C. B. Lee

Improvement in the properties of high Tc films fabricated in situ by laser ablation of YBa2Cu3O7-Ag targets

We have studied the microstructural and superconducting properties of YBa2Cu3O7 films fabricated by pulsed laser evaporation of YBa2Cu3O7‐Ag targets, containing approximately 30% by weight of silver. The films produced by excimer laser evaporation/ablation of YBa2Cu3O7‐Ag targets were found to possess better superconducting and microstructural properties than films prepared under the same conditions from bulk YBa2Cu3O7 targets. YBa2Cu3O7 films deposited on yttria stabilized zirconia (YSZ) substrates from YBa2Cu3O7‐Ag targets at 650 °C showed helium ion channeling yields of approximately 9%–12%, while films deposited from YBa2Cu3O7 targets had channeling yields in the range of 18%–25%. The critical current density of YBa2Cu3O7Agx films on YSZ substrates had values of approximately 1.8×106 A/cm2 at 77 K which corresponds to approximately an 80% improvement from the values obtained from films deposited from cold pressed YBa2Cu3O7 targets. Based on the structural and chemical information, it appears that two ...

Magnetic properties of self-assembled nanoscale La2/3Ca1/3MnO3 particles in an alumina matrix

We have investigated the processing and properties of La2/3Ca1/3MnO3 self-assembled nanodots formed in a nonmagnetic alumina matrix, which were produced by a pulsed-laser deposition process. The size of the nanodots was found to be in the range of 10–15 nm using high-resolution transmission electron microcopy. The average interlayer separation between two dots has been found to be 2–5 nm, which is sufficient to decouple the magnetic grains. The decoupling of the grains is supported by the zero-field-cooled and field-cooled magnetization (M) data. The coercivity of the La2/3Ca1/3MnO3 nanodots has been measured using magnetization measurements as a function of field (H) at different temperatures above and below the blocking temperature of the samples. The coercivity is found to vary from 600 Oe at 10 K to 400 and 200 Oe at 20 and 50 K, respectively. Above the blocking temperature, the sample is found to transform to a superparamagnetic magnetic state, resulting in the disappearance of any hysteresis in the M–H loops.We have investigated the processing and properties of La2/3Ca1/3MnO3 self-assembled nanodots formed in a nonmagnetic alumina matrix, which were produced by a pulsed-laser deposition process. The size of the nanodots was found to be in the range of 10–15 nm using high-resolution transmission electron microcopy. The average interlayer separation between two dots has been found to be 2–5 nm, which is sufficient to decouple the magnetic grains. The decoupling of the grains is supported by the zero-field-cooled and field-cooled magnetization (M) data. The coercivity of the La2/3Ca1/3MnO3 nanodots has been measured using magnetization measurements as a function of field (H) at different temperatures above and below the blocking temperature of the samples. The coercivity is found to vary from 600 Oe at 10 K to 400 and 200 Oe at 20 and 50 K, respectively. Above the blocking temperature, the sample is found to transform to a superparamagnetic magnetic state, resulting in the disappearance of any hysteresis in the ...

Growth, characterization, and electrical properties of PbZr0.52Ti0.48O3 thin films on buffered silicon substrates using pulsed laser deposition

Epitaxial thin films of PbZr 0 . 5 2 Ti 0 . 4 8 O 3 (PZT) were synthesized successfully on SrRuO 3 /SrTiO 3 /MgO/TiN/Si heterostructures by pulsed laser deposition. The films were single phase and had (001) orientation. The deposition parameters were varied to obtain the best epitaxial layer for each of the compounds. Transmission electron microscopy indicated good epitaxy for the entire heterostructure and sharp interfaces between the epilayers. Dielectric and P-E hysteresis loop measurements were carried out with evaporated Ag electrodes. The dielectric constant for the films was found to be between 400-450. The value of saturation polarization P s was between 55-60 μC/cm 2 , and the coercive field E c varied from 60-70 kV/cm. Integration of PZT films with silicon will be useful for future memory and micromechanical devices.

