Browsing by Author "Lo, Chih-Chieh"

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Competing magnetic interactions and interfacial frozen-spins in Ni-NiO core-shell nano-rods
(2012-03-15)
Hsu, Hao-Chun
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Lo, Chih-Chieh
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Tseng, Yuan-Chieh
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材料科學與工程學系
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Department of Materials Science and Engineering
This paper investigates the subtle interfacial magnetism of highly-aligned, free-standing Ni-NiO core-shell rods on a Si substrate, fabricated by electroless-plating and an anodic aluminum oxide template. Transmission electron microscopy found that the NiO shell was uniformly present along the entire rod. Vertical magnetization shift, arising from opposite field cooling conditions, suggests frozen spins (FS) at the Ni-NiO interface. The FS were related to the pinning effects of the NiO on the Ni. The pinning strength depended on the NiO thickness, displaying a tunable fashion from 6 to 10 nano-meters with thermal annealing. The FS mediated the antiferromagnetic (AFM)-ferromagnetic (FM) interfacial coupling, leading to the temperature-dependent properties of the rods. FS were evident below 100 K, at which the NiO-AFM dominated the properties with a suppressed coercive field and non-saturated magnetization. At 100 K, however, the Ni-FM was superior to the NiO-AFM with a restored FM phase. Meanwhile, the interfacial magnetic frustration occurred due to the disappearance of FS. These two factors resulted in the coercivity enhancement at 100 K. The uniqueness of the structure opens opportunities to tailoring the properties of the rods by manipulating the core-shell inter-dependency, as well as inspiring further researches concerning its applications in spintronics. (C) 2012 American Institute of Physics. [ http://dx.doi.org/10.1063/1.3699039]
Magnetic properties of electroless-deposited Ni and Ni-NiO core-shell nano-arrays
(2011-08-01)
Lo, Chih-Chieh
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Huang, Chun-Chao
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Liu, Chien-Min
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Chen, Chih
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Kuo, Chang-Yang
Ni and Ni-NiO core-shell nano-arrays were fabricated by means of electroless deposition, where the latter was covered by a NiO shell of similar to 10 nm by annealing the former at 350 degrees C for 30 min in an atmospheric condition. HRTEM showed that the NiO shell was developed at the expense of Ni at the array's surface. Ferromagnetic ordering of the Ni-NiO arrays was found to be suppressed compared with those of the less oxidized reference Ni arrays. This is attributed to the screening effect of the NiO shell, and weak ferromagnetism of inner Ni arrays resulted from the development of the NiO. X-ray absorption spectrum reveals that the reference Ni is partially oxidized. Also, X-ray magnetic circular dichroism suggests that the magnetic suppression of the Ni-NiO arrays is associated with a reduced spin moment. (C) 2011 Elsevier B.V. All rights reserved.
Magnetostructural phase transition in electroless-plated Ni nanoarrays
(2011-06-01)
Huang, Chun-Chao
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Lo, Chih-Chieh
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Tseng, Yuan-Chieh
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Liu, Chien-Min
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Chen, Chih
Ni nanoarrays were synthesized by electroless-plating and shaped by an anodic aluminum oxide template. The as-plated arrays exhibited superparamagnetic (SM) ordering resulting from nanocrystalline microstructure. Ferromagnetic (FM) ordering was found to be restored as the arrays' crystallinity was enhanced upon post-annealing. The microstructure (crystallinity) and the FM ordering are strongly coupled, revealing a magneto-structural correlation for the arrays. The magnetostructural properties of the arrays can be controlled by post-annealing, where the magnetization is proportional to the annealing temperature. The electroless-plated arrays synthesized in this work display magnetic anisotropy not found in electroplated ones. This is likely attributed to the nature of the clusterlike microstructure, whose cluster-boundaries may confine the FM rotation within the cluster. The spin-polarization was probed by x-ray magnetic circular dichroism while the arrays underwent the SM -> FM phase transition. The sum-rules results reveal that the total magnetization of the arrays is dominated by spin moment (m(spin)). The change in m(spin) is responsible for the SM -> FM phase transition upon annealing, as well as for the loss of magnetization upon temperature increase that we observed macroscopically. VC 2011 American Institute of Physics. [doi:10.1063/1.3594692]