Effects of Hf and CNTs on structure and magnetic properties of TbCu7-type Sm–Co magnets

SmCo7, SmCo6.9Hf0.1, SmCo6.9Hf0.1(CNTs)0.05 and SmCo6.9Hf0.1(CNTs)0.1 as-cast alloys and ribbons melt-spun at the speed of 40 m/s are prepared. The X-ray diffractometer, electron microscope and magnetometer are employed to investigate the effects of Hf and CNTs on structure and magnetic properties of the as-cast alloys and ribbons. The results show that Hf and CNTs additions in SmCo7 alloys can stabilize TbCu7 structure. Hf addition can form HfCo2 grain boundary phase. Fine dispersed Hf(CNTs) grain boundary phase can be derived from simultaneous addition of Hf and CNTs. The four types of ribbons all consist of TbCu7-type Sm(Co,Hf)7 main phase. Cooling rate is the key factor of forming TbCu7-type phase. It is attempted that the grain boundaries with CNTs addition mainly consist of Sm(Co,Hf)7(CNTs)x. The coercivity, remanence and remanence ratio of the ribbons all increase with Hf and CNTs additions. The comprehensive magnetic properties of SmCo6.9Hf0.1(CNTs)0.05 ribbons are Hci = 13.95 kOe, Mr = 48.85 emu/g, M2T = 61.09 emu/g and Mr/M2T = 0.80. Excessive CNT addition may worsen the magnetic properties.