S. Y. Dai

High sensitivity of positive magnetoresistance in low magnetic field in perovskite oxide p–n junctions

Large positive magnetoresistance (MR) and high MR sensitivity in low magnetic fields have been discovered in the Sr-doped LaMnO3 and Nb-doped SrTiO3 p-n junctions fabricated by laser molecular-beam epitaxy. The MR ratios, defined as DeltaR/R-0, DeltaR = R-H-R-0, are observed as large as 11 % in 5 Oe, 23 % in 100 Oe, and 26 % in 1000 Oe at 290 K; 53% in 5 Oe, 80 % in 100 Oe, and 94 % in 1000 Oe at 255 K. The MR sensitivities are 85 Omega/Oe at 290 K, 246 Omega/Oe at 255 K, and 136 Omega/Oe at 190 K, respectively, with the applied magnetic field changed from 0 to 5 Oe. The positive MR ratios and high MR sensitivities of the p-n junctions are very different from that of the LaMnO3 compound family

Picosecond photoelectric characteristic in La0.7Sr0.3MnO3∕Si p-n junctions

Ultrafast photoelectric effects have been observed in La0.7Sr0.3MnO3∕Si p-n junctions fabricated by laser molecular-beam epitaxy. The rise time was ∼210ps and the full width at half-maximum was ∼650ps for the photovoltaic pulse when the junction was irradiated by a 1064nm laser pulse of 25ps duration. The photovoltaic sensitivity was as large as 435mV∕mJ for a 1064nm laser pulse. No such photovoltaic signal was observed with irradiation from a 10.6μm CO2 laser pulse. The results reveal that this phenomenon is an ultrafast photoelectric effect.

Positive colossal magnetoresistance in a multilayer p–n heterostructure of Sr-doped LaMnO3 and Nb-doped SrTiO3

A positive colossal magnetoresistance (CMR) has been discovered in an epitaxial multilayer p–n heterostructure fabricated with Sr-doped LaMnO3 and Nb-doped SrTiO3 by laser molecular-beam epitaxy. In contrast to the negative CMR of the LaMnO3 compound family, positive CMR is observed in the temperature range from 100 to 300 K. The largest value of the magnetoresistance (MR) ratio (ΔR/R0,ΔR=RH−R0), 517%, is one order of magnitude larger than that of simple p–n junctions of the same materials previously reported. A very large MR ratio, 297%, remains in a low field of 0.01 T. Even at a temperature as high as 300 K, a MR ratio as large as 17.3% is still observed.

Reduction of leakage current by Co doping in Pt(Ba0.5Sr0.5TiO3/Nb-SrTiO3 capacitor

Effect of Co doping on leakage current has been investigated in capacitor consisting of Ba0.5Sr0.5Ti1−xCoxO3 (BSTC, x=0, 0.002, 0.010) thin film, Pt top electrode, and Nb-doped SrTiO3 (STON) bottom electrode. Co doping remarkably decreases the leakage current in BSTC thin film, such as from 9×10−7 A in undoped thin film to 8×10−11 A in 1.0 at. % Co-doped BSTC thin film at bias voltage of 6 V. In the case of the Pt electrode positively biased, the leakage current shows space-charge-limited-current behavior. The trap-filled-limit voltage and the calculated trapped electron density increase with Co concentration in BSTC thin film. The mechanism of the reduction of the leakage current by Co doping is discussed.

Structural, electric and magnetic properties of the electron-doped manganese oxide: La1−xTexMnO3 (x=0.1, 0.15)

In this study, the electrical and magnetic properties of La1−xTexMnO3 (x=0.1, 0.15), which is a new material and shows good colossal magnetoresistance behavior, have been investigated. These compounds have rhombohedral structure. In this material, Te replaced a part of La ions, which induced the lattice cell constriction and Mn–O–Mn bond angle widening. X-ray photoemission spectroscopy measurement revealed that the Te ions were in the tetravalent state and the manganese ions could be considered as in a mixture state of Mn2+ and Mn3+. Thus the material could be viewed as an electron-doped compound. The Curie temperature (Tc) was about 240 and 255 K for x=0.1, 0.15, respectively. The maximum magnetoresistance ratio MR=[ρ(0)−ρ(H)]/ρ(0) was about 51% at 200 K and in the applied magnetic field of 40 kOe.

