Effect of Heat Treatment on the Magnetic Properties for Nd–Fe–B/Nd–Cu Multilayer Films

The control of the grain boundary phase is of importance for understanding the coercivity mechanism of Nd-Fe-B magnets. In this study, in order to see the effect of Cu on the magnetic properties for Nd-Fe-B thin films, alternate deposition of Nd-Fe-B and Nd-Cu layers has been performed and the effects of the deposition temperature T_s and the postannealing temperature T_s have been investigated. With increasing T_s, the squareness of the magnetization curve was improved and also remarkable change of the initial magnetization curve was observed. Large coercivity H_c of 28.7 kOe was obtained for the [Nd-Fe-B(6 nm)/Nd-Cu(0.25 nm)]₂ thin film deposited at T_s = 300°C and postannealed at T_a = 550°C. With increasing of T_s upto 500°C, the maximum value of H_c = 29.2 kOe was obtained at T_a = 550°C. The possible reason for the enhancement of H_c and the squareness is thought to arise from a diffusion of Cu into the grain boundary phase, and hence, the domain wall is pinned at the grain boundary, since the initial magnetization curves were changed from a steep increase to a gentle one. These results suggest that the magnetization process of the films prepared by the alternate deposition of Nd-Fe-B and Nd-Cu layers followed by the appropriate heat treatment is proposed to be a “pinning-type” magnetization behavior.