Browsing by Author "Chang, W. J."

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Anisotropic magnetoresistance of La0.7Ca0.3MnO3 thin film biepitaxial step junctions
(2006-12-01)
Chen, S. F.
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Chang, W. J.
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Hsieh, C. C.
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Liu, S. J.
;
Juang, J. Y.
The angular dependence of magnetoresistance (MR) of the La0.7Ca0.3MnO3 thin film biepitaxial step junction (BSJ) shows a simple sin(2)(theta) dependence in the in-plane high-field magnetoresistance, with theta being the angle between the applied field and current. This behavior is similar to the spin-orbit coupling-induced anisotropic magnetoresistance (AMR) commonly observed in transition ferromagnetic metals, except for two salient features. First, the maximum MR in the present case occurs at an oblique angle between the applied field (H) and electric current (I), while it is usually observed to occur when H parallel to I. Second, the AMR in the plane perpendicular to the film surface displays a remarkable value (Delta rho/rho similar to 8%), which is about an order of magnitude larger than that of the in-plane AMR. Such a large AMR cannot be solely explained by spin-orbit coupling effect. We suggest instead that the metallic and ferromagnetic inhomogeneous granules existing in the BSJ region might have acted as the source of spin-polarized scattering giving rise to the enhanced AMR when the colossal magnetoresistance was measured across the biepitaxial step boundaries. (c) 2006 American Institute of Physics.
Electronic structure of pyrochlore Cd(2)Re(2)O(7)
(2009-05-13)
Huang, Shih-Wen
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Jeng, Horng-Tay
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Lin, J-Y
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Chang, W. J.
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Chen, J. M.
Detailed band structure calculations have been performed for Cd(2)Re(2)O(7) in high-, middle- and low-temperature (T) phases. The calculations are based on the observed lattice structures from x-ray diffraction measurements. The spin-orbit interaction is incorporated self-consistently in both the generalized gradient approximation (GGA) and the GGA plus Hubbard U (GGA + U) approaches. It is found that the on-site U has negligible effects on the Re 5d band structures; therefore both the GGA and GGA + U Re 5d band energies agree well with the observed O K-edge x-ray absorption spectroscopy (XAS) spectrum, whereas the Cd 4d band energy observed from photoemission spectroscopy can only be correctly reproduced by GGA + U calculations, indicating the relatively itinerant Re 5d and localized Cd 4d electrons. On the other hand, the spin-orbit coupling gives rise to nontrivial spin and orbital magnetic moments for the middle-T phase. Most unexpectedly, we found that the low-T phase exhibits quasi-two-dimensional Fermi surfaces. The calculated carrier numbers for the three phases are, at least qualitatively, consistent with the measured Hall coefficient.
Electronic structure of pyrochlore Cd2Re2O7
(2009-05-13)
Huang, Shih-Wen
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Jeng, Horng-Tay
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Lin, J-Y
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Chang, W. J.
;
Chen, J. M.
Detailed band structure calculations have been performed for Cd2Re2O7 in high-, middle- and low-temperature (T) phases. The calculations are based on the observed lattice structures from x-ray diffraction measurements. The spin-orbit interaction is incorporated self-consistently in both the generalized gradient approximation (GGA) and the GGA plus Hubbard U (GGA + U) approaches. It is found that the on-site U has negligible effects on the Re 5d band structures; therefore both the GGA and GGA + U Re 5d band energies agree well with the observed O K-edge x-ray absorption spectroscopy (XAS) spectrum, whereas the Cd 4d band energy observed from photoemission spectroscopy can only be correctly reproduced by GGA + U calculations, indicating the relatively itinerant Re 5d and localized Cd 4d electrons. On the other hand, the spin-orbit coupling gives rise to nontrivial spin and orbital magnetic moments for the middle-T phase. Most unexpectedly, we found that the low-T phase exhibits quasi-two-dimensional Fermi surfaces. The calculated carrier numbers for the three phases are, at least qualitatively, consistent with the measured Hall coefficient.
Fabrication and low temperature thermoelectric properties of NaxCoO2 (x=0.68 and 0.75) epitaxial films by the reactive solid-phase epitaxy
(2007-02-05)
Chang, W. J.
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Hsieh, C. C.
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Chung, T. Y.
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Hsu, S. Y.
