Effects of the Hf content in Co-Hf-Ta thin films on the microstructure and magnetic properties were investigated in this study. It was found that appropriate Hf addition can effectively refine the Co grain size. Co grain sizes sharply decreased from 50nm down to 2.3nm with increasing the Hf content from 1.02 at.% to 2.81 at.%, leading to the reduced magneto-crystalline anisotropy. The Co-Hf-Ta thin films with small Co grains reveal low anisotropy field, low coercivity, and high resistivity. By optimizing the Hf content, the film with Hf concentration of 2.81 at.% exhibits excellent soft magnetic properties: high saturation magnetization (4 pi M-S similar to 13.6 kG), and low coercivity (H-C similar to 0.6 Oe). The effective permeability of the film reaches 800 and remains constant up to 1 GHz. (C) 2010 Elsevier B.V. All rights reserved.