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The magnetism-induced ferroelectricity in the E-type orthorhombic HoMnO3 (o-HMO) films grown on Nb-doped and undoped SrTiO3(001) substrates was studied. The temperature dependent magnetization in these films evidently exhibits an antiferromagnetic ordering around 44 K and an anomalous second magnetic ordering along the c-axis near 35 K. Interestingly, at this anomalous transition temperature, an associated ferroelectric polarization was probed by the P-E hysteresis measurements, indicating the intimated coupling between the ferroelectric response and the reordering of Mn spins, which has never been disclosed previously by dielectric constant measurements. [ABSTRACT FROM AUTHOR]

In this work, we use ultrafast optical pump-optical probe spectroscopy to probe the polaron dynamics in La0.7Ca0.3MnO3 (LCMO) thin films. The temporal evolution in transient reflectivity change ΔR/R exhibits two relaxing components: a fast component with a time constant of subpicosecond and a slow component with time constant ranging from tens of picoseconds to hundreds of picoseconds. The amplitude of the fast component, though has been seldom discussed before, exhibits the similar temperature dependence with that of the resistivity and the neutron scattering intensity due to nanoscale correlated polarons. The results strongly suggest that the fast photoinduced reflectivity change may have been due to the photoexcitation and trapping process of correlated Jahn–Teller polarons in the paramagnetic and ferromagnetic phases and, thus, implies the presence of electronic inhomogeneity in LCMO manganites. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Orthorhombic YMnO3 thin films with (200), (020), and (001) orientations were, respectively, obtained by pulsed laser deposition on SrTiO3(110), LaAlO3(110), and SrTiO3(001) substrates. The results demonstrate that the strain between film and substrate can serve as an alternative in transforming the thermodynamically stable hexagonal YMnO3 into the orthorhombic phase, which previously could be obtained with high-pressure high temperature syntheses and epitaxy-stabilized thin film processes. More importantly, these films allow us, for the first time, to unambiguously disclose the intrinsic magnetic property along different crystallographic orientations. Our results show that, although the antiferromagnetic (AFM) ordering remains the same, there is an additional spin reordering transition which is very much dependent on the crystallographic orientation along which the measuring field was applied and on the in-plane crystallographic alignment of the films. Detailed analyses indicate that the origin of the observed second reordering may be due to some strain-induced canted AFM state instead of the incommensurate lock-in transitions conceived previously. [ABSTRACT FROM AUTHOR]

The angular dependence of magnetoresistance (MR) of the La0.7Ca0.3MnO3 thin film biepitaxial step junction (BSJ) shows a simple sin2(θ) dependence in the in-plane high-field magnetoresistance, with θ 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∥I. Second, the AMR in the plane perpendicular to the film surface displays a remarkable value (Δρ/ρ∼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. [ABSTRACT FROM AUTHOR]

Single-phase electron-doped manganite thin films with nominal composition of La0.7Ce0.3MnO3 (LCeMO) have been prepared on SrTiO3 (100) substrates by pulsed laser deposition. The conditions for obtaining purely single-phase LCeMO films lie within a very narrow window of substrate temperature (Ts∼720 °C) and laser energy density (ED∼2 J/cm2) during deposition. In situ postdeposition annealing, mainly to relax the possible epitaxial in-plane tensile strain between the film and the substrate, leads to an increasing c-axis lattice constant accompanied by the formation of secondary CeO2 phase and higher metal-insulator transition temperature. This is indicative of a strong coupling between the electron and lattice degree of freedom. [ABSTRACT FROM AUTHOR]

Unoccupied states of the (110) YBa[sub 2]Cu[sub 3]O[sub 6.9] (YBCO) thin films were investigated by the Eab and Ec polarized O 1 s x-ray absorption near edge structure (XANES) spectroscopy. With correction of the self-absorption effect, the Eab O 1 s XANES spectra of the (110) YBCO films obtained from the normal incidence geometry and grazing incidence geometry are very similar. After the self-absorption correction, the Ec spectrum of the (110) YBCO film obtained from the normal incidence geometry is almost identical to that of the (001) YBCO film obtained from the grazing incidence geometry. Hole distribution over the different oxygen sites has been derived. The hole number in the apical sites in some thin films obtained in the present investigation is larger than that from the previous x-ray absorption studies of single crystals and from the theoretical calculations. The comparison of the XANES spectra between the (001) and (110) YBCO thin films is also presented. [ABSTRACT FROM AUTHOR]

