Kenji Morii

Structural, dielectric, and magnetic properties of epitaxially grown BaFeO3 thin films on (100) SrTiO3 single-crystal substrates

We have prepared single-crystalline films of BaFeO3, which may contain high valent Fe4+ ions, on (100) SrTiO3 substrates by pulsed laser-beam deposition. The reflection high-energy electron diffraction patterns for the films apparently show a C4 symmetry operation, suggesting that the sample had a pseudocubic perovskite structure rather than hexagonal unit cell. The films were found to be highly insulating, and showed a high value of dielectric constant of e=59. At 300 K, the magnetization loop of the film apparently shows hysteresis, as well as small remanent magnetization. These characteristics are totally different from those of bulk BaFeO3.

Microstructure and some properties of TiAl-Ti2AlC composites produced by reactive processing

The TiAl‐Ti2AlC composites with and without impurities, Ni, Cl and P, were prepared by combustion reaction from the elemental powders and cast after arc melting. The resulting composites had about 18vol% Ti2AlC in the lamellar matrix consisting of -TiAl and Ti3Al (2). In the homogenized specimens, the2 phase decomposed to-TiAl and Ti2AlC. The composite material had a high strength both at ambient and elevated (1173K) temperatures; about 800 and 400MPa, respectively, with an ambient temperature ductility of 0.7% at bending test. The fracture toughness test also proved that the homogenized composite has higher toughness than the as cast one. The toughness value reached to 17.8MPam 1/2 . The zigzag cracks propagated in the homogenized composite and the reinforcement Ti2AlC particles and the finely precipitated Ti2AlC particles were main obstacles to the crack propagation. The composite with impurities showed a marginal improvement in the oxidation resistance over the composites without impurities. # 1999 Elsevier Science Ltd. All rights reserved.

Dielectric relaxation in amorphous thin films of SrTiO3 at elevated temperatures

Temperature and frequency dispersion of dielectric permittivity was investigated on thin amorphous films of SrTiO3 prepared by a sputtering method using neutralized argon‐ion beams. The amorphous SrTiO3 films deposited on glass substrates exhibited a marked dielectric relaxation at temperatures 500–800 K in a frequency range 0.1–50 kHz. This behavior was explained based on a dipolar relaxation of the Cole–Cole type with the static dielectric constant e’s≂380, the constant at high frequency e’∞≂35, and the distribution parameter of the relaxation time β≂0.8. The analysis of the temperature dependence of relaxation time gave the activation energy for the relaxation of about 1.08 eV and the characteristic relaxation time of the order of 10−12 s. In the temperature range where strong relaxation occurred, a semiconductor‐type conduction having the activation energy of about 0.84 eV became dominant. A correlation between the mechanisms of the dielectric relaxation and the thermally activated motions of ionized ...

Magnetic properties of highly resistive BaFeO3 thin films epitaxially grown on SrTiO3 single-crystal substrates

We have synthesized single-crystalline films of BaFeO3 (BFO), which may contain unusual tetravalent Fe ions, on (100) and (111) SrTiO3 substrates by pulsed laser-beam deposition. The epitaxially grown BFO is considered to have a pseudocubic perovskite-type crystal structure with some oxygen vacancies. The films were found to be highly insulating, and showed a high value of dielectric constant of e=59. The magnetization curve of the films exhibited hysteresis, as well as small remanent magnetization. The origin of the observed small magnetization can be ascribed to the mixed valence state of the Fe4+ and Fe3+ ions. The present epitaxially grown BFO apparently had some different magnetic and dielectric properties from those for bulk BFO as well as for the previously reported other Fe4+ oxides: coexisting high resistivity and spontaneous magnetization at room temperature.

Oxidation-resistant coating for gamma titanium aluminides by pack cementation

Gamma titanium aluminides ({gamma}-TiAl alloys), having an L1{sub 0}-type structure, are candidate materials for use in future gas turbine aero-engines and automotive engines because of their low density, high specific strength and high stiffness. In air, however, it is well known that titanium aluminide oxidizes at a more rapid rate at temperatures above 1,123 K; therefore, the oxidation resistance becomes a critical factor for TiAl alloys to be used at high temperatures (perhaps above 1,073 K). Coatings for the TiAl alloy are essential to high temperature oxidation resistance. The Al-Ti-Cr ternary phase diagram at 1,423 K indicates that the L1{sub 2} phase field is in equilibrium with the TiAl(L1{sub 0}) phase field. Furthermore, the L1{sub 0}- and L1{sub 2}-alloys in this ternary system resemble each other very closely in crystal structure and lattice parameter (TiAl(L1{sub 0}); a = 0.4005, c = 0.4070 nm (27), Ti-67Al-8Cr(L1{sub 2}); a = 0.3960nm (25)). Therefore, the L1{sub 2}-(Al,Cr){sub 3}Ti alloy can be considered applicable to L1{sub 0}-TiAl alloys as an optimum material for coatings. The purpose of this study is to apply L1{sub 2} alloy coatings to the TiAl alloys by pack cementation techniques, and to evaluate the oxidation resistance of the coated alloys.

