Browsing by Author "Chang, Shu-Jui"

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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
;
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.
Competing Anisotropy-Tunneling Correlation of the CoFeB/MgO Perpendicular Magnetic Tunnel Junction: An Electronic Approach
(2015-11-24)
Yang, Chao-Yao
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Chang, Shu-Jui
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Lee, Min-Han
;
Shen, Kuei-Hung
;
Yang, Shan-Yi
We intensively investigate the physical principles regulating the tunneling magneto-resistance (TMR) and perpendicular magnetic anisotropy (PMA) of the CoFeB/MgO magnetic tunnel junction (MTJ) by means of angle-resolved x-ray magnetic spectroscopy. The angle-resolved capability was easily achieved, and it provided greater sensitivity to symmetry-related d-band occupation compared to traditional x-ray spectroscopy. This added degree of freedom successfully solved the unclear mechanism of this MTJ system renowned for controllable PMA and excellent TMR. As a surprising discovery, these two physical characteristics interact in a competing manner because of opposite band-filling preference in space-correlated symmetry of the 3d-orbital. An overlooked but harmful superparamagnetic phase resulting from magnetic inhomogeneity was also observed. This important finding reveals that simultaneously achieving fast switching and a high tunneling efficiency at an ultimate level is improbable for this MTJ system owing to its fundamental limit in physics. We suggest that the development of independent TMR and PMA mechanisms is critical towards a complementary relationship between the two physical characteristics, as well as the realization of superior performance, of this perpendicular MTJ. Furthermore, this study provides an easy approach to evaluate the futurity of any emerging spintronic candidates by electronically examining the relationship between their magnetic anisotropy and transport.
Complex magnetic interactions and charge transfer effects in highly ordered NixFe1-x nano-wires
(2013-04-01)
Chang, Shu-Jui
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Yang, Chao-Yao
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Ma, Hao-Chung
;
Tseng, Yuan-Chieh
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材料科學與工程學系
This work investigates the subtle magnetic interactions within the NixFe1-x (x=0.3, 0.5, and 1.0) nano-wires by probing spin-dependent behaviors of the two constituted elements. The wires were fabricated by electro-deposition and an anode aluminum oxide template to produce free-standing nature, and the Ni-Fe interactions were probed by x-ray magnetic spectroscopy across a BCC -> FCC structural transition. The wires' magneto-structural properties were predominated by Ni, as reflected by a decrease but an increase in total magnetization and FCC x-ray intensity with increasing x, even if the Fe moment increased simultaneously. Upon annealing, a prominent charge transfer, together with the changes of spin-dependent states, took place in the Ni and Fe 3d orbitals, and a structural disordering was also obtained, for the wires at x=0.3. The charge transfer led to a local magnetic-compensation for the two elements, explaining the minor change in total magnetization for x=0.3 probed by a vibrational sample magnetometer. When x was increased to 0.5, however, the charge transfer became inactive due to persistent structural stability supported by Ni, albeit resulting in nearly invariant magnetization similar to that of x=0.3. The complexity of the Ni-Fe interactions varied with the composition and involved the modifications of the coupled magnetic, electronic and structural degrees of freedom. The study identifies the roles of Ni and Fe as unequally-influential in NixFe1-x, which provides opportunities to re-investigate the compound's properties concerning its technological applications. (C) 2012 Elsevier B.V. All rights reserved.
GdFe0.8Ni0.2O3: A Multiferroic Material for Low-Power Spintronic Devices with High Storage Capacity
(2019-08-28)
Chang, Shu-Jui
;
Chung, Ming-Han
;
Kao, Ming-Yi
;
Lee, Shang-Fan
;
Yu, Yi-Hsing
Multiferroic materials are strong candidates for reducing the energy consumption of voltage-controlled spintronic devices because of the coexistence of ferroelectric (FE) and magnetic orders in a single phase. In this article, we present a new multiferroic perovskite, GdNixFe1-xO3 (GFNO), produced via sputtering on a SrTiO3 substrate. The proposed GFNO is FE and canted antiferromagnetic (AFM) within a monoclinic framework at room temperature. The FE polarization of the GFNO is up to 37 mu C/cm(2). When capped with a Co layer, the resulting heterostructure exhibits voltage-controlled magnetism (VCM). The heterostructured device exhibits two distinct features. First, its VCM depends on the magnitude as well as the polarity of the applied bias, thereby doubling the number of available magnetic readout states under a fixed voltage. Furthermore, the magnetic order of the device can be controlled very effectively within +/- 1 V. These two characteristics satisfy the requirements for low-power and high-storage technology. Theoretical analysis and experimental results indicate the importance of Ni dopant in regulating the polarity-dependent multiferroicity of this gadolinium ferrite system.
