Browsing by Author "Chen, Chien-Te"

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A Metal-Insulator Transition of the Buried MnO2 Monolayer in Complex Oxide Heterostructure
(2016-11-02)
Liu, Heng-Jui
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Lin, Jheng-Cyuan
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Fang, Yue-Wen
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Wang, Jing-Ching
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Huang, Bo-Chao
A novel artificially created MnO2 monolayer system is demonstrated in atomically controlled epitaxial perovskite heterostructures. With careful design of different electrostatic boundary conditions, a magnetic transition as well as a metal-insulator transition of the MnO2 monolayer is unveiled, providing a fundamental understanding of dimensionality-confined strongly correlated electron systems and a direction to design new electronic devices.
Antiferromagnetic Interfacial Coupling and Giant Magnetic Hysteresis in La0.5Ca0.5MnO3-SrRuO3 Superlattices
(2018-10-01)
Kumar, Vandrangi Suresh
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Zhou, Shu-Liang
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Liu, Rui-Rui
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Zhu, Yuan-Min
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Liu, Heng-Jui
Superlattices are of great importance due to their potential as new materials genome to synthesize new functional materials. Thus, tuning of the ground state of superlattices is crucial to further control their physical properties. In this study, superlattices (SLs) consisting of alternating layers of SrRuO3 (SRO) (5 nm) and La0.5Ca0.5MnO3 (LCMO) (5 nm) are epitaxially grown on SrTiO3 (STO) and LaAlO3 (LAO) substrates with 10-unit-cell periods. A variation in the substrate-induced-strain for this choice of SLs triggers observation of remarkable properties, such as magnetic anisotropy and large magnetic hysteresis. The strain states experienced by LCMO and SRO in these SLs result in strong ferromagnetic interlayer coupling and weak antiferromagnetic interlayer coupling at low temperatures in SLs of LCMO-SRO/STO and a strong antiferromagnetic interlayer coupling in SLs of LCMO-SRO/LAO. Besides, a large magnetic hysteresis resulting from the predominant magnetic anisotropy of SRO together with the strength of magnetic coupling is observed in SLs of LCMO-SRO/LAO along the out-of-plane direction of the LAO substrate. These four different magnetic behaviors along four different directions of substrate orientations are interpreted in terms of preferential orbital occupation and competing magnetic exchange coupling together with magnetic anisotropy. This study demonstrates the subtleties in controlling the strength of magnetic coupling at the interface and stands as a model system to realize fascinating magnetic phenomena in layer-by-layer hetero-epitaxial oxide films.
Complex Oxide-Noble Metal Conjugated Nanoparticles
(2013-04-11)
Guo, Jun-Ling
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Chiou, Yao-De
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Liang, Wen-I
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Liu, Heng-Jui
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Chen, Ying-Jiun
Hybrid nanoparticles (NPs) composed of multiple components offer new opportunities for next-generation materials. In this study, a paradigm for the noble metal/ternary complex oxide hybrid NPs is reported by adopting pulsed laser ablation in liquids. As model hybrids, gold-spinel heterodimer (Au-CoFe2O4) and gold-pervoskite heterodimer (Au-SrTiO3) NPs are investigated. This work has demonstrated the diverse playgroup of NP conjugation enlarged by complex oxides.
Correlation among photoluminescence and the electronic and atomic structures of Sr2SiO4:xEu(3+) phosphors: X-ray absorption and emission studies
(2020-07-29)
Zheng, Shi-Yan
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Chiou, Jau-Wern
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Li, Yueh-Han
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Yang, Cheng-Fu
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Ray, Sekhar Chandra
A series of Eu3+-activated strontium silicate phosphors, Sr2SiO4:xEu(3+) (SSO:xEu(3+), x=1.0, 2.0 and 5.0%), were synthesized by a sol-gel method, and their crystalline structures, photoluminescence (PL) behaviors, electronic/atomic structures and bandgap properties were studied. The correlation among these characteristics was further established. X-ray powder diffraction analysis revealed the formation of mixed orthorhombic alpha'-SSO and monoclinic beta-SSO phases of the SSO:xEu(3+) phosphors. When SSO:xEu(3+) phosphors are excited under ultraviolet (UV) light (lambda=250 nm,similar to 4.96 eV), they emit yellow (similar to 590 nm), orange (similar to 613 nm) and red (similar to 652 and 703 nm) PL bands. These PL emissions typically correspond to 4f-4f electronic transitions that involve the multiple excited D-5(0)-> F-7(J) levels (J=1, 2, 3 and 4) of Eu3+ activators in the host matrix. This mechanism of PL in the SSO:xEu(3+) phosphors is strongly related to the local electronic/atomic structures of the Eu3+-O2- associations and the bandgap of the host lattice, as verified by Sr K-edge and Eu L-3-edge X-ray absorption near-edge structure (XANES)/extended X-ray absorption fine structure, O K-edge XANES and K-alpha X-ray emission spectroscopy. In the synthesis of SSO:xEu(3+) phosphors, interstitial Eu2O3-like structures are observed in the host matrix that act as donors, providing electrons that are nonradiatively transferred from the Eu 5d and/or O 2p-Eu 4f/5d states (mostly the O 2p-Eu 5d states) to the D-5(0) levels, facilitating the recombination of electrons that have transitioned from the D-5(0) level to the F-7(J) level in the bandgap. This mechanism is primarily responsible for the enhancement of PL emissions in the SSO:xEu(3+) phosphors. This PL-related behavior indicates that SSO:xEu(3+) phosphors are good light-conversion phosphor candidates for use in near-UV chips and can be very effective in UV-based light-emitting diodes.