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Wu, K. H.
The authors have fabricated NaxCoO2 thin films via lateral diffusion of sodium into Co3O4 (111) epitaxial films (reactive solid-phase epitaxy [Ohta , Cryst. Growth Des. 5, 25 (2005)]). The environment of thermal diffusion is key to the control of the sodium content in thin films. From the results of x-ray diffraction and in-plane rho(ab), the epitaxial growth and the sodium contents of these films were identified. The thermoelectric measurements show a large thermoelectric power similar to that observed in single crystals. The quasiparticle scattering rate is found to approach zero at low temperatures, consistent with the small residual resistivity, indicating high quality of the NaxCoO2 thin films. (c) 2007 American Institute of Physics.
Fabrication and X-ray absorption NaxCoO2 spectroscopy in layered cobaltate thin films
(2007-03-01)
Chang, W. J.
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Lin, J.-Y.
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Chung, T. Y.
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Chen, J. M.
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Hsu, C.-H.
NaxCoO2 (x similar to 0.68) thin films were fabricated on sapphire (0 0 0 1) substrates via the lateral diffusion of sodium into Co3O4 (1 1 1) films, which were grown by pulsed-laser deposition. From the results of X-ray diffraction and in-plane resistivity rho(ab), the single phase and the metallic behaviors of these NaxCoO2 films can be identified. For the same sodium content x, rho(ab) is consistent with that of single crystals. In addition, the O 1s X-ray absorption near edge spectra of thin films are measured and compared with those of single crystals. (c) 2006 Elsevier B.V. All rights reserved.
Optical and magneto-optical studies of (La(0.7)A(0.3))MnO(3) (A = Sr, Ca, and Ce) thin films
(2006-09-01)
Liu, H. L.
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Zhang, Kenneth Y. -J.
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Wu, L. C.
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Uba, L.
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Uba, S.
The optical and magneto-optical (MO) properties of (La(0.7)A(0.3))MnO(3) (A = Sr, Ca, and Ce) thin films have been investigated. It is found that the value shape of spectra is strongly dependent on different cation dopings. The Sr-doped thin film exhibits a strong Drude response and maximum Kerr rotation angle of 0.32 degrees at similar to 3.4 eV. Interestingly, the replacement of Sr ions with the smaller Ca ions and the tetravalent Ce ions all lead to a remarkable suppression of the carrier density and the MO response. Such differences are interpreted as due to their different electronic band structures. (C) 2006 Elsevier B.V. All rights reserved.
Optical properties of Na0.68CoO2 thin film
(2007)
Hsu, M. H.
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Liu, H. L.
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Chang, W. J.
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Lin, J. Y.
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電子物理學系
The optical reflectance of single-crystalline Na0.68CoO2 (thickness around 3000 angstrom) thin film has been measured over a wide frequency (40-55000 cm(-1)) and at temperatures between 20 and 300 K. The room-temperature optical conductivity spectrum displays a metallic character. In addition, one E-1u phonon mode at about 518 cm(-1) is identified, while the other four phonon peaks are due to the sapphire substrate. As the temperature is lowered, the Drude plasma frequency exhibits little temperature dependence, whereas the scattering rate monotonically decreases with decreasing temperature. Notably, the Drude scattering rate at 20 K is found to approach zero, consistent with the small residual resistivity, indicating high quality of Na0.68CoO2 thin film. (C) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Phase separation and persistent magnetic memory effect in La(0.625)Ca(0.375)MnO(3) and La(0.375)Pr(0.25)Ca(0.375)MnO(3) films
(2009-01-01)
Tsai, Y. T.
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Chang, W. J.
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Hsieh, C. C.
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Luo, C. W.
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Wu, K. H.
