Shinobu Ohya

Electromotive force and huge magnetoresistance in magnetic tunnel junctions

The electromotive force (e.m.f.) predicted by Faraday’s law reflects the forces acting on the charge, –e, of an electron moving through a device or circuit, and is proportional to the time derivative of the magnetic field. This conventional e.m.f. is usually absent for stationary circuits and static magnetic fields. There are also forces that act on the spin of an electron; it has been recently predicted that, for circuits that are in part composed of ferromagnetic materials, there arises an e.m.f. of spin origin even for a static magnetic field. This e.m.f. can be attributed to a time-varying magnetization of the host material, such as the motion of magnetic domains in a static magnetic field, and reflects the conversion of magnetic to electrical energy. Here we show that such an e.m.f. can indeed be induced by a static magnetic field in magnetic tunnel junctions containing zinc-blende-structured MnAs quantum nanomagnets. The observed e.m.f. operates on a timescale of approximately 102–103 seconds and results from the conversion of the magnetic energy of the superparamagnetic MnAs nanomagnets into electrical energy when these magnets undergo magnetic quantum tunnelling. As a consequence, a huge magnetoresistance of up to 100,000 per cent is observed for certain bias voltages. Our results strongly support the contention that, in magnetic nanostructures, Faraday’s law of induction must be generalized to account for forces of purely spin origin. The huge magnetoresistance and e.m.f. may find potential applications in high sensitivity magnetic sensors, as well as in new active devices such as ‘spin batteries’.

Magneto-optical and magnetotransport properties of heavily Mn-doped GaMnAs

The authors have studied the magneto-optical and magnetotrasnport properties of Ga1−xMnxAs thin films with high Mn concentrations (x=12.2%–21.3%) grown by molecular-beam epitaxy. These heavily Mn-doped GaMnAs films were formed by decreasing the growth temperature to as low as 150–190°C and by reducing the film thickness to 10nm in order to prevent precipitation of hexagonal MnAs clusters. Magnetic circular dichroism and anomalous Hall effect measurements indicate that these GaMnAs films have the nature of intrinsic ferromagnetic semiconductors with high ferromagnetic transition temperature up to 170K.

Recent progress in III-V based ferromagnetic semiconductors: Band structure, Fermi level, and tunneling transport

Spin-based electronics or spintronics is an emerging field, in which we try to utilize spin degrees of freedom as well as charge transport in materials and devices. While metal-based spin-devices, such as magnetic-field sensors and magnetoresistive random access memory using giant magnetoresistance and tunneling magnetoresistance, are already put to practical use, semiconductor-based spintronics has greater potential for expansion because of good compatibility with existing semiconductor technology. Many semiconductor-based spintronics devices with useful functionalities have been proposed and explored so far. To realize those devices and functionalities, we definitely need appropriate materials which have both the properties of semiconductors and ferromagnets. Ferromagnetic semiconductors (FMSs), which are alloy semiconductors containing magnetic atoms such as Mn and Fe, are one of the most promising classes of materials for this purpose and thus have been intensively studied for the past two decades. He...

Quantum size effect and tunneling magnetoresistance in ferromagnetic-semiconductor quantum heterostructures

We report on the resonant tunneling effect and the increase of tunneling magnetoresistance (TMR) induced by it in ferromagnetic-semiconductor GaMnAs quantum-well heterostructures. The observed quantum levels of the GaMnAs quantum well were successfully explained by the valence-band kp model with the p-d exchange interaction. It was also found that the Fermi level of the electrode injecting carriers is important to observe resonant tunneling in this system.

Unveiling the impurity band induced ferromagnetism in the magnetic semiconductor (Ga,Mn)As

(Ga,Mn)As is a paradigm diluted magnetic semiconductor which shows ferromagnetism induced by doped hole carriers. With a few controversial models emerged from numerous experimental and theoretical studies, the mechanism of the ferromagnetism in (Ga,Mn)As still remains a puzzling enigma. In this Letter, we use soft x-ray angle-resolved photoemission spectroscopy to positively identify the ferromagnetic Mn 3d-derived impurity band in (Ga,Mn)As. The band appears hybridized with the light-hole band of the host GaAs. These findings conclude the picture of the valence band structure of (Ga,Mn)As disputed for more than a decade. The non-dispersive character of the IB and its location in vicinity of the valence-band maximum indicate that the Mn 3d-derived impurity band is formed as a split-off Mn-impurity state predicted by the Anderson impurity model. Responsible for the ferromagnetism in (Ga,Mn)As is the transport of hole carriers in the impurity band.