Defect induced ferromagnetic ordering in epitaxial Zn0.95Mn0.05O films on sapphire (0001)
(2019-12-04)
Liu, Wei-Rein
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Pao, Chih-Wen
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Yu, Doris Weijyun
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Chin, Yi-Ying
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Wu, Wen-Bin
We report the influence of Mn dopant on magnetic properties of Zn0.95Mn0.05O (ZMO)/Al2O3(0001) hetero-epitaxial systems grown by using pulsed-laser deposition. The room temperature (RT) intrinsic ferromagnetic (FM) ordering verified by superconducting quantum interference device magnetometer and x-ray magnetic circular dichroism spectrum of Mn L-2(,3) edges is ascribed to the substitutional Mn atoms in the Zn site of ZnO. Mn in ZMO has a tetrahedral local symmetry instead of the octahedral symmetry of MnO, after verifying the absence of the Mn-related impurities or clusters in ZMO epitaxial film by Mn K-edge and Zn K-edge x-ray absorption spectroscopy spectrum, as well as the analysis of long-range structural ordering on Renninger scan of forbidden (0005) reflection in x-ray diffraction, transmission electron microscopy and Raman spectrum. Comparison of x-ray absorption spectra of ZMO with those of ZnO epilayers at O K-, Zn K-, and L-3-edges indicates that the substitution of the Zn site with Mn enhances the charge-transfer (CT) transition and the presence of Zn vacancies (V-Zn) also dominate the photoluminescence (PL) spectrum, implying that the formation of numerous V-Zn defects plays an important role in activating FM interactions. The strong CT effect and the existence of high-density V-Zn suggest that the intrinsic RT FM ordering of insulating ZMO is a result of the formation of the bound magnetic polarons (BMPs) that interact with each other via intermediate magnetic impurities.
Deterministic optical control of room temperature multiferroicity in BiFeO3 thin films
(2019-06-01)
Liou, Yi-De
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Chiu, Yu-You
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Hart, Ryan Thomas
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Kuo, Chang-Yang
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Huang, Yen-Lin
Controlling ferroic orders (ferroelectricity, ferromagnetism and ferroelasticity) by optical methods is a significant challenge due to the large mismatch in energy scales between the order parameter coupling strengths and the incident photons. Here, we demonstrate an approach to manipulate multiple ferroic orders in an epitaxial mixed-phase BiFeO3 thin film at ambient temperature via laser illumination. Phase-field simulations indicate that a light-driven flexoelectric effect allows the targeted formation of ordered domains. We also achieved precise sequential laser writing and erasure of different domain patterns, which demonstrates a deterministic optical control of multiferroicity at room temperature. As ferroic orders directly influence susceptibility and conductivity in complex materials, our results not only shed light on the optical control of multiple functionalities, but also suggest possible developments for optoelectronics and related applications.
Easy-cone magnetic structure in (Cr0.9B0.1)Te
(2020-03-09)
He, Yangkun
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Fecher, Gerhard H.
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Kroder, Johannes
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Borrmann, Horst
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Wang, Xiao
Although stoichiometric CrTe is difficult to synthesize because of the appearance of Cr vacancies, ferromagnetic Cr1-x Te compounds have attracted increasing attention. This work investigates single crystalline (Cr-0.9 B-0.1)Te with the Cr vacancies filled by B to stabilize the hexagonal crystal structure and shift the Fermi energy. The structural and magnetic properties have been characterized by experimental measurements and ab initio calculations. A collinear spin structure with an easy axis along c is observed at high temperature, whereas the magnetic moments localized at the Cr atoms gradually tilt away from the c-axis below 140K. A maximum tilt angle of 30 degrees is observed at a temperature of 2K. (C) 2020 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http:// creativecommons.org/licenses/by/4.0/).
