Systematic investigation of the Fermi level position and band structure of ferromagnetic semiconductor GaMnAs (Tanaka group)

　The origin of ferromagnetism in the prototype ferromagnetic semiconductor GaMnAs is controversial due to the insufficient understanding of its band structure and Fermi level position. This is a major issue for further development of this material for future semiconductor spintronics. Tanaka group of the Univ. of Tokyo found that a unique method with precise etching technique and resonant tunneling spectroscopy for a variety of surface GaMnAs layers elucidates the universal valence-band (VB) picture of GaMnAs. They found that the VB structure of GaAs is almost perfectly maintained and that it is not merged with the impurity band in all the GaMnAs samples with the Mn concentrations from 6 to 15%. Furthermore, the exchange splitting of VB is found to be quite small (only several meV) even in GaMnAs with a high Curie temperature (154 K). These findings shed light on the veiled mechanism of the ferromagnetism in GaMnAs in dispute for more than a decade.
　This work has been published online in Nature Physics, on 6 February, 2011 (DOI 10.1038/NPHYS1905).
URL: http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys1905.html

Spin-Seebeck effect in insulators and insulator-based thermoelectric generation (Saitoh group, Maekawa group)

　A spin-Seebeck effect, generation of spin currents as a result of temperature differences, was found to appear in magnetic insulators, which has been measured only in ferromagnetic metals. Furthermore, the group has showed that, by combining this phenomenon with the relativity effect in a solid (inverse spin-Hall effect), thermoelectric power can be generated from heat flowing in an insulator, which has seemed to be impossible. The achievement would enable the use of insulators, with less energy loss due to heat transfer, for thermoelectric conversion elements, which could result in widening design possibilities and installation sites of such elements and contribute to environment-friendly electric technologies.
　The research result was published in the scientific journal “Nature Materials” as an Advanced Online Publication (September 27, 2010), and was reported in news papers including Nikkan Kogyo Shimbun and Nikkei-Sangyo Shimbun. 　[K. Uchida, J. Xiao, H. Adachi, J. Ohe, S. Takahashi, J. Ieda, T. Ota, Y. Kajiwara, H. Umezawa, H. Kawai, G. E. W. Bauer, S. Maekawa, and E. Saitoh, “Spin Seebeck insulator”Nature Materials 9 (2010) 894 - 897. ]

Spin current and electric signal transmission in insulator (Saitoh group, Takanashi group, Maekawa group)

　A spin current was successfully injected into a Mott insulator and can propagate over a long distance. We have shown that even an insulator can transmit electric signals via these processes. This has enabled, for the first time, to use spin current in insulator, making a good use of the relativity effect in a solid (spin-Hall effect) and exchange interaction at the interface between a metal and a Mott insulator.
　The research achievement was published in the British Science Journal "Nature" on March 11, 2010, and was reported in the top pages of Mainichi Shimbun, Kahoku Shimpo, and others.　[Y. Kajiwara, K. Harii, S. Takahashi, J. Ohe, K. Uchida, M. Mizuguchi, H. Umezawa, H. Kawai, K. Ando, K. Takanashi, S. Maekawa, and E. Saitoh,“Transmission of electrical signals by spin-wave interconversion in a magnetic insulator”Nature 464 (2010) 262-266. ]

Valence-Band Structure of Ferromagnetic-Semiconductor GaMnAs Studied by Spin-Dependent Resonant Tunneling Spectroscopy (Tanaka Group)

　The valence-band (VB) structure and the Fermi level (EF) position of ferromagnetic-semiconductor GaMnAs are in dispute for more than a decade. Tanaka group has quantitatively investigated these problems by electrically detecting the resonant tunneling levels of a GaMnAs quantum well (QW) in double-barrier heterostructures. The resonant level from the heavy-hole first state is clearly observed in the metallic GaMnAs QW, indicating that holes have high coherency and that EF exists in the bandgap. Clear enhancement of tunnel magnetoresistance induced by resonant tunneling is demonstrated in these double-barrier heterostructures.
　This work has been published in Physical Review Letters. [S. Ohya, I. Muneta, P.N. Hai, and M. Tanaka. Physical Review Letters 104, 167204/1-4 (2010).]

