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ABSTRACT: CaCu3Ti4O12 thin films were epitaxially grown on (001) LaAlO3 substrates by pulsed laser deposition under optimized growth conditions. The crystal structure and sample morphology were characterized by x-ray diffraction, AFM, TEM, ellipsometry, and Raman spectroscopy. The dielectric responses of the films were investigated in a large temperature range (5 to 375 K) by infrared reflectivity and impedance spectroscopies. The films exhibited a colossal dielectric response, with the dielectric permittivity reaching 104 at 100 Hz. The results obtained in a broad frequency range allowed us to investigate the behavior of intrinsic and extrinsic dielectric responses of this material. The room temperature centrosymmetrical cubic structure remains stable down to 5 K, though softening of the lower frequency infrared phonon modes indicates an incipient ferroelectric character. The radio frequency dielectric response reveals two relaxations of extrinsic origin, a primary higher frequency relaxation responsible for the colossal dielectric behavior and a secondary one of lower frequency. The activation energies of these processes are compatible with the presence of shallow defect levels created by oxygen vacancies.
Journal of Applied Physics 10/2011; 110(7):074102-074102-8. · 2.17 Impact Factor
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ABSTRACT: La0.7Sr0.3MnO3 (LSMO) thin films grown by pulsed laser deposition on SrTiO3 (STO) substrates with (1 0 0) and (1 1 0) crystallographic orientations are characterized by means of magneto-optic (MO) 'Kerr' spectroscopy at polar and longitudinal magnetizations combined with spectroscopic ellipsometry (SE) in the photon energy range between 1.2 and 4.8 eV. The LSMO film thickness, t, ranges from 10 to 50 nm. Amplitudes in the permittivity tensor spectra, determined from the MO and SE data, depend on t and substrate orientation. The magnetization onset is faster in the films grown on (1 1 0) substrates, probably due to lower strain induced by (1 1 0) substrates. In the films grown on (1 0 0) substrates, the polar and longitudinal MO 'Kerr' spectra can be reasonably explained assuming the diagonal elements of the permittivity tensor to be equal to each other and the off-diagonal elements deduced from the MO polar 'Kerr' spectroscopy. This approach applied to the films grown on (1 1 0) substrates also provides the expected spectral shape; the computed amplitudes are stronger than the observed ones which suggests that the surface morphology affects the permittivity tensor elements in some way. The agreement of the permittivity tensor spectra with those acquired on sintered samples and thick films confirms a perfectly developed perovskite structure even at 10 nm. The study shows that MO spectroscopy and SE can be used for a fast, cheap and efficient diagnostics of the LSMO films in the thickness range relevant for device applications. A low noise level in MO polar 'Kerr' spectra on the thinnest samples (t ≈ 10 nm) indicates that the presence of the room temperature magnetization and LSMO characteristics in MO spectra can safely be detected in films of t ≈ 5 nm.
Journal of Physics D Applied Physics 09/2009; 42(19):195002. · 2.54 Impact Factor
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ABSTRACT: Calcium cobaltite thin films with a nominal ratio Ca/Co=1 were grown on (101)-NdGaO3 substrate by the pulsed laser deposition technique. The structure of the as-deposited metastable phase is solved using a precession electron diffraction three-dimensional data set recorded from a cross-sectional sample. We found that an ordered oxygen-deficient Ca2Co2O5 perovskite of the brownmillerite type with lattice parameters a≈ap√2≈5.46 Å, b≈4ap≈14.88 Å, and c≈ap√2≈5.46 Å (space group: Ibm2) has been stabilized using the substrate-induced strain. The structure and microstructure of this metastable cobaltite are further discussed and compared to related bulk materials based on our transmission electron microscopy investigations. The present results open the route to the resolution of metastable phases prepared in the form of a thin film using the precession electron diffraction and should be useful for the solid state chemistry community.
Phys. Rev. B. 05/2009; 79(18).
