J.B. Dadson

Magnetic and electrical properties of single-phase multiferroic BiFeO3

We have reported the structural, thermal, microscopic, magnetization, polarization, and dielectric properties of BiFeO3 ceramics synthesized by a rapid liquid-phase sintering technique. Optimum conditions for the synthesis of single-phase BiFeO3 ceramics were obtained. Temperature-dependent magnetization and hysteresis loops indicate antiferromagnetic behavior in BiFeO3 at room temperature. Although saturated ferroelectric hysteresis loops were observed in single-phase BiFeO3 ceramic synthesized at 880 °C, the reduced polarization is found to be due to the high loss and low dielectric permittivity of the ceramic, which is caused by higher leakage current.

High-temperature ferromagnetism in pulsed-laser deposited epitaxial (Zn,Mn)O thin films: Effects of substrate temperature

We report on the observation of remarkable room-temperature ferromagnetism in epitaxial (Zn,Mn)O films grown by a pulsed-laser deposition technique using high-density targets. The optimum growth conditions were demonstrated from x-ray measurements, microstructure, Rutherford backscattering, micro-Raman, and magnetic studies. Superior ferromagnetic properties were observed in (Zn,Mn)O films grown at a substrate temperature of 500°C and with an oxygen partial pressure of 1mTorr. Ferromagnetism becomes weaker with increasing substrate temperature due to the formation of isolated Mn clusters irrespective of higher crystalline quality of the film.

Integration of epitaxial colossal magnetoresistive films onto Si(100) using SrTiO3 as a template layer

We report on the integration of epitaxial colossal magnetoresistive La0.67Ba0.33MnO films on Si(100) semiconductor using SrTiO3 template layer by pulsed-laser deposition. X-ray diffraction reveals the superior quality of the manganite film that grows epitaxially on heteroepitaxially grown SrTiO3 template layer on Si substrate. The epitaxial films demonstrate remarkable surface morphology, magnetic transition and hysteresis, magnetoresistance, and ferromagnetic resonance, illustrating the ferromagnetic nature of the film and possible device applications at room temperature.

Ferromagnetic properties of epitaxial manganite films on SrTiO3∕Si heterostructures

We report on the magnetic properties of epitaxial La0.7Ba0.3MnO3 and La0.7Sr0.3MnO3 films on Si (100) and Si (111) substrates using SrTiO3 template layer, which demonstrate magnetic and electrical properties at and above room temperature. The magnetization data show magnetic transition and magnetic hysteresis at and above room room temperature. The films show well-defined magnetic domains. The ferromagnetic resonance studies show anisotropic effects related to ferromagnetic properties of films. The smaller grain size of about 20nm in manganite films on SrTiO3∕Si may be one of the reasons to minimize the strain effect through strain relaxation at the interface between SrTiO3 and manganites through the formation of three-dimensonal islands.

Luminescence and spectroscopic behavior of Eu3+-doped Y2O3 and Lu2O3 epitaxial films grown by pulsed-laser deposition

We report on the fabrication of highly epitaxial Y2O3:Eu3+ and Lu2O3:Eu3+ films on sapphire and Si single crystalline, and quartz substrates under different growth and optimization conditions using pulsed-laser deposition technique. The films were characterized using x rays, microstructure, and spectroscopic measurements. Our results demonstrate superior spectroscopic performance on the films grown on sapphire substrates due to the high quality and epitaxial nature of the film. The surface morphology of the epitaxial films controls the photoluminescence property of these phosphors film.

Effects of As and Mn doping on microstructure and electrical conduction in ZnO films

We report the synthesis of epitaxial As-doped ZnO and Mn-doped (ZnAs)O films by pulsed-laser deposition technique. The grain size in (ZnAs)O films decreases from 40 to less than 10nm upon Mn doping, illustrating that Mn acts as a potential catalyst to create nanosize grains. Temperature dependent electrical resistance shows metal-insulator transition and metal-semiconductor transition (MST) at 165 and 115K, respectively, in (ZnAs)O, although Mn doping suppresses MST completely. Both ionization efficiency on oxygen vacancies and percolation of charge carriers may be responsible for such transitions. In addition, electrical conduction in these films shows strong aging effects.

Ferromagnetism in nanocrystalline epitaxial Co:TiO2 thin films

We report on the observation of remarkable room-temperature ferromagnetism in nanocrystalline epitaxial Co:TiO2 films grown on sapphire (0001) substrates by a pulsed-laser deposition technique using high-density targets. The films were characterized by x-ray measurements, atomic force microscopy, micro-Raman, electron-paramagnetic resonance, and magnetization studies. The films exhibit three-dimensional islandlike growth that contains nanocrystalline particles. Our experimental results suggest that the remarkable ferromagnetism in Co:TiO2 films is controlled either by the interstitial Co2+ ions or small clusters, which are mainly present at the interface and on the surface of the films. Our work clearly indicates that Co interstitials and nanoclusters cause room-temperature ferromagnetism in Co-doped TiO2.

Ferromagnetic resonance studies in ZnMnO dilute ferromagnetic semiconductors

We report on the ferromagnetic resonance studies on ZnMnO films grown by the pulsed-laser deposition technique. ZnMnO films were annealed in different atmospheres. The films grown and annealed in oxygen demonstrate ferromagnetic behavior at room temperature and below. However, annealing in either nitrogen or argon deteriorates the ferromagnetic response of the films. Further annealing the films in oxygen recovers the ferromagnetic response. Our results suggest that oxygen plays a major role for controlling the ferromagnetic properties in ZnMnO films.

Self-assembled nanocrystalline epitaxial manganite films on SrTiO3/Si heterostructures

We report the growth and magnetic characterizations of La0.7Ba0.3MnO3 and La0.7Sr0.3MnO3 films on SrTiO3-buffered Si (100) and Si (111) substrates by pulsed-laser deposition. The structural studies show the epitaxial nature of the films. The microscopic studies show that the films consist of nanocrystalline particles. All films display sharp magnetic and electrical transitions associated with the colossal magnetoresistance behavior at and above room temperature, illustrating the superior quality of the films.

Oxide-based dilute ferromagnetic semiconductors: ZnMnO and Co:TiO2

We report on ferromagnetic properties of ZnMnO and Co:TiO2 films grown by pulsed laser deposition with varying growth conditions. We have demonstrated that ZnMnO films show ferromagnetic properties at room temperature. However, oxygen plays a dominant role in the occurrence of ferromagnetism. Introducing carriers into ZnMnO films did not improve the ferromagnetic properties. Our experimental results indicate that the mechanism for ferromagnetism lies, probably, within the perspective of charge transfer between Mn ions through oxygen. On the other hand, our experimental results suggest that the ferromagnetism in Co:TiO2 films is controlled by the presence of small metal Co2+ clusters in the rutile TiO2 matrix, which are mainly present at the interface and on the surface of the films.