Hiromi Shima

Enhancement of ferroelectric and magnetic properties in BiFeO3 films by small amount of cobalt addition

Both the ferroelectric and magnetic properties of polycrystalline BiFeO3 films fabricated by chemical solution deposition were enhanced by adding small amounts of cobalt. Addition of 3at.% cobalt to BiFeO3 films increased the remanent polarization from 49to72μC∕cm2 and decreased the electric coercive field from 0.54to0.44MV∕cm. The ferroelectricity degraded when the cobalt concentration exceeded 9at.% due to the formation of the secondary phases of Bi2Pt. The saturation magnetization was drastically enhanced by the addition of cobalt up to 12at.%. This is because the magnetic moments are not canceled locally since the differences of magnetic moment between B-sites. The saturation magnetization decreased when the cobalt content exceeded 15at.%, thereby attributing to the formation of a nonmagnetic secondary phase of Bi2Pt. It is concluded that both ferroelectric and magnetic properties were enhanced, provided only small amount of cobalt were added to the films.

Annealing Temperature Dependences of Ferroelectric and Magnetic Properties in Polycrystalline Co-Substituted BiFeO3 Films

Multiferroic Co-substituted BiFeO3 films were fabricated by chemical solution deposition method followed by post deposition annealing at various temperatures. The substitution of cobalt of B-sites for iron in BiFeO3 was promoted at relatively high temperatures. The B-site substitution by cobalt promoted increases in saturation magnetization and spontaneous magnetization. By substitution, leakage current density was suppressed in a high-electric-field region, and ferroelectric hysteresis (P–E) loops became measurable even at room temperature. The optimal annealing temperature for the coexistence of a high remanent polarization and a high remanent magnetization was 923 K having a high B-site substitution ratio of cobalt.

La Content Dependence of Electrooptic Properties of Polycrystalline (Pb,La)(Zr0.65,Ti0.35)O3 Thick Films

The systematic investigation of the electrical, optical and electrooptic properties of lanthanum-substituted lead zirconate titanate (PLZT) films was carried out. Polycrystalline PLZT films with various La and Pb contents were prepared at various sintering temperatures by chemical solution deposition (CSD). Among the 135 conditions we examined, a sintering temperature of 700 °C and a Pb content ratio of 125% relative to a stoichiometric value were optimum for preparing well-filled films. The maximum polarization monotonically decreased with increasing La content, while the dielectric constant change was maximum at a La content of 6 mol %. The refractive indexes of the PLZT films were estimated to be in the range from 2.35 to 2.42 at 630 nm. The refractive index change due to the application of a DC bias voltage was maximum at a La content of 0 mol %, monotonically decreased with increasing La content, and strongly correlated with the maximum polarization change. Therefore, we concluded that the electrooptic effect in the PLZT films is mainly caused by the reconfiguration of the domain structure in the films.

Optical Properties of BiFeO3-System Multiferroic Thin Films

The optical properties of the chemical solution deposition (CSD)-derived BiFeO3 (BFO) film and the pulsed laser deposition (PLD)-derived (Bi,Nd)(Fe,Mn)O3 (BNFM) film were evaluated by spectroscopic ellipsometry, and their optical constants (n, k) and band gaps were determined. At a wavelength of 600 nm, the refractive indexes of 3.22 and 2.87 were estimated for the BFO and BNFM films, respectively, although the existence of a refractive index gradient was suggested in the BFO film. In addition, at a wavelength of 1550 nm, which is generally used for optical communication, the refractive indexes of 2.91 and 2.59 were estimated for the BFO and BNFM films, respectively. The band gaps of the BFO and BNFM films were estimated to be 2.79 and 2.72 eV, respectively, and it was confirmed that the extinction coefficients of both films were almost zero at wavelengths larger than 600 nm. These results suggest that the BFO-system multiferroic films have a high potential as an optical material with a high refractive index.

Preparation and characterization of Ba(ZrxTi1?x)O3 thin films for high-frequency applications

Ba(ZrxTi1−x)O3 (BZT) thin films were deposited on a (100)MgO substrate by RF-magnetron reactive sputtering using metal targets. In this paper, a model of the reactive sputtering process using a metal composite target at different reactive gas flow rates is presented. 500-nm-thick BZT thin (x = 0.26) films for high frequency performance showed a single perovskite phase and a high crystallinity on the MgO substrate with only a (001)/(100) orientation at an optimum reactive gas flow rate in Ar + O2. The BZT films had a stoichiometric Ba/Ti ratio and epitaxially grew on the MgO substrate. They showed a dense microstructure without cracks or voids. Their low capacitance and loss tangent showed little dispersion at 1–18 GHz. These results indicated that we succeeded in depositing high-quality and potentially tunable ferroelectric BZT films for high-frequency applications by RF-magnetron reactive sputtering using metal targets.

