Understanding of the enhanced coercivity and energy product in Sm(Co_balFe_0.27Cu_0.05Zr_0.02)_z permanent magnets with lower z value

2:17-type Sm(CoFeCuZr)_z based permanent magnets has been widely studied due to its high Curie temperature, good magnetic property and better corrosion resistance. Magnets with high Fe content exhibiting enhanced remanence (B_r) have attracted great attention due to the low cost. However, especially for high temperature magnets with small z values (i.e., z < 7.1), the room temperature low coercivity and limited energy product ((BH)_max < 30 MGOe) has limited their application. In this work, a large room temperature (BH)_max∼ 32.47 MGOe with H_cj∼20.06 kOe was achieved in a Fe-rich Sm(Co_balFe_0.27Cu_0.05Zr_0.02)_7.07 magnet. Through a comparative study between samples with z = 7.07 and 7.47, the enhanced coercivity and energy product with lower z value can be understood by more pinning sites and stronger pinning strength provided by 1:5 H cell boundary phase. A large volume fraction of 1:5 H phase and smaller magnetic domains with more zig-zag sawtooth prove that there exist more pinning sites. While the large phase difference between two adjacent magnetic domains around domain walls in MFM results directly proves the stronger pinning strength in z = 7.07 sample, which is related to the higher Cu concentration in the 1:5 H cell boundary. Our work may shed lights on how to improve the magnetic performance of high temperature magnets with high Fe concentration.