In this study, the relationship between the microstructural and coercivity changes of Zn-bonded Sm-Fe-N magnets was investigated. A Zn-bonded Sm-Fe-N magnet prepared using a fine Zn powder with a low O content exhibited a high coercivity of 2.7 MA m−1, whereas a Zn-free magnet displayed a low coercivity of only 0.31 MA m−1. The raw commercial Sm-Fe-N powder contained an oxidized layer at the surface of the powder with a thickness of approximately 13 nm. In the Zn-free Sm-Fe-N magnet, O diffused into interior of Sm2Fe17N3 grains to generate coarse α-Fe grains with diameters of several tens of nanometers in the oxidized layer. In the high-coercivity Zn-bonded magnet, a Zn-rich region existed at the surface of the Sm2Fe17N3 grains. In the Zn-diffused region, Fe, Zn, Sm, and O were detected via transmission electron microscopy / energy-dispersive X-ray analysis, and this region contained fine grains of Γ-FeZn, α-FeZn, and Sm-O. The non-magnetic Γ-FeZn phase served to magnetically isolate the soft magnetic α-FeZn phase from the Sm2Fe17N3 phase, resulting in the Zn-bonded Sm-Fe-N magnet with high coercivity.
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