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Field: Physics/Astrophysics Session Topic: Spintronics Speaker: Yuzo Ohno, Tohoku University Title: Spin Injection and Spin Control in Semiconductors Semiconductor spin-electronics (spintronics), where both charge and spin degrees of freedom are utilized, is currently of great interest. This is because it is expected to enable integration of magnetic functionalities and semiconductor circuitry as well as quantum information technology based on coherent nature of spins in semiconductors. For developing practical semiconductor "spintronics" devices, injection of spin-polarized electrical current into non-magnetic semiconductors is one of key elements. For a number of spin-based device schemes, electrical spin injection through high quality semiconductor heterojunctions in the absence of magnetic field is preferable. Ferromagnetic semiconductor that can be epitaxially grown on high-quality nonmagnetic heterostructures is a candidate to induce spin polarized current in nonmagnetic semiconductors. (Ga,Mn)As, which exhibits a ferromagnetic phase at relatively high temperatures (Curie temperature TC~160 K), is a natural choice as it can be combined with existing devices made of III-V compounds. In order to demonstrate spin injection into semiconductors, we fabricated light emitting diodes (LEDs) which consist of pn junctions of p-type ferromagnetic semiconductor (Ga,Mn)As and non-magnetic n-GaAs with a strained (In,Ga)As quantum well (QW) inserted as an active region. Here, p-type (Ga,Mn)As is used as a spin polarizer: Spin-polarization of the injected holes is determined directly from the electroluminescence (EL) polarization emitted after the recombination with unpolarized electrons according to the optical selection rule. EL is collected from the cleaved facet to minimize magneto-optical effects due to the nearby (Ga,Mn)As, and its polarization was investigated with variable magnetic field applied parallel to the easy axis of the (Ga,Mn)As layer, i.e. in Faraday configuration. We observed that the EL polarization below TC draws a clear hysteresis loop: the remanence EL polarization at zero magnetic field is about 1% at T = 6 K. It follows the magnetization of (Ga,Mn)As which is independently measured by a Superconducting Quantum Interference Device (SQUID) type of magnetometer. The presence of hysteretic EL polarization indicates that the hole spins can be injected and transported in non-magnetic GaAs. Injection of spin polarized electrons, not holes, is more preferable from the application point of view as electrons usually exhibit longer spin lifetime due to small spin-orbit coupling. The known ferromagnetic semiconductors compatible with high quality heterostructures are, however, all p-type. This is believed to be due to the small exchange interaction between magnetic spins and conduction band electron spins. Because of the lack of n-type ferromagnetic semiconductor, we employed a spin Esaki diode and demonstrated electrical electron spin injection from the valence band of a p-type ferromagnetic semiconductor (Ga,Mn)As into the conduction band of a non-magnetic semiconductor via interband tunneling. Clear hysteresis loop with ±6.5% remanence is observed in the magnetic field dependence of EL polarization from an integrated p-(Ga,Mn)As/n-GaAs/(In,Ga)As/p-GaAs LED. It is also of great importance to understand the spin dephasing mechanism in semiconductor quantum structures. The spin relaxation time of electron spin must be sufficiently long to process information stored in the form of the polarization of spins. The lack of inversion symmetry of zinc-blende structure, like GaAs, results in spin splitting of the conduction band via spin-orbit coupling, which is the driving force for the spin relaxation. For a two-dimensional electron systems confined in a QW potential, the spin relaxation due to spin-orbit interaction has been predicted to have strong dependence on the growth axis, although in most experiments QWs formed on a (100) plane were investigated. We thus investigated electron spin dynamics in GaAs/AlGaAs QWs grown on (110)-oriented substrates with n-doping, in which the spin relaxation mechanism predominant for conventional (100) QWs is shown to be substantially suppressed. It was demonstrated that the spin relaxation time is found to reach nanosecond order in wide temperature range by optical time-resolved measurements. Compared to the conduction electron spins, nuclear spins in semiconductors have several orders of magnitude longer lifetimes and are thus favorable candidates for storing quantum information. Local manipulation of nuclear spin can be achieved by controlling hyperfine interaction with electron spins. Our time-resolved measurements of electron spin precession provided unambiguous signatures of all optical nuclear magnetic resonance, in which electron spins excited by repetitive optical pulses not only polarize but also tip the nuclear spins. This enables spatially selective manipulation of nuclear spin through confinement of the tipped field. Furthermore, we demonstrated the gate-control of dynamic nuclear polarization in an n-type GaAs/AlGaAs QW very recently, using the sensitivity of hyperfine interaction on the metal-insulator phase transition. We have investigated and demonstrated the electrical spin injection and local control of electron and nuclear spins in semiconductor quantum wells. Further developments in fabrication of magnetic/non-magnetic semiconductor nanostructures and manipulation of electron and nuclear spins will pave the way to future spintronics devices.