Engineering spin propagation across a hybrid
organic/inorganic interface using a polar layer
L. Schulz
1
, L. Nuccio
2
, M. Willis
2
, P. Desai
2
, P. Shakya
2
, T. Kreouzis
2
, V. K. Malik
1
, C. Bernhard
1
,
F. L. Pratt
3
, N. A. Morley
4
, A. Suter
5
, G. J. Nieuwenhuys
5
, T. Prokscha
5
, E. Morenzoni
5
, W. P. Gillin
2
*
and A. J. Drew
1,2
*
Spintronics has shown a remarkable and rapid development, for
example from the initial discovery of giant magnetoresistance
in spin valves
1
to their ubiquity in hard-disk read heads in
a relatively short time. However, the ability to fully harness
electron spin as another degree of freedom in semiconductor
devices has been slower to take off. One future avenue that may
expand the spintronic technology base is to take advantage
of the flexibility intrinsic to organic semiconductors (OSCs),
where it is possible to engineer and control their electronic
properties and tailor them to obtain new device concepts
2
. Here
we show that we can control the spin polarization of extracted
charge carriers from an OSC by the inclusion of a thin interfacial
layer of polar material. The electric dipole moment brought
about by this layer shifts the OSC highest occupied molecular
orbital with respect to the Fermi energy of the ferromagnetic
contact. This approach allows us full control of the spin band
appropriate for charge-carrier extraction, opening up new
spintronic device concepts for future exploitation.
The development and understanding of new hybrid or-
ganic/inorganic interfaces will enable considerable progress in
organic spintronics for technological purposes, including process-
ing elements, sensors, memories and conceptually different future
applications. In addition to the ‘standard’ spintronic applications,
newly developed interfaces could bring spintronic effects to the
field of organic light-emitting diodes (OLEDs), as well as in the
fast progressing field of organic field-effect transistors. For example,
the injection of carriers with a controlled spin state could enable
the amplification of either singlet or triplet exciton states
2
leading
to a significant increase in the efficiency of the electrolumines-
cence in OLEDs. Although these considerations are conceptually
straightforward, no efficiency amplification has yet been reported
in the literature, despite several attempts
3
. The failure of those
approaches was caused by the simple reason that light emission
can be detected starting from an applied voltage of a few volts,
whereas state-of-the-art spin injection in organic materials persists
to a maximum of around 1 V (refs 4–6). As yet, this is unexplained.
Further complications arise from the fact that various reports on
working devices show a wide spread of performances for apparently
similar structures, highlighting the issue of reproducibility
7–9
. The
poor reproducibility is mainly due to the unknown interplay
between processing and spin transfer performance and there is
little deterministic control of the interface properties. However,
it has recently been demonstrated that the insertion of a barrier
1
Department of Physics and Fribourg Center for Nanomaterials, University of Fribourg, Chemin du Musée 3, CH-1700 Fribourg, Switzerland,
2
Queen Mary
University of London, School of Physics, Mile End Road, London E1 4NS, UK,
3
ISIS Pulsed Neutron and Muon Source, Rutherford Appleton Laboratory,
Chilton, Didcot OX11 0QX, UK,
4
Department of Materials Science and Engineering, University of Sheffield, Mappin Street, Sheffield S1 3JD, UK,
5
Laboratory
for Muon-Spin Spectroscopy, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland. *e-mail: W.Gillin@qmul.ac.uk; A.J.Drew@qmul.ac.uk.
layer on top of an organic material can increase reproducibility for
aluminium tris(8-hydroxyquinoline) (Alq
3
)-based spin valves
10
.