Low temperature synthesis and electrical properties of epitaxial Sr0.8Bi2.2Ta2O9 thin films

High quality epitaxial thin films of Sr0.8Bi2.2Ta2O9 (SBT) were grown on LaNiO3 (LNO) bottom electrodes. The SBT/LNO heterostructure was fabricated on (001) oriented LaAlO3 substrates by pulsed laser ablation. X-ray diffraction and high resolution transmission electron microscopy revealed epitaxial growth of SBT and LNO layers along the (001) direction and sharp interfaces between the epilayers. The SBT films exhibited a dielectric constant of ∼270 and the loss tangent varied from 0.02 to 0.04. The dielectric constant measured as a function of bias field revealed that the films were not ferroelectric in nature. The room temperature frequency response of the dielectric constant was observed to obey Curie–von Schweidler power law with an exponent of 0.02 in the range of 10 kHz–1 MHz.

MICROSTRUCTURE, MAGNETORESISTANCE, AND MAGNETIC PROPERTIES OF PULSED-LASER-DEPOSITED EXTERNAL, INTERNAL, AND MIXED-DOPED LANTHANUM MANGANITE FILMS

In this article we report our studies on the microstructure, magnetoresistance (MR) behavior, and magnetic properties of the La1−x−yCaxMnO3 system in thin-film form. By varying the values of x and y in La1−x−yCaxMnO3, we have synthesized an external- (x=0.3, y=0), an internal- (x=0, y=0.3), and a mixed-doped (x=0.2, y=0.1) system with the same Mn3+/Mn4+ ratio. Thin films of these materials have been grown in situ on (100) LaAlO3 substrates using a pulsed-laser-deposition technique. Atomic force microscopy, x-ray diffraction and high-resolution transmission electron microscopy measurements carried out on these films have shown that the films are smooth, highly crystalline, and epitaxial on the (100) LaAlO3 substrates. Electrical resistance and magnetoresistance have been measured in the 10–300 K range in magnetic fields up to 5 T using a superconducting quantum interference device magnetometer. The MR ratios (calculated using the expression, [R0−RH]/RH, where R0 and RH are resistances in zero and applied f...

Oxygen Content And Crystallinity Effects in Pulsed Laser Deposited Lanthanum Manganite Thin Films

Eventhough colossal magnetoresistance in Lanthanum calcium manganite (LCMO) thin films was known for a long time, the effect of oxygen content and crystallinity on the properties of these films is not clearly understood. It is in this context that we have performed a systematic study of these effects by annealing the films in various ambients. A series of LCMO thin films have been grown in situ on (100) LaAlO 3 substrates using a pulsed laser deposition technique under identical conditions. Microstructural characterization carried out on these films has shown that the films are smooth, single phase and highly textured. The films were subjected to the following post deposition treatments: (i) annealing in oxygen at 900°C for 4 hrs, (ii) annealing in argon at 900°C for 4 hrs, (iii) annealing in oxygen at 500°C for 12 hrs, (iv) annealing in argon for 12 hrs and (v) annealing in vacuum at 850°C for half-an-hour. As deposited LCMO films show a transition temperature of 260 K and a magnetoresistance ratio (defined as [R(0)−R(H)/R(H)]) of 190% at 260 K in 5T magnetic field. The samples subjected to 500°C oxygen and Ar anneal have shown no change in the transition temperature and MR ratio. The films subjected to a 900°C annealing in Ar ambient have shown marginal improvement in transition temperature but a drastic improvement in the MR ratio (525%). 900°C oxygen annealed films have shown an improvement in the transition temperature (290 K) and MR ratio (225%) over as deposited films. Vacuum annealed samples have shown deteriorated properties. These results indicate that the metal-insulator transition is related to the oxygen content of the films while the MR ratio is related to the domain size.

