Chemical segregation mechanism and magnetic properties of Sm2Co17-based permanent magnets: experimental investigation and first-principles calculations

Abstract

The magnetic properties and chemical segregation have led to an increased interest in Sm2Co17-based magnets. This investigation employs experiments and first-principles calculations to obtain more profound comprehension of segregation and coercivity mechanism. The Sm(CobalFe0.25Cu0.06Zr0.02)7.5 alloy exhibits a chemically heterogeneous cellular structure with Fe-rich 2:17R cells (60–70 nm) and Fe- and Cu-rich 1:5 cell boundaries (8–10 nm), and achieves excellent magnetic properties with a Br of 11.69 kGs, an Hcj of 30.96 kOe and a (BH)max of 33.0 MGOe. First-principles calculations are performed on Sm6Co51–xMx and SmCo5–yMy (M = Fe, Cu) unit cells to identify their substitution energy (Es) and electronic structure. In Sm6Co51–xMx, Fe atoms preferentially occupy the 6c and 18f Co sites, whereas Cu replacement is unlikely to occur due to its positive Es values. The segregation of Fe into the 2:17R structure is due to the decrease in the total density of states (DOS) at the Fermi level (EF) and the introduction of Fe-Co covalent-like bonding. By comparison, Fe and Cu atoms are both helpful for stabilizing the 1:5 phase, with Cu atoms first occupying the 2c site (Es = −1.697 eV/f.u.) and Fe atoms first occupying the 3g site (Es = −1.627 eV/f.u.). This small energy difference further explains the existence of Fe atoms in the 1:5 phase during ageing, which could significantly increase the magnetization. The segregation of Cu to the 1:5 phase is attributed to the formation of a pseudogap or bandgap at EF. Ultimately, we furnish an extensive elucidation of the commonly acknowledged domain wall pinning coercivity mechanism at ambient temperature according to SmCo5–yCuy cell boundary phases.