Long spin-relaxation time in a single metal nanoparticle

Spin-relaxation time is key to the performance of spin-based devices. Although the spin-relaxation times of semiconductor materials are typically approximately 100 ns (ref. 3), they are on the order of picoseconds in bulk metals due to the high density of scattering centres. In metallic nanoparticles, the spin-relaxation times can be strongly enhanced due to the quantum size effect, reaching 150 ns in cobalt nanoparticles. Here, we show that for extra electrons confined in a single ferromagnetic-metal MnAs nanoparticle embedded in a GaAs semiconductor matrix, the spin-relaxation time can reach 10 micros at 2 K, which is seven orders of magnitude longer than those of conventional metallic thin film or bulk systems, and the longest value ever reported for metallic nanoparticles. This long relaxation time is made possible by using epitaxially grown single-crystal devices with abrupt interfaces, and by avoiding surface contamination of the MnAs nanoparticle. Such a long spin-relaxation time can be very useful in nanoscale spintronic devices.

Magnetic properties of heavily Mn-doped quaternary alloy ferromagnetic semiconductor (InGaMn)As grown on InP

We have studied magnetic properties of heavily Mn-doped [(In0.44Ga0.56)0.79Mn0.21]As thin films grown by low-temperature molecular-beam epitaxy on InP substrates. (InGaMn)As with high Mn content (21%) was obtained by decreasing the growth temperature to 190 °C. When the thickness of the [(In0.44Ga0.56)0.79Mn0.21]As layer is equal or thinner than 10 nm, the reflection high-energy electron diffraction pattern and transmission electron microscopy show no MnAs clustering, indicating that a homogeneous single crystal was grown. Magnetic circular dicroism characterizations, as well as transport and magnetization measurements, indicate that the Curie temperature is 125–130 K.

Tunneling magnetoresistance in GaMnAs∕AlAs∕InGaAs∕AlAs∕GaMnAs double-barrier magnetic tunnel junctions

We have studied the tunneling magnetoresistance (TMR) of Ga0.94Mn0.06As∕AlAs(dnm)∕In0.4Ga0.6As(0.42nm)∕AlAs(dnm)∕Ga0.94Mn0.06As double-barrier magnetic tunnel junctions with various AlAs thicknesses (d=0.8–2.7nm) grown on p+GaAs (001) substrates by low-temperature molecular-beam epitaxy. In some junctions, unusual inverse TMR, in which the tunnel resistance in antiparallel magnetization is lower than that in parallel magnetization, was observed. The TMR ratio oscillated between positive and negative values with increasing the AlAs thickness, suggesting the existence of the resonant tunneling effect in the InGaAs quantum well.

Room temperature deposition of sputtered TiN films for superconducting coplanar waveguide resonators

We present a systematic study of the properties of TiN films by varying the deposition conditions in an ultra-high-vacuum reactive magnetron sputtering chamber. By increasing the deposition pressure from 2 to 9 mTorr while keeping a nearly stoichiometric composition of Ti(1-x)N(x) (x=0.5), the film resistivity increases, the dominant crystal orientation changes from (100) to (111), grain boundaries become clearer, and the strong compressive strain changes to weak tensile strain. The TiN films absorb a high concentration of contaminants including hydrogen, carbon, and oxygen when they are exposed to air after deposition. With the target-substrate distance set to 88 mm the contaminant levels increase from ~0.1% to ~10% as the pressure is increased from 2 to 9 mTorr. The contaminant concentrations also correlate with in-plane distance from the center of the substrate and increase by roughly two orders of magnitude as the target-substrate distance is increased from 88 mm to 266 mm. These contaminants are found to strongly influence the properties of TiN films. For instance, the resistivity of stoichiometric films increases by around a factor of 5 as the oxygen content increases from 0.1% to 11%. These results suggest that the sputtered TiN particle energy is critical in determining the TiN film properties, and that it is important to control this energy to obtain high-quality TiN films. Superconducting coplanar waveguide resonators made from a series of nearly stoichiometric films grown at pressures from 2 mTorr to 7 mTorr show an increase in intrinsic quality factor from ~10^4 to ~10^6 as the magnitude of the compressive strain decreases from nearly 3800 MPa to approximately 150 MPa and the oxygen content increases from 0.1% to 8%. The films with a higher oxygen content exhibit lower loss, but the nonuniformity of the oxygen incorporation hinders the use of sputtered TiN in larger circuits.

Growth and properties of quaternary alloy magnetic semiconductor (InGaMn)As

We have studied the growth and properties of quaternary alloy magnetic semiconductor (InGaMn)As grown on both GaAs substrates and InP substrates by low-temperature molecular-beam epitaxy (LT-MBE). (InGaMn)As thin films were ferromagnetic below ~ 30 K, exhibiting a strong magneto-optical effect. The lattice constant of [(InyGa1-y)1-xMnx]As, whose Mn concentration x is below 4%, is slightly smaller than that of InyGa1-yAs with the same In/Ga content ratio. We have also obtained very smooth surface morphology of nearly lattice-matched (InGaMn)As thin films grown on InP substrates, which is important for application to thin-film-type magneto-optical devices integrated with III–V opto-electronics.