Tamotsu Koyano

X-ray diffraction study of Fe/MgO multilayered films with an enhanced magnetization

Fe atoms in Fe(16 AA)/MgO(60 AA) multilayered films, prepared in UHV conditions, have been identified as having the same BCC structure as for the bulk specimens, using X-ray diffraction. The lattice parameter for the Fe atoms was found to be larger than the bulk value by 1.0%. Such a lattice dilation is related to the enhancements in magnetization and hyperfine field of Fe atoms in Fe/MgO multilayered films.

Magnetization of α′ iron nitride produced through the fcc→bct martensitic transformation in high magnetic field

The γ iron nitride (nitrogen austenite) was subjected to high magnetic field process in order to drive the fcc→bct martensitic transformation. Molar fraction of martensite monotonically increased with increasing the magnetic field and reached 94% at 35T. With a combination of magnetization and Fe57 Mossbauer spectroscopy data, magnetization of bulk processed α′ phase with 9.6at.% N is determined to be 229emu∕g, the same as that for dc sputtered thin films.

Surface Roughness Evaluation of Multilayer Coated X-ray Mirrors by Scanning Tunneling Microscope

Multilayer X-ray mirrors have been deposited by electron beam evaporation in a high-vacuum system, and the surface irregularities have been investigated by means of a scanning tunneling microscope (STM), the X-ray reflectivity measurement and a stylus profilometer. A good correlation between the STM and the peak X-ray reflectivity has been demonstrated.

Fabrication of Magnetic Tunneling Junctions with NaCl Barriers

We fabricated magnetic tunneling junctions (MTJs) with epitaxial NaCl(001) barriers grown on Fe(001). Thin magnetic layers were grown using a vacuum deposition system and junction patterns were formed using shadow masks installed in the deposition system. We confirmed that a barrier layer of NaCl(001) grows epitaxially on an Fe(001) bottom electrode layer but that the top layer of Fe is polycrystalline. The MTJs show nonlinear current–voltage characteristics and tunneling magneto-resistance (TMR) of ~3% at room temperature. The observed TMR ratio is smaller than that expected for an epitaxial barrier with a fourfold symmetric crystal structure. Although the reason for this large discrepancy in TMR ratio between the MTJs with MgO barriers and those with NaCl barriers is unclear at this stage, novel materials such as a family of alkali halides have the potential to function as an applicable barrier for MTJs.

Comprehensive studies for the crystal structures and electronic properties of the superconducting system Fe1 + δSe1 − xTex with \delta \simeq 0.037 and x \simeq 0.55

Structural aspects and electronic properties for the Fe1 + δSe1 − xTex system with 0.04 ≤ δ ≤ 0.08 and 0.5 ≤ x ≤ 1, especially with the superconducting composition and , are investigated comprehensively with x-ray four-circle diffraction and through measurements of electrical resistivity, thermoelectric power, magnetic susceptibility and nuclear magnetic resonance for 125Te. The crystal structures with an excess Fe site are refined precisely with obvious constraints. For the superconducting composition, the transport properties are explained in terms of the two-band model, where an electron carrier band gives a linear-in-T resistivity and another hole band leads to nearly temperature-independent behaviour. The magnetic susceptibility and the Knight shift are explained with the idea that the electron correlation is enhanced with increasing x and it is reduced with annealing. The spin–lattice relaxation rates for the normal state that show the apparent Korringa relation may also be understood in this framework. These evidences suggest that the superconductivity may emerge in a regime where the correlation is relatively weak in this system.Structural aspects and electronic properties for the Fe(1 + δ)Se(1 - x)Te(x) system with 0.04 ≤ δ ≤ 0.08 and 0.5 ≤ x ≤ 1, especially with the superconducting composition δ is approximately equal to 0.037 and x is approximately equal to 0.55, are investigated comprehensively with x-ray four-circle diffraction and through measurements of electrical resistivity, thermoelectric power, magnetic susceptibility and nuclear magnetic resonance for (125)Te. The crystal structures with an excess Fe site are refined precisely with obvious constraints. For the superconducting composition, the transport properties are explained in terms of the two-band model, where an electron carrier band gives a linear-in-T resistivity and another hole band leads to nearly temperature-independent behaviour. The magnetic susceptibility and the Knight shift are explained with the idea that the electron correlation is enhanced with increasing x and it is reduced with annealing. The spin-lattice relaxation rates for the normal state that show the apparent Korringa relation may also be understood in this framework. These evidences suggest that the superconductivity may emerge in a regime where the correlation is relatively weak in this system.

Calibration of a Superconducting Quantum Interference Device Magnetometer

Calibration factor for magnetization scale depends on sample size along the scan direction in a superconducting quantum interference device (SQUID) magnetometer. A reference material with the same shape and size as a sample or the proper correction of apparent values is necessary for the accurate determination of magnetization.

