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A large coercivity and anisotropy enhancement in perpendicular NdDyFeB (120 nm)/Dy (t(Dy)) films has been realized by a Dy grain-boundary diffusion process. The coercivity H-C and the ratio M-r/M-s reach their maxima at t(Dy) 50 nm, and the magnetic domain sizes increase with increasing t(Dy). The H-C and M-r/M-s increasing with t(Dy) is due to the enhancement of the anisotropy of (Nd, Dy)(2)Fe14B grains by Dy substitution for Nd. The coercivity mechanism is a nucleation-type mechanism. Dy and Nd elements coexist at grain boundaries, forming a (Nd, Dy)-rich phase, which may promote the nucleation of reversal domains. (C) 2012 American Institute of Physics. [doi: 10.1063/1.3679074]