Single phase TiO[sub 2] thin films, of either rutile or anatase structure, have been prepared on SrTiO[sub 3](STO)(100) substrates by in situ pulsed laser deposition (PLD). Thermodynamically unfavorable, for films deposited on STO(100) substrate directly, pure anatase TiO[sub 2](001) films were formed even when a rutile TiO[sub 2] (110) substrate was used as a target. On the other hand, pure rutile TiO[sub 2](110) films were obtained by oxidizing PLD TiN films in-situ at temperatures higher than 700 °C. The optimized deposition conditions for preparing TiN and TiO[sub 2] films were reported. The crystalline structure, surface morphology, and electronic structure of these films were examined. A mechanism of the process of film formation is also proposed. [ABSTRACT FROM AUTHOR]

Investigates the electrical properties of ultrathin silicon oxides grown through the oxidation kinetics of microwave plasma afterglow oxidation. Characteristics of ultrathin oxides; Description of the breakdown field distributions of the as-grown and fluorinated oxides; Inherent properties of oxides grown by microwave plasma afterglow oxidation.

The dichotomy of photoinduced quasiparticle dynamics in the anisotropic high Tc superconductors (YBa2Cu3O7) is revealed by using the orientation-resolved femtosecond reflection spectroscopy in the specially designed thin films. The transient reflectivity ΔR/R along the ab diagonal and b axis are totally different in the slow relaxation process which relates to the opening of the superconducting gap. In addition, the distribution of the temperature evolution of ΔR/R on the CuO2 planes of YBa2Cu3O7 may indicate the d-wave symmetry of the superconducting gap in high Tc superconductors. [ABSTRACT FROM AUTHOR]

Superconducting NdBa[sub 2] Cu[sub 3] O[sub 7-δ] (NBCO) thin films with T[sub c] =89 K have been successfully grown on bare (1&1macr;02) sapphire substrates by pulsed laser deposition (PLD). The X-ray diffraction results show that the as-grown NBCO films are all c-axis oriented with no observable second phase. The c-axis parameter decreases monotonically with increasing the deposition tempera- ture (T[sub s]), suggesting that the corresponding degradation of T[sub c] for T[sub s] > 790 o C might not originate from oxygen deficiency. It is conceived that the film- substrate reaction that occurring at the interface may cause Ba-deficiency and hence enhance Nd-Ba antisite substitutions. [ABSTRACT FROM AUTHOR]

Highly in-plane aligned a-axis oriented YBa[sub 2]Cu[sub 3]O[sub 7-δ] (YBCO) thin films with unprecedented transition temperature (T[sub c] ∼ 91.3 K) have been deposited on (100) LaSrGaO[sub 4] substrates using a continuously grown PrBa[sub 2]Cu[sub 3]O[sub 7-δ] (PBCO) template layer by pulsed laser. The anisotropies between b- and c-axis were investigated by transport measurements and atomic force microscopy (AFM). Although, the onset T[sub c]'s along b- and c-axis appeared approximately the same, a significant filing was observed in the resistive transition along c-axis. The temperature-dependent of the tailing effect, however, cannot be explained by thermally activated phase-slippage (TAPS) commonly encountered in grain-boundary weak links. [ABSTRACT FROM AUTHOR]

The characteristics of photogenerated terahertz radiation from a current-biased superconducting YBa[sub 2] Cu[sub 3] O[sub 7-δ] (YBCO) bow-tie antenna were investigated using a free-space electrooptic sampling technique. Picosecond electromagnetic pulses about 450 fs wide were obtained. The frequency spectrum derived by Fourier transforming the picosecond pulses spans over 0.1-4 THz. The dynamics of the quasi-particles optically induced by the uitrafast laser pulse primarily determines the performances of the transient terahertz radiation generated under different operating parameters. The results indicate a characteristic quasi-particle relaxation time of about 2.5 ps close to the critical temperature T[sub c], and a faster time at lower temperatures. [ABSTRACT FROM AUTHOR]

