Hiroshi Sakakima

Giant Magnetoresistanc of Dual Spin-Valves Using Antiferromagnetic .ALPHA.-Fe2O3 as a Pinning Layer.

We report the large magnetoresitance (MR) ratio of a dual spin-valve structure in α-Fe2O3(50 nm)/Co/Cu/Co/Cu/Co/α-Fe2O3(50 nm) films using the insulating antiferromagnetic α-Fe2O3 as a pinning layer. The MR ratio of a dual spin-valve is obtained as 27.8%, the highest value ever reported. This large MR ratio may indicate that the specular reflection of the conduction electrons occurs at the metal/insulator (Co/α-Fe2O3) interfaces.

Spin-Valve Memory Elements Using [{Co-Pt/Cu/Ni-Fe-Co}/Cu] Multilayers

A new type of magnetoresistive memory using spin-valve multilayers composed of semi-hard magnetic layers and soft magnetic layers separated with nonmagnetic layers was developed. The fabricated memory element was composed of a word line of Au film and a magnetoresistive sense line of [{CoPt/Cu/NiFeCo}/Cu]N multilayers. This spin-valve memory has nonvolatile and nondestructive readout properties.

Low-Field Giant Magnetoresistance in [Ni-Fe-Co/Cu/Co/Cu] Superlattices

[Ni-Fe-Co/Cu/Co/Cu] superlattices were prepared by RF sputtering. The superlattices showed low-field (<500 Oe) giant magnetoresistance (Δρ/ρ=15%) at RT when the Cu layer thickness was about 2 nm. The MR (magnetoresistance) ratio was found to oscillate with the Cu spacer layer thickness with a period of 1.1 nm, which implies RKKY (Ruderman-Kittel-Kasuya-Yosida)-like interlayer exchange coupling between the magnetic layers.

Ferromagnetic Resonance in (NiFeCo/Cu/Co)-Multilayers.

Ferromagnetic resonance (FRM) measurements were made on [Ni 0.8 Fe 0.15 Co 0.05 /Cu/Co] multilayers in order to observe the coupling between ferromagnetic layers across Cu spacer. The resonant fields vary oscillatory with Cu spacer thickness. The FMR results were analyzed by the resonant model of a magnetically coupled two-layer system. By using a modified RKKY theory for interlayer exchange interaction and FMR results, the coupling constant, J , was determined as a function of Cu spacer thickness. The resonant field can be predicted for Cu spacer thickness and are well compared to the experimental values. In samples of antiferromagnetic coupling, the resonance mode of the magnetization flopping were observed at small applied fields.

Theoretical and experimental studies on wide-band-gap p-type conductive BaCuSeF and related compounds

BaCuSeF and related compounds, MCuQF (M = Ba, Sr; Q = Se, S), are known to show p-type conduction. The formation energies of the Cu vacancy ΔH[VCu] in a MCuQF system were computed by first-principles calculation with a generalized gradient approximation (GGA) of the Perdew–Burke–Ernzerhof (PBE) functional as an electron exchange and correlation functional. The density of states (DOS) of BaCuSeF was calculated with the hybrid functional of Heyd–Scuseria–Ernzerhof (HSE) 06. ΔH[VCu] was found to be very small under both the Cu- and Q-rich conditions, which probably contributes to p-type conduction. The electronic structure of BaCuSeF was studied by X-ray photoelectron spectroscopy (XPS) with UV photoelectron yield spectroscopy (UVPYS) and photoemission yield spectroscopy (PYS). The determined depth of the top of the valence band relative to the vacuum level was about 4.9 eV. This value is desirable for applications in compound semiconductor thin-film tandem solar cells since the absorbers of polycrystalline thin-film solar cells, such as CdTe and Cu(In,Ga)Se2, are p-type semiconductors. The DOS of BaCuSeF calculated with the HSE06 functional was almost consistent with the XPS spectrum.

