Browsing by Author "Chou, FC"

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    Antiferromagnetic spin structure and negative thermal expansion of Li2Ni(WO4)2
    (American Physical Society, 2015-07-10) Karna, SK; Wang, CW; Sankar, R; Avdeev, M; Singh, A; Panneer Muthuselvam, I; Singh, VN; Guo, GY; Chou, FC
    We report the results of a study on the crystal and magnetic structure of Li2Ni(WO4)2 with a neutron diffraction technique. The Ni2+ spins of S = 1 for NiO6 octahedra are coupled via corner-sharing, nonmagnetic double tungstate groups in a super-superexchange route. Two magnetic anomalies at TN1∼ 18 K and TN2∼ 13 K are revealed from the measured magnetic susceptibility χ(T), and TN2 is confirmed to be the onset of a commensurate long-range antiferromagnetic (AF) ordering through neutron diffraction. A negative thermal expansion phenomenon is observed below TN2, which has been interpreted as a result of competing normal thermal contraction and long-range AF spin ordering through counterbalanced WO4 and NiO6 polyhedral local distortion. The AF spin structure has been modeled and used to show that Ni spins with a saturated magnetic moment of ∼1.90(27)μB that lies in the a−c plane approximately 46∘(±10∘) off the a axis. The experimental results are compared and found to be consistent with theoretical calculations using density-functional theory with a generalized gradient approximation plus on-site Coulomb interaction. ©2015 American Physical Society
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    Commensurate to incommensurate magnetic phase transition in the type-II multiferroic YBaCuFeO5
    (Australian Institute of Physics, 2015-02-06) Lai, YC; Chen, WT; Wang, CW; Rule, KC; Chou, FC; Du, CH
    The simultaneous existence of magnetic and ferroelectric ordering is a characteristic of multiferroic materials. The search for new multiferroics is partly motivated by the need for non-volatile random access memories for which the electric polarisation (magnetisation) is controlled by magnetic field (electric field) or vice versa. The use of such materials would be extremely beneficial for the next generation of electronic devices (mobile phones, tablets etc) whereby compact size constraints are important as well as optimising reading/writing speeds and power consumption. YBaCuFeO5 has been classified as a type-II multiferroic due to its complex magnetic interactions and low temperature feroelectricity. Two magnetic phase transitions of antiferromagnetic nature have been found near TN1 = 450 K and TN2 = 170 K. The first represents the ordering of the Fe3+ ions into a commensurate antiferromagnetic state, while the second represents the ordering of the Cu2+ ions giving an overall incommensurate antiferromagnetic ground state. For the first time, using modified traveling solvent floating zone growth method, we have been able to grow a centimeter-sized, high-quality, singlecrystal of YBaCuFeO5. Magnetisation and neutron diffraction results indicate a complex magnetic phase diagram in this material with a strong hysteresis effect and a chiral magnetic ground state.
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    Determination of hole distribution in Sr14-xCaxCu24O41 using soft x-ray absorption spectroscopy at the Cu L3 edge.
    (American Physical Society, 2013-07-22) Huang, MJ; Deng, G; Chin, YY; Hu, ZW; Cheng, JG; Chou, FC; Conder, K; Zhou, JS; Pi, TW; Goodenough, JB; Lin, HJ; Chen, CT
    The physical properties of Sr14-xCaxCu24O41 are determined by the hole distribution between the edge-shared CuO2 chain and the corner-shared Cu2O3 two-leg ladder, but inconsistent results on the hole distribution were obtained in various experimental works in recent decades. In this work we reinvestigate the hole distribution by soft x-ray absorption spectroscopy at the Cu L-3 edge. By comparing with the pure ladder and the pure chain systems, we can unambiguously distinguish between the hole distributions in the chain and the ladder. We have found that there are 5.3 holes in the edge-shared chain and 0.7 holes in the corner-shared ladder on average for Sr14Cu24O41. Upon Ca substitution, the holes gradually transfer from the edge-shared chain to the corner-shared ladder, and there are up to 1.21 holes in the corner-shared ladder for Sr1.8Ca12.2Cu24O41. © 2013, American Physical Society.