The perovskite CaBi4Ti4O15 thin film for resistive random access memory device application has been investigated in this work. Experimentally, the CaBi4Ti4O15 thin films were deposited on the indium-tin oxide/glass substrates by radio-frequency (RF) magnetron sputtering and then annealed by rapid thermal annealing. The annealing effects on the bipolar resistive switching behaviours and the conduction mechanisms of the Al/CaBi4Ti4O15/indium-tin oxide resistive random access memory structure with a compliance current of 0·1 A were discussed in this work. The results show a stable bipolar resistive switching with a retention time of over 1 × 104 s and a high resistance state/low resistance state ratio larger than 5 × 102. [ABSTRACT FROM AUTHOR]

The ferromagnetic properties of erbium-doped GaN (GaN:Er) epilayers grown by metal-organic chemical vapor deposition were studied. It is found that the different tensile strains produced by the respective lattice mismatch for different substrates used (GaN/Al2O3, AlN/Al2O3, GaN/Si (111), and c-GaN bulk) correlate well with the observed room-temperature saturation magnetization. Under application of a magnetic field, the photoluminescence of the erbium dopant, which causes the ferromagnetism, indicates that the magnetic states of the ions are coupled to the electronic states of the host. These results hold promise for the use of strain to control the magnetic properties of GaN:Er films for spintronic applications. [ABSTRACT FROM AUTHOR]

Erbium doped InGaN alloys (InGaN:Er) were grown on Si (001) substrates using metal organic chemical vapor deposition. The growth of epitaxial films was accomplished by depositing InGaN:Er on GaN templates deposited on 7.3° off-oriented Si (001) substrates which were prepared by etching and subsequent selective area growth. X-ray diffraction measurements confirmed the formation of wurtzite InGaN [formula] epilayers, which exhibit strong photoluminescence emission at 1.54 μm. The observed emission intensity at 1.54 μm was comparable to that from similar alloys grown on GaN/AlN/Al2O3 templates. These results indicate the high potential for on-chip integration of erbium based photonic devices with complementary metal oxide semiconductor technology. [ABSTRACT FROM AUTHOR]

The authors have fabricated NaxCoO2 thin films via lateral diffusion of sodium into Co3O4 (111) epitaxial films (reactive solid-phase epitaxy [Ohta et al., 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 ρab, 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. [ABSTRACT FROM AUTHOR]

The authors report on the excitation dynamics of the photoluminescence (PL) emission of Er doped GaN thin films synthesized by metal organic chemical vapor deposition. Using the frequency tripled output from a Ti:sapphire laser, the authors obtained PL spectra covering the ultraviolet (UV) to the infrared regions. In the UV, a dominant band edge emission at 3.23 eV was observed at room temperature; this is redshifted by 0.19 eV from the band edge emission of undoped GaN. An activation energy of 191 meV was obtained from the thermal quenching of the integrated intensity of the 1.54 μm emission line. This value coincides with the redshift of the band edge emission and is assigned to the ErGa-VN complex level. [ABSTRACT FROM AUTHOR]

Deep ultraviolet emission from gadolinium (Gd)-implanted AlN thin films has been observed using photoluminescence (PL) spectroscopy. The AlN epilayers were ion implanted with Gd to a total dose of ∼6×1014 cm-2. Using the output at 197 nm from a quadrupled Ti:sapphire laser, narrow PL emission was observed at 318 nm, characteristic of the trivalent Gd ion. A broader emission band, also centered at 318 nm, was measured with excitation at 263 nm. The PL emission intensity decreased by less than a factor of 3 over the sample temperature range of 10–300 K and decay transients were of the order of nanoseconds. [ABSTRACT FROM AUTHOR]

The strain effects on the electronic structures of La0.67Ca0.33MnO3 (LCMO) and La0.8Ba0.2MnO3 (LBMO) thin films have been studied by O K-edge x-ray absorption near edge structure (XANES) spectroscopy. For LCMO, the first-principles calculations reveal that the features in the XANES spectra are associated with hybridized states between O 2p and Mn minority-spin 3d t2g and eg, La 5d/Ca 3d, and Mn 4s/Ca 4p states. An analysis of these features shows that the tensile strain decreases substantially La–O and Ca–O hybridization and TC for LCMO. For LBMO, the small compressive strain enhances slightly La–O and Ba–O hybridization and TC. [ABSTRACT FROM AUTHOR]

