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/SiO_2 and Al_2O_3 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 Nd_2Fe_(14)B 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 Nd_2Fe_(14)B grains with the redistributed Nd-rich phase.
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