Takashi Manago

Material Design of Half-Metallic Zinc-Blende CrAs and the Synthesis by Molecular-Beam Epitaxy

A new class of half-metallic ferromagnets has been found in the zinc-blende crystal structure. The previously nonexistent zinc-blende CrAs thin films have been synthesized on GaAs (001) substrates by molecular-beam epitaxy, and show a ferromagnetic behavior at room temperature. The zinc-blende CrAs has been designed by ab initio calculations based on the local spin-density approximation, and the calculation predicts the highly spin-polarized electronic band structure.

Spin-polarized light-emitting diode using metal/insulator/semiconductor structures

We have succeeded in growing ferromagnetic metals (Co, Fe, and NiFe)/Al2O3/AlGaAs heterostructures with homogeneous flat interfaces. The electroluminescence from a light-emitting diode with a metal/insulator/semiconductor (MIS) structure depends on the magnetization direction of the ferromagnetic electrode. This fact shows that a spin injection from the ferromagnetic metal to the semiconductor is achieved. The spin-injection efficiency is estimated to be the order of 1% at room temperature.

Epitaxial growth of zinc-blende CrAs/GaAs multilayer

Zinc-blende CrAs/GaAs multilayers were grown by molecular beam epitaxy. It was certified that each CrAs layer maintains an epitaxial relationship with the zinc-blende GaAs structure judging from the reflection high-energy electron diffraction observation. The film contains thicker zinc- blende CrAs layers in total than the CrAs thin film directly grown on the GaAs substrate which has the critical thickness of 3 nm. It was clarified that the optimum thicknesses of CrAs and GaAs to keep a good epitaxial relationship are 2 ML and 2 ML, respectively. The electronic structure of the multilayer is thought to be close to that of the (Ga, Cr)As thin film which has 50% of Cr content judging from x-ray absorption spectroscopy measurements.

Electrical Spin Injection from Out-of-Plane Magnetized FePt/MgO Tunneling Junction into GaAs at Room Temperature

We have succeeded in demonstrating zero-magnetic-field spin injection in metal–insulator–semiconductor (MIS) structure at room temperature using FePt/MgO/GaAs-based light-emitting diode heterojunction with out-of-plane magnetization. The spin polarization was investigated by spin-polarized electroluminescence (EL). The lower estimate injected at remanence was 1.5% and that injected at 1 T was reached up to 11.5%. The spin injection efficiency was estimated at least 29%. The bias dependence of the EL circular polarization showed that it decreases with increasing bias voltage for both at 1 T and at remanence.

Magnetic-field-controllable avalanche breakdown and giant magnetoresistive effects in Gold/semi-insulating-GaAs Schottky diode

Gold (Au)∕semi-insulating (SI)-GaAs Schottky diode was fabricated by the standard photolithography method using wet etching. Magnetic-field-dependent avalanche breakdown phenomena were observed in the current–voltage curves measured under magnetic field. The avalanche breakdown due to impact ionization was postponed to higher electrical field under applied magnetic field. Accordingly, threshold voltages of avalanche breakdown increased with the applied magnetic field. Above 0.2T, avalanche breakdown was totally quenched. When Au‐SI‐GaAs Schottky diode was operated above the threshold voltage, giant mangetoresistive effects up to 100 000% were achieved under magnetic field of 0.8T.

Growth condition dependence of spin-polarized electroluminescence in Fe∕MgO∕light-emitting diodes

We compared the electroluminescence (EL) polarization of two Fe∕MgO∕light-emitting-diode (LED) structures grown at different substrate temperatures for MgO growth: room temperature and 400°C. Two spin-LED wafers were prepared on molecular beam epitaxy grown LEDs by e-beam evaporation: one was LED∕MgO (RT)∕Fe (RT)∕Au cap (RT), and the other was LED∕MgO (400°C)∕Fe (150°C)∕Au cap (90°C). Spin-polarized EL was clearly observed in the latter sample, while the EL polarization was hardly observed in the former sample. The reasons for the near absence of EL polarization in the former sample are considered to be the degradation of the tunneling junction resulting from the crystallinity and the As-rich surface of the LED.

Thickness-dependent magnetic domain change in epitaxial MnAs films on GaAs(001)

The authors report the change of the magnetic domain structure, dependent on the film thickness of MnAs films epitaxially grown on GaAs(001), investigated by magnetic force microscopy. Interestingly, as the film thickness decreases, the domain structure within the ferromagnetic α-MnAs stripes changes from a head-on domain structure to a simple 180° one around a thickness of 250nm. This result is understood by the change in the demagnetizing factor of the ferromagnetic stripes with the film thickness.

Transport properties of ferromagnet/insulator/semiconductor tunnel junctions

Spin-dependent transport of the photoexcited electrons in the semiconductor (p-GaAs)/insulator (Al2O3)/ferromagnetic metal (permalloy) junctions was investigated. As samples, homogeneous tunneling junctions were prepared on the flat and As-defect-free GaAs(111)B homoepitaxial surface by in situ oxidation of the Al layer and successive metallization by permalloy deposition. Spin-polarized electrons were excited in the GaAs by circularly polarized light and injected into the permalloy layer. Since the permalloy has almost zero magnetic circular dichroism at the vicinity of the band gap energy of GaAs, we can detect spin-dependent current exclusively. As a result, the energy dependence of the observed helicity asymmetry (1.44–3.05 eV) of the photoinduced current shows the absence of the spin-dependent tunneling in the sample. The importance of controlling the electron lifetime to obtain the spin-dependent tunneling was discussed.

Spin Wave Excitation and Propagation Properties in a Permalloy Film

Spin wave excitation and propagation properties in a permalloy were investigated using a vector network analyzer for the magnetostatic surface wave (MSSW) and magnetostatic backward volume wave (MSBVW) configurations. In the MSSW configuration, the excitation and transmission spectra show many peaks. They originate at the distance of antenna lines of the coplanar waveguide, and the waveguide design is important for selecting the excitation and transmission wave vectors of the spin wave. The attenuation length of the MSSW was estimated to be 7.1 µm, and the group velocity of the MSSW with a wave number of 0.26 µm-1 was estimated to be about 8.6 µm/ns for an external magnetic field of 20 mT. In the MSBVW configuration, however, the excitation spin wave spectrum shows a single peak, since many quantized peaks overlap. A transmission signal with a single peak was also detected, but this could be an artifact such as an induced current.

Epitaxial growth of NiO/Pd superlattices by reactive evaporation method

NiO/Pd epitaxial superlattices were successfully grown by a reactive evaporation method and their structures were studied by various X-ray diffraction (XRD) methods. We prepared (111) oriented films and (100) oriented films on α-Al2O3(00·1) and MgO(100) substrates, respectively. The XRD patterns of 2θ/θ scans were analyzed by an extended step model calculation. In-plane alignment was investigated by pole figure method and reciprocal space mapping. The specimens can be epitaxially grown up to a certain thickness of the NiO layer tNiO [tNiO≤20 A for (111) oriented film and tNiO≤10 A for (100) oriented film] with the lattice shrinking of the NiO of approximately 10%. On the other hand, those beyond the critical thickness have texture structure with the lattice spacing of bulk NiO.