Despite the reproducibility issues, the potential for organic
spintronic devices seems enormous, with reports of very large
spin-valve magnetoresistance at low temperature
11
. One clear goal,
whatever the application, is to be able to select and control the
injection and extraction of spins in organic materials. This in turn
requires the exquisite control of the electronic and structural states
at the hybrid organic/inorganic interface
12
, which until recently
has been only passively determined through experiments, rather
than proactively and deterministically controlled. One of the key
advantages of OSCs is the ease with which their electronic nature
can be altered, and one such way is the use of polar materials
to tune the alignment between the electrode Fermi level and the
OSC molecular levels
13,14
. This has recently been demonstrated
by covering a TiO
2
electrode with an oriented ionic molecular
12,000
10,000
8,000
6,000
4,000
2,000
0
Particles (arb. units)
4.25 keV
6.23 keV
9.23 keV
NiFe/LiF
Alq
3
FeCo
Cathode AnodeOrganic transport layer
¬40
¬20
0
20
40
Current density (μA cm
¬2
)
¬100 ¬50 0 50 100
Bias voltage (mV)
With LiF
Without LiF
0 500
1,000
1,500
Depth (A)
˚
Figure 1 | A schematic of the device structure. The muon’s stopping profile
is plotted for sample A, calculated with a Monte Carlo algorithm (see the
Methods section). A similar profile is obtained in sample B. Inset: The IV
characteristics measured on the two small-area devices—with LiF and
without LiF (sample A and B, respectively).
1
Published in
which should be cited to refer to this work.
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With LiF
With LiF
With LiF
25 26 27 28
P
V
(B) ¬ P
0
(B)
29 30
Magnetic field (mT)
Magnetic field (mT)
31 32
0.02
0
0
0
Bias voltage (mV)
40 80 120 160
0.1
0.2
0.04
150 mV
150 mV
6 mV
1.5 mV
6 mV
1.5 mV
0 mV
0.06
0.08
0.10
0.12
P(B)
¬0.02
0
26 28 30 32
0.02
a
With LiF
Without LiF
27.0
27.1
27.2
27.3
0 40 80 120 160
Bias voltage (mV)
Peak field (mT)
bcd
V
¬
0
ΔΔ
With LiF
Without LiF
¬0.6
¬0.4
¬0.2
0
0.2
0.4
0.6
0 20 40 60 80 100
Bias voltage (mV)
% magnetoresistance
Figure 2 | Spin-polarized charge carriers are present in the OSC, close to the top NiFe interface. a, Probability distribution of magnetic field inside the
device with LiF (sample A) at several different voltages. Inset: The difference between the data with and without an applied voltage, where a clear increase
in the difference signal amplitude is observed at higher voltages. b,c, The change in line-shape skewness (b) and peak field (c) clearly saturates at higher
voltages. A clear reversal in the voltage dependence of the peak field is observed in c when the LiF layer is omitted. d, The magnetoresistance for devices
with and without LiF. The reduction of magnetoresistance occurs at similar voltages to the saturation of the LE-μSR data shown in b and c. Muon
measurements were taken at a temperature of 10 K and at an energy of 6.23 keV. The magnetoresistance was taken at 20 K. In b and c, the error bars
represent one standard deviation, calculated from the Poissonian statistics of the muon data. For d, the error bars represent an estimate of the scatter
present in the magnetoresistance data. In b and c, the lines are guides to the eye.
monolayer of amphiphilic molecules
15
. Unfortunately, the resultant
energy level shift is not well understood for any organic/inorganic
interface, especially when the electrode material is ferromagnetic
13
.
Here we show, using the direct spectroscopic technique low-
energy muon spin rotation
16,17
(LE-μSR), that the polar material LiF
reverses the spin polarization of carriers at the NiFe interface with
Alq
3
. LiF has the advantage that it can be vacuum-deposited over
large areas, using thermal evaporation. It is a standard material used
to achieve a vacuum level shift of up to 1 eV in OLED devices
18,19
,
via the electric dipole moment that develops as a result of the termi-
nation of the polar material at the interfaces. In our LE-μSR experi-
ment, two devices were measured with an active area of 16×16 mm,
comprising FeCo 17 nm/Alq
3
150 nm/LiF 1 nm/NiFe 20 nm
(sample A) and FeCo 17 nm/Alq
3
150 nm/NiFe 20 nm (sample B).
Two identical devices were grown for the electrical and magne-
toresistance measurements with an active area of 2 × 2 mm. All
samples were grown sequentially, using the same conditions in
the same deposition system; further details can be found in the
Methods section. A schematic of the device structure of sample A is
shown in Fig. 1, which also shows the muon stopping profile for the
implantation energies used in our experiments. These energies were
chosen to ensure that most muons stop inside the organic layer. The
inset of Fig. 1 shows the current–voltage characteristics of the two
smaller-area devices.