Electrically enhanced magnetization in highly strained BiFeO3 films
(2016-05)
Yang, Jan-Chi
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Kuo, Chang-Yang
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Liu, Heng-Jui
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Ding, Hang-Chen
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Duan, Chun-Gang
The control of magnetism via an electric field has attracted substantial attention because of potential applications in magnetoelectronics, spintronics and high-frequency devices. In this study, we demonstrate a new approach to enhance and control the magnetization of multiferroic thin film by an electric stimulus. First, to reduce the strength of the antiferromagnetic superexchange interaction in BiFeO3, we applied strain engineering to stabilize a highly strained phase. Second, the direction of the ferroelectric polarization was controlled by an electric field to enhance the Dzyaloshinskii-Moriya interaction in the highly strained BiFeO3 phase. Because of the magnetoelectric coupling in BiFeO3, a strong correlation between the modulated ferroelectricity and enhanced magnetization was observed. The tunability of this strong correlation by an electric field provides an intriguing route to control ferromagnetism in a single-phase multiferroic.
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.
Enhanced Magnetocaloric Effect Driven by Interfacial Magnetic Coupling in Self-Assembled Mn3O4-La0.7Sr0.3MnO3 Nanocomposites
(2015-12-09)
Vandrangi, Suresh K.
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Yang, Jan-Chi
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Zhu, Yuan-Min
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Chin, Yi-Ying
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Lin, Hong-Ji
Magnetic refrigeration, resulting from the magneto caloric effect of a material around the magnetic phase-transition temperature, is a topic of great interest as it is considered to be an alternate energy solution to conventional vapor-compression refrigeration. The viability of a magnetic refrigeration system for magnetic cooling can be tested by exploiting materials in various forms, from bulk to nanostrucutres. In this study, magnetocaloric properties of self-assembled Mn3O4-La0.7Sr0.3MnO3 nanocomposites, with varying doping concentrations of Mn3O4 in the form of nanocrystals embedded in the La0.7Sr0.3MnO3 matrix, are investigated. The temperatures corresponding to the paramagnetic-to-ferromagnetic transitions are higher, and the values of change in magnetic entropy under a magnetic field of 2 T show an enhancement (highest being similar to 130%) for the nanocomposites with low doping concentrations of Mn3O4, compared to that of pure La0.7Sr0.3MnO3 thin films. Relative cooling power remain close to those of La0.7Sr0.3MnO3. The enhanced magnetic phase-transition temperature and magnetocaloric effect are interpreted and evidenced in the framework of interfacial coupling between Mn3O4 and La0.7Sr0.3MnO3. This work demonstrates the potentiality of self-assembled nanostructures for magnetic cooling near room temperature under low magnetic fields.
Enhancing the magnetic moment of ferrimagnetic NiCo2O4 via ion irradiation driven oxygen vacancies
(2018-06-01)
Pandey, Parul
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Bitla, Yugandhar
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Zschornak, Matthias
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Wang, Mao
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Xu, Chi
Ion irradiation has emerged as a powerful tool for the efficient control of uniaxial lattice expansion to fine tune and modulate the otherwise inaccessible complex correlated phases in oxide thin-films. We report the fine tuning of the magnetic moment, ferromagnetic-paramagnetic and metal-insulator transition temperatures in the NiCo2O4 inverse-spinel oxide by creating oxygen deficiencies, employing high energy He-ion irradiation. Tailoring of oxygen vacancies and consequently a uniaxial lattice expansion in the out-of-plane direction drives the system toward the increase of the magnetic moment by two-times in magnitude. The magnetic moment increases with the He-ion irradiation fluence up to 2.5 x 10(16)/cm(2) . Our results are corroborated well by spin-polarized electronic structure calculations with density functional theory and X-ray absorption spectroscopic data, which show peak-height change and energy shift of Co-L-2,L-3 and Ni-L-2,L-3 edges driven by the oxygen vacancies. These results demonstrate a new pathway of tailoring oxygen vacancies via He-ion irradiation, useful for designing new functionalities in other complex oxide thin-films. (C) 2018 Author(s).
