Crystal-field effects and the magnetic properties of rare-earth rhodium borides

The magnetic properties of the $R{\mathrm{Rh}}_4{\mathrm{B}}_4$ ($R$ represents rare-earth metal) compounds are discussed in terms of a combined crystalline-electric-field and molecular-field approach. Magnetization data taken on single-crystal Er${\mathrm{Rh}}_4$ ${\mathrm{B}}_4$ in five orientations are used to obtain the crystal-field and molecular-field coefficients, resulting in a complete analysis for the paramagnetic properties of this compound. The crystal-field parameters are additionally used to discuss all previously published magnetic data for polycrystalline $R{\mathrm{Rh}}_4{\mathrm{B}}_4$ compounds. For these highly anisotropic materials, the effect of preferential crystallite orientation on the observed data is pointed out. Consideration of the variation of the magnetic transition temperature $T_m$ between different constituent rare earths has verified a previous conclusion that the crystal field has a pronounced effect on $T_m$, but this alone does not fully explain the observations.