Unintentionally doped Al0.67Ga0.33N epilayers were grown on AlN/sapphire templates by metalorganic chemical vapor deposition. Optimized undoped Al0.67Ga0.33N epilayers exhibited an n-type conductivity as confirmed by Hall-effect measurement with a room-temperature resistivity of about 85 Ω cm. Variable temperature Hall-effect measurement revealed the existence of a shallow donor level with activation energy of about 90 meV. The photoluminescence (PL) spectra exhibited an emission peak at 4.13 eV (4.06 eV) related to an impurity transition at 10 K (300 K). Temperature dependent PL measurement also confirmed the existence of a shallow donor with comparable activation energy as that obtained by Hall-effect measurement. Isolated oxygen impurities are believed to be a strong candidate of the donor that remains as a shallow state in AlxGa1-xN up to x∼0.7. Compensating defects and the nature of the O donor in Al0.67Ga0.33N epilayers are also discussed. [ABSTRACT FROM AUTHOR]

Discusses the X-ray diffraction analysis of the defect structure in aluminum gallium nitride films grown by metalorganic chemical vapor deposition. Relation between the dislocation densities and aluminum content of the crystal; Suggestion that the edge dislocation density is significantly influenced by the average grain diameter; Claim that atomic diffusion appears to be a dominant process associated with the film's growth allowing aluminum atoms to diffuse and fill the gallium sites in the near-interface region in the film.

A1N epilayers with high optical qualities have been obtained by metalorganic chemical vapor deposition on sapphire substrates. Deep UV picosecond time-resolved photoluminescence (PL) spectroscopy has been employed to study the optical transitions in A1N epilayers. Two PL emission lines associated with the donor bound exciton (D[sup 0]X, or I[sub 2]) and free exciton (FX) transitions have been observed, from which the binding energy of the donor bound excitons in A1N epilayers was determined to be around 16 meV. Time-resolved PL measurements revealed that the recombination lifetimes of the I[sub 2] and free exciton transitions in A1N epilayers were around 80 and 50 ps, respectively. The temperature dependencies of the free exciton radiative decay lifetime and emission intensity were investigated, from which a value of about 80 meV for the free exciton binding energy in A1N epilayer was deduced. This value is believed to be the largest free exciton binding energy ever reported in semiconductors, implying excitons in A1N are an extremely robust system that would survive well above room temperature. This together with other well-known physical properties of A1N may considerably expand future prospects for the application of III-nitride materials. [ABSTRACT FROM AUTHOR]

We observed strong green luminescence (around 2.37 eV) in two InAlGaN thin film samples, which were originally prepared for applications in the UV range. Based on the observation of the InN peaks and the extended distributions, corresponding to InGaN compounds, in the x-ray diffraction patterns, such green luminescence was attributed to the formation of indium-rich clusters, which formed localized states, in the quaternary films of low indium content (2%). Carrier trapping of such localized states, particularly when carriers received thermal energy such that they could overcome certain potential barriers around the clusters, resulted in strong green luminescence. The observed optical behaviors, including the blueshift phenomenon of photoluminescence spectral peak position in varying temperature, showed that the green luminescence was quite different from the previously reported yellow luminescence, which resulted from defects. [ABSTRACT FROM AUTHOR]

Epitaxial CrO[sub 2] (110)-oriented films were fabricated on Si (100) substrates buffered by rutile TiO[sub 2] derived from oxidation of a pulsed-laser-deposited TiN layer. The epitaxial films of CrO[sub 2] were prepared by chemical vapor deposition in a two-zone furnace with oxygen flow from a CrO[sub 3] precursor. The transport measurements show that the CrO[sub 2] films are metallic with a Curie temperature of about 380 K. The temperature dependence of resistivity was best described by a phenomenological expression ρ(T) = ρ[sub 0] +AT²e[sup (-δ/T)] over the range of 5-350 K with &de;ta= 94 K. The magnetic measurements show the in-plane coercive fields are about 30 and 60 Oe at 300 and 5 K, respectively. The temperature dependent spontaneous magnetization follows Bloch's T[sup 3/2] law and the slope suggests a critical wavelength of λ[sub δ] ∼ 30.6 Å beyond which spin-flip scattering becomes important. [ABSTRACT FROM AUTHOR]