Long spin-relaxation time in a single metal nanoparticle (Tanaka Group)

　Spin relaxation time is the key to determine the performance of spin-based devices. While the spin-relaxation times of semiconductor materials are typically 100 ns, they are on the order of picosecond in bulk metals due to the high density of scattering centers. In metallic nanoparticles, the spin relaxation times can be strongly enhanced due to the quantum size effect, and have reached 150 ns in Co nanoparticles. Tanaka group has shown 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 microseconds at 2 K, which is 7 orders of magnitude longer than those of conventional metallic thin film or bulk systems. 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 for nano-scale spintronic devices.
　This work has been published in Nature Nanotechnology [P. N. Hai, S. Ohya, and M. Tanaka, Nature Nanotechnology 5, pp.593-596 (2010)], and other media [Nikkan Kogyo Shinbun, Nikkei Press Release, Nanotech Japan].

Realization of “spin motive force”and huge magnetoresistance
(Tanaka group and Barnes-Maekawa group)
　The electromotive force (e.m.f.) predicted by Faraday's law reflects the forces acting on the charge 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 by Barnes and Maekawa 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. (spin motive force) 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. Tanaka group (University of Tokyo), Barnes (Miami University) and Maekawa (Tokhoku University) showed 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 % is observed for certain bias voltages. These 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”.
　This work was published in Nature 458, 489-492 (2009) [online on March 8, 2009] and reported in many newspapers (Yomiuri, Nikkan-Kogyo, Kagaku-Kogyo, Nikkei).

Controllable magnetization switching by using pure spin currents
(Otani group)
　Otani group developed a non-air exposure fabrication procedure for lateral spin valves consisting of ferromagnetic nano-pillars bridged by a copper nano-wire. Thereby the spin injection efficiency was so improved that the non-local spin valve signal increased by a factor of 10 compared to the previously reported value. This enabled them to control the switching of the nano-pillar magnetization by using pure spin currents. This achievement shows that spin currents carrying no electronic charge can switch the magnetization of the ferromagnetic nano-pillar submicron away from the spin injector; and hence expected to be an elemental technology for large-scale integration of spintronic devices.
　This work was published in Nature Physics 4 (2008) 851-854. The related article entitled “Switching by electronic spin” was also reported in the journal [Nikkei Sangyo Shinbun 9th Oct. 2008]　

Observation of the spin Seebeck effect
(Saitoh group and Maekawa group)
　Saitoh's group and Maekawa's group have successfully observed the spin Seebeck effect, the generation of a spin current and spin voltage from a heat flow. We utilized the inverse spin Hall effect in a Pt wire as a spin-current probe and demonstrated, in a NiFe film, spin voltage generation from a heat flow. This method allows us to make a versatile thermal spin-current generator and may also be used a method for measuring spin entropy of conduction electrons.
　This work was published in Nature 455 (2008) 778.　

Electric manipulation of magnetization relaxation using spin-Hall effect
(Saitoh group and Maekawa group)
　Saitoh's group and Maekawa's group have successfully observed current-induced modulation of magnetization relaxation in a NiFe/Pt film; we demonstrates that, in terms of the ferromagnetic-resonance spectroscopy, an application of currents to the film generates a spin current via the spin-Hall effect and modulates the magnetization relaxation rate in the NiFe layer by exerting spin torque on the magnetization. The method enables the electric manipulation of magnetization relaxation and allows us to measure a spin current in a quantitative manner.
　This work was published in Phys.Rev.Lett.98 (2008) 036601.

Observation of giant spin Hall effect in Au at room temperature (Takanashi & Maekawa groups)
　Takanashi's group and Maekawa's group have successfully observed a giant spin Hall effect in Au using the multi-terminal device consisting of the FePt perpendicular spin injector and the nano-sized Au Hall cross. The spin Hall effect has attracted much attention as a technique to convert charge current into spin current and vice versa without a ferromagnetic material. The present electrical signal of the spin Hall effect at room temperature is two order of magnitude larger than those reported previously, which opens up the new way for writing/reading the information in spin-electronics　devices.
　This work was published in Nature Materials 7 (2008) 125, and　also introduced in Nihon-Keizai Shimbun and Nikkan-Kogyo Shimbun(January 14th, 2008).