Browsing by Author "Liang, Jaw-Yeu"

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Atomic origin of the spin-polarization of the Co2FeAl Heusler compound
(2016-02-25)
Liang, Jaw-Yeu
;
Lam, Tu-Ngoc
;
Lin, Yan-Cheng
;
Chang, Shu-Jui
;
Lin, Hong-Ji
Using synchrotron x-ray techniques, we studied the Co2FeAl spin-polarization state that generates the half-metallicity of the compound during an A(2) (low-spin) -> B-2 (high-spin) phase transition. Given the advantage of element specificity of x-ray techniques, we could fingerprint the structural and magnetic cross-reactions between Co and Fe within a complex Co2FeAl structure deposited on a MgO (0 0 1) substrate. X-ray diffraction and extended x-ray absorption fine structure investigations determined that the Co atoms preferably populate the (1/4,1/4,1/4) and (3/4,3/4,3/4) sites during the development of the B-2 phase. X-ray magnetic spectroscopy showed that although the two magnetic elements were ferromagnetically coupled, they interacted in a competing manner via a charge-transfer effect, which enhanced Co spin polarization at the expense of Fe spin polarization during the phase transition. This means that the spin-polarization of Co2FeAl was electronically dominated by Fe in A(2) whereas the charge transfer turned the dominance to Co upon B-2 formation. Helicity-dependent x-ray absorption spectra also revealed that only the minority state of Co/Fe was involved in the charge-transfer effect whereas the majority state was independent of it. Despite an overall increase of Co2FeAl magnetization, the charge-transfer effect created an undesired trade-off during the Co-Fe exchange interactions, because of the presence of twice as many X sites (Co) as Y sites (Fe) in the Heusler X(2)YZ formula. This suggests that the spin-polarization of Co2FeAl is unfortunately regulated by compromising the enhanced X (Co) sites and the suppressed Y (Fe) sites, irrespective of the development of the previously known high-spin-polarization phase of B-2. This finding provides a possible cause for the limited half-metallicity of Co2FeAl discovered recently. Electronic tuning between the X and Y sites is necessary to further increase the spin-polarization, and likely the half-metallicity as well, of the compound.
Phase-driven magneto-electrical characteristics of single-layer MoS2
(2016)
Yang, Chao-Yao
;
Chiu, Kuan-Chang
;
Chang, Shu-Jui
;
Zhang, Xin-Quan
;
Liang, Jaw-Yeu
Magnetism of the MoS2 semiconducting atomic layer was highlighted for its great potential in the applications of spintronics and valleytronics. In this study, we demonstrate an evolution of magneto-electrical properties of single layer MoS2 with the modulation of defect configurations and formation of a partial 1T phase. With Ar treatment, sulfur was depleted within the MoS2 flake leading to a 2H (low-spin) -> partial 1T (high-spin) phase transition. The phase transition was accompanied by the development of a ferro-magnetic phase. Alternatively, the phase transition could be driven by the desorption of S atoms at the edge of MoS2 via O-2 treatment while with a different ordering magnitude in magnetism. The edge-sensitive magnetism of the single-layer MoS2 was monitored by magnetic force microscopy and validated by a first-principle calculation with graded-Vs (sulfur vacancy) terminals set at the edge, where band-splitting appeared more prominent with increasing Vs. Treatment with Ar and O-2 enabled a dual electrical characteristic of the field effect transistor (FET) that featured linear and saturated responses of different magnitudes in the I-ds-V-ds curves, whereas the pristine MoS2 FET displayed only a linear electrical dependency. The correlation and tuning of the Vs-1T phase transition would provide a playground for tailoring the phase-driven properties of MoS2 semiconducting atomic layers in spintronic applications.
Realization of an H-2/CO dual-gas sensor using CoPd magnetic structures
(2018-10-29)
Liang, Jaw-Yeu
;
Chou, Yu-Jun
;
Yin, Chiao-Wen
;
Lin, Wen-Chin
;
Lin, Hong-Ji
We demonstrated the application of CoPd magnetic devices in single (H-2) and dual (H-2 and CO) gas detection by varying the thin-film structure. The anomalous Hall effect provided the read-out signal of the devices. The dual-gas sensor was particularly useful for examining whether methane reformation by steam was completed during H-2 production where CO is a byproduct. Through in situ Fourier transform infrared and X-ray absorption spectroscopy, we discovered that H-2 detection involved a charge-transfer effect between Co and Pd in addition to a surface-adsorption process, for both multilayer and alloy structures; however, CO detection was solely associated with a surface-adsorption process, which only occurred in the alloy structure. The proposed devices demonstrated performances exceeding those of current commercial models (solid-state based) regarding power consumption (<5 mW) because they were not required to undergo a heating process. An Arduino prototype platform was employed to test the modularization of the device, and adequate feasibility was demonstrated. Published by AIP Publishing.