We studied ultrafast dynamics and photoinduced mechanical strain of BiFeO3 thin films by dual-color transient reflectivity measurements (ΔR/R). Anisotropic photostriction in BiFeO3 is found to be mainly driven by the optical rectification effect. Results of the photostriction at various thicknesses show that the estimated sound velocity along [110] direction of BiFeO3 is 4.76 km/s. [ABSTRACT FROM AUTHOR]

Abstract: Na x CoO2 (x∼0.68) thin films were fabricated on sapphire (0001) substrates via the lateral diffusion of sodium into Co3O4 (111) films, which were grown by pulsed-laser deposition. From the results of X-ray diffraction and in-plane resistivity ρ ab, the single phase and the metallic behaviors of these Na x CoO2 films can be identified. For the same sodium content x, ρ 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. [Copyright &y& Elsevier]

Abstract: 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 250–300nm. Temperature-dependent resistivity ρ(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(↑) and t 2g(↓) energy levels at low temperatures in La1− x Ca x MnO3 and La1− x Sr x MnO3 (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. [Copyright &y& Elsevier]

Abstract: Single-phase La0.7Sn0.3MnO3 (LSnMO) thin films were fabricated on SrTiO3 (STO) substrates by pulsed laser deposition (PLD). The as-deposited films, though were insulating with no sign of insulator-–metal transition (IMT), did display paramagnetic–ferromagnetic transition (PFT) around 150K. Ex situ annealing of the films at 850°C in 250Torr oxygen for 4h, nonetheless, not only significantly improves the crystallographic quality but also raises IMT and PFT at 315K and 320K, respectively. The manifestations of spin-glass-like behavior in both the as-deposited and post-annealed films suggest that it could be intrinsic LSnMO, albeit transport properties can be drastically changed by the strain originating from large ion-size misfit between Sn and La and film/substrate epitaxial relation. The preliminary X-ray absorption spectroscopy shows signature of Mn3+/Mn2+ mixed-valence indicating that tetravalent Sn ions does result in electron-doping into the band of Mn. [Copyright &y& Elsevier]

The biepitaxial La0.7Ca0.3MnO3 (LCMO) thin films grown on SrTiO3 substrates using a buffer layer of anatase TiO2 were fabricated. The magnetoresistance (MR) of biepitaxial step junction (BSJ) across the boundary layer of biepitaxial LCMO (0 0 1) and LCMO (1 1 0) film was investigated. The temperature and field dependence of MR for BSJ are qualitatively similar to those obtained in other type of artificial grain boundaries with a comparable MR ratio at low temperatures. However, the observed linear current–voltage characteristic across BSJ is in sharp contrary to the commonly reported non-ohmic characteristics. The results are consistent with features predicted by the model of spin-dependent transport across a depressed magnetic ordering and metallic-like junction layer. [Copyright &y& Elsevier]

The strain effects on the electronic structures of La0.67Ca0.33MnO3 (LCMO) and La0.8Ba0.2MnO3 (LBMO) thin films have been studied by O K-edge x-ray absorption near edge structure (XANES) spectroscopy. For LCMO, the first-principles calculations reveal that the features in the XANES spectra are associated with hybridized states between O 2p and Mn minority-spin 3d t2g and eg, La 5d/Ca 3d, and Mn 4s/Ca 4p states. An analysis of these features shows that the tensile strain decreases substantially La–O and Ca–O hybridization and TC for LCMO. For LBMO, the small compressive strain enhances slightly La–O and Ba–O hybridization and TC. [ABSTRACT FROM AUTHOR]

The photoexcited carrier relaxation dynamics in a-axis oriented YBa2Cu3O7−δ (YBCO) thin films has been investigated by femtosecond time-resolved spectroscopy. Distinct responses along b-axis (CuO3 chain) and c-axis of YBCO have been separated by the polarization-dependent femtosecond pump–probe measurements. It is found that the transient reflectivity (ΔR/R) curves for the electric field E of the polarized light parallel to the b- or c-axes of the a-axis oriented YBCO film (E∥b or E∥c) are significantly different. The opening of superconducting gap in CuO3 chain was obviously observed from the measurements of the ΔR/R curves for E∥b at temperatures below Tc. However, the ΔR/R curves for E∥c did not exhibit the similar characteristics. [Copyright &y& Elsevier]

Abstract: The optical and magneto-optical (MO) properties of (La0.7A0.3)MnO3 (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° at ∼3.4eV. 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. [Copyright &y& Elsevier]