Figure 2a shows the distribution of local magnetization, P(B),
in sample A at T = 10 K for four bias voltages of 0, 1.5, 6 and
150 mV, obtained from our LE-μSR experiments. On applying a
spin-polarized current through the device, small but significant
changes in the distribution P(B) are observed. These changes due
to the spin-polarized current can more easily be observed by
taking the difference of ‘voltage on’ and ‘voltage off’ P
V
(B)− P
0
(B).
This is shown in the inset of Fig. 2a, where it can be seen
that the magnetization in the sample increases as a higher spin-
polarized current is passed. A quantitative description of the voltage
dependence of the changes in the muon line shapes shown in Fig. 2a
can be obtained by fitting the muon’s time-dependent asymmetry to
a skewed Lorentzian relaxation function
16
, comprising a skewness
parameter and peak field (corresponding to the mode of the field
distribution). It has previously been shown that the skewness
parameter,
Δ, is a very sensitive probe of the polarization of the
injected charge carriers
16
. To understand how a change in injected
spin polarization alters
Δ, we must first define spin-majority holes
or electrons to be those that extract to or inject from the spin-
majority band of the relevant ferromagnetic contact, and vice versa
for the spin-minority carriers. If
Δ increases on the application
of a spin-polarized current, the muons are measuring a higher
magnetic field due to the magnetization resulting from a population
imbalance of the two spin channels in favour of spin-majority
electrons, either by extraction of spin-majority holes in the highest
occupied molecular orbital (HOMO) or injection of spin-majority
electrons in the lowest unoccupied molecular orbital (LUMO). On
the other hand, if
Δ decreases, the muons measure a lower magnetic
field from the imbalance in favour of spin-minority carriers.
The obtained peak field and skewness, from the time-domain
fits, are then plotted as a function of voltage in Fig. 2b,c. The
peak field that the muons experience increases as the bias voltage
2
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¬0.3
¬0.2
¬0.1
0
16012080400
Bias voltage (mV)
¬0.10
¬0.08
¬0.06
¬0.04
¬0.02
0
¬100 ¬50 0 50 100
NiFe Alq
3
FeCo
Sample B (without LiF)
0.2
0.1
0
160
6.23 keV
4.25 keV
6.23 keV
9.23 keV
12080400
Bias voltage (mV)
0.20
0.15
0.10
0.05
0
% magnetoresistance
% magnetoresistance
¬100 ¬50 0 50 100
Magnetic field (mT) Magnetic field (mT)
Spin polarization Spin polarization
Magnetic field Magnetic field
Magnetization
Magnetization
Magnetization
Magnetization
b
e
ac
d
f
NiFe
Alq
3
FeCo
Sample A (with LiF)
V
¬
0
ΔΔ
V
¬
0
ΔΔ
LiF
Figure 3 | A comparison of the device magnetoresistance and spin polarization close to the top interface. a, The magnetoresistance (a) and the change in
μSR line-shape skewness (b) for sample A, with the LiF layer. c,d, The magnetoresistance (c) and the change in μSR line-shape skewness (d) for sample B,
without the LiF layer. Muon measurements were taken at a temperature of 10 K, with the magnetoresistance measurements taken at a bias of 20 mV and at
20 K. As can be seen from a to d, there is a clear reversal of spin polarization as a result of the presence of the LiF layer. This is due to a change of extracted
spin polarization brought about by a vacuum level shift due to the electric dipole moment induced by the LiF. e, For spin-majority hole extraction there is an
increase in magnetization close to the interface. f, For spin-minority hole extraction, there is a decrease in magnetization close to the interface. In e and f,
the red shaded area and spins correspond to hole injection, whereas the blue corresponds to hole extraction. In b and d, the error bars represent one
standard deviation and the lines are guides to the eye. In a and c, the blue and red points correspond to different magnetic field sweep directions, defined
by the coloured arrows.
on the device increases and there is a noticeable enhancement
of the skewness of the line shape. All observed effects here are
consistent with spin-polarized charge carriers being injected into
the organic layer, with this sample showing very similar behaviour
to one previously studied, which had a very similar structure also
including a LiF interfacial layer
16
. However, as is evident from all
of the data presented in Fig. 2, the magnetization resulting from
the spin-polarized current seems to saturate at higher voltages. It
is interesting to note that spin injection into organic materials has
so far been demonstrated only for voltages below approximately
1 V, with the largest magnetoresistance observed typically at 100 mV
and below
4,20,21
. Indeed, as can be seen from Fig. 2d, this very
trend is observed in our devices, with the saturation voltage
observed in our muon experiments corresponding well to the loss of
magnetoresistance. Clearly, there is a loss of spin polarization with
increasing voltage, as the μSR line-shape skewness and peak field
should scale with current if the polarization remains unchanged,
whereas the magnetoresistance as plotted in Fig. 2d should scale
with polarization and be independent of current.
