K. Inomata

Tunnel magnetoresistance for junctions with epitaxial full-Heusler Co2FeAl0.5Si0.5 electrodes with B2 and L21 structures

The tunnel magnetoresistance (TMR) effect has been investigated for magnetic tunnel junctions with epitaxial Co2FeAl0.5Si0.5 Heusler electrodes with B2 and L21 structures on a Cr-bufferd MgO substrate. The epitaxially grown Co2FeAl0.5Si0.5 has B2 structure when annealed below 400°C, and has L21 structure for annealing above 450°C. The TMR ratio of 76% at room temperature and 106% at 5K were obtained for a MgO(001)∕Cr∕B2-type Co2FeAl0.5Si0.5∕Al oxide/Co75Fe25∕IrMn∕Ta. The TMR ratio is larger than that of magnetic tunnel junction with an L21-type electrode, which may be due to the smoother surface of the B2 structure and disordered L21 structure due to the Cr atom interdiffusion.

Structure, magnetic property, and spin polarization of Co2FeAlxSi1−x Heusler alloys

We report the spin polarization of Co2FeAlxSi1−x (x=0.0, 0.3, 0.5, 0.7) bulk alloys measured by the point contact Andreev reflection method. All the Co2FeAlxSi1−x alloys had an L21 structure along with A2- and B2-type disorder. Several off-stoichiometric alloys (CoxFeyAl0.5Si0.5) were prepared to understand the effect of the compositional deviation from the stoichiometry on the spin polarization. By substituting Al for Si, the spin polarization changed from 0.57±0.01 for x=0.0 to a maximum value of 0.60±0.01 for x=0.5. The off-stoichiometric alloys had spin polarizations of 0.57−0.60±0.01. Ab initio calculations were performed to interpret the effect of Al addition as well as the effect of disorder on the magnetic properties and on the electronic structure.

Current-perpendicular-to-plane giant magnetoresistance in spin-valve structures using epitaxial Co2FeAl0.5Si0.5/Ag/Co2FeAl0.5Si0.5 trilayers

A current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) spin valve using epitaxial layers of Co2FeAl0.5Si0.5 (CFAS) Heusler alloy as ferromagnetic electrodes is reported. A multilayer stack of Cr/Ag/CFAS/Ag/CFAS/Co75Fe25/Ir22Mn78/Ru was deposited on a MgO (001) single crystal substrate. Epitaxial growth of the Cr, Ag, and CFAS layers in the (001) orientation up to the top CFAS layer was confirmed. Large MR ratios of 6.9% at room temperature and 14% at 6 K were observed for the CPP-GMR device. High spin polarization of epitaxial CFAS is the possible reason for high MR ratios.

175% tunnel magnetoresistance at room temperature and high thermal stability using Co2FeAl0.5Si0.5 full-Heusler alloy electrodes

The authors have fabricated epitaxially grown spin-valve-type magnetic tunnel junctions with L21-Co2FeAl0.5Si0.5 full-Heusler alloys for top and bottom electrodes and a MgO barrier. For MgO thickness tMgO=1.5nm, tunnel magnetoresistance (TMR) ratio and resistance and area product (RA) initially increase up to around 350°C and then decrease by annealing, while for tMgO=2.0 and 2.5nm, the TMR ratio increases with annealing temperature and peaks around 500°C. The TMR ratio up to 175% at RT and thermal stability up to 500°C have been achieved for tMgO=2.0nm, suggesting the large tunneling spin polarization and high thermal stability for Co2FeAl0.5Si0.5 with L21 structure.

High spin-filter efficiency in a Co ferrite fabricated by a thermal oxidation

Co ferrites fabricated by a thermal oxidation have been tested as a spin filter. Spin-filter efficiencies of 44% and 4.3% were confirmed at 10 K and RT, respectively, using a magnetic tunnel junction of Pt/CoFe2O4/MgO/Co. By increasing the bias voltage, the tunneling magnetoresistance (TMR) at 10 K increases then decreases. This is interpreted as a tunneling via spin-down conduction band in the Co ferrite. Since the electron at RT is attributed to a hopping conduction on the defect states in the Co ferrite, the reduction of the defects will be the key to achieving a higher spin-filter effect.

