arXiv subjects

Ryosho Nakane

Publications and source records attributed to Ryosho Nakane.

26 records · Page 2Linked to original sources

arXiv2017

Electronic structure and magnetic properties of magnetically dead layers in epitaxial CoFe2O4/Al2O3/Si(111) films studied by X-ray magnetic circular dichroism

Epitaxial CoFe2O4/Al2O3 bilayers are expected to be highly efficient spin injectors into Si owing to the spin filter effect of CoFe2O4. To exploit the full potential of this system, understanding the microscopic origin of magnetically dead layers at the CoFe2O4/Al2O3 interface is necessary. In this paper, we study the crystallographic and electronic structures and the magnetic properties of CoFe2O4(111) layers with various thicknesses (thickness d = 1.4, 2.3, 4, and 11 nm) in the epitaxial CoFe2O4(111)/Al2O3(111)/Si(111) structures using soft X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) combined with cluster-model calculation. The magnetization of CoFe2O4 measured by XMCD gradually decreases with decreasing thickness d and finally a magnetically dead layer is clearly detected at d = 1.4 nm. The magnetically dead layer has frustration of magnetic interactions which is revealed from comparison between the magnetizations at 300 and 6 K. From analysis using configuration-interaction cluster-model calculation, the decrease of d leads to a decrease in the inverse-to-normal spinel structure ratio and also a decrease in the average valence of Fe at the octahedral sites. These results strongly indicate that the magnetically dead layer at the CoFe2O4/Al2O3 interface originates from various complex networks of superexchange interactions through the change in the crystallographic and electronic structures. Furthermore, from comparison of the magnetic properties between d = 1.4 and 2.3 nm, it is found that ferrimagnetic order of the magnetically dead layer at d = 1.4 nm is restored by the additional growth of the 0.9-nm-thick CoFe2O4 layer on it.

cond-mat.mtrl-sci↗

arXiv2014

Carrier transport properties of the Group-IV ferromagnetic semiconductor Ge1-xFex with and without boron doping

We have investigated the transport and magnetic properties of group-IV ferromagnetic semiconductor Ge1-xFex films (x = 1.0 and 2.3 %) with and without boron doping grown by molecular beam epitaxy (MBE). In order to accurately measure the transport properties of 100-nm-thick Ge1-xFex films, (001)-oriented silicon-on-insulator (SOI) wafers with an ultra-thin Si body layer (~5 nm) were used as substrates. Owing to the low Fe content, the hole concentration and mobility in the Ge1-xFex films were exactly estimated by Hall measurements because the anomalous Hall effect in these films was found to be negligibly small. By boron doping, we increased the hole concentration in Ge1-xFex from ~1018 cm-3 to ~1020 cm-3 (x = 1.0%) and to ~1019 cm-3 (x = 2.3%), but no correlation was observed between the hole concentration and magnetic properties. This result presents a contrast to the hole-induced ferromagnetism in III-V ferromagnetic semiconductors.

cond-mat.mtrl-sci↗

arXiv2011

Influence of anisotropic magnetoresistance on nonlocal signals in Si-based multi-terminal devices with Fe electrodes

We have investigated the influence of anisotropic magnetoresistance (AMR) on nonlocal signals in Si-based multi-terminal devices with ferromagnetic Fe electrodes. The AMR of the Fe electrodes was found to have a significant influence on nonlocal signals when the in-plane device structure is not optimized. Moreover, realization of a pure spin current by spin diffusion was found to be virtually impossible because of the electric potential distribution in the depth direction in the Si channel. Although apparent signals indicating the spin-valve effect were not detected, we mainly present structural influence on the electric potential distribution which is indispensable for the analyses of spin-dependent transport.

cond-mat.mtrl-sci↗

arXiv2010

Epitaxial germanidation of full-Heusler Co2FeGe alloy thin films formed by rapid thermal annealing

The authors demonstrated that a full-Heusler Co2FeGe (CFG) alloy thin film was epitaxially grown by rapid-thermal-annealing-induced germanidation of an Fe/Co/pseudo-Ge(001)-on-insulator (GOI) multilayer formed on a Si-on-insulator (SOI) substrate. X-ray diffraction (XRD) measurements with the out-of-plane and in-plane configurations revealed that the CFG film was epitaxially grown along the [001] direction with the in-plane epitaxial relation of CFG[100]||GOI[100], although the film slightly contained a texture component. The strong (111) and (200) superlattice diffraction intensities indicated that the CFG film had a high degree of order for the L21 structure. Cross-sectional high-resolution transmission electron microscopy images of the film implied that the film had the dominant epitaxial and slight texture components, which was consistent with the XRD measurements. The epitaxial component was grown directly on the BOX layer of the SOI substrate without the formation of any interfacial layer.

cond-mat.mtrl-sci↗

arXiv2010

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

cond-mat.mtrl-sci↗

arXiv2009

A new spin-functional MOSFET based on magnetic tunnel junction technology: pseudo-spin-MOSFET

We fabricated and characterized a new spin-functional MOSFET referred to as a pseudo-spin-MOSFET (PS-MOSFET). The PS-MOSFET is a circuit using an ordinary MOSFET and magnetic tunnel junction (MTJ) for reproducing functions of spin-transistors. Device integration techniques for a bottom gate MOSFET using a silicon-on-insulator (SOI) substrate and for an MTJ with a full-Heusler alloy electrode and MgO tunnel barrier were developed. The fabricated PS-MOSFET exhibited high and low transconductance controlled by the magnetization configurations of the MTJ at room temperature. This is the first observation of spin-transistor operations for spin-functional MOSFETs.

cond-mat.mtrl-sci↗

arXiv2009

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 relatively high degree of L21-ordering of 83 % under a high degree of B2-ordering of 97 %.

cond-mat.mtrl-sci↗

arXiv2007

Characterization of half-metallic L2_1-phase Co_2FeSi full-Heusler alloy films formed by rapid thermal annealing

The authors developed a preparation technique of Co_2FeSi full-Heusler alloy films with the L2_1-ordered structure on silicon-on-insulator (SOI) substrates, employing rapid thermal annealing (RTA). The Co_2FeSi 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 L2_1-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. This 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.

cond-mat.mtrl-sci↗