Exchange Coupling and Stability of SmCo_7-xHf_x

Electronic structure of SmCo_7-xHf_x compound is calculated by using the multi-scattering Xα method. It is shown that a few of electrons can transfer to the Sm 5d orbital due to orbital hybridization between Sm and Co atoms. The 3d-5d coupling is stronger, which is the main reason to result in the long-range ferromagnetic order between Sm and Co atoms in SmCo_7-xHf_x. According to the Stoner criterion, the result of spin-unpolarized calculation for the Sm₅Co₃₂Hf₂ cluster could lead to a better understanding of why the ferromagnetic SmCo_7-xHf_x is a stable phase. For the Sm₅Co₃₂Hf₂ cluster the Fermi level is situated at the overall maximum of the density of states. Moreover the cluster wavefunctions at E_F are antibonding and hence highly localized in real space, which would lead to a large value for the cluster Stoner integral. Thus a rationalization for the magnetic stability of SmCo_7-xHf_x has been obtained.