The authors have developed a magnetic tunnel junction of Co2FeAl0.5Si0.5 electrodes and a MgO barrier fabricated by molecular beam epitaxy and observed that this device had a tunnel magnetoresistance ratio of 386% at approximately 300 K and 832% at 9 K. The lower Co2FeAl0.5Si0.5 electrode was annealed during and after deposition resulting in a highly ordered structure with small roughness. This highly ordered structure could be obtained by annealing treatment even at low temperatures. Furthermore, a weak temperature dependence of the tunnel magnetoresistance ratio was observed for the developed magnetic tunnel junction.

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The authors fabricated Co 2 FeAl 0.5 Si 0.5 full Heusler alloy thin films by using a molecular beam epitaxy system on Cr buffered MgO single crystal substrates and investigated their structural and magnetic properties. It was revealed that Co 2 FeAl 0.5 Si 0.5 films formed ordered L2 1 and B2 structures after annealing above 500°C and below 400°C, respectively. Then magnetic tunnel junctions with these electrodes and Al-oxide or MgO barriers were also fabricated, and the bias voltage dependence of tunnel magnetoresistance ratio was investigated. It was found that the tunnel magnetoresistance ratio of the junction with Al-oxide barrier has a larger asymmetric bias voltage dependence compared with that with MgO barrier, and the negative tunnel magnetoresistance ratio at high negative bias voltage is observed for both junctions.Half metallic ferromagnetic ͑HMF͒ thin films, which have spin polarization of 1 at the Fermi energy, are promising candidates for spintronics devices such as magnetic tunnel junctions ͑MTJs͒ or spin injection devices. Such a large spin polarization can result in a large magnetoresistance of tunnel magnetoresistance ͑TMR͒ or a current perpendicular to plane giant magnetoresistance and perfect spin filter devices containing HMFs. Co-based full Heusler alloys attract much attention because it is predicted to possess a high spin polarization 1 even at room temperature. In recent years, high TMR ratio was reported for the MTJs with Co-based full Heusler alloy electrodes 2-4 in conjunction with high crystallographic quality. In these reports, Co-based full Heusler electrodes were fabricated by a sputtering system. In our previous work, the maximum tunneling spin polarization of 0.72 at room temperature was obtained for a Co 2 FeAl 0.5 Si 0.5 ͑CFAS͒ full Heusler alloy fabricated by sputtering. 5 In this work, we have investigated the TMR effect using CFAS electrodes fabricated by a molecular beam epitaxy ͑MBE͒ system, with Al-oxide barriers fabricated by sputtering or MgO single crystal barriers by MBE.The samples were fabricated on a Cr buffered MgO ͑001͒ single crystal substrate by a MBE system with a base pressure of 3 ϫ 10 −8 Pa combined with sputtering system with a base pressure of 2 ϫ 10 −5 Pa. The crystalline structure and magnetic property for MgO substrate/Cr/CFAS films were determined by x-ray diffraction ͑XRD͒ patterns using excitation by Cu K␣ radiation and a vibrating sample magnetometer. The magnetoresistance effect of the MTJs was measured by a four point probe technique at room temperature.Typical diffraction patterns in ͑a͒ in-plane geometry and ͑b͒ pole figure of ͑111͒ for CFAS are shown in Fig. 1. In Fig. 1͑a͒, only ͑200͒ and ͑400͒ CFAS peaks are exhibited for all samples, except for the peaks due to the MgO substrate, and in Fig. 1͑b͒, clear ͑111͒ peaks are shown for CFAS annealed at 500 and 600°C. The ordered structure of the films is characterized whether there exists either a diffraction peak from the ͑111͒ ͓or ͑311͔͒ superlattice reflection corresponda͒

The interface condition between the graphene channel and aluminum oxide (Al2O3) gate insulator in a graphene field-effect transistor (FET) has been analyzed. The hard X-ray photoelectron spectroscopy technique was employed to analyze the interface. In the obtained C1s spectra, a small peak was found at 284.2 eV, which was considered to be derived from a covalent bond between the graphene and Al2O3. In the pulsed S-parameters measurements, it was found that the direction of the Dirac voltage shift matched the polarity of the applied voltage stress. The Dirac voltage shift demonstrated that there were electron traps at the interface, degrading the FET performance such as the cutoff frequency. It was concluded that the unexpected bond at the interface formed electron traps whose energy level located near the conduction band minimum and that the Dirac voltage shifted in accordance with carrier capturing or emitting by the traps.

A novel method to fabricate uniform epitaxial graphene on C-face SiC substrates was investigated. Graphene was grown on the C-face 6H-SiC substrates with a sputtered SiC film by annealing temperatures ranging from 1400 to 1900 °C under an Ar ambient. The fractional area of the graphene having the layer number of two was about 95% in a 75×75 μm square by a Raman mapping and a low energy electron microscopy. Graphene on the C-face SiC fabricated by this method is quite uniform compared to that made by a conventional method without the sputtered SiC films and is thus suitable for high frequency analog devices.