Size Dependent Magnetization and High-Vacuum Annealing Enhanced Ferromagnetism in Zn1−x Co x O Nanowires
Diameter controllable ZnO nanowires have been fabricated by thermal evaporation (vapor transport) with various sizes of gold nanoparticles as catalysts. Diluted magnetic semiconductor (DMS) Zn1−x Co x O nanowires were then made by high energy Co ion implantation. The as-implanted and the argon-annealed Zn1−x Co x O nanowires displayed weak ferromagnetism while the high-vacuum annealed nanowires exhibited strong ferromagnetic ordering at room temperature. Size dependent behavior has been observed in the magnetic field and temperature dependences of magnetization. The shrinkage of the nanowire diameter reduced the spontaneous magnetization as well as the hysteresis loops. Field cooled and zero-field cooled magnetization and coercivity measurements were performed between 2 and 300 K to study the evolution of magnetism from the weak to the strong ferromagnetic states. In particular, superparamagnetic features were observed and shown to be intrinsic characteristics of the DMS Zn1−x Co x O nanowires. The room-temperature spontaneous magnetization of individual Zn1−x Co x O nanowires was also established by using magnetic force microscope measurements.
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Document Type: Research Article
Publication date: 2008-01-01
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