Strong electrical dipoles are present at many OSC/metal
interfaces and these interfacial dipoles can significantly alter the
non-interacting equilibrium energy levels
11,13,14,22–28
. Thus far, little
is experimentally known about the role of these vacuum level
shifts on spin injection and extraction. Clearly, a spectroscopic
study of spin propagation investigating the effect of such an
energy shift is crucial for the understanding of the spin transport
properties of hybrid ferromagnetic/organic devices. For this reason
we carried out LE-μSR measurements on a second sample—
nominally identical to the first one, other than the absence of the
thin LiF layer at the cathode interface. Plotted in Fig. 3a–d is the
bias-voltage dependence of the change in skewness for both samples
and the corresponding magnetoresistance. Also plotted in Fig. 3c
and d are the voltage-dependent peak field and magnetoresistance,
respectively, for the sample without the LiF layer. It is immediately
clear that the presence of the LiF layer reverses the spin polarization
in the Alq
3
. As this phenomenon is probably due to a vacuum level
shift at the interface changing the relevant spin band, we must first
understand which molecular orbitals are responsible for the current
in our devices before we can discuss the origin of the spin reversal.
From the IV characteristics (inset of Fig. 1), it can be seen that the
contacts are almost ohmic with only a very small and symmetrical
built-in potential. Given that the devices were being operated with
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Fermi level
E
HOMO
c
Fermi level
E
Vacuum level
δ
Vacuum level
HOMO
b
Alq
3
Cathode
(NiFe)
LiF
Cathode
(NiFe)
HOMO
LUMO
Fermi level
Fermi level
a
Injection
Extraction
Cathode
(NiFe)
Anode
(FeCo)
Alq
3
d e
x
x
x
x
Alq
3
Figure 4 | Schematic of hole transport in the OSC and how a vacuum level
shift leads to a change of extracted spin polarization. a, Hole injection
occurs close to the Fermi level, as there must be vacant states in the
ferromagnet for injection to occur. Conversely, hole extraction can take
place anywhere below the Fermi level. b,d, As holes are extracted, the
probability of one particular spin state dominating the extraction is related
to the spin DOS at the extraction energy. For the case of the device with LiF
where there is a vacuum level shift δ, this results in spin-majority electron
accumulation, as spin-majority holes are extracted more efficiently. c,e,For
the device without the LiF layer, the vacuum level shift is not present. This
results in a probability of extraction such that the most probable extracted
hole polarization is spin minority, leading to an accumulation of
spin-minority electrons close to the interface.
the FeCo contact as the anode, the current in the device is due to one
of the two following phenomena. Either holes are being transported
in the HOMO, entering the Alq
3
from the anode and exiting at the
cathode, or electrons are being transported in the LUMO, entering
at the cathode and exiting at the anode. As the workfunctions of the
transition metals are very high (∼4.5 eV for Fe and ∼5 eV for Co
and Ni; ref. 29), the possibility of electron injection into the Alq
3
is unlikely, particularly given that the devices operate at less than
100 mV. It seems likely that the current in the device is therefore
carried predominantly by holes traversing the HOMO (ref. 30).
Injection into an OSC can occur only from within a few k
B
T
of the Fermi energy of the electrode; in the case of holes, as in
our devices, there need to be unoccupied states in the FeCo anode.
The spin polarization of the injected holes is therefore determined
by the spin density of states (DOS) at the Fermi energy of the
ferromagnet. In contrast, any energy below the Fermi energy can
accept a hole being extracted, provided there is a non-zero DOS
in the cathode at the energy corresponding to the HOMO of the
OSC. This process is schematically shown in Fig. 4a, where hole
injection occurs near the Fermi energy and extraction below it.
