Lanthanide exchange fields and intermultiplet transitions in permanent-magnet materials
Résumé
Intermultiplet (spin-orbit) transitions in lanthanides, observed by means of high-energy inelastic neutron scattering are well-suited for the determination of exchange field H$_{ex}$ in permanentmagnet materials. The situation is particularly favorable in Nd-based magnets, where H$_{ex}$ is given by µB$_{ex}$ = 143/296 (E$_{inter}$ − ∆$_{so}$), Einter being the observed transition energy and ∆$_{so}$ the spin-orbit splitting between the barycenters of the two lowest-lying multiplets. The latter can be regarded as a known atomic constant, ∆$_{so}$ = 232 ± 1 meV. A more accurate value of ∆$_{so}$ is obtainable by way of relativistic densityfunctional calculations, however, it proves material-dependent. Thus, ∆$_{so}$ = 231.4 meV for Nd2Fe14B. Two obstacles in the way of the direct determination of H$_{ex}$ are J-mixing and crystal electric field. In order to allow for the J-mixing, approximate formulas are proposed that enable a definitive solution of the problem. By contrast, crystal-field corrections have to be treated on a case-by-case basis.
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