Yasuhiro Shirahata

Manipulation of magnetic coercivity of Fe film in Fe/BaTiO3 heterostructure by electric field

The manipulation of magnetism at Fe/BaTiO3 interfaces is demonstrated via lattice distortion induced by thermal and electrical means. We find that the magnetic coercivity shows similar electric field dependence for positive and negative electric fields in the tetragonal phase of BaTiO3, whereas those in the orthorhombic and rhombohedral phases vary asymmetrically with respect to the polarity. The temperature dependent magnetization also reveals that the effect has its origin in the strong magnetoelastic coupling at the interface. The underlying mechanisms of the electric field dependence of the coercivity are discussed, associated with the different ferroelectric poling processes of BaTiO3.

Switching of the symmetry of magnetic anisotropy in Fe/BaTiO3 heterostructures

Switching of the symmetry of magnetic anisotropy is clearly demonstrated in an epitaxial Fe layer on BaTiO3, in association with the interface lattice distortion occurred at the structural phase transition of BaTiO3. The polar plot of the normalized remanent magnetization shows the fourfold symmetry with the magnetization easy axis along [100] of Fe at room temperature, while that at 230 K exhibits the twofold symmetry with the easy axis along [110]. Spatially resolved micro-Raman inspection corroborates the fact that the change in the magnetic symmetry arises from the magnetoelastic coupling at the interface.

Efficient spin injection into GaAs quantum well across Fe3O4 spin filter

We demonstrate efficient spin injection into GaAs across an Fe3O4 electrode. Spin polarization of electrons injected into a GaAs quantum well becomes significantly large below 120 K, reaching a value of 33% at 10 K. The large spin polarization is likely due to spin filtering effect across the insulating ferrimagnetic Fe3O4 layer at the interface. The results indicate that spin filtering effect across Fe3O4 is a very promising means to enhance the spin injection efficiency into semiconductors.

Alternating domains with uniaxial and biaxial magnetic anisotropy in epitaxial Fe films on BaTiO3

We report on domain formation and magnetization reversal in epitaxial Fe films on ferroelectric BaTiO3 substrates with ferroelastic a–c stripe domains. The Fe films exhibit biaxial magnetic anisotropy on top of c domains with out-of-plane polarization, whereas the in-plane lattice elongation of a domains induces uniaxial magnetoelastic anisotropy via inverse magnetostriction. The strong modulation of magnetic anisotropy symmetry results in full imprinting of the a–c domain pattern in the Fe films. Exchange and magnetostatic interactions between neighboring magnetic stripes further influence magnetization reversal and pattern formation within the a and c domains.

Strain-induced reversible and irreversible magnetization switching in Fe/BaTiO3 heterostructures

Magnetization switching of an Fe film in Fe/BaTiO3 heterostructures is demonstrated due to the interface lattice distortion caused by the structural phase transition of BaTiO3. The temperature dependence of in-plane magnetization of the Fe film in both zero and a small negative applied magnetic field clearly reveals that the reversible and irreversible magnetization switching processes occur in the Fe/BaTiO3 heterostructures. The variation in the magnetization orientation is corroborated by the fact that the symmetry of the magnetic anisotropy changes as the BaTiO3 undergoes the structural phase transition from the tetragonal to orthorhombic phases.Magnetization switching of an Fe film in Fe/BaTiO3 heterostructures is demonstrated due to the interface lattice distortion caused by the structural phase transition of BaTiO3. The temperature dependence of in-plane magnetization of the Fe film in both zero and a small negative applied magnetic field clearly reveals that the reversible and irreversible magnetization switching processes occur in the Fe/BaTiO3 heterostructures. The variation in the magnetization orientation is corroborated by the fact that the symmetry of the magnetic anisotropy changes as the BaTiO3 undergoes the structural phase transition from the tetragonal to orthorhombic phases.

Structural and electrical characterization of AgInSe2 crystals grown by hot-press method

Undoped polycrystalline AgInSe2 crystals were successfully grown at low temperature (650°C) using a hot-press method. The sizes of the samples were 2 cm in diameter. The largest grain size was approximately 90 nm. The presence of lattice defects such as Ag interstitials might lead to an enhancement in n-type electrical conductivity. The crystals had a resistivity of 2.2 Ω cm, a carrier concentration of 4.2 × 1016 cm3 and a mobility of 70 cm2V-1s-1 obtained by Hall measurement at RT.

Evaluation of photovoltaic power generation system using spherical silicon solar cells and SiC-FET inverter

A photovoltaic power generation system using spherical silicon (Si) solar cells and silicon carbide (SiC) field effect transistor (FET) inverter for photovoltaic applications was constructed and evaluated. The spherical Si solar cells were connected to the SiC-FET inverter and were used as a power source. Comparing the photovoltaic power generation system using an ordinary Si-FET inverter, direct current-alternating current conversion efficiencies of the SiC-FET inverter were improved due to reduction of power loss in the SiC-FET inverter.

Fabrication and characterization of CH3NH3(Cs)Pb(Sn)I3(Br) perovskite solar cells

Perovskite-type CH3NH3(MA)PbI3−based photovoltaic devices were fabricated and characterized. Doping effects of cesium iodide (CsI), cesium bromide (CsBr) and tin bromide (SnBr2) on the photovoltaic properties and surface microstructures of the perovskite phase were investigated. Short-circuit current densities, open-circuit voltages and fill factors increased by CsI and SnBr2 addition. The surface coverage of the perovskite crystals was also improved by SnBr2 doping, which resulted in improvement of the fill factor. The cell prepared by a starting composition of MA0.95Cs0.05Pb0.95Sn0.05I2.90Br0.10 showed the best photovoltaic performance in the present work.

Perpendicularly magnetized spin filtering Cu/Ni multilayers

Spin filtering at perpendicular magnetized Cu/Ni multilayer/GaAs(001) interfaces is demonstrated at remanence using optical spin orientation method. [Cu(9 nm)/Ni(tNi nm)]n multilayers are found to show a crossover from the in-plane to out-of-plane magnetic anisotropy at the Cu/Ni bilayer repetition n = 4 and the Ni layer thickness tNi = 3. For a perpendicularly magnetized Cu/Ni multilayer/n-GaAs(001) interface, circular polarization dependent photocurrent shows a clear hysteretic behavior under optical spin orientation conditions as a function of magnetic field out-of-plane while the bias dependence exhibits a substantial peak at a forward bias, verifying that Cu/Ni multilayers work as an efficient spin filter in the remanent state.

Electric field driven variation in magnetoresistance of Co/Cu/Fe/BaTiO3 heterostructure

We demonstrate control of the magnetoresistance of Co/Cu/Fe giant magnetoresistance structures on BaTiO3(001) by electric field. The magnetoresistance clearly increases with electric field strength in the three different ferroelectric phases of BaTiO3, while the electric field dependence is more significant in the orthorhombic and rhombohedral phases with hysteretic behaviors, associated with the different ferroelectric domain reversal processes of BaTiO3. The results clearly show that electrically induced lattice distortion at the Fe/BaTiO3 interface can provide an alternative means to manipulating the electronic structure of the Fe layer by electric field.