Both La(0.375)Pr(0.25)Ca(0.375)MnO(3) (LPCMO) and La(0.625)Ca(0.375)MnO(3) (LCMO) were found to exhibit coexistence of competing orders (phase separation) over a wide temperature range. However, substantial hysteretic behaviors in both of the temperature-dependent resistance [R(T)] and magnetization [M(T)] also known as the persistent magnetic memory effect (PMME)] are only displayed in LPCMO. The results indicate that, in LPCMO, the size distribution of the coexisting charge-ordered insulating and metallic ferromagnetic (FM) phases plays a determinant role in the PMME effects in different temperature regimes. Moreover, due to the direct competition between the two coexisting phases, the system is most susceptible to the external applied field in the hysteretic temperature region. On the other hand, in LCMO, the phase transition between paramagnetic and FM is more like an isomorphic transition in pure materials, and thus does not show significant hysteresis. c 2009 American Institute of Physics. [DOI: 10.1063/1.3055802]
Phase separation and persistent magnetic memory effect in La0.625Ca0.375MnO3 and La0.375Pr0.25Ca0.375MnO3 films
(2009-01-01)
Tsai, Y. T.
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Chang, W. J.
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Hsieh, C. C.
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Luo, C. W.
;
Wu, K. H.
Both La0.375Pr0.25Ca0.375MnO3 (LPCMO) and La0.625Ca0.375MnO3 (LCMO) were found to exhibit coexistence of competing orders (phase separation) over a wide temperature range. However, substantial hysteretic behaviors in both of the temperature-dependent resistance [R(T)] and magnetization [M(T)] also known as the persistent magnetic memory effect (PMME)] are only displayed in LPCMO. The results indicate that, in LPCMO, the size distribution of the coexisting charge-ordered insulating and metallic ferromagnetic (FM) phases plays a determinant role in the PMME effects in different temperature regimes. Moreover, due to the direct competition between the two coexisting phases, the system is most susceptible to the external applied field in the hysteretic temperature region. On the other hand, in LCMO, the phase transition between paramagnetic and FM is more like an isomorphic transition in pure materials, and thus does not show significant hysteresis. c 2009 American Institute of Physics. [DOI: 10.1063/1.3055802]
Temperature-dependent X-ray absorption near edge spectroscopy of doped LaMnO(3): Ion-size and the Jahn-Teller distortion effects
(2009-05-01)
Tsai, Y. T.
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Chang, W. J.
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Huang, S. W.
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Lin, J. -Y.
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Lee, J. Y.
The temperature-dependent X-ray absorption near-edge spectroscopy (XANES) of LaMnO(3) thin films doped with various divalent ions was systematically carried out to investigate the effects of the ion size and associated Jahn-Teller distortions (JTDs) on the electronic structure modifications. Doped LaMnO(3) thin films were grown on (100) SrTiO(3) and (110) NdGaO(3) substrates by pulsed-laser deposition with typical thickness of 250-300 nm. Temperature-dependent resistivity p(T) and magnetization M(T) were measured. The results indicate that the drastic changes in magneto-transport properties induced by the substitution of A-site ion and the splitting of the O K-edge pre-edge peak associated with the e(g)(up arrow) and t(2g)(down arrow) energy levels at low temperatures in La(1-x)Ca(x)MnO(3) and La(1-x)Sr(x)MnO(3) (x = 0.3, 0.375) may both relate to the lattice disorder and the associated JTD effects on modifying the electronic structure of doped-LaMnO(3). (C) 2009 Elsevier B.V. All rights reserved.
Temperature-dependent X-ray absorption near edge spectroscopy of doped LaMnO3: Ion-size and the Jahn-Teller distortion effects
(2009-05-01)
Tsai, Y. T.
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Chang, W. J.
;
Huang, S. W.
;
Lin, J. -Y.
;
Lee, J. Y.
The temperature-dependent X-ray absorption near-edge spectroscopy (XANES) of LaMnO3 thin films doped with various divalent ions was systematically carried out to investigate the effects of the ion size and associated Jahn-Teller distortions (JTDs) on the electronic structure modifications. Doped LaMnO3 thin films were grown on (100) SrTiO3 and (110) NdGaO3 substrates by pulsed-laser deposition with typical thickness of 250-300 nm. Temperature-dependent resistivity p(T) and magnetization M(T) were measured. The results indicate that the drastic changes in magneto-transport properties induced by the substitution of A-site ion and the splitting of the O K-edge pre-edge peak associated with the e(g)(up arrow) and t(2g)(down arrow) energy levels at low temperatures in La1-xCaxMnO3 and La1-xSrxMnO3 (x = 0.3, 0.375) may both relate to the lattice disorder and the associated JTD effects on modifying the electronic structure of doped-LaMnO3. (C) 2009 Elsevier B.V. All rights reserved.