Microstructual properties of Co thin films grown on p-GaAs (100) substrates at room temperature by ion beam-assisted deposition were investigated. An atomic force microscopy image showed that the root mean square of the average surface roughness of the Co film was 32.2 Angstrom, and X-ray diffraction and selected area diffraction pattern measurements showed that Co film layers grown on GaAs (100) substrates were polycrystalline. A bright-field transmission electron microscopy image showed that the Co/p-GaAs (100) heterointerface grown at room temperature was sudden. These results provide important information on the microstructural properties for Co thin films grown on p-GaAs (100) substrates at room temperature. (C) 2004 Elsevier Ltd. All rights reserved.