Crystal-field analysis of the magnetization process in a series ofNd2Fe14B-type compounds

A systematic study of the magnetic properties of a series of $R_2$ ${\mathrm{Fe}}_{14}$B (R denotes a rare-earth element) compounds has been made using a combined molecular-field and crystalline-electric-field (CEF) approximation. The Hamiltonian for an R ion is (in units of ${\mu }_B$) ${\mathrm{scrH}}_R$=λL⋅S+${\mathrm{scrH}}_{\mathrm{CEF}}$ +2S⋅${\mathrm{H}}_m$+(L+2S)⋅H, where ${\mathrm{scrH}}_{\mathrm{CEF}}$ is the CEF Hamiltonian and ${\mathrm{H}}_m$ is the molecular field acting on the R ion due to the Fe-R exchange interaction. We have calculated the spin structure and magnetization curves using the Racah method, taking the excited J multiplets into account. The experimental results are explained satisfactorily in terms of almost the same CEF parameters. For the heavy-R compounds the Stevens method can be applied. The sixth-order CEF parameters are indispensable to explain the anomalous magnetization increase observed in the Sm compound and the first-order magnetization process (FOMP) observed in the Nd and Pr compounds. The excited J multiplets are shown to have an appreciable influence on the FOMP.