Raman spectroscopy studies of Ce-doping effects on Ba0.5Sr0.5TiO3 thin films

Ba0.5Sr0.5TiO3 (BST) thin films are among the best-known ferroelectric and dielectric materials. Ce-doped BST films have been fabricated by pulsed laser deposition in order to enhance their dielectric properties. X-ray diffraction, atomic force microscopy, and Raman spectroscopy have been used to study variations of crystal structure, surface morphologies, and phase stability of Ce-doped BST films, respectively. A strong influence of Ce doping on the properties of the BST films has been observed. First, a small amount of Ce dopant makes easy epitaxial growth of a BST film with a smooth surface on a MgO substrate. Second, residual stress in a BST film on a MgO substrate can be reduced by Ce doping, as demonstrated by the blueshift of phonon peaks in Raman spectroscopy.

Dielectric property studies of multiferroic GaFeO3

Magnetic and dielectric properties of a polycrystalline GaFeO3 sample prepared by a solid-state reaction have been investigated. A ferrimagnetic phase transition TN of the material was identified at a temperature of 225 K. An anomaly on the permittivity as a function of temperature has been observed near its magnetic transition point TN, which could be induced by the magnetoelectric coupling between its electric and magnetic orders. At low temperature, T< T N, the permittivity variation �e depends on magnetization M and shows clearly a linear relationship between �e and M 2 , which indicates that a dielectric and magnetic correlation exists in the compound. Furthermore, its dielectric relaxation process is discussed in the temperature interval of 77–300 K. (Some figures in this article are in colour only in the electronic version)

Sb-doped SrTiO3 transparent semiconductor thin films

Optically transparent Sb-doped SrTiO3 thin films with a transmittance higher than 95% in most of the visible region have been grown on SrTiO3 (001) substrate by pulsed laser deposition. The films behave as an n-type semiconductor between 10 K and room temperature. The carrier concentration and mobility of the films at room temperature are ∼5.8×1017 cm−3 and ∼6.4 cm2/V s, respectively. X-ray photoelectron spectroscopy measurement reveals that the delocalized electrons from the Sb dopants give rise to deep impurity levels within the band gap of the parent compound, which are responsible for the electrical conduction observed. The wide band gap and low density of states in the conduction band account for transparency of the films.

Structural magnetic properties and spin-glass behavior in La0.9Te0.1MnO3

We report the structural, magnetic, and electrical transport properties of pervoskite oxide La0.9Te0.1MnO3. This material, with space group R-3C, shows spin-glasslike features at low temperature and has a good colossal magnetoresistance behavior. The magnetoresistance ratio MR=[ρ(0)−ρ(H)]/ρ(0) is about 51% at 200 K in a field of 4 T. X-ray photoemission spectroscopy measurements suggest that Te ions are in the Te4+ state, while the Mn ions may be in the Mn2+ and Mn3+ valence state.

Optical and transport properties of Sb-doped SrTiO3 thin films

Optically transparent SrTi1−xSbxO3 (=0.05, 0.10, 0.15, and 0.20) thin films with transmittances higher than 85% in the visible region have been grown on SrTiO3 substrates by pulsed-laser deposition. Unless overdoped, the films possess a single-crystal phase and impurity conduction. The temperature dependence of the resistivities shows a metal–semiconductor transition for the film with x=0.05, and semiconducting behaviors for the films with x=0.10 and 0.15. The overdoped film with x=0.20 is an insulator. Sb concentration has a dominant effect on the electrical properties of the films, and the Anderson localization is probably the mechanism. X-ray photoelectron spectroscopy results indicate that the Sb impurity atoms provide donor electrons to form impurity states within the band gap, which is responsible for the electrical localized impurity. The wide band gap and the low density of states in the conduction band result in the transparency of the films. The disorder increases with Sb concentration, which is...