Heteroepitaxy of Co-Based Heusler Compound/Muscovite for Flexible Spintronics
(2019-09-25)
Chen, Yi-Cheng
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Yen, Min
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Lai, Yu-Hong
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Markou, Anastasios
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Zhang, Liguo
Materials with high spin-polarization play an important role in the development of spintronics. Co-based Heusler compounds are a promising candidate for practical applications because of their high Curie temperature and tunable half-metallicity. However, it is a challenge to integrate Heusler compounds into thin film heterostructures because of the lack of control on crystallinity and chemical disorder, critical factors of novel behaviors. Here, muscovite is introduced as a growth substrate to fabricate epitaxial Co2MnGa films with mechanical flexibility. The feature of heteroepitaxy is evidenced by the results of X-ray diffraction and transmission electron microscopy. Moreover, high chemical ordering with superior properties is delivered according to the observation of large Hall conductivity (680 Omega(-1) cm(-1)) and highly saturated magnetic moment (similar to 3.93 mu(B)/f.u.), matching well with bulk crystals. Furthermore, the excellence of magnetic and electrical properties is retained under the various mechanical bending conditions. Such a result suggests that the development of Co2MnGa/muscovite heteroepitaxy provides not only a pathway to the thin film heterostructure based on high-quality Heusler compounds but also a new aspect of spintronic applications on flexible substrates.
Large Magnetoresistance in Magnetically Coupled SrRuO3-CoFe2O4 Self-Assembled Nanostructures
(2013-09-14)
Liu, Heng-Jui
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Vu-Thanh Tra
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Chen, Ying-Jiun
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Huang, Rong
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Duan, Chun-Gang
Magnetic Mesocrystal-Assisted Magnetoresistance in Manganite
(2014-11-01)
Yang, Jan-Chi
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He, Qing
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Zhu, Yuan-Min
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Lin, Jheng-Cyuan
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Liu, Heng-Jui
Mesocrystal, a new class of crystals as compared to conventional and well-known single crystals and polycrystalline systems, has captured significant attention in the past decade. Recent studies have been focused on the advance of synthesis mechanisms as well as the potential on device applications. In order to create further opportunities upon functional mesocrystals, we fabricated a self-assembled nanocomposite composed of magnetic CoFe2O4 mesocrystal in Sr-doped manganites. This combination exhibits intriguing structural and magnetic tunabilities. Furthermore, the antiferromagnetic coupling of the mesocrystal and matrix has induced an additional magnetic perturbation to spin-polarized electrons, resulting in a significantly enhanced magnetoresistance in the nanocomposite. Our work demonstrates a new thought toward the enhancement of intrinsic functionalities assisted by mesocrystals and advanced design of novel mesocrystal-embedded nanocomposites.
Mechanical Modulation of Colossal Magnetoresistance in Flexible Epitaxial Perovskite Manganite
(1970-01-01)
Yen, Min
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Lai, Yu-Hong
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Kuo, Chang-Yang
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Chen, Chien-Te
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Chang, Chun-Fu
Heteroepitaxially flexible oxide systems have been intensely developed and considered as the most promising materials for leading the creation of next-generation flexible electronic devices. Among them, perovskite manganites have attracted significant attention with their abundant and novel properties such as colossal magnetoresistance (CMR) and metal-insulator transition. However, the requirement of high quality samples hampers this field, not to mention the advanced nanoengineering. In this study, fluorophlogopite mica (F-mica) is selected as a flexible substrate to fabricate heteroepitaxial Pr0.5Ca0.5MnO3(PCMO) with a nanocolumn structure. Through a precise control of thickness, different morphologies are realized to manipulate the magnetotransport properties (reduction of melting field). Moreover, thanks to the excellent flexibility of F-mica, mechanical modulation of CMR (approximate to 1000%) can be achieved in different flex modes while the magnetic properties remain unaffected. Detailed bending tests are performed to study the behavior of resistive change (approximate to 30%). Through the combination of high flexibility, high quality PCMO, and well-designed nanocolumn structure, the study exhibits the significant controllability of CMR via mechanical bending, and manifests the potential of such a heteroepitaxially flexible oxide system which can be applied on flexible magnetoresistive devices and sensors.
Mechanically tunable exchange coupling of Co/CoO bilayers on flexible muscovite substrates
(2020-02-07)
Thai Duy Ha
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Yen, Min
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Lai, Yu-Hong
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Kuo, Chang-Yang
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Chen, Chien-Te
The employment of flexible muscovite substrates has given us the feasibility of applying strain to heterostructures dynamically by mechanical bending. In this study, this novel approach is utilized to investigate strain effects on the exchange coupling in ferromagnetic Co and anti-ferromagnetic CoO (Co/CoO) bilayers. Two different Co/CoO bilayer heterostructures were grown on muscovite substrates by oxide molecular beam epitaxy, with the CoO layer being purely (111)- and (100)-oriented. The strain-dependent exchange coupling effect can only be observed on Co/CoO(100)/mica but not on Co/CoO(111)/mica. The origin of this phenomenon is attributed to the anisotropic spin re-orientation induced by mechanical bending. The strain-dependent magnetic anisotropy of the bilayers determined by anisotropic magnetoresistance measurements confirms this conjecture. This study elucidates the fundamental understanding of how magnetic exchange coupling can be tuned by externally applied strain via mechanical bending and, hence, provides a novel approach for implementing flexible spintronic devices.
