Magnetic properties, phase evolution, and microstructure of melt spun Hf-substituted Sm(Co0.97Hf0.03)xCy (x = 5–9; y = 0–0.1) ribbons quenched at the wheel speed of 40 m/s are investigated. X-ray diffraction analysis shows that the main phases existed in Sm(Co0.97Hf0.03)x ribbons are 1:5 phase for x = 5–5.5; 1:5 and 1:7 phases for x = 6; 1:7 phase for x = 6.5–7.5; 1:7 and 2:17 phases for x = 8; and only 2:17 phase for x = 8.5–9, respectively. For Sm(Co0.97Hf0.03)x (x = 5–9) ribbons, the optimum magnetic properties of Br = 5.6 kG, iHc = 15.6 kOe and (BH)max = 7.1 MGOe are obtained for Sm(Co0.97Hf0.03)6.5 ribbons. Furthermore, a slight amount of C addition in Sm(Co0.97Hf0.03)x ribbons slightly modify phase constitution and effectively refine the grain size from 200–700 nm for C free ribbons to 10–70 nm, strengthening the exchange coupling effect between magnetic grains of the ribbons. As a result, magnetic properties are further improved. The magnetic properties of Br = 6.9 kG, iHc = 9.2 kOe and (BH)max = 10.0 MGOe can be achieved for Sm(Co0.97Hf0.03)7.5C0.1 nanocomposites.
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