Influence of Cu distribution on the coercivity of Sm(CobalFe0.25Cu0.06Zr0.02)7.5 magnets during microwave aging process

Abstract

In this study, we investigated the magnetic properties, microstructures, and Cu distribution on the mechanism of coercivity enhancement of Sm(CobalFe0.25Cu0.06Zr0.02)7.5 magnets during two-step and multistage microwave aging treatments. The intrinsic coercivity, Hcj, and maximum energy product, (BH)max, increase from 15.9 to 24.4 kOe and from 24.3 to 26.5 MGOe, respectively, upon transitioning from two-step to multistage aging. The aged alloys possess a cellular microstructure comprising rhombohedral 2:17 R cell phase and hexagonal 1:5 cell boundary phase after a short microwave aging time of 3 h. The multistage aged alloy exhibits a larger cell size, thicker cell walls, and higher Cu concentration in the 1:5 phase than the two-step aged sample. Elemental mapping reveals Cu atom enrichment at the cell boundary centers and at the 2:17 R/1:5 triple junction of the cell boundary phase, but depletion at the cell boundary edges during multistage aging. First-principles calculations indicate that Cu atoms occupy the Co(2c) sites of a SmCo5 unit cell and weaken not only the strong Co(2c)–Co(3g) direct exchange-coupling interaction, but also the exchange-coupling interaction between Sm 4f and Co 3d electrons, giving rise to low 1:5 phase exchange constant values. Furthermore, the enrichment of Cu atoms at the center of the cell wall and at the 2:17 R/1:5 triple junction of the cell boundary phase produces a gradual increase in the domain wall energy difference as a function of the edge-to-center distance in the cell boundary phase. This enrichment enhances the domain wall pinning strength and increases coercivity. A high concentration and optimal distribution of Cu are critical for achieving a high Hcj. The results of this study provide theoretical guidance to future studies on the mechanism controlling the coercivity of Sm2Co17-based permanent magnets.