Effect of oxygen deficiencies on magnetic properties of epitaxial grown BaFeO/sub 3/ thin films on (100) SrTiO/sub 3/ substrates

BaFeO/sub 3/ (BFO) epitaxial thin films on [001] SrTiO/sub 3/ substrates have been successfully produced by pulsed laser beam deposition. The epitaxially grown BFO was found to have tetragonal crystal structure with some amount of oxygen deficiencies. The magnetic susceptibilities of the samples strongly depended on the (hkl) directions of BFO varied in accordance with the crystalline directions, i.e., /spl chi//sub (100)/</spl chi//sub [110]/</spl chi//sub [001]/. The magnetic spin structure of the epitaxially grown BFO was speculated to be conventional antiferromagnetic. The remanent magnetization of the samples was found to depend on the amount of oxygen deficiencies significantly. This implies that the 180/spl deg/ superexchange interaction of Fe/sup 4+/-O/sup 2-/-Fe/sup 4+/ may possibly produce some kinds of ferromagnetic ordering.

Improvement of magnetization and leakage current properties of magnetoelectric BaFeO3 thin films by Zr substitution

We report on the synthesis of the magnetoelectric BaFe1−xZrxO3−δ (x=0.5 and 0.7) single-crystalline films whose magnetic and leakage properties have been greatly improved. The films were found to be excellent insulators, up to the leakage current density of about 1.1×10−5A∕cm2 at the bias electric field of 150kV∕cm for the x=0.7 sample. This is much less than 10−2 of that for the BaFeO3−δ single-crystalline film, which means that the leakage current properties have been distinctly improved by zirconium substitution. As for the magnetic properties, the hysteresis loops measured at 5K for the x=0.7 samples apparently show huge saturation magnetization of 0.98μB∕Fe ion, in contrast to 0.09μB∕Fe ion for the x=0.5 sample. This implies that the magnetic ordering for the x=0.7 sample has been greatly enhanced, possibly due to the ferromagnetic spin alignment of Fe ions.

Structural and electric properties of TiB2 thin films by RF sputtering

Abstract We have investigated the structural and electric properties of TiB 2 thin films produced by RF sputtering on glass substrates. The films are composed of the TiB 2 small crystallites as well as the Brich amorphous phase, which is resulting from the Brich composition of the films. The resistivities of the films vary from 239 to 323 μΩ cm depending on the RF power and the sputter time. These values are considerably larger than that of a bulk sample. The temperature dependence of the resisitivities is found to be carrier concentration dependent. However, the very high carrier densities (0.7–1.2×10 22 /cm 3 ) and the relatively small mobilities (2.3–5.7 cm 2 V −1 s −1 ) were obtained, suggesting that the conduction carrier might behave like nearly free electrons. We discuss these features in conjunction with the microstructure of the films.

Meyer–Neldel rule in amorphous strontium titanate thin films

In this letter, we report the experimental result indicating that the electrical conductivity in thin films of amorphous strontium titanate (a-STO) is well fit to the Meyer–Neldel (MN) rule over the temperature range 300–470 K. The films were ion-beam sputtered and annealed in two different atmospheres: a vacuum and flowing oxygen. The MN plots for the films show two parallel straight lines depending on the annealing atmosphere, which give the identical MN parameters of about 35 meV with the conductivity prefactors of 9.3×10−10 and 2.3×10−14(Ω cm)−1 for the vacuum- and oxygen- annealed films, respectively.

Influence of antimony doping on electrical properties of barium titanate (BaTiO3) thin films

BaTiO3 thin films having a fine grain size <50 nm were prepared by crystallization of an ion-beam sputtered amorphous phase at 923 K for 1 h in air. The effects of Sb-doping on the electrical properties of the films were investigated in the concentration range 0.04–0.6 at%. The overall resistivity of the films depended markedly on the doping concentration. The doping with Sb contributed to enhance the semiconductive character; the resistivity as well as the activation energy for conduction decreased with increasing concentration of Sb. The minimum room-temperature resistivity was attained in samples doped with 0.38 at% Sb. On further increase in doping, the films showed a positive temperature coefficient of resistivity at elevated temperatures. The results suggest that a similar doping effect, such as found in bulk electronic ceramics, occurs in thin dielectric films, and are discussed in relation to doping mechanisms and Heywangs model.