Current-perpendicular-to-plane giant magnetoresistance of a spin valve using Co2MnSi Heusler alloy electrodes

We report the current-perpendicular-to-plane (CPP) giant magnetoresistance (GMR) of a spin valve with Co2MnSi (CMS) Heusler alloy ferromagnetic electrodes. A multilayer stack of Cr/Ag/Cr/CMS/Cu/CMS/Fe25Co75/Ir28Mn72/Ru was deposited on a MgO (001) single crystal substrate. The bottom CMS layer was epitaxially grown on the Cr/Ag/Cr buffer layers and was ordered to the L21 structure after annealing at 673 K. The upper CMS layer was found to grow epitaxially on the Cu spacer layer despite the large lattice mismatch between Cu and CMS. The highest MR ratios of 8.6% and 30.7% for CPP GMR were recorded at room temperature and 6 K, respectively. The high spin polarization of the epitaxial CMS layers is the most likely origin of the high MR ratio.We report the current-perpendicular-to-plane (CPP) giant magnetoresistance (GMR) of a spin valve with Co2MnSi (CMS) Heusler alloy ferromagnetic electrodes. A multilayer stack of Cr/Ag/Cr/CMS/Cu/CMS/Fe25Co75/Ir28Mn72/Ru was deposited on a MgO (001) single crystal substrate. The bottom CMS layer was epitaxially grown on the Cr/Ag/Cr buffer layers and was ordered to the L21 structure after annealing at 673 K. The upper CMS layer was found to grow epitaxially on the Cu spacer layer despite the large lattice mismatch between Cu and CMS. The highest MR ratios of 8.6% and 30.7% for CPP GMR were recorded at room temperature and 6 K, respectively. The high spin polarization of the epitaxial CMS layers is the most likely origin of the high MR ratio.

Ferromagnetic Mesostructured Alloys: Design of Ordered Mesostructured Alloys with Multicomponent Metals from Lyotropic Liquid Crystals

Mesoporous materials have attracted worldwide interest because of their outstanding structural characteristics, such as high surface area, narrow pore size distribution, and wellordered arrangement of mesopores, which are suitable for a wide variety of applications. Significant advances in synthetic approaches have made it possible to synthesize mesoporous materials with various components, such as non-siliceous metal oxides, organosilicas, carbon meterials, and even polymers. The synthesis of mesoporous materials with magnetic properties has been subject to extensive research. In the presence of an external magnetic field, magnetic mesoporous materials are attracted to the magnet. This magnetic motor effect is indeed attractive for the development of separation technology. Traditionally, magnetic nanoparticles were loaded after the synthesis of mesoporous materials. However, blocking of the mesopores was observed, which prevented effective incorporation of guest species into the mesopores. Several strategies have been reported to overcome this problem. A simple block-copolymer-based “one-pot” selfassembly approach was developed to incorporate magnetic giron oxide nanoparticles in the mesopore walls. However, the amount of the embedded nanoparticles in the framework is limited, which is a serious problem for higher magnetizations. Microspheres consisting of a magnetic nanoparticle core and mesoporous silica shell have attracted particular attention. These characteristics endow them with significant application potential in various fields, such as bioseparation, enzyme immobilization, and diagnostic analysis. Another route is hard-templating, which involves the deposition of metal oxides within original templates, such as mesoporous silica or carbon, and the subsequent removal of the templates. This nanocasting approach is widely applicable to the preparation of metal oxides that are difficult to synthesize by conventional pathways. So far, various kinds of magnetic metal oxides, such as CoO, Co3O4, [13] Mn3O4, [14] NiO, a-Fe2O3, g-Fe2O3, Fe3O4, [16] and Gd2O3, [17] have been prepared. Although these mesoporous metal oxides have magnetic properties, the saturation magnetization has a lower value than that of pure ferromagnetic metals, such as Fe, Co, and Ni. The saturation magnetization of mesoporous materials depends on the compositions and amounts of magnetic phases. Higher magnetization and greater control are much in demand for current separation technology. Furthermore, if we can proportionally change the saturation magnetization just by changing the framework compositions, we can selectively collect particular ferromagnetic mesoporous materials by changing the external magnetic field. Fe group elements (Ni, Co, and Fe) and Gd are the only ferromagnetic metallic elements at ambient conditions. From a Slater–Pauling curve, Fe has the highest magnetic moment (fcc Ni: 0.6 mB/atom, fcc Co: 1.7 mB/atom, bcc Fe: 2.2 mB/ atom). With such multicomponent alloying, more precise control of the magnetization over a wide range and higher [*] Prof. Dr. Y. Yamauchi, Dr. Y. Nemoto, Dr. K. Sato World Premier International (WPI) Research Center for Materials Nanoarchitectonics (MANA) National Institute for Materials Science (NIMS) Namiki 1-1, Tsukuba, Ibaraki 305-0044 (Japan) Fax: (+ 81)29-860-4706 E-mail: yamauchi.yusuke@nims.go.jp Homepage: http://www.nims.go.jp/mana/members/ independent_scientist/y_yamauchi/index.html

