Browsing by Author "Lam, Tu-Ngoc"

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Atomic origin of the spin-polarization of the Co2FeAl Heusler compound
(2016-02-25)
Liang, Jaw-Yeu
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Lam, Tu-Ngoc
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Lin, Yan-Cheng
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Chang, Shu-Jui
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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.
Comparing Cyclic Tension-Compression Effects on CoCrFeMnNi High-Entropy Alloy and Ni-Based Superalloy
(2019-08-01)
Lam, Tu-Ngoc
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Chou, You-Shiun
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Chang, Yao-Jen
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Sui, Tsung-Ruei
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Yeh, An-Chou
An equal-molar CoCrFeMnNi, face-centered-cubic (fcc) high-entropy alloy (HEA) and a nickel-based superalloy are studied using in situ neutron diffraction experiments. With continuous measurements, the evolution of diffraction peaks is collected for microscopic lattice strain analyses. Cyclic hardening and softening are found in both metallic systems. However, as obtained from the diffraction-peak-width evolution, the underneath deformation mechanisms are quite different. The CoCrFeMnNi HEA exhibits distinct lattice strain and microstructure responses under tension-compression cyclic loadings.
Enhanced Magnetic Anisotropy via Quasi-Molecular Magnet at Organic-Ferromagnetic Contact
(2013-01-17)
Hsu, Yao-Jane
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Lai, Yu-Ling
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Chen, Chih-Han
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Lin, Ying-Chang
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Chien, Hsiu-Yun
To realize the origin of efficient spin injection at organic-ferromagnetic contact in organic spintronics, we have implemented the formation of quasi-molecular magnet via surface restructuring of a strong organic acceptor, tetrafluoro-tetracyanoquinodimethane (F4-TCNQ), in contact with ferromagnetic cobalt. Our results demonstrate a spin-polarized F4-TCNQ layer and a remarkably enhanced magnetic anisotropy of the Co film. The novel magnetic properties are contributed from strong magnetic coupling caused by the molecular restructuring that displays an angular anchoring conformation of CN and upwardly protruding fluorine atoms. We conclude that the pi bonds of CN, instead of the lone-pair electrons of N atoms, contribute to the hybridization-induced magnetic coupling between CN and Co and generate a superior magnetic order on the surface.
Hardening steels by the generation of transient phase using additive manufacturing
(2019-06-01)
Huang, E-Wen
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Lee, Soo Yeol
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Jain, Jayant
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Tong, Yang
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An, Ke
In the present work, the tensile properties of 15-5 PH steel fabricated by selective laser melting (SLM) were examined with respect to the transient austenite phase. Compared with the 8%-transient-phase sample, the 18%-transient-phase one shows higher ultimate tensile strength and relatively low yield strength, as well as hardening behavior. We conducted in-situ neutron-diffraction study to examine the microstructure evolution for mechanistic understanding. After applying the external load, most non-equilibrium, retained austenite in the 8% transient-phase sample transforms before the yield strength, whereas in the 18%-transient-phase sample only 50% of the austenite transforms. Accompanying with the phase transformation, a decrease in the dislocation density and the dislocation strain energy-assisted phase transformation of the ferrite phase are found in the 8%-transient-phase sample even before yielding, which, however, is not the case in the 18%-transient-phase sample. The study demonstrates the SLM enables tuning the amount of transient-phase ratio and coherency between phases to realize a mechanical property control for additive manufactured steel.
Interfacial symmetry of Co-Alq(3)-Co hybrid structures for effective spin filtering
(2015-11-01)
Lam, Tu-Ngoc
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Lai, Yu-Ling
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Chen, Chih-Han
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Chen, Po-Hung
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Wei, Der-Hsin
Understanding the interfacial behavior at FM-OSC-FM hybrid structures for both the bottom contact (Alq(3) adsorption on Co, Co/Alq(3)) and the top contact (Co atop Alq(3), Alq(3)/Co) is crucial for efficient spin filtering with transport of spin-polarized charge carriers through these interfaces. X-ray photoelectron spectroscopy (XPS) spectra indicate a symmetry of charge transfer from Co to Alq(3) and the corresponding orbital hybridization to a certain extent at both contacts. The alignment of energy levels at both Alq(3)/Co and Co/Alq(3) heterostructures is depicted with ultraviolet photoelectron spectroscopy (UPS). Through magnetic images acquired with a X-ray photoemission electron microscope (XPEEM), the strong hybridization of the top contact presents no micromagnetic domain but still shows magnetic coupling, to some extent, to the bottom contact in the Co-Alq(3)-Co trilayer structure. Measurements of X-ray magnetic circular dichroism (XMCD) demonstrate the induced spin-polarization of non-magnetic Alq(3) at both contacts, proving Alq(3) a unique and promising organic material for spin filtering in OSV. (C) 2015 Elsevier B.V. All rights reserved.
