Browsing by Author "Chen, I-Jan"

Now showing 1 - 2 of 2

Size dependent magnetization and high-vacuum annealing enhanced ferromagnetism in Zn(1-x)Co(x)O nanowires
(2008-01-01)
Jian, Wen-Bin
;
Chen, I-Jan
;
Liao, Tai-Ching
;
Ou, Yi-Ching
;
Nien, Cheng-Hsun
Diameter controllable ZnO nanowires have been fabricated by thermal evaporation (vapor transport) with various sizes of gold nanoparticles as catalysts. Diluted magnetic semiconductor (DMS) Zn(1-x)Co(x)O nanowires were then made by high energy Co ion implantation. The as-implanted and the argon-annealed Zn(1-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 Zn(1-x)Co(x)O nanowires. The room-temperature spontaneous magnetization of individual Zn(1-x)Co(x)O nanowires was also established by using magnetic force microscope measurements.
Size effects on thermal treatments and room-temperature ferromagnetism in high-vacuum annealed ZnCoO nanowires
(2008-06-26)
Chen, I-Jan
;
Ou, Yi-Ching
;
Wu, Zhong-Yi
;
Chen, Fu-Rong
;
Kai, Ji-Jung
Zn0.92Co0.08O nanowires with two different average diameters, 19 and 38 nm, have been made by the vapor transport and Co ion implantation method. The as-implanted nanowires were thermally annealed through multiple steps in a high vacuum. The morphology and crystal structure of the nanowires were inspected by use of scanning and transmission electron microscopes. Electron microscopy analysis was used to ensure the absence of Co nanocrystals in the annealed nanowires. Measurements of temperature-dependent hysteresis were carried out to demonstrate a strong magnetic state in the high-vacuum annealed samples. The coercive field obeying a square-root temperature dependence and the room-temperature ferromagnetism have been discovered. Moreover, by comparison of magnetic properties between different-diameter nanowires, field-dependent magnetizations reveal considerably stronger ferromagnetism in the smaller diameter (19 nm) Zn0.92Co0.08O nanowires than that in the larger diameter (38 nm) nanowires. We argue that the generation of point defects by thermal annealing is the origin for the enhanced ferromagnetism in our high-vacuum annealed nanowires.