Size dependence of Curie temperature in Co+ ion implanted 14ZnO nanowires

ZnO nanowires with different diameters were grown on Si substrates, then implanted with 80 keV Co+ ions at a dose of 3 x 10(16) cm(-2), and subsequently annealed at 700 degrees C for 5 min. No Co precipitates appeared in the ZnO nanowires according to the results observed using a high-resolution transmission electron microscope, revealing that the magnetism of the ZnO nanowires was independent of the Co precipitates. The RKKY theory could be used to explain the phenomenon. Hysteresis was observed at 10 K for all samples of ZnO nanowires with different sizes. The Curie temperature decreased (from above 300 to 70 K) with the reduction in the ZnO nanowires' diameters. The mechanism of this phenomenon is also discussed with the RKKY theory. It is very valuable to show that the quantum-confined effect appears gradually with decreasing size of the nanowires.