We report on the preparation and detailed characterization of ferromagnetic (FM) Zn1-xCrxO thin films deposited on Si substrates using reactive co-sputtering of Cr and Zn in controlled oxygen atmosphere. X-ray diffraction (XRD) data showed wurtzite ZnO peaks in the FM films prepared with lower Cr sputter powers, whose position and intensities are influenced by Cr doping. However, samples prepared with higher Cr powers did not show ferromagnetism but displayed evidence of Cr2O3 and ZnCr2O4 phases with no zinc oxide (ZnO) phase. The magnetization is higher (saturation magnetization M-s = 18 emu/cm(3)) for lower Cr concentrations and decreases for higher Cr doping. The samples were investigated extensively to understand the film composition, dopant distribution, homogeneity and potential origin of the observed ferromagnetism. Particle-induced X-ray emission (PIXE) studies were employed to determine the chemical composition as well as the Cr/Zn ratio in the films. Film uniformity and homogeneity, investigated using Rutherford backscattering spectrometry, showed a relatively uniform ZnO layer in the as-prepared samples but, in a sample annealed at 800 degrees C, showed some diffusion of Si from the substrate. X-ray photoelectron spectroscopy (XPS) studies indicated that Cr ions are in the oxidized state, but showed changes in the binding energy and Cr concentration when measured after removing 10 nm from the surface using Ar ion sputtering. Possible origins of the observed FM behavior are discussed based on the comprehensive characterization results. (c) 2007 Elsevier B.V. All rights reserved.