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    Fe-excess ions as electronic charge suppliers for zero thermal expansion in the normal state of Fe1.16Te0.6Se0.4
    (The Physical Society of Japan, 2015-08-27) Karna, SK; Lee, CH; Li, WH; Sankar, R; Chou, FC; Avdeev, M
    We report on the observation of a zero thermal expansion of the crystalline lattice of Fe1.16Te0.6Se0.4 in the normal state, using neutron and x-ray diffraction, ac magnetic susceptibility, magnetization and resistivity measurements. Superconductivity develops below 15 K. Magnetic hysteresis loops are revealed at all temperatures studied, with the loop opening at 5 K being noticeably larger than that at 300 K. An extremely large thermal expansion of the lattice is observed in the superconducting state. Thermal expansion coefficients of the lattice are quenched upon loss of superconductivity. Zero thermal expansion is retained over a very broad temperature range from 20 to 200 K. These behaviors are understood as being due to the electronic charge redistribution, in which the excess Fe ions on the interstitial sites act as electronic charge suppliers that strengthen the electronic connections between the Te/Se and Fe ions on the lattice sites once the temperature is raised. ©2015 The Physical Society of Japan
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    Large magnetoresistance and charge transfer between the conduction and magnetic electrons in layered oxyselenide BiOCu0.96Se
    (Royal Society of Chemistry, 2013-8-27) Karna, SK; Hung, CH; Wu, CM; Wang, CW; Li, WH; Sankar, R; Chou, FC; Avdeev, M
    The electrical and magnetic properties of slightly Cu-deficient BiOCu0.96Se have been investigated using neutron and X-ray diffraction, ac magnetic susceptibility, magnetization and electric resistivity measurements. The layered BiOCu0.96Se crystallizes into a tetragonal lattice with a P4/nmm symmetry. Thermal profiles of the electrical resistivity reveal a semiconductor type behavior, but depart from its course at low temperatures when antiferromagnetic coupling becomes thermally loosened at 140 K. Positive magnetoresistances are obtained at all temperatures studied. With an applied magnetic field of 0.5 kOe, the magnetoresistance reaches 235% at 2 K. It decreases with increasing temperature, but stabilizes to 70% above 60 K. Both ferromagnetic and antiferromagnetic coupling are detected between the Cu spins in the SeCu4 pyramidal blocks, which results in a non-collinear spin arrangement at low temperatures. The antiferromagnetic component becomes disordered above TN = 140 K, whereas the ferromagnetic moment persists up to TC = 300 K. Interlayer charge transfer between the conduction and magnetic electrons gives rise to an anomaly in the magnetic order parameter. © 2013, Royal Society of Chemistry.
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    Magnetic properties and magnetic structure of the frustrated quasi-one-dimensional antiferromagnet SrCuTe2O6
    (American Physical Society, 2020-10-07) Saeaun, P; Zhao, Y; Piyawongwatthana, P; Sato, TJ; Chou, FC; Avdeev, M; Gitgeatpong, G; Matan, K
    Magnetization measurements on single-crystal cubic SrCuTe2O6 with an applied magnetic field along three inequivalent high symmetry directions [100], [110], and [111] reveal weak magnetic anisotropy. The fits of the magnetic susceptibility to the result from a quantum Monte Carlo simulation on the Heisenberg spin-chain model, where the chain is formed via the dominant third-nearest-neighbor exchange interaction J3, yield the intrachain interaction (J3/kB) between 50.12(7) K for the applied field along [110] and 52.5(2) K along [100] with about the same g factor of 2.2. Single-crystal neutron diffraction unveils the transition to the magnetic ordered state as evidenced by the onset of the magnetic Bragg intensity at TN1=5.25(9)K with no anomaly of the second transition at TN2 reported previously. Based on irreducible representation theory and magnetic space group analysis of powder and single-crystal neutron diffraction data, the magnetic structure in the Shubnikov space group P4132, where the Cu2+S=1/2 spins antiferromagnetically align in the direction perpendicular to the spin chain, is proposed. The measured ordered moment of 0.52(6)μB, which represents 48% reduction from the expected value of 1μB, suggests the remaining influence of frustration resulting from the J1 and J2 bonds. ©2020 American Physical Society
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    Sodium layer chiral distribution and spin structure of Na2Ni2TeO6 with a Ni honeycomb lattice
    (American Physical Society, 2017-03-08) Karna, SK; Zhao, Y; Sankar, R; Avdeev, M; Tseng, PC; Wang, CW; Shu, GJ; Matan, K; Guo, GY; Chou, FC
    The nature of Na ion distribution, diffusion path, and the spin structure of P2-type Na2Ni2TeO6 with a Ni honeycomb lattice has been explored. The nuclear density distribution of Na ions reveals a two-dimensional (2D) chiral pattern within Na layers without breaking the original 3D crystal symmetry, which has been achieved uniquely via an inverse Fourier transform (iFT)-assisted neutron-diffraction technique. The Na diffusion pathway described by the calculated isosurface of the Na ion bond valence sum (BVS) map is found consistent to a chiral diffusion mechanism. The Na site occupancy and Ni2+ spin ordering were examined in detail with the neutron diffraction, magnetic susceptibility, specific heat, thermal conductivity, and transport measurements. Signatures of both strong incommensurate (ICM) and weak commensurate (CM) antiferromagnetic (AFM) spin ordering were identified in the polycrystalline sample studied, and the CM-AFM spin ordering was confirmed by using a single-crystal sample through the k scan in the momentum space corresponding to the AFM peak of (12,0,1). © 2017 American Physical Society