Hidefumi Asano

Development of half-metallic ultrathin Fe3O4 films for spin-transport devices

We attempted to fabricate a high-quality Fe3O4 film while satisfying both low-thermal preparation (≦573 K) and film thinness (≦500 A). X-ray diffractometry showed that our prepared Fe3O4 film was epitaxially grown onto a MgO (100) substrate. The saturation magnetization, resistivity, and Verwey point were, respectively, ∼438 emu/cm3, ∼10 000 μ Ω cm, and ∼110 K. These values were comparable to those of the Fe3O4 bulk. Our experimental results suggested that a high-quality Fe3O4 film could be obtained even under the crucial conditions of the deposition temperature being low (∼523 K) and the film being ultrathinned (∼100 A).

Relative permittivity and dielectric loss tangent of substrate materials for high-Tc superconducting film

We measure the relative permittivityεr and dielectric loss tangent tanδ of substrate materials for high-Tc superconducting films at 18–300 K and at 5–10 GHz using the cavity-resonator method. The materials measured are single crystals of MgO, LaAlO3, YAlO3, LaSrGaO4, NdGaO3, sapphire, and ZrO2 ceramic. Theεr values are 10–30 and become almost constant below about 50 K. The tanδ values decrease with decreasing temperature and are below 1×10−5 at 77 K except for those of NdGaO3 and ZrO2 ceramic. This suggests that the tanδ values of MgO, LaAlO3, YAlO3, LaSrGaO4, and sapphire do not limit the quality factors of microwave passive components fabricated using high-Tc superconducting films. It is also demonstrated that the tanδ of the substrate material is strongly affected by impurities.

Giant magnetoresistance of a two‐dimensional ferromagnet La2−2xCa1+2xMn2O7

Bulk samples of La2−2xCa1+2xMn2O7 with the layered Sr3Ti2O7‐type perovskite structure have been successfully synthesized and investigated with respect to their magnetic and electrical properties. It is found that La2−2xCa1+2xMn2O7 (x=0.25) is a metallic ferromagnet with a magnetic transition temperature Tc of 215 K. The large magnetoresistance (MR) effect with Δρ/ρ0 of ∼60% at 1.8 T was observed in a wide temperature range below a cusp temperature in resistivity of 96 K, which is well below the magnetic Tc. This behavior is quite different from that of the well‐known double‐exchange ferromagnets such as La1−xCaxMnO3, where large MR effects are restricted to a narrow temperature range around the ferromagnetic transition. The present result could be interpreted by using the double‐exchange theory incorporating the anisotropy resulting from the two‐dimensional Mn‐O‐Mn networks in La2−2xCa1+2xMn2O7.

Synthesis of Y-Ba-Cu-O Thin Films on Sapphire Substrates by RF Magnetron Sputtering

Y-Ba-Cu-O thin films have been prepared on sapphire substrates by RF magnetron sputtering from sintered targets. The substrate temperature dependence of Ba and Cu concentrations against Y of as-sputtered films has been clarified. Film composition deviated from target composition. Cu concentration decreased significantly with increasing Ts, while Ba concentration was independent of the substrate temperature. Using the CuO-compensated Y1Ba6Cu10Ox target, assputtered high-Tc films (Tc onset=87 K, Tc endpoint=40 K) have been successfully obtained as a Ts of about 580°C.

Superconducting and Structural Properties of EuBa2Cu3O7-δ Ultrathin Films Deposited on MgO(100) Substrates Using Magnetron Sputtering

EuBa2Cu3O7-δ thin films were produced on MgO(100) substrates at 580°C using planar-type magnetron sputtering and were studied by X-ray diffraction and RHEED. The c-oriented films were epitaxially grown with a crystal orientation of [110]EuBaCuO//[010]MgO. Films more than 1000 A thick exhibited Tc endpoints of ~90 K. As film thickness decreased, superconducting properties deteriorated. However, even films with 2-unit-cell thickness became superconducting. The cause of the deterioration of superconducting properties in ultrathin films is discussed on the basis of the relationship between lattice matching and structural properties.

