Yota Takamura

Analysis of L21-ordering in full-Heusler Co2FeSi alloy thin films formed by rapid thermal annealing

The authors developed a new analysis approach for evaluation of atomic ordering in full-Heusler alloys, which is extension of the commonly used Webster model. Our model can give accurate physical formalism for the degree of atomic ordering in the L21 structure, including correction with respect to the fully disordered A2 structure, i.e., the model can directly evaluate the degree of L21 ordering under a lower ordering structure than the complete B2-ordering structure. The proposed model was applied to full-Heusler Co2FeSi alloy thin films formed by rapid thermal annealing. The film formed at TA=800 °C showed a high degree of L21 ordering of 83% under a high degree of B2 ordering of 97%.

Quantitative analysis of atomic disorders in full-Heusler Co2FeSi alloy thin films using x-ray diffraction with Co Kα and Cu Kα sources

The authors developed a new analysis technique for atomic disorder structures in full-Heusler alloys using x-ray diffraction (XRD) with Co-Ka and Cu-Ka sources. The developed technique can quantitatively evaluate all the atomic disorders for the exchanges between X, Y, and Z atoms in full-Heusler X2YZ alloys. In particular, the technique can treat the DO3 disorder that cannot be analyzed by ordinary Cu-Ka XRD. By applying this technique to full-Heusler Co2FeSi alloy thin films formed by rapid thermal annealing (RTA), RTA-temperature (TA) dependence of the atomic disorders was revealed. The site occupancies of Co, Fe, and Si atoms on their original sites were 98 %, 90 %, and 93 %, respectively, for the film formed at TA = 800 degree C, indicating that the RTA-formed Co2FeSi film had the L21 structure with the extremely high degree of ordering.

Characterization of half-metallic L21-phase Co2FeSi full-Heusler alloy thin films formed by rapid thermal annealing

The authors developed a preparation technique of Co2FeSi full-Heusler alloy thin films with the L21-ordered structure on silicon-on-insulator (SOI) substrates, employing rapid thermal annealing (RTA). The Co2FeSi full-Heusler alloy films were successfully formed by RTA-induced silicidation reaction between an ultrathin SOI (001) layer and Fe∕Co layers deposited on it. The highly (110)-oriented L21-phase polycrystalline full-Heusler alloy films were obtained at the RTA temperature of 700°C. Crystallographic and magnetic properties of the RTA-formed full-Heusler alloy films were qualitatively the same as those of bulk full-Heusler alloy. The proposed technique is compatible with metal source∕drain formation process in advanced complementary metal-oxide semiconductor technology and would be applicable to the fabrication of the half-metallic source∕drain of metal-oxide-semiconductor field-effect transistor type of spin transistors.

Full-Heusler Co2FeSi alloy thin films with perpendicular magnetic anisotropy induced by MgO-interfaces

A 100-nm-thick L21-ordered full-Heusler Co2FeSi (CFS) alloy film was fabricated using the facing targets sputtering (FTS) method at a substrate temperature TS of 300 °C. The degrees of L21- and B2-order for the film were 37% and 96%, respectively. In addition, full-Heusler CFS alloy thin films with perpendicular magnetic anisotropy (PMA) induced by the magnetic anisotropy of MgO-interfaces were also successfully fabricated using the FTS method. The CFS/MgO stacked layers exhibited PMA when the CFS layer had a thickness of 0.6 nm ≤ dCFS ≤ 1.0 nm. The PMA in these structures resulted from the CFS/MgO interfacial perpendicular magnetic anisotropy.

Magnetic anisotropy of [Co2MnSi/Pd]n superlattice films prepared on MgO(001), (110), and (111) substrates

Superlattice films with full-Heusler Co2MnSi (CMS) alloy and Pd layers prepared on Pd-buffered MgO(001), (110), and (111) substrates were investigated. Crystal orientation and epitaxial relationship of Pd and CMS layers were analyzed from x-ray diffraction, pole figure measurements, and transmission electron microscope observation. Formation of the L21-ordered structure in the CMS layers was confirmed by observation of CMS(111) diffraction. Perpendicular magnetic anisotropy (PMA) was obtained in the [CMS (0.6 nm)/Pd (2 nm)]6 superlattice film formed using MgO(111) substrates although other superlattice films prepared using MgO(001) and (110) substrates showed in-plane and isotropic magnetic anisotropy, respectively. The perpendicular magnetic anisotropy energy constant K for the superlattice films prepared using MgO(111) substrate was estimated to be 2.3 Mergs/cm3, and an interfacial anisotropy constant Ki per one CMS-Pd interface in the superlattice films was estimated to be 0.16 ergs/cm2. Ki in superlat...

