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Cr-doped AlN thin films were epitaxially grown on Al2O3(001) substrates at low temperature by reactive magnetron sputtering,and their magnetic properties were investigated. Extensive x-ray diffraction studies indicated that the films have a wurtzite-type hexagonal structure and are (001) oriented, with an epitaxial relationship of the  direction of the films along the  direction of Al2O3. The c axis lattice parameter of the films showed a linear dependence on the Cr concentration for Cr concentrations below 0.15. Room temperature ferromagnetism was observed, and the magnetic properties showed strong dependence on the Cr concentration over a wide range.