Micro-Layer and Lattice Structure Effects on Impedance of Titanium Oxide Phthalocyanine
(2018-07-01)
Lam, Tu-Ngoc
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Huang, Wei-Jie
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Wang, Chun-Chieh
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Chuang, Wei-Tsung
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Su, Ya-Wen
In this study, the authors employ synchrotron techniques to characterize multi-scale microstructure in elucidating how the modified crystal lattices at angstrom level and molecular stacking at micrometer scale of titanium oxide phthalocyanine (TiOPc) thin film tuning the impedance. The a and c lattice lengths of the TiOPc reveal a positive correlation with the impedance because of the - stacking distances of TiOPc molecules arrangements. Our results suggest a promising dip coating method to tailor the charge storage capability of TiOPc through controlling the crystal lattice sizes of TiOPc.
Multi-scale mapping for collagen-regulated mineralization in bone remodeling of additive manufacturing porous implants
(2019-05-15)
Tsai, Pei-I.
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Lam, Tu-Ngoc
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Wu, Meng-Huang
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Tseng, Kuan-Ying
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Chang, Yuan-Wei
Long term success of metallic fusion cages depends on mechanobiological processes through the bone incorporation and rich osseointegration. An optimal configuration of porous titanium-aluminum-vanadium (Ti-6Al-4V) implant fabricated via the additive manufacturing was evaluated in complimentary structure examinations to investigate the growth of autologous osseous at multi-length scales. X-ray microcomputed tomography (micro-CT) and transmission X-ray microscopy (TXM) using newly-built analysis method indicate the porous Ti-6Al-4V is much better for bone ingrowth compared to commercially non-porous titanium (Ti) and porous tantalum (Ta) implants at the ultramicrostructural level. The evolution of bone formation and remodeling acquired by nano X-ray Laue diffraction mapping exhibits the isotropic orientation and low crystallinity of all newly formed bone whereas mature bone in Ti-6Al-4V discloses the preferential alignment and higher crystallinity volumes of constituent hydroxyapatite (HA) crystallites. The high degree in mineral crystallinity of the fully mature bone suggests additive manufactured Ti-6Al-4V pores enhance the collagen-regulated mineralization.
Spin filtering of a termination-controlled LSMO/Alq(3) heterojunction for an organic spin valve
(2017-09-21)
Lam, Tu-Ngoc
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Huang, Yen-Lin
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Weng, Ke-Chuan
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Lai, Yu-Ling
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Lin, Ming-Wei
La1-xSrxMnO3 (LSMO) is weLL known as an efficient spin-injection electrode in ferromagnetic organic hybrid-based organic spin valves (OSV). Herein, we examine the influence of three terminating Layers of LSMO fiLms (mixed-, MnO2-, and LaSrO- (LSO-) terminated) on the spin filter at the bottom interface of tri (8-hydroxyquinoLine) aluminum (ALq(3)) deposited on these substrates. Angle-dependent X-ray photoelectron spectra (XPS) demonstrated the presence of SrO segregation at the outermost Layer for three types of LSMO fiLms but in different amounts that might contribute to their magnetic properties. A weak surface coupling between LSMO and Alq(3) was obtained through a small increase of Mn L-edge X-ray magnetic circular dichroism (XMCD) but it does not vary the bulk magnetic properties of LSMO fiLms upon Alq(3) adsorption (LSMO/Alq(3)). The surface dipoles due to the interaction between LSMO fiLms and Alq(3) were found to be 0.5, 0.5, and 0.7 eV for the mixed-ter/Alq(3), MnO2-ter/Alq(3), and LSO-ter/Alq(3) interfaces, respectively. Our results indicate that the segregated SrO Layer Ways an important role in the surface electronic and magnetic properties at LSMO/Alq(3) interfaces, which contributes to a fundamental understanding of the spin injection and transport in LSMO-Alq(3) hybrid-based OSV devices.
Using magnetic structure of Co40Pd60/Cu for the sensing of hydrogen
(2017-07-10)
Lang, Jaw-Yeu
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Pai, Yun-Chieh
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Lam, Tu-Ngoc
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Lin, Wen-Chin
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Chan, Ting-Shan
This paper reports on a magnetic device with a [Co40Pd60/Cu](10)/Fe structure for use in the detection of H-2. In a magneto-optical, transport, and gas-detection system, the proposed device presented sharp, reproducible H-2-dependent magnetic/electrical properties. The device's saturation magnetization (induced resistance change) dropped (increased) by a factor of similar to 5 when a H-2 pressure of 75 kPa is given. Besides, the electrical signal-to-noise ratio of the device can be restored similar to 50% by exposing it to a magnetic field of 1000 Oe, even when the sensitivity of the device dropped at a low H-2 pressure (0.7 KPa). This demonstrates the applicability of the device for use as a low-pressure H-2 detector. Operando x-ray spectroscopy revealed that changes in H-2-induced magnetism arose from a Co-Pd charge transfer effect coupled to changes in the local-structure symmetry. Published by AIP Publishing.