Formation of Co2FeSi/SiOxNy/Si tunnel junctions for Si-based spin transistors

The authors developed a fabrication technique of Co2FeSi/SiOxNy/Si tunnel junctions, employing rapid thermal annealing (RTA). The full-Heusler Co2FeSi (CFS) alloy thin film was directly formed on the ultrathin SiOxNy barrier layer without a buffer layer by RTA-induced silicidation reaction of a Co/Fe/amorphous-Si multilayer deposited on the barrier layer. The ultrathin SiOxNy layer formed on a Si substrate effectively blocked the diffusion of Co and Fe atoms into the Si substrate, leading to the formation of a high quality tunnel junction. It was found from crystallographic analyses that the CFS film on SiOxNy has a highly (110)-oriented texture structure with the L21 ordering and the CFS/SiOxNy interface was atomically flat without intermixing and crystallinity degradation.

Formation of Si- and Ge-based Full-Heusler Alloy Thin Films using SOI and GOI Substrates for the Half-metallic Source and Drain of Spin Transistors

The paper presents a novel preparation technique for Si- and Ge-based half-metallic full-Heusler alloy thin films, utilizing silicon-on-insulator (SOI) and germanium-on-insulator (GOI) substrates, respectively. Full-Heusler Co2FeSi (Co2FeGe) alloy thin films were successfully formed by thermally activated silicidation (germanidation) reaction between an ultra-thin SOI (GOI) layer and Co/Fe layers deposited on it. This technique can easily produce fully ordered L21 structure that is necessary for the half-metallicity of full-Heusler alloys. The proposed technique is compatible with metal source/drain formation process in advanced CMOS technology and would be applicable to the fabrication of the half-metallic source/drain of MOSFET type of spin transistors.

Compact magnetooptical isolator with cobalt ferrite on silicon photonic circuits

In the telecom wavelength range, the magnetooptical effect of cobalt ferrites is approximately 10 times larger than that of conventional magnetooptical materials such as yttrium iron garnets. In this study, we focus on an application of cobalt ferrite to a magnetooptical isolator that is to be miniaturized and made suitable for integration. First, we prepare polycrystalline cobalt ferrite films deposited on a silicon substrate using a MgO buffer layer. Next, we fabricate a waveguide optical isolator of silicon waveguides by the partial deposition of the cobalt ferrite films. An optical isolation ratio of 5.5 dB is demonstrated.

Fabrication of High-Quality Co2FeSi/SiOxNy/Si(100) Tunnel Contacts Using Radical-Oxynitridation-Formed SiOxNy Barrierfor Si-Based Spin Transistors

A high-quality Co2FeSi (CFS)/SiOxNy/Si tunnel junction was fabricated, in which the SiOxNy barrier layer was formed by radical oxynitridation of an Si(100) substrate and the CFS electrode was formed by silicidation of an Fe/Co/amorphous-Si multilayer deposited on the barrier layer. The ultrathin SiOxNy barrier layer completely blocked diffusion of Co and Fe atoms into the Si substrate during rapid thermal annealing (RTA) for the silicidation. X-ray diffraction investigations clarified that the CFS film on the ultrathin SiOxNy barrier layer exhibited a highly (110)-oriented texture structure and that the film had the L21 structure with a high degree of L21 order. High resolution cross-sectional transmission electron microscopy investigations revealed that the CFS/SiOxNy interface was atomically flat and that the crystal lattice of the CFS film was directly grown on the SiOxNy surface without degradation of the crystallinity at the interface.

Analysis and control of the Hanle effect in metal–oxide–semiconductor inversion channels

The authors theoretically analyzed the output characteristics of a proposed Hanle-effect spin transistor based on a spin-MOSFET. The device can easily create oscillating Hanle-effect signals by applying an accelerating bias voltage. The behavior of the magnetic field interval of the oscillatory Hanle-effect signals for a sufficiently high accelerating bias is well correlated with the universality of the effective electron mobility in the Si MOS inversion channel, which is useful for revealing spin transport dynamics in the MOS inversion channel.