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ABSTRACT: Calcium cobaltite thin films with a ratio Ca/Co=1 were grown on (101)-NdGaO3 substrate by the pulsed laser deposition technique. The structure of the deposited metastable phase is solved using a precession electron diffraction 3D dataset recorded from a cross-sectional sample. It is shown that an ordered oxygen-deficient Ca2Co2O5+d perovskite of the brownmillerite-type with lattice parameters a= 0.546nm, b=1.488nm and c=0.546nm (SG: Ibm2) has been stabilized using the substrate induced strain. The structure and microstructure of this metastable cobaltite is further discussed and compared to related bulk materials based on our transmission electron microscopy investigations Comment: 13 pages, 10 figures, 2 tables, accepted in Phys. Rev. B
04/2009;
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ABSTRACT: The thermal hysteresis of the electronic transport properties were studied for V2O3 thin films. The temporal evolution of the resistance shows the out-of-equilibrium nature of this hysteresis with a very slow relaxation. Partial cycles reveal not only a behavior consistent with phase coexistence, but also the presence of spinodal temperatures which are largely separated. The temperature spreading of phase coexistence is consistent with the bulk phase diagram in the pressure-temperature plane, confirming that the film is effectively under an effective pressure induced by the substrate. Comment: Accepted for publication in Europhysics Letters
EPL (Europhysics Letters) 10/2008; · 2.17 Impact Factor
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ABSTRACT: We explore the charge dynamics of a 40-nm-thick strained single-crystal La2/3Sr1/3MnO3 film as a function of temperature. A coherent-incoherent crossover, occurring when the resistivity value approaches the Ioffe-Regel-Mott limit, is observed at T⋆≈200 K in both the dc resistivity and the optical conductivity. This crossover, located far below the metal-insulator transition and the Curie temperature (TC=338 K), is associated to a rapid decrease in the mobile charge-carrier concentration above T⋆. A semiphenomenological model, whose activation energy decreases with the square of magnetization, is used to fit the experimental change of resistivity ln ρ/ρmax and thus determine the film metal-insulator (MI) transition that arises 100 K above T⋆ close to room temperature.
Phys. Rev. B. 09/2008; 78(11).
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ABSTRACT: The spectacular metal-to-insulator transition of V2O3 can be progressively suppressed in thin film samples. Evidence for phase separation was observed using microbridges as a mesoscopic probe of transport properties where the same film possesses domains that exhibit a metal-to-insulator transition with clear first order features or remain metallic down to low temperatures. A simple model consisting of two parallel resistors can be used to quantify a phase coexistence scenario explaining the measured macroscopic transport properties. The interaction between film and substrate is the most plausible candidate to explain this extended phase coexistence as shown by a correlation between the transport properties and the structural data. Comment: extended version accepted in Journal of Physics: Condensed Matter. More references and figures. More details are given on the structural part
Journal of Physics Condensed Matter 07/2008; · 2.55 Impact Factor
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ABSTRACT: A series of epitaxial (LaVO 3) 6m (SrVO 3) m superlattices having the same nominal composition as La 6/7 Sr 1/7 VO 3 , a Mott-Hubbard insulator, were grown with pulsed-laser deposition on [001]-oriented SrTiO 3 substrates, and their superlattice period was varied. When m = 1, the insulating resistivity of bulk-like La 6/7 Sr 1/7 VO 3 is obtained; however, an increase in the periodicity (m ≥ 2) results in metallic samples. Comparison of the superlattice periodicity with the coherence length of charge carriers in perovskite oxide heterostructures are used to understand these observations. A filling-controlled insulator-metal transition was induced by placing a single dopant layer of SrVO 3 within LaVO 3 layers of varying thickness.
11/2007; 73.
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ABSTRACT: A series of epitaxial (LaVO3)6m(SrVO3)m superlattices having the same nominal composition as La6/7Sr1/7VO3, a Mott-Hubbard insulator, were grown with pulsed-laser deposition on [001]-oriented SrTiO3 substrates, and their superlattice period was varied. When m=1, the insulating resistivity of bulk-like La6/7Sr1/7VO3 is obtained; however, an increase in the periodicity (m>=2) results in metallic samples. Comparison of the superlattice periodicity with the coherence length of charge carriers in perovskite oxide heterostructures are used to understand these observations. A filling-controlled insulator-metal transition was induced by placing a single dopant layer of SrVO3 within LaVO3 layers of varying thickness. Comment: Submitted to Applied Physics Letters
10/2007;
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ABSTRACT: High quality vanadium sesquioxide V2O3 films (170-1100 {\AA}) were grown using the pulsed laser deposition technique on (0001)-oriented sapphire substrates, and the effects of film thickness on the lattice strain and electronic properties were examined. X-ray diffraction indicates that there is an in-plane compressive lattice parameter (a), close to -3.5% with respect to the substrate and an out-of-plane tensile lattice parameter (c) . The thin film samples display metallic character between 2-300 K, and no metal-to-insulator transition is observed. At low temperature, the V2O3 films behave as a strongly correlated metal, and the resistivity (\rho) follows the equation \rho =\rho_0 + A T^2, where A is the transport coefficient in a Fermi liquid. Typical values of A have been calculated to be 0.14 \mu\Omega cm K^{-2}, which is in agreement with the coefficient reported for V2O3 single crystals under high pressure. Moreover, a strong temperature-dependence of the Hall resistance confirms the electronic correlations of these V2O3 thin films samples.