Enhancement of Giant Magnetoresistance in NiCoFe/Cu/Co with a Ag Capping Layer Prepared by Sputtering

Enhancement of giant magnetoresistance (GMR) in NiCoFe/Cu/Co with the addition of Cu/Ag capping layers prepared by sputtering on Si(001) single crystal substrates with Cu buffer layer was observed. This enhancement of magnetoresistance (MR) ratio may be attributed to the specular scattering of conduction electrons at the surface of the Ag capping layer. The dependence of the MR ratio on the Ag capping layer thickness was visible in the broad peaks at around t(Ag)=2 nm.

Spin-Valve Effect in [{Co–Pt/Cu/Ni–Fe–Co}/Cu] Multilayers

Spin-valve multilayers using hard magnetic layers such as Co–Pt, instead of antiferromagnetic layers such as Fe–Mn, were proposed. [Co–Pt/Cu/Ni–Fe–Co/Cu]N multilayers were prepared by RF sputtering. The multilayers showed typical spin-valve type magnetoresistance (MR) characteristics with a considerably large MR ratio ( ∼4%) and a small switching field ( ∼5 Oe). The MR ratio increased with the repetition number N and showed a saturation tendency for N ≥5.

Magnetoresistance in [Cu/{Fe-Mn/M/Cu/M}] Spin-Valve Multilayers (M=Ni80Fe15Co5, Ni80Co20, Co90Fe10)

[Cu/{Fe-Mn/M/Cu/M}]N spin-valve multilayers (M=Ni-Fe-Co, Ni-Co, Co-Fe) were prepared by RF sputtering. The multilayers showed a considerably large magnetoresistance (MR) ratio (~5%) at RT with a small transient field (≤30 Oe). The MR ratio increased with the repetition number N, whereas the MR ratio of Fe-Mn-capped multilayers such as Fe-Mn/[Ni-Fe-Co/Cu]N decreased with N, and the value was smaller than that of [Ni-Fe-Co/Cu]N or spin-valve multilayers [Cu/{Fe-Mn/Ni-Fe-Co/Cu/Ni-Fe-Co}]N.

Fabrication of CdS/CdTe solar cells with transparent p-type conductive BaCuSeF back contact

Transparent and p-type conductive SrCuSeF films were deposited by pulsed laser deposition (PLD) and their optical and electronic properties were characterized. The SrCuSeF films deposited at TS≥200°C showed high transmittance of more than 50% in the visible light region (400<;λ<;800 nm). The band gaps of these SrCuSeF films were all about 2.8 eV. All the SrCuSeF films showed p-type conductivity. The conductivity and carrier concentration of the SrCuSeF films were a little smaller than those of BaCuSeF film. SrCuSeF films were applied to the back contact of CdS/CdTe solar cells. The solar cell with SrCuSeF back contact deposited at substrate temperature of 200°C showed an efficiency of 10.3%. Then, the CdTe surface was etched in bromide-bromate solution before depositing the SrCuSeF back. We obtained the higher conversion efficiency of 11.6% (Voc=771 mV, Jsc=26.4 mA/cm2, and FF=0.570) for a solar cell with CdTe etched in the bromide-bromate solution.

Fabrication of transparent p-type conductive BaCuSeF films by pulsed laser deposition and their application to CdS/CdTe solar cells

Transparent p-type conductive BaCuSeF films prepared by pulsed laser deposition (PLD) were studied for application to tandem configuration solar cells. The BaCuSeF films were deposited at low substrate temperatures (TS) of 150, 200, 250, and 300 °C. The films prepared at TS ≥ 200 °C showed considerably high transmittance in the visible light region. The highest transmittance of 63% was obtained for the film deposited at TS = 300 °C. All of the films showed p-type conductivities of more than 1 S/cm. These BaCuSeF films were deposited on the CdTe surface of CdS/CdTe solar cells. A high conversion efficiency of 2.82% was obtained for the solar cell with the transparent p-type conductive BaCuSeF film deposited at TS = 200 °C. The higher efficiency of 3.12% was obtained by inserting a thin Ni0.97Li0.03O buffer layer between the BaCuSeF and CdTe layers.