Structure and transport properties of current-perpendicular-to-plane spin valves using Co2FeAl0.5Si0.5 and Co2MnSi Heusler alloy electrodes

We report the structure and transport properties of current-perpendicular-to-plane spin valves (CPP SVs) with Co2FeAl0.5Si0.5 (CFAS) or Co2MnSi (CMS) Heusler alloy magnetic layers and an Ag or Cu spacer layer. A multilayer stack of sub/Cr/Ag/Heusler/(Ag or Cu)/ Heusler/Co75Fe25/Ir22Mn78/Ru was deposited on a MgO(001) single crystalline substrate by magnetron sputtering. Transmission electron microscopy observations showed epitaxial growth from the substrate to the top Heusler layer. The CPP SV with a CFAS/Ag/CFAS trilayer showed relatively large magnetoresistance (MR) ratios of 12% at room temperature and 31% at 12 K, with monotonous temperature dependence. However, the MR values of the SV with the CMS/Ag/CMS trilayer showed a different temperature dependence with a maximum value of 22% at 100 K. This might be related to the 90° couplings between the two CMS layers.

Monolithic integration of pseudo-spin-MOSFETs using a custom CMOS chip fabricated through multi-project wafer service

We demonstrated monolithic integration of pseudo-spin-MOSFETs (PS-MOSFETs) using vendor-made MOSFETs fabricated in a low-cost multi-project wafer (MPW) product and lab-made magnetic tunnel junctions (MTJs) formed on the topmost passivation film of the MPW chip. The tunneling magnetoresistance (TMR) ratio of the fabricated MTJs strongly depended on the surface roughness of the passivation film. Nevertheless, after the chip surface was atomically flattened by SiO2 deposition and successive chemical-mechanical polish (CMP) process, the fabricated MTJs on the surface exhibited a sufficiently large TMR ratio (> 140 %) adaptable to the PS-MOSFET application. The implemented PS-MOSFETs showed clear modulation of the output current controlled by the magnetization configuration of the MTJs, and a maximum magnetocurrent ratio of 90 % was achieved. These magnetocurrent behaviors were quantitatively consistent with those predicted by HSPICE simulations. The developed integration technique using a MPW CMOS chip would also be applied to monolithic integration of CMOS devices/circuits and other various functional devices/materials, which would open the door for exploring CMOS-based new functional hybrid circuits.

Interlayer exchange coupling in Co2FeAl0.5Si0.5/Cr/Co2FeAl0.5Si0.5 trilayers

Interlayer exchange couplings were examined for Co2FeAl0.5Si0.5(CFAS)/Cr/CFAS trilayered films grown on MgO (001) single crystal and thermally oxidized Si substrates. The films were (001) epitaxial on MgO and (110) textured polycrystalline on SiO2. Strong exchange couplings were observed for the films with the 1.5 nm thick Cr spacer layer. A 90° coupling is dominant in the (001) epitaxial film. In contrast, an antiparallel coupling exists in the polycrystalline one. The relationship of interlayer couplings with the structure is discussed.