10/2007;
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ABSTRACT: Epitaxial La2NiMnO6 thin films have been grown on (001)-oriented SrTiO3 using the PLD technique. The thin films are semiconducting and FM with a TC close to 270K, a coercive field of 920Oe, and a saturation magnetization of 5muB per f.u. TEM, conducted at RT, reveals a majority phase having "I-centered" structure with a=c=1.4asub and b=2asub along with a minority phase-domains having "P-type" structure (asub being the lattice parameter of the perovskite structure). A discusion on the presence of Ni/Mn long-range ordering, in light of recent literature on double perovskites La2NiMnO6 is presented. Comment: To be published in Applied Physics Letters
06/2007;
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ABSTRACT: We have measured the contribution of magnetic domain walls (DWs) to the electric resistance in epitaxial manganite films patterned by electron-beam lithography into a track containing a set of notches. We find a DW resistance-area (RA) product of ~2.5 10^(-13) Ohm/m^2 at low temperature and bias, which is several orders of magnitude larger than the values reported for 3d ferromagnets. However, the current-voltage characteristics are highly linear which indicates that the DWs are not phase separated but metallic. The DWRA is found to increase upon increasing the injected current, presumably reflecting some deformation of the wall by spin-transfer. When increasing temperature, the DWRA vanishes at ~225K which is likely related to the temperature dependence of the film anisotropy.
11/2006;
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ABSTRACT: V2O3 thin films about 10 nm thick were grown on Al2O3 (0001) by pulsed laser deposition. The XRD analysis is in agreement with R-3c space group. Some of them exhibit the metal / insulator transition characteristic of V2O3 bulk material and others samples exhibit a metallic behavior. For the latter, the XPS analysis indicates an oxidation state of +III for vanadium. There is no metal / insulator transition around 150 K in this sample and a strongly correlated Fermi liquid rho = AT2 behavior of the resistivity at low temperature is observed, with a value of A of 1.2 10-4 ohm cm, 3 times larger than the bulk value at 25 kbar.
09/2006;
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J. Mistrik,
T. Yamaguchi,
M. Veis,
E. Liskova,
S. Visnovsky,
M. Koubaa,
A. M. Haghiri-Gosnet,
Ph. Lecoeur,
J. P. Renard,
W. Prellier, B. Mercey
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ABSTRACT: Magneto-optical (MO) spectroscopy combined with spectroscopic ellipsometry has been used to learn about the electronic transitions in La2/3Sr1/3MnO3 (LSMO) film pulse laser deposited onto SrTiO3 (100) substrates. The dispersion function of LSMO films was parametrized by the sum of three damped Lorentz oscillators and adjusted together with the film thickness numerically. With the knowledge of film optical properties the off-diagonal elements of the LSMO permittivity tensor (magneto-optical constants) were calculated from the complex polar Kerr effect measured on the thickest (60 nm) LSMO film (considered as optically thick). The considered approximation as well as the reliability of the obtained optical and MO constants was verified by the consistence of the calculated and experimental MO spectra for the thinner LSMO films (where the propagation effect plays an important role). The resolved electronic transition centered at about 4 eV features the spectral dependences of diagonal and off-diagonal elements of LSMO permittivity in the studied spectral range. In the molecular orbital picture this could be assigned to the charge transfer electronic transition from the O 2p states to the Mn 3d t2g states, split by the spin-orbit interaction.