Heterostructured ferromagnet-topological insulator with dual-phase magnetic properties
(2018-01-01)
Chang, Shu-Jui
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Chuang, Pei-Yu
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Chong, Cheong-Wei
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Chen, Yu-Jung
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Huang, Jung-Chun Andrew
The introduction of ferromagnetism at the surface of a topological insulator (TI) produces fascinating spin-charge phenomena. It has been assumed that these fascinating effects are associated with a homogeneous ferromagnetic (FM) layer possessing a single type of magnetic phase. However, we obtained phase separation within the FM layer of a Ni80Fe20/Bi2Se3 heterostructure. This phase separation was caused by the diffusion of Ni into Bi2Se3, forming a ternary magnetic phase of Ni:Bi2Se3. The inward diffusion of Ni led to the formation of an FeSe phase outward, transforming the original Ni80Fe20/Bi2Se3 into a sandwich structure comprising FeSe/Ni:Bi2Se3/Bi2Se3 with dual-phase magnetic characteristics similar to that driven by the proximity effect. Such a phenomenon might have been overlooked in previous studies with a strong focus on the proximity effect. X-ray magnetic spectroscopy revealed that FeSe and Ni:Bi2Se3 possess horizontal and perpendicular magnetic anisotropy, respectively. The overall magnetic order of the heterostructure can be easily tuned by adjusting the thickness of the Bi2Se3 as it compromises the magnetic orders of the two magnetic phases. This discovery is essential to the quantification of spin-charge phenomena in similar material combinations where the FM layer is composed of multiple elements.
Instrument for x-ray absorption spectroscopy with in situ electrical control characterizations
(2013-12-01)
Huang, Chun-Chao
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Chang, Shu-Jui
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Yang, Chao-Yao
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Chou, Hsiung
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Tseng, Yuan-Chieh
We report a synchrotron-based setup capable of performing x-ray absorption spectroscopy and x-ray magnetic circular dichroism with simultaneous electrical control characterizations. The setup can enable research concerning electrical transport, element-and orbital-selective magnetization with an in situ fashion. It is a unique approach to the real-time change of spin-polarized electronic state of a material/device exhibiting magneto-electric responses. The performance of the setup was tested by probing the spin-polarized states of cobalt and oxygen of Zn1-xCoxO dilute magnetic semiconductor under applied voltages, both at low (similar to 20 K) and room temperatures, and signal variations upon the change of applied voltage were clearly detected. (C) 2013 AIP Publishing LLC.
Phase-driven magneto-electrical characteristics of single-layer MoS2
(2016)
Yang, Chao-Yao
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Chiu, Kuan-Chang
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Chang, Shu-Jui
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Zhang, Xin-Quan
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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.
