character of quadrupolar moments, the influence of induced moments and the form of Zeeman term. Considering available experimental results, we introduce an improved model with asymmetric interactions and discuss the phase diagram of CeB 6 . The connection between magnetostriction and uniform quadrupolar moments is discussed.

Magnetic-Field Effects on Quadrupolar Ordering in a Γ8-Quartet System CeB6

Magnetostriction and internal stresses in thin films: the cantilever method revisited

Magnetic and quadrupolar properties of PrPb3

The magnetoelastic properties of the ferromagnetic cubic compounds of heavy rare earths with zinc ($R\mathrm{Zn}$) are investigated. Lattice distortions occuring below the ordering point are particularly strong when tetragonal: For instance, DyZn exhibits at low temperature a spontaneous tetragonal strain expressed as $\frac{c}{a}\ensuremath{-}1=+8.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$. All of these magnetoelastic properties are chiefly due to the crystalline electric field splitting of the ground-state $J$ multiplet. But the tetragonal distortion observed in GdZn below the ordering point reveals pseudodipolar magnetoelastic effects to be important also; this contribution is taken into account for all the compounds. From our strain gauges and capacitance dilatometer experiments we deduce the one-ion and two-ion magnetoelastic coefficients across the series. The former ones are compared with determinations from ultrasonic measurements and their variation through the series is interpreted in terms of crystal-field theory. Finally, an attempt was made to connect these coefficients to the present knowledge of the conduction band in these CsCl-type intermetallic compounds.

E. du Tremolet de Lacheisserie

Papers

Magnetostriction and internal stresses in thin films: the cantilever method revisited

Magnetic and quadrupolar properties of PrPb3

Magnetoelastic properties of R Zn equiatomic compounds

Magnetoelastic properties of amorphous alloys

From bulk to film magnetostrictive actuators