Browsing by Author "Kuo, Chang-Yang"

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Canted Antiferromagnetism on Rectangular Layers of Fe2+ in Polymorphic CaFeSeO
(2017-04-17)
Lai, Kwing To
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Komarek, Alexander Christoph
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Fernandez-Diaz, Maria Teresa
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Chang, Pi-Shan
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Huh, Sungjoon
From stoichiometric amounts of CaO, Fe, and Se, pure powders and single crystals of quaternary Ca-infinity(2)[FeSe2/2O2/2] can be obtained by solid-state reaction and self-flux growth, respectively. The as-synthesized compound exhibits a polymorphic crystal structure, where the two modifications have different stacking sequences of (2)(infinity)[FeSe2/2O2/2](2-) layers. The two polymorphs have similar unit cells but different crystal symmetries (Cmc2(1), and Pnma), of which the former is non-centrosymmetric. Fe is divalent (d(6)) and high-spin, as proven by X-ray spectroscopy, Mossbauer spectroscopy, and powder neutron diffraction data. The latter two, in combination with magnetic susceptibility and specific heat data, reveal a long-range antiferromagnetic spin order (T-N = 160 K) with a minor spin canting. CaFeSeO is an electronic insulator, as confirmed by resistivity measurements and density functional theory calculations. The latter also suggest a relatively small energy difference between the two polymorphs, explaining their intimate intergrowth.
Complex strain evolution of polar and magnetic order in multiferroic BiFeO3 thin films
(2018-09-21)
Chen, Zuhuang
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Chen, Zhanghui
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Kuo, Chang-Yang
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Tang, Yunlong
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Dedon, Liv R.
Electric-field control of magnetism requires deterministic control of the magnetic order and understanding of the magnetoelectric coupling in multiferroics like BiFeO3 and EuTiO3. Despite this critical need, there are few studies on the strain evolution of magnetic order in BiFeO3 films. Here, in (110)-oriented BiFeO3 films, we reveal that while the polarization structure remains relatively unaffected, strain can continuously tune the orientation of the antiferromagnetic-spin axis across a wide angular space, resulting in an unexpected deviation of the classical perpendicular relationship between the antiferromagnetic axis and the polarization. Calculations suggest that this evolution arises from a competition between the Dzyaloshinskii-Moriya interaction and single-ion anisotropy wherein the former dominates at small strains and the two are comparable at large strains. Finally, strong coupling between the BiFeO3 and the ferromagnet Co0.9Fe0.1 exists such that the magnetic anisotropy of the ferromagnet can be effectively controlled by engineering the orientation of the antiferromagnetic-spin axis.
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.
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.
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.
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]
Synthesis and Characterization of Ba[CoSO]: Magnetic Complexity in the Presence of Chalcogen Ordering
(2015-07-20)
Valldor, Martin
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Roessler, Ulrich K.
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Prots, Yurii
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Kuo, Chang-Yang
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Chiang, Jen-Che
Barium thio-oxocobaltate(II), Ba[CoS2/2O2/2], was synthesized by the reaction of equimolar amounts of BaO, Co, and S in closed silica ampoules. The title compound (Cmcm, a=3.98808(3), b=12.75518(9), c=6.10697(4)angstrom) is isostructural to Ba[ZnSO]. The use of soft X-ray absorption spectroscopy confirmed that cobalt is in the oxidation state +2 and tetrahedrally coordinated. Its coordination consists of two sulfur and two oxygen atoms in an ordered fashion. High-temperature magnetic susceptibility data indicate strong low-dimensional spin-spin interactions, which are suggested to be closely related to the layer-type crystal structure and perhaps the ordered distribution of sulfur and oxygen. Antiferromagnetic ordering below T-N=222K is observed as an anomaly in the specific heat, coinciding with a significant lowering of the magnetic susceptibility. Density functional theory calculations within a generalized-gradient approximation (GGA)+U approach identify an antiferromagnetic ground state within the square-like two-dimensional layers of Co, and antiferromagnetic correlations for nearest and next nearest neighbors along bonds mediated by oxygen or sulfur. However, this magnetic state is subject to frustration by relatively strong interlayer couplings.