Pulse-Driven Nonvolatile Perovskite Memory with Photovoltaic Read-Out Characteristics
(2019-09-18)
Chang, Shu-Jui
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Chen, Syuan-Ye
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Chen, Po-Wen
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Huang, Szu-Jung
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Tseng, Yuan-Chieh
This paper presents a unique GdFe08Ni0.2O3 perovskite thin film for use in pulse-controlled nonvolatile memory devices (combined with a SrTiO3 (STO) substrate) without the need for an electrical-stressing read-out process. The use of pulse voltage imposes permanent downward/upward polarization states on GFNO, which enables greater energy density and higher energy efficiency than the unpoled state for memory. The two polarization states produce carrier migrations in opposing directions across the GFNO/STO interface, which alter the depletion region of the device, as reflected in photovoltaic short-circuit current density (J(sc)) values. Modulating the duration (varying the number of sequential pulses but fixing the pulse width and delay time) and direction of continuous pulse voltage is an effective method for controlling J(sc), thereby allowing the fabrication of nondestructive, light-tunable, nonvolatile memory devices. In experiments, J(sc) in the downward polarized state was approximately 6 times greater than that in the upward polarized state. It is promising that more memory states can be enabled by the proposed heterostructure by selecting appropriate pulse trains. Real-time interfacial changes (relative to the nonvolatile characteristics of the device) were obtained by applying synchrotron X-ray techniques simultaneously with pulse characterization. This made it possible to separately probe the electronic and chemical states of the GFNO (a p-type-like semiconductor) and STO (an n-type-like semiconductor) while varying the pulse direction, thereby making it possible to identify the mechanisms underlying the observed phenomena.
Soft and hard natures of Nd2Fe14B permanent magnet explored by first-order-reversal-curves
(2014-12-01)
Chen, Po-An
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Yang, Chao-Yao
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Chang, Shu-Jui
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Lee, Min-Han
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Tang, Nai-Kuang
Two commercial Nd2Fe14B samples, MQP-B and sintered-NdFeB were investigated using synchrotron based x-ray diffraction and first-order-reversal-curves (FORCs). Despite differing in magnetic and structural properties, the two samples were found to comprise two major ferromagnetic components in FORCs. For the sintered-NdFeB case, the soft component may originate from the intrinsically soft Nd-f site which was coupled with its local Fe atomic environment that differs in magnetic anisotropy from the Nd-g site (intrinsically hard). It may directly originate from the Nd-rich phase or microstructural imperfection, while the former possibility (Nd-f sire) appears greater than the latter While for the MQP-B, the minor second phase resulting from high structural disorder was likely in charge of the presence of the soft component. Sophisticated FORCs analyses revealed the natures of the soft and hard components, soft hard coupling and switching reversibility of the two cases, irrespective of the origins of their two components. This provides insights to the origin of magnetic stability and reversal dynamics of Nd2Fe14B that have not been fully understood by conventional magnetic analyses. The coexistence of the two components led to an incoherent reversal undermining the magnetic stability of Nd2Fe14B. This is a fundamental problem as to why the performance extremity can only be improved finitely through extrinsic tuning. From FORCs simulation we understand that the soft-hard coupling was moderate in a real Nd2Fe14B compound. A stronger soft-hard coupling is necessary to conquer the anisotropic competition to enable a coherent reversal that will promote the magnetic hardness. (C) 2014 Elsevier B.V. All rights reserved,
Voltage-induced Interface Reconstruction and Electrical Instability of the Ferromagnet-Semiconductor Device
(2017-03-23)
Chang, Shu-Jui
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Chang, Po-Chun
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Lin, Wen-Chin
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Lo, Shao-Hua
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Chang, Liang-Chun
Using x-ray magnetic spectroscopy with in-situ electrical characterizations, we investigated the effects of external voltage on the spin-electronic and transport properties at the interface of a Fe/ZnO device. Layer-, element-, and spin-resolved information of the device was obtained by cross-tuning of the x-ray mode and photon energy, when voltage was applied. At the early stage of the operation, the device exhibited a low-resistance state featuring robust Fe-O bonds. However, the Fe-O bonds were broken with increasing voltage. Breaking of the Fe-O bonds caused the formation of oxygen vacancies and resulted in a high-resistance state. Such interface reconstruction was coupled to a charge-transfer effect via Fe-O hybridization, which suppressed/enhanced the magnetization/coercivity of Fe electronically. Nevertheless, the interface became stabilized with the metallic phase if the device was continuously polarized. During this stage, the spin-polarization of Fe was enhanced whereas the coercivity was lowered by voltage, but changes of both characteristics were reversible. This stage is desirable for spintronic device applications, owing to a different voltage-induced electronic transition compared to the first stage. The study enabled a straightforward detection of the spin-electronic state at the ferromagnet-semiconductor interface in relation to the transport and reversal properties during operation process of the device.