Journal of Applied Physics 04/2006; 99(8):08Q317-08Q317-3. · 2.17 Impact Factor
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ABSTRACT: The prediction of ferromagnetism at room temperature in Co-ZnO thin films has generated a large interest in the community due to the possible applications. However, the results are controversial, going from ferromagnetism to non-ferromagnetism, leading to a large debate about its origin (secondary phase, Co clusters or not). By carefully studying the micro-structure of various Co-ZnO films, we show that the Co2+ partly substitutes the ZnO wurtzite matrix without forming Co clusters. Surprisingly, the ferromagnetism nature of the films disappears as the Co content increases. In addition, our results suggest that the observed ferromagnetism is likely associated to a large amount of defects- close to the interface and strongly depending on the growth temperature- which may explained the spreading of the results. Comment: 4 pages, 4 figures, to be published in Journal of Applied Physics (2006)
04/2006;
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ABSTRACT: Thin films of perovskitelike CaCu2Mn5O12 have been grown by pulsed laser deposition. These films are ferromagnetic with a Curie temperature (TC) of 292 K which is very close to that expected for the bulk material (TC = 310 K). Increasing the oxygen pressure during cooling does not markedly affect the out-of-plane lattice parameter (c) or TC of the films, but enhances the magnetization at low temperature. Transport properties of films exhibit metallic behavior in the temperature range of 10–360 K. The most important difference between these films and the bulk material was observed in their magnetotransport characteristics. The magnetoresistance effect is important at low temperatures for the bulk material, while for the thin films, this effect is large (−8%) close to TC, as expected for a regular ferromagnetic material.
Applied Physics Letters 12/2005; 87(24):242502-242502-3. · 3.84 Impact Factor
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ABSTRACT: Sm0.5Ca0.5MnO3 manganite thin films with charge ordering (CO) and colossal magnetoresistance were synthesized by pulsed laser deposition on (100)‐SrTiO3 and (100)‐LaAlO3 substrates. We first compare the structural modifications as functions of the substrate and film thickness. Secondly, transport properties in magnetic fields up to 24 T were studied and the temperature-field phase diagram describing the stability of the CO state was established. This enables us to compare the thin film to the bulk material. We show that some substrate-induced structural modifications exist as a result of which the CO melting magnetic field is greatly reduced. Moreover, no modification of the lattice parameters is observed with temperature decrease. We then propose an explanation based on the quenching of the thin film’s unit cell which then adopts the in-plane lattice parameters of the substrate and thus prevents the complete setting in of the CO state at low temperatures.
Journal of Applied Physics 11/2005; 98(9):093903-093903-6. · 2.17 Impact Factor
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ABSTRACT: Thin films of Sm0.5Ca0.5MnO3 manganites with charge ordering (CO) properties and colossal magnetoresistance were synthesized by pulsed laser deposition technique on (100)-SrTiO3 and (100)-LaAlO3 substrates. We first compare the structural modifications as function of the substrate and film thickness. Secondly, measuring transport properties in magnetic fields up to 24T, we establish the temperature-field phase diagram describing the stability of the CO state and compare it to bulk material. We show that some structural modification induced by the substrate occurs and that the CO melting magnetic field is greatly reduced. Moreover, with the temperature decrease, no modification of the lattice parameters is observed. We then propose an explanation based on the quenching of the unit cell of the film that adopts the in-plane lattice parameters of the substrate and thus, prevents the complete growth of the CO state at low temperature.
10/2005;
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ABSTRACT: A series of superlattices and trilayers composed of ferromagnetic and ferroelectric or paraelectric layers were grown on (100) SrTiO3 by the pulsed laser deposition technique. Their structural and magneto-electric properties were examined. The superlattices made of ferromagnetic Pr0.85Ca0.15MnO3 (PCMO) and a ferroelectric, namely Ba0.6Sr0.4TiO3 (BST) or BaTiO3, showed enhanced magnetoresistance (MR) at high applied magnetic field, whereas such enhancement was absent in Pr0.85Ca0.15MnO3/SrTiO3 superlattices, which clearly demonstrates the preponderant role of the ferroelectric layers in this enhanced MR. Furthermore, the absence of enhanced MR in trilayers of PCMO/BST indicates that the magneto-electric coupling which is responsible for MR in these systems is stronger in multilayers than in their trilayer counterparts. Comment: to be published in J. Appl. Phys
03/2005;
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ABSTRACT: We report a systematic study of Pr0.5Sr0.5MnO3 epitaxial thin films grown on (100)–LaAlO3 single-crystal substrates using the pulsed laser deposition technique. Two different phases are observed depending on the deposition temperature (TS) of the film, leading respectively to an out-of-plane lattice parameter of 0.3846 nm at low temperature (TS < 600°C) and 0.3941 nm at higher temperatures (TS > 670°C). Moreover, we find that the field-induced insulator-to-metal transition is correlated to the smaller lattice parameter and, thus, can be directly tuned by changing the deposition temperature of the film. Finally, we conclude that the growth conditions of the film (in particular the deposition temperature) directly influence the substrate-induced strain and, therefore, the transport properties.
Journal of Physics D Applied Physics 12/2004; 38(1):172. · 2.54 Impact Factor
