High-coercivity Nd–Fe–B thick films without heavy rare earth additions

Abstract

The magnetic properties and microstructures of two Nd–Fe–B thick films with different Nd contents have been studied. The films were deposited in the amorphous state and were crystallized by post-deposition annealing. Both films show a strong 〈001〉 fibre texture out-of-plane. The film with the higher Nd content has a large room temperature coercivity of 2.7T, while the one with the lower Nd content has a room temperature coercivity of only 0.7T. The difference in coercivity may be explained by the fact that the film with the higher Nd content exhibits a continuous Nd-rich grain boundary phase, giving better isolation of the Nd2Fe14B grains with respect to magnetic exchange interactions. The extrusion of Nd-rich liquid to the top surface of the film with high Nd content during post-deposition annealing led to the formation of ripples in the Ta capping layer, indicating that the films are under compressive stress. This stress-induced flow of the Nd-rich material up through the film explains the excellent distribution of the Nd-rich grain boundary phase. Atom probe tomography has revealed the presence of Cu in the Nd-rich grain boundary phase, explaining the formation of the liquid phase at the relatively low temperature of 550°C due to the eutectic reaction of Nd and Cu.