As stated earlier, the interface dipole introduced by LiF produces
a vacuum level shift, which moves the HOMO energy relative to
the Fermi level of the metal contact. This is schematically shown in
Fig. 4b,c, which compares spin-dependent hole extraction with and
without the LiF-induced vacuum level shift. As the spin-dependent
hole extraction probability depends on the spin polarization in the
cathode at the OSC HOMO energy, a shift in the HOMO would
change the spin polarization of extracted holes
13
(see Fig. 4). This
can easily explain the data presented in Fig. 3. We would like to
note that it has already been suggested that changes in coupling at
the interface between the OSC and ferromagnet can alter the spin
polarization of injected electrons
11
.
For the device without the LiF layer (Sample B, Fig. 3c,d), at
the magnetic fields where the LE-μSR measurements were carried
out the magnetization of the two contacts are aligned. The change
in skewness close to the cathode is negative, indicating that the
total field is lowered by the spin-polarized hole extraction (upper
schematic plot in Fig. 3f). This must mean that the extracted holes
are spin minority, as there must be an excess of electron spins
opposed to the applied field as shown in Fig. 4e, which would result
in a lowering of the magnetic field observed by the muons (lower
schematic plot in Fig. 3f). As the sample is in a high-resistance
state when the ferromagnets have their magnetization aligned, the
two electrodes must inject/extract opposite spins and therefore
the anode must be injecting spin-majority holes (lower schematic
plot, Fig. 3f). For the device with a LiF layer (Sample A, see
Fig. 3a,b), the anode is unchanged and so should still be injecting
spin-majority holes. However, if the extraction spin band is altered
at the cathode, one would expect spin-majority hole extraction (see
lower schematic plot, Fig. 3e). This would lead to a spin-majority
electron accumulation at the cathode interface as shown in Fig. 4d,
which results in an increased magnetization (see upper schematic
plot, Fig. 3e) and consequently a positive change in skewness—as
is observed in Fig. 3b. As both electrodes would be efficiently
injecting/extracting spin-majority holes, one would expect the
device to be in a low-resistance state when the two ferromagnetic
layers have aligned magnetizations, as is observed in Fig. 3a. It is
worth noting that changes to the tunnelling matrix element, if this
is the relevant spin transfer mechanism into the OSC, could also
contribute to a different spin transfer across the interface
31,32
. This
can arise if one considers the matching condition for the k-vectors
of the evanescent wave in a tunnelling barrier and the one in the
ferromagnet. However, it can also lead to a bias-dependent reversal
of the injected spin polarization
31
, which has never been observed
in OSC spin-valve structures.
These results are particularly exciting for spintronics appli-
cations as they demonstrate that the dominant spin band for
charge-carrier extraction can be modified through the introduction
of an interfacial layer, as has already been predicted
11,13
. They
also highlight the possibility for the engineering of more complex
devices where spins can be manipulated. For example, for a metal
sandwiched between two organic materials, it would be possible
to extract spins with one polarization from an OSC and inject the
opposite spin polarization into another OSC, and this could be
switchable. If the polar layers were ferroelectrics, then it could be
possible to switch the polarization of the electric dipole moment
with an electric field; thus, the device could act as a spin filter
or switch. Furthermore, spatially patterning alternate orientations
of the polar materials may yield interference of spin-polarized
currents, which could be used as a spin interferometer. Clearly, it
should be possible to enhance the effects observed here, by growing
epitaxial or self-assembled films with a preferred ionic orientation.
It would also be interesting to carry out LE-μSR experiments for
4
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both positive and negative bias, at both injection and extraction
electrodes. These could shed light on the unsymmetrical nature
of the voltage-dependent reduction in magnetoresistance
5
.Itis
unlikely that the polarization of extracted holes is symmetrical with
positive/negative voltage as the extraction polarization depends on
the DOS at the HOMO energy of the extracting electrode. The inclu-
sion of a LiF layer should not affect the spin polarization of injected
holes, because these are injected within a few k
B
T of the Fermi
surface. If this study was repeated with a La
1−x
Sr
x
MnO
3
electrode,
it would be possible to differentiate between our model and the
recently suggested localized ‘hot spot’ model
11
. Finally, we would
like to note that the magnitude of the magnetoresistance and muon
signal is similar for both of our devices, indicating that there may be
no fundamental obstacle to injecting or extracting polarized charge
carriers from transition-metal ferromagnets at higher voltages. In
our case, we were able to access states with different polarization
through a vacuum level shift, but these may equally well be accessed
by the choice of OSC or an increased bias voltage.
