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 degrees C and postannealed at T(a) = 550 degrees C. With increasing of T(s) upto 500 degrees C, the maximum value of H(c) = 29.2 kOe was obtained at T(a) = 550 degrees 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.