Preparation and Characterization of Ba(ZrxTi1-x)O3 Thin Films Using Reactive Sputtering Method

Ba(ZrxTi1-x)O3 (BZT) thin films with different Zr contents were deposited on (100) MgO and (100) Pt/(100) MgO substrates by RF-magnetron reactive sputtering using metal targets. The BZT (0 < x < 0.89) thin films had a single perovskite phase with only (001)/(100) orientation. In all cases, the ratio of Ba/Ti was stoichiometric according to X-ray fluorescence spectrometry (WDX) measurement. Atomic force microscopy (AFM) study proved that BZT films possess a dense microstructure without cracks or voids. The grain size was found to decrease with increasing of Zr content. The effect of Zr content on the dielectric constant and leakage current was studied. BZT thin films showed ferroelectric-to-paraelectric properties with increasing Zr content and excellent leakage properties according to measurements of electrical properties. These results indicated that we succeeded in depositing high-quality and low-sputter-damage BZT thin films by reactive sputtering using metal targets.

Electrooptic and Piezoelectric Properties of (Pb,La)(Zr,Ti)O3 Films with Various Zr/Ti Ratios

A systematic investigation of the electrooptic properties of (Pb,La)(Zr,Ti)O3 (PLZT) films was carried out. 700-nm-thick polycrystalline PLZT films with 2 mol % La and various Zr/Ti ratios were formed on Pt/Ti/SiO2/Si substrates, and their reflectance spectra were measured. Zr/Ti ratio significantly affected the surface morphology of the films, and PLZT films with what Ti/(Zr+Ti) ratios ranging from 40 to 70% showed less reflectance light loss that because of their smooth surface. The maximum resonant wavelength shift was attained at a Ti/(Zr+Ti) ratio of 40%. These results suggest that the PLZT film with a Ti/(Zr+Ti) ratio of 40% is optimum for application in optical devices such as a spatial light modulator (SLM). The piezoelectric properties of the PLZT films were also evaluated because their resonant wavelength shift was caused by changes in not only refractive index but also film thickness. The piezoelectric displacement showed a maximum Ti/(Zr+Ti) ratio of 10% and monotonically decreased with increasing Ti/(Zr+Ti) ratio in our PLZT films. The exact Pockels coefficient of the PLZT(2/60/40) film was estimated to be 104 pm/V at 600 nm by subtracting the effect of the change in film thickness from the resonant wavelength shift.

THE OPTICAL PROPERTY OF MULTIFERROIC BiFeO3 FILMS

ABSTRACT We fabricated a multiferroic BiFeO3 (BFO) film and evaluated its optical properties. A polycrystalline BFO film with a thickness of 650 nm was formed on a Pt/Ti/SiO2/Si substrate. The refractive index of the BFO film was estimated to be approximately 3.0 at a wavelength of 600 nm. This value is the largest in the visible light range for oxide films such as rutile-type TiO2. Furthermore, the extinction coefficient was estimated to be zero in the wavelength range of 600 to 1670 nm. These results suggest that the produced BFO film is a promising material for optical communication devices.

Structural analysis of interfacial strained epitaxial BiMnO3 films fabricated by chemical solution deposition

An interfacial epitaxial BiMnO3 layer was fabricated by chemical solution deposition on SrTiO3 (100) substrate, and the microstructure of the film was analyzed by x-ray diffraction (XRD) and cross-sectional transmission electron microscopy (TEM). The TEM observation revealed the epitaxial growth of BiMnO3 on the SrTiO3 substrate as follows: ([110](001))BiMnO3∥[0-10](001) SrTiO3. XRD and TEM analyses revealed that the mismatch between the epitaxial BiMnO3 and the SrTiO3 substrate causes a distortion in lattice parameters of BiMnO3 and, consequently, a large compressive strain in the BiMnO3 layer.

Evaluation of oxygen vacancy in ZnO using Raman spectroscopy

Oxygen vacancies in zinc oxide (ZnO) are intrinsic defects, which are easily generated during crystal growth or device processing. Investigations of defects, such as oxygen vacancies, are important to understand the properties of ZnO. In this study, we attempt to quantify oxygen vacancies in ZnO powders using Raman spectroscopy. ZnO powder is reduced in a hydrogen atmosphere at 300-600 °C for 30-90 min. The peak position of the E2(high) mode, which is related to the oxide ion vibration, shifts toward a lower frequency as the oxygen vacancies increase. Upon re-oxidation, the initial E2(high) peak position is restored. Because the E2(high) peak shift is scaled with the amount of oxygen vacancies, this relationship can be used to estimate the amount of oxygen vacancies.