Electrical conduction and tunneling magnetoresistance of Fe, Co/MgF2 granular multilayers

Electrical transport was investigated in Fe/MgF2 and Co/MgF2 granular thin films prepared by alternating deposition in ultrahigh vacuum. Temperature dependence of the resistivity changed from metallic to semiconductor-like when Fe (Co) layer thickness was less than 45 A (40 A) with a fixed MgF2 layer of 30 A. Discontinuous ferromagnetic layers were formed in films with semiconductor-like conduction and were confirmed with transmission electron micrography. The largest room-temperature magnetoresistance of 8.5% at 15 kOe (11% at 50 kOe) was found in a [Fe(15 A)/MgF2(20 A)] film, and that of 5.4% was found in a [Co(15 A)/MgF2(30 A)] film. The resistivity of the Fe,Co/MgF2 films, which show large tunneling magnetoresistance (TMR), was one order higher than those of other systems prepared with co-deposition. Tunnelling activation energy, estimated to be from 200 to 1700 K, did not markedly differ from that of other systems.

Characterization of Highly Concentrated Bi Donors Wire-δ-Doped in Si

We studied the Bi wire-δ-doping process to achieve a high concentration of Bi donors in Si. Our process has two steps: (i) burial of Bi nanowires in Si by molecular beam epitaxy, and (ii) activation of Bi atoms in the δ-doped layer by laser annealing. The peak concentration of Bi atoms in the δ-doped layer is controlled by two parameters: the coverage of surfactant layer, and the growth temperature during the Si cap-layer growth, whose maximum concentration is larger than 1020 cm-3. Photoluminescence and electrical carrier transport measurements reveal that dense Bi atoms are activated upon heating the area at close to the melting point of Si. As a result, our doping process results in Bi donors in the wire-δ-doped layer with concentration of >1018 cm-3. This will be useful for establishing next-generation, quantum information processing platform.

Development of Shape Memory Actuator for Cryogenic Application

Shape memory alloy (SMA) is one of actuator materials which converts thermal energy into mechanical energy. Since it has much larger output strain and stress compared to piezoelectric materials or magnetostrictive materials. Thus SMA is suitable as actuator materials for extreme environment and in the areas which require a large motion and high energy density. The purpose of present study is to develop the shape memory alloys which can operate at temperatures below liquid nitrogen temperature. Effect of grain size and chemical composition on martensitic transformation were assessed in TiNi and Cu-Al-Mn alloys.

Comprehensive studies for the crystal structures and electronic properties of the superconducting system Fe(1 + δ)Se(1 - x)Te(x) with δ is approximately equal to 0.037 and x is approximately equal to 0.55.

Structural aspects and electronic properties for the Fe1 + δSe1 − xTex system with 0.04 ≤ δ ≤ 0.08 and 0.5 ≤ x ≤ 1, especially with the superconducting composition and , are investigated comprehensively with x-ray four-circle diffraction and through measurements of electrical resistivity, thermoelectric power, magnetic susceptibility and nuclear magnetic resonance for 125Te. The crystal structures with an excess Fe site are refined precisely with obvious constraints. For the superconducting composition, the transport properties are explained in terms of the two-band model, where an electron carrier band gives a linear-in-T resistivity and another hole band leads to nearly temperature-independent behaviour. The magnetic susceptibility and the Knight shift are explained with the idea that the electron correlation is enhanced with increasing x and it is reduced with annealing. The spin–lattice relaxation rates for the normal state that show the apparent Korringa relation may also be understood in this framework. These evidences suggest that the superconductivity may emerge in a regime where the correlation is relatively weak in this system.Structural aspects and electronic properties for the Fe(1 + δ)Se(1 - x)Te(x) system with 0.04 ≤ δ ≤ 0.08 and 0.5 ≤ x ≤ 1, especially with the superconducting composition δ is approximately equal to 0.037 and x is approximately equal to 0.55, are investigated comprehensively with x-ray four-circle diffraction and through measurements of electrical resistivity, thermoelectric power, magnetic susceptibility and nuclear magnetic resonance for (125)Te. The crystal structures with an excess Fe site are refined precisely with obvious constraints. For the superconducting composition, the transport properties are explained in terms of the two-band model, where an electron carrier band gives a linear-in-T resistivity and another hole band leads to nearly temperature-independent behaviour. The magnetic susceptibility and the Knight shift are explained with the idea that the electron correlation is enhanced with increasing x and it is reduced with annealing. The spin-lattice relaxation rates for the normal state that show the apparent Korringa relation may also be understood in this framework. These evidences suggest that the superconductivity may emerge in a regime where the correlation is relatively weak in this system.