Time resolved electrical injection of coherent spin packets through a Schottky barrier

  • Zeitaufgelöste elektrische Injektion kohärenter Spinpakete durch eine Schottky-Barriere

Schreiber, Lars Reiner; Güntherodt, Gernot (Thesis advisor)

Aachen : Publikationsserver der RWTH Aachen University (2008)
Dissertation / PhD Thesis

Aachen, Techn. Hochsch., Diss., 2008

Abstract

During the last few years, the manipulation of the electron’s spin degree of freedom for information processing was explored in the new field of spintronics. Future spintronic devices rely on the generation of a spin imbalance in a semiconductor. Electrical spin injection from a ferromagnet into a semiconductor has been demonstrated for various material systems and recently even high injection efficiency for electron spins has been achieved by exploiting a polarized current tunnelled through a Schottky barrier. Despite this progress, an essential ingredient for coherent spintronic devices is still missing: electrical injection of a phase-coherent spin packet. With all-optical time-resolved methods a phase-coherent spin packet can be readily oriented in a semiconductor using circularly polarized laser pulses. Phase-coherence means that the spins within the packet are initially generated with the same orientation, which can be proven by the precession of the spin induced net magnetization about an external magnetic field. The evolution of the spin precession was probed by means of time-resolved optical Faraday rotation (TRFR). However, no time-resolved measurement of electrical spin injection and coherent spin manipulation has been successfully performed yet, although all-electrical phase-sensitive spintronic devices are aimed at. Here, we introduce a novel time-resolved technique based on electrical pumping and optical probing. As a pump we apply nanosecond voltage pulses in order to electrically inject phase-coherent spin packets from a 3.5 nm thick epitaxially grown Fe injector layer through a reverse biased Schottky barrier into a 5 µm thick bulk n-GaAs layer, which exhibits spin dephasing times exceeding 50 ns at 20 K. The electrically injected spins are probed by TRFR using a picosecond pulsed probe laser, which is phase-locked to the voltage pulses. This technique allows measuring the net magnetization of electrically injected spins in a time-interval of 125 ns with picosecond resolution. For the first time, spin precession of electrically injected spin packets is obtained in a transverse magnetic field as is evident from the following observations: Firstly, the sign-dependence of the TRFR angle follows the Fe magnetization hysteresis loop, which proves electrical injection of the spin packet. Secondly, spin precession demonstrates the phase-coherence of the electrically injected spins probed in the n-GaAs layer as confirmed by the characteristic effective g-factor. We present a model for the time-evolution of the electrical spin injection through a Schottky barrier. Applying an equivalent network of the Schottky junction that consists of a capacitance parallel to a resistance, we assume the tunnel current to be partially spin polarized, while the displacement current is unpolarized. These assumptions predict an exponentially damped tail (~ 8 ns) of the spin polarized current after the voltage pulse, which corresponds to the discharging of the capacitance. Changing the voltage pulse width in a range from 0.2 ns to 11 ns, the TRFR signal as a function of the magnetic field and the pump-probe delay matches simulations based on the model. The result points to the speed limitation of a Schottky junction for electrical spin injection, which has to be taken into account for the design of high-frequency spintronic devices. Electrical injection by repetitive voltage pulses leads to interference of the injected spins yielding resonant spin amplification (RSA), if the Larmor frequency is in resonance with the pump repetition frequency. Sophisticated pulse patterns demonstrate, how a voltage pulse destructively interferes with a resonantly built up spin ensemble. Moreover, the voltage pulses can be superimposed with a dc-bias that leads to a constant flow of polarized carriers through the sample. This alters the dynamic equilibrium of both the electron and the nuclear spin systems on a slow time scale exceeding seconds. The nuclear polarization gives rise to an effective nuclear magnetic field of several mT, which alters the Larmor precession frequency as observed by an Overhauser shift of the RSA peaks. The sign of the dc-voltage determines, whether the precession frequency of the phase-coherent spin packets is speeded up or slowed down. The dynamic nuclear polarization (DNP) is confirmed by resonant depolarisation of the nuclear system at the characteristic frequencies using a radio-frequency coil. Polarization and depolarization times (~ 450 s) are determined and compared to the DNP induced by optical spin pumping, which turned out to be faster (~ 10 s).

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