Pham Nam Hai

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’.

Spinodal nanodecomposition in semiconductors doped with transition metals

T. Dietl, 2, 3 K. Sato, ∗ T. Fukushima, A. Bonanni, † M. Jamet, A. Barski, S. Kuroda, M. Tanaka, Pham Nam Hai, and H. Katayama-Yoshida Institute of Physics, Polish Academy of Sciences, PL-02-668 Warszawa, Poland Institute of Theoretical Physics, Faculty of Physics, University of Warsaw, PL-02-093 Warszawa, Poland WPI-Advanced Institute for Materials Research (WPI-AIMR), Tohoku University, Sendai 980-8577, Japan Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan Institut für Halbleiter-und-Festkörperphysik, Johannes Kepler University, A-4040 Linz, Austria Commissariat à l’Energie Atomique, INAC/SP2M-UJF, F-38054 Grenoble, France Institute of Materials Science, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan Department of Electrical Engineering and Information Systems, The University of Tokyo, Tokyo 113-8656, Japan Department of Physical Electronics, Tokyo Institute of Technology, Tokyo 152-8552, Japan

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.

Semiconductor waveguide optical isolator based on nonreciprocal loss induced by ferromagnetic MnAs

We fabricated TM mode InGaAlAs∕InP active waveguide optical isolators based on the magnetically induced nonreciprocal loss. We used epitaxially grown MnAs thin films as ferromagnetic electrodes of the semiconductor active waveguide optical isolators. We demonstrated TM mode nonreciprocal propagation (8.8dB∕mm) at 1540nm with an excellent ferromagnetic electrode contact, which has greater semiconductor active waveguide optical isolator performance than that of our previously reported devices with Ni∕Fe polycrystalline electrodes.

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.

1.54-μm TM-mode waveguide optical isolator based on the nonreciprocal-loss phenomenon: device design to reduce insertion loss

We developed a 1.5-microm band TM-mode waveguide optical isolator that makes use of the nonreciprocal-loss phenomenon. The device was designed to operate in a single mode and consists of an InGaAlAs/InP ridge-waveguide optical amplifier covered with a ferromagnetic MnAs layer. The combination of the optical waveguide and the magnetized ferromagnetic metal layer produces a magneto-optic effect called the nonreciprocal-loss phenomenon--a phenomenon in which the propagation loss of light is larger in backward propagation than it is in forward propagation. We propose the guiding design principle for the structure of the device and determine the optimized structure with the aid of electromagnetic simulation using the finite-difference method. On the basis of the results, we fabricated a prototype device and evaluated its operation. The device showed an isolation ratio of 7.2 dB/mm at a wavelength from 1.53 to 1.55 microm. Our waveguide isolator can be monolithically integrated with other waveguide-based optical devices on an InP substrate.

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.

Growth and characterization of n-type electron-induced ferromagnetic semiconductor (In,Fe)As

We show that by introducing isoelectronic iron (Fe) magnetic impurities and Beryllium (Be) double-donor atoms into InAs, it is possible to grow a n-type ferromagnetic semiconductor (FMS) with the ability to control ferromagnetism by both Fe and independent carrier doping by low-temperature molecular-beam epitaxy. We demonstrate that (In,Fe)As doped with electrons behaves as an n-type electron-induced FMS. This achievement opens the way to realize novel spin-devices such as spin light-emitting diodes or spin field-effect transistors, as well as helps understand the mechanism of carrier-mediated ferromagnetism in FMSs.

GaMnAs-based magnetic tunnel junctions with an AlMnAs barrier

We investigate the spin-dependent transport of GaMnAs-based magnetic tunnel junctions (MTJs) containing a paramagnetic AlMnAs barrier with various thicknesses. The barrier height of AlMnAs with respect to the Fermi level of GaMnAs is estimated to be 110 meV. We observe tunneling magnetoresistance (TMR) ratios up to 175% (at 2.6 K), which is higher than those of the GaMnAs-based MTJs with other barrier materials in the same temperature region. These high TMR ratios can be mainly attributed to the relatively high crystal quality of AlMnAs and the suppression of the tunneling probability near at the in-plane wave-vector k||=0.