Epitaxy and orientation of Eu1Ba2Cu3O7−y films grown in situ by magnetron sputtering

Superconducting films of Eu1Ba2Cu3O7-y have been grown in situ onto SrTiO3 and MgO (100) substrates by magnetron sputtering from a stoichiometric oxide target. The sputtering process results in the growth of epitaxial films with perfect a-axis orientation as well as perfect c-axis orientation. The orientation of the films can be controlled by the proper choices of substrate temperature, oxygen pressure, and film growth rate. The routine production of films showing Tc zero of 88–90 K is accomplished. In situ growth of the films with the particular orientation is of importance in the fabrication of multilayer structures such as tunnel junctions.

Growth and properties of high-Curie-temperature Sr2CrReO6 thin films

Thin films of ordered double-perovskite Sr2CrReO6 have been grown epitaxially onto SrTiO3 substrates by sputtering, and their microstructural, electrical, and magnetic properties are investigated. It is found that a wide growth-temperature range of at least 700–800°C can yield epitaxial films with superior magnetic properties and atomically flat surfaces. These films exhibit Curie temperatures Tc of up to 620K, and saturation magnetizations Ms of 0.9μB∕formula unit (f.u.), which is comparable to the predicted value of 1μB∕f.u. for the fully ordered half-metallic material. Owing to the wide temperature window for growth of high-quality films, the surface morphology and coercivity of Sr2CrReO6 films are highly controllable, which is favorable for fabrication of magnetic tunnel junctions.

Magnetoresistance in thin films and bulks of layered-perovskite La2−2xCa1+2xMn2O7

Epitaxial thin-film and polycrystalline bulk samples of ferromagnetic La2−2xCa1+2xMn2O7 (x=0.3) with a layered perovskite structure have been examined with respect to their magnetotransport properties. In addition to a large magnetoresistance (MR) effect at temperatures around the metal–insulator transition, a-axis thin films exhibit unusual low-temperature MR behavior with apparent hysteresis in applied magnetic fields. The features in the bulk samples are a low-temperature MR effect without such hysteresis, and also an MR effect at high temperatures well above the metal–insulator transition temperature. The unique MR behavior depending on the sample form would reflect an anisotropic nature in the layered-perovskite ferromagnet with two-dimensional Mn-O networks.

PREPARATION AND PROPERTIES OF TRIPLE PEROVSKITE LA3-3XCA1 +3XMN3O10 FERROMAGNETIC THIN FILMS

The ferromagnetic compound La3−3xCa1+3xMn3O10 with a triple perovskite structure can be successfully synthesized by using a thin-film growth method. Sputtered a-axis thin films with x=0.3 have been examined with respect to their magnetotransport properties. For the triple perovskite compound, we have observed the features, including the enhanced magnetoresistene (MR) effect and the characteristic low-temperature MR effect resulting from intragrain spin-polarized tunneling, which were reported for the double perovskite manganites. A comparison of the magnitude of these features in triple and double perovskite manganites suggests that the features are actually determined by the c-axis Mn–O bond configuration in a layered-perovskite ferromagnet.

30-kV spin-polarized transmission electron microscope with GaAs–GaAsP strained superlattice photocathode

A spin-polarized electron beam has been used as the probe beam in a transmission electron microscope by using a photocathode electron gun with a photocathode made of a GaAs–GaAsP strained superlattice semiconductor with a negative electron affinity (NEA) surface. This system had a spatial resolution of the order of 1 nm for at 30 keV and it can generate an electron beam with an energy width of 0.24 eV without employing monochromators. This narrow width suggests that a NEA photocathode can realize a high energy resolution in electron energy-loss spectroscopy and a longitudinal coherence of 3 × 10−7 m.