Periodic magnetic nanodot arrays have been produced on an area as large as 1 cm×1 cm by direct nanolithography using interferometric laser radiation. The dots are formed by the local annealing of sputtered amorphous Co–C films in regions where the laser intensity is highest. At room temperature the dots exhibit ferromagnetic order and are embedded in a paramagnetic matrix. The onset of room-temperature ferromagnetism is caused by nanoscale chemical and morphological changes during dot formation and reflects the phase separation of magnetic Co-rich clusters. The present single-step nanolithography is potentially an efficient method for fabrication of patterned magnetic arrays. © 2001 American Institute of Physics.