The influence of buffer/capping-layer-mediated stress on the coercivity of NdFeB films

The influence of buffer/capping-layer-mediated stress on the coercivity of NdFeB films is demonstrated. NdFeB films, 5 μm thick and rich in Nd, were deposited on Si/SiO2 and Al2O3 substrates, with or without buffer/capping layers of Ta. The coercivity of Ta-free samples (∼0.5 T) is significantly less than that achieved in samples with Ta present as a buffer and/or capping layer (1.7–1.8 T). The as-sputtered Ta layers are under strong compressive stress due to peening. During post-deposition annealing to crystallize the Nd2Fe14B phase, stresses are relieved in the Ta layer. This leads in turn to a compressive stress in the NdFeB layer, inducing extrusion of a Nd-rich phase up through the NdFeB layer. The high values of coercivity achieved in Ta-containing structures are attributed to good coverage of individual Nd2Fe14B grains with the redistributed Nd-rich phase.