Origin of metallic behavior in NiCo2O4 ferrimagnet
(2015-10-15)
Bitla, Yugandhar
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Chin, Yi-Ying
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Lin, Jheng-Cyuan
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Van, Chien Nguyen
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Liu, Ruirui
Predicting and understanding the cation distribution in spinels has been one of the most interesting problems in materials science. The present work investigates the effect of cation redistribution on the structural, electrical, optical and magnetic properties of mixed-valent inverse spinel NiCo2O4(NCO) thin films. It is observed that the films grown at low temperatures (T < 400 degrees C) exhibit metallic behavior while that grown at higher temperatures (T > 400 degrees C) are insulators with lower ferrimagnetic-paramagnetic phase transition temperature. So far, n-type Fe3O4 has been used as a conducting layer for the spinel thin films based devices and the search for a p-type counterpart still remains elusive. The inherent coexistence and coupling of ferrimagnetic order and the metallic nature in p-type NCO makes it a promising candidate for spintronic devices. Detailed X-ray Absorption and X-ray Magnetic Circular Dichroism studies revealed a strong correlation between the mixed-valent cation distribution and the resulting ferrimagnetic-metallic/insulating behavior. Our study clearly demonstrates that it is the concentration of Ni3+ ions and the Ni3+-O-2-Ni2+ double exchange interaction that is crucial in dictating the metallic behavior in NCO ferrimagnet. The metal-insulator and the associated magnetic order-disorder transitions can be tuned by the degree of cation site disorder via growth conditions.
Phase evolution of magnetite nanocrystals on oxide supports via template-free bismuth ferrite precursor approach
(2012-11-15)
Cheung, Jeffrey
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Bogle, Kashinath
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Cheng, Xuan
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Sullaphen, Jivika
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Kuo, Chang-Yang
This report investigates the phase evolution pathway of magnetite nanocrystal synthesis on oxide-supported substrates. A template-free phase separation approach, which exploits the thermodynamic instability of ternary perovskite BiFeO3 and inherent volatility of bismuth oxide in low oxygen pressure and high temperature is presented. The formation of an intermediate hematite nanocrystal phase is found as a key step that controls the eventual size and morphology of the magnetite nanocrystals. X-ray absorption spectra measurements and X-ray magnetic circular dichroism confirm that the spectral fingerprints of the magnetite nanocrystals match with that of bulk crystals. Magnetic measurements show that magnetic anisotropy is directly attributed to the nanocrystal morphology. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4766748]
Self-Assembled Epitaxial Core-Shell Nanocrystals with Tunable Magnetic Anisotropy
(2015-09-02)
Liao, Sheng-Chieh
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Chen, Yong-Lun
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Kuo, Wei-Cheng
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Cheung, Jeffrey
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Wang, Wei-Cheng
Strong magnetic enhancement in self-assembled multiferroic-ferrimagnetic nanostructures
(2013)
Chen, Ying-Jiun
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Hsieh, Ying-Hui
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Liao, Sheng-Chieh
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Hu, Zhiwei
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Huang, Meng-Jie
In the past decade, self-assembled vertical nano-heterostructures have drawn considerable attention because a high interface-to-volume ratio can be used to tailor or create functionalities. We have systematically investigated the magnetic properties of oxide heterostructures consisting of the CoFe2O4 nanopillars embedded in the BiFeO3 matrix using macroscopic magnetization measurements and element-selective soft X-ray absorption magnetic circular dichroism (XMCD) at the Co- and Fe-L-2,L-3 edge. The magnetization and XMCD data show that the total ordered magnetic moment of Co2+ in CoFe2O4-BiFeO3 nano-heterostructures is strongly enhanced. This study clearly indicates that the high interface-to-volume ratio vertical nanostructure creates a strong ferromagnetic and antiferromagnetic magnetic coupling via an interface. Furthermore, the magnetic coupling can be tuned in the multiferroic-ferrimagnetic self-assembled heterostructures by controlling the spacing between nanopillars.