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@PHDTHESIS{Schreiber:50067,
author = {Schreiber, Lars R.},
othercontributors = {Güntherodt, Gernot},
title = {{T}ime resolved electrical injection of coherent spin
packets through a {S}chottky barrier},
address = {Aachen},
publisher = {Publikationsserver der RWTH Aachen University},
reportid = {RWTH-CONV-112629},
pages = {IX, 188, XXIII S. : Ill., graph. Darst.},
year = {2008},
note = {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).},
keywords = {Magnetoelektronik (SWD) / Gaas (SWD) / Pump-Probe-Technik
(SWD) / Spinrelaxation (SWD) / Kohärenz (SWD) / Optische
Messung (SWD) / Hochfrequenz (SWD) / Schottky-Kontakt (SWD)
/ Magnetische Kernreso (SWD)},
cin = {132210 / 130000 / 132110},
ddc = {530},
cid = {$I:(DE-82)132210_20140620$ / $I:(DE-82)130000_20140620$ /
$I:(DE-82)132110_20140620$},
shelfmark = {85.75.Hh * 33.35.+r * 73.30.+y * 85.75.-d * 78.47.J-},
typ = {PUB:(DE-HGF)11},
urn = {urn:nbn:de:hbz:82-opus-23162},
url = {https://publications.rwth-aachen.de/record/50067},
}
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