Magnetotransport and magnetic properties of La0.7MnO3−δ and Pr0.65Ba0.05Ca0.3MnO3−δ superlattices

Superlattice structures consisting of La0.7MnO3−δ (LMO) and Pr0.65Ba0.05Ca0.3MnO3−δ (PBCMO) systems, in which the thickness of La0.7MnO3−δ is fixed and that of Pr0.65Ba0.05Ca0.3MnO3−δ varied from 1 to 8 unit cells, have been grown in situ on (100) LaAlO3 substrates using a pulsed-laser deposition technique. Microstructural characterization carried out on these films shows the presence of characteristic intense satellite peaks, indicating the chemical modulation of the superlattice structure. The insulator-to-metal transition and the magnetoresistance (MR) ratio are found to vary with the number of unit cells. The samples with 1, 2, 5, and 8 unit cells of Pr0.65Ba0.05Ca0.3MnO3−δ show transition temperatures of 240, 230, 150, and 160 K and MR ratios of 540%, 592%, 3150%, and 2875%, respectively. We have observed an enhancement of magnetoresistance ratios in the case of superlattices with a thickness of PBCMO greater than 5 unit cells, which may be attributed to a ferromagnetic biasing provided by the LMO la...

Magnetotransport and Magnetic Properties of La 0.7 MnO 3−δ and Pr 0.65 Ba 0.05 Ca 0.3 MnO 3−δ Superlattices

In an effort to achieve high magnetoresistance ratios at high temperature and low fields, we have fabricated superlattice structures consisting of La 0.7 MnO 3−δ (LMO) and Pr 0.65 Ba 0.05 Ca 0.3 MnO 3−δ (PBCMO) systems where La 0.7 MnO 3−δ is believed to act as a ferromagnetic biasing source to Pr 0.65 Ba 0.05 Ca 0.3 MnO 3−Δ . LMO and PBCMO individually transform to ferromagnetic states at 240 K and 60 K respectively. A series of samples, in which the thickness of La 0.7 MnO 3−δ is fixed and that of Pr 0.65 Ba 0.05 Ca 0.3 MnO 3−δ varied from 1 to 8 unit cells, have been grown in situ on (100) LaAlO 3 substrates using a pulsed laser deposition technique. Microstructural characterization carried out on these films show the presence of characteristic intense satellite peaks indicating the chemical modulation of the superlattice structure. The insulator-to-metal transition and the MR ratio, defined as [R(0)-R(H)/R(H)], where R(0) and R(H) are resistances in zero and applied fields, is found to vary with the number of unit cells. The samples with 1, 2, 5 and 8 unit cells of Pr0.65Ba 0.05 Ca 0.3 MnO 3−δ show a transition temperature of 240 K, 230 K, 150 K and 160 K and MR ratio of 540%, 592%, 3150% and 2875 % respectively. We have observed an enhancement of magnetoresistance ratio in case of superlattices with thickness of PBCMO greater than 5 unit cells. We attribute this enhancement to a ferromagnetic biasing provided by the LMO layers acting as a ferromagnetic film below its transition temperature.

Interfacial Diffusion in a MOCVD Grown Barium Titanate Film

A combination of two nondestructive techniques, Grazing Incidence X-ray Reflectivity and High Resolution X-ray Diffraction, is used to study (at around 10{angstrom} resolution) the composition profile across a 500{angstrom} thick film of BaTiO{sub 3} grown epitaxially on (100) MgO by MOCVD. Results form both studies indicate diffusion of Mg to about 250{angstrom} into the film at film-substrate interface, consistent with the diffuse ferroelectric phase transition observed in this film. The lattice parameter a shows a progressive decrease as the authors move into the film from the interface, and an anomalously low value in the Mg-free portion of the film.

Zero-bias conductance and energy gap measurements in LaNiO3-YBa2Cu3O7-x junctions

Conductance measurements of junctions between a high- superconductor and a metallic oxide have been carried out along the a-b plane to examine the tunnel-junction spectra. For these measurements, in situ films have been grown on c-axis oriented thin films using the pulsed laser deposition technique. Two distinctive energy gaps have been observed along with conductance peaks around zero bias. The analysis of zero-bias conductance and energy gap data suggests the presence of midgap states located at the centre of a finite energy gap. The results obtained are also in accordance with the d-wave nature of high- superconductors.