Effect of Co and W additions on the structure and magnetic properties of Nd2Fe14B/alpha-Fe nanocomposite magnets

The effect of Co addition combined with W on the microstructure and magneti c properties of nanocomposite magnets with composition of (1 - x wt%)Nd8.55 Fe84.49W0.60 B-0.636 + x wt% CO, prepared by mechanical alloying, has been investigated. The addition of W can increase the unit cell volume of soft a nd hard phases in nanocomposite: magnets, and cause the anisotropy field of the hard phase component to increase. Therefore, the addition of W can imp rove coercivity. On the other hand, addition of Co leads to a decrease of t he grain size of the alpha-Fe soft phase component in nanocomposite magnets annealed at temperatures lower than 670 degrees C. It leads further to a d ecrease of the optimal annealing temperatures, an increase of the remanence and maximum magnetic energy product, and a slight decrease of the coercivi ty. With increasing the content of Co, the dependence of remanence J(r) on annealing temperature T-a changes from a convex shape, giving a maximum, to a monotonically decreasing one. These phenomena have been explained in ter ms of the variation of grain growth and exchange coupling between the grain s, due to the addition of Co and W.