Influence of a large amount of Co substitution on the magnetic properties of NdFeCoGaB magnets (abstract)

We have found that a NdFeCoGaB sintered magnet containing 30 at. % Co still shows high coercive force in spite of such a large amount of Co substitution. The Curie temperature of the magnet is higher than that of the ternary NdFeB magnet by 280°C. As a result, the following magnetic properties have been attained for Nd_14.5Fe₄₆Co_{3 0}Ga₁B_8.5: T_c=590°C, B _r =11.7 kG, H_ci =14.3 kOe, and (BH)_max=32 MG Oe. Furthermore, in this magnet there exists an additional phase, Nd ₁(FeCoGa)₄B₁ with Ce₁Co ₄B₁ structure, which, as far as authors know, has not been reported to exist in the NdFeB-based magnets. In this study we investigated the magnetic properties and microstructure of Nd_14.5Fe _76-xCo_xGa₁B_8.5 (x=16-50) sintered magnets. The high coercive force can be obtained in the Co-content region from 16 to 30 at. %. In particular, the magnet with 30 at. % Co shows coercive force as high as 14.3 kOe. However, further substitution of Co drastically deteriorates the magnetic properties. The coercive force of the magnet with 50 at. % Co is less than 1 kOe. From the metallographical point of view, the above-mentioned Nd₁(FeCoGa)₄B₁ phase is not observed in the magnets containing less than 30 at. % Co. This phase abruptly appears in the magnet with 30 at. % Co and its amount increases with increasing Co content. The strongest x-ray-diffraction peak observed in the magnet with 50 at. % Co comes from the Nd₁(FeCoGa)₄B₁ phase. The demagnetization-curve measurements suggest that reverse magnetic domains are nucleated in the Nd₁(FeCoGa)₄B₁ phase at a low reverse magnetic field. It is noteworthy that the magnet with 30 at. % Co maintains the high coercive force in spite of the existence of such a soft magnetic phase. Details will be discussed in the coming session.