Methods
Samples. The devices were grown on a high-purity fused-quartz substrate with
an r.m.s. roughness of less than 2 nm. Alq
3
(99.995% pure) was purchased from
Aldrich and purified by train-sublimation under a 10
−6
mbar vacuum. The Ni, Co
and Fe were purchased from Aldrich (>99.9% pure) and pressed into pellets in
the proportions 80:20 Ni/Fe and 50:50 Fe/Co, which were subsequently thermally
evaporated. High-purity aluminium, purchased from Aldrich, was evaporated at
the edges of the sample to enable contacts to be made effectively; the contacts were
well away from the centre of the beam, such that less than 2% of the beam hit the
contacts
16
. The deposition of the Alq
3
and LiF layers was carried out using a Kurt
J. Lesker SPECTROS evaporation system under ∼10
−7
mbar. Magnetic layers were
evaporated in a separate system under ∼10
−6
mbar vacuum. A calibrated oscillating
quartz-crystal monitor was used to determine the rate and thickness of all deposited
layers. X-ray reflectivity was used to estimate the thickness of each layer and the
interface roughness. The deposition rate of the Alq
3
, Al and LiF was maintained at
0.2nms
−1
and that of the magnetic contacts at around 0.1nms
−1
. Shadow masks
were used to define the device geometry.
LE-μSR. Positive muons decay to a positron, a muon antineutrino and an electron
neutrino. The angular emission of positrons is well characterized, with the emission
direction being correlated with the muon’s spin at the time of decay. Thus, by
measuring the direction and the timing of a statistically significant number of
decay positrons, it is possible to follow directly the evolution of the spin of the
ensemble of muons as a function of time after implantation. Muons can act as
passive local magnetic microprobes, by directly measuring the magnetic field
distribution at the implanted site with very high sensitivity (less than 0.1 mT). Being
able to follow the evolution of the spin with time means that the local magnetic
field experienced by the muon can be determined through the measurement of
the Larmor precession of the muon spin, which is obtained using two positron
counters mounted on opposite sides of the sample. We used a bespoke floating
power supply/volt meter that could bias the sample to a high degree of accuracy
(±0.1 nA and ±0.1 μV) while floating the sample at ±10 kV. The high voltage
is necessary for controlling the muon implantation energy and thus the muon
stopping distribution within the device. Electrical contacts were made using
spring-loaded electrodes supported by polytetrafluoroethylene blocks and the
whole assembly was mounted on a high-purity Ag-coated Al plate. The magnetic
field was applied parallel to the layers and perpendicular to the muon’s initial spin
direction and momentum. The measurements proceeded by first applying a field
of 100 mT to ensure that the ferromagnetic layers were saturated, after which the
magnetic field was reduced to 27 mT. The μSR spectra were first obtained with the
current on and then with current off.
In our LE-μSR experiment, 200–300 nm of a weakly bound van der Waals
cryosolid multilayer (solid-Ar/solid-N
2
) was deposited on the downstream side
of a cold metal substrate, which moderates a fraction of an intense surface muon
beam to ∼15 eV (with a similar r.m.s. energy spread) while conserving the initial
full polarization. The epithermal muons are extracted (by applying up to +20 kV
to the moderator substrate), transported and focused by electrostatic elements to
the sample. A trigger detector provided a muon start signal by detecting secondary
electrons, released by the muons when passing through a 2 μgcm
−2
carbon foil onto
a microchannel plate detector. The mean implantation energies were 4.25, 6.23 and
9.23 keV, controlled by choosing the appropriate moderator, transport and sample
voltages. The muon’s stopping profile can be calculated using a Monte Carlo
algorithm TRIM.SP (ref. 32). This is shown in relation to our devices in Fig. 1. By
varying the muon’s stopping profile, we were able to probe the depth profile of
the induced magnetization due to injected spin-polarized charge carriers. Further
information regarding the technique can be found in refs 16,17,33,34.
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