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PHYSICAL REVIEW LETTERS

Charge-to-Spin Conversion and Spin Diffusion in Bi/Ag Bilayers
Observed by Spin-Polarized Positron Beam
1

H. J. Zhang,1,* S. Yamamoto,2 B. Gu,3 H. Li,1 M. Maekawa,1 Y. Fukaya,1 and A. Kawasuso1

Advanced Science Research Center, Japan Atomic Energy Agency, 1233 Watanuki, Takasaki, Gunma 370-1292, Japan
Quantum Beam Science Directorate, Japan Atomic Energy Agency, 1233 Watanuki, Takasaki, Gunma 370-1292, Japan
3
Advanced Science Research Center, Japan Atomic Energy Agency, 2-4 Shirakata Shirane, Tokai-mura, Ibaraki 319-1195, Japan(Received 22 January 2015; revised manuscript received 12 March 2015; published 22 April 2015)
2

Charge-to-spin conversion induced by the Rashba-Edelstein effect was directly observed for the firsttime in samples with no magnetic layer. A spin-polarized positron beam was used to probe the spinpolarization of the outermost surface electrons of Bi=Ag=Al2 O3 and Ag=Bi=Al2 O3 when charge currents
were only associated with the Ag layers. An opposite surface spin polarization was found between
Bi=Ag=Al2 O3 and Ag=Bi=Al2 O3 samples with the application of a charge current in the same direction.
The surface spin polarizations of both systems decreased exponentially with the outermost layer thickness,suggesting the occurrence of spin diffusion from the Bi/Ag interface to the outermost surfaces. This work
provides a new technique to measure spin diffusion length.
DOI: 10.1103/PhysRevLett.114.166602

PACS numbers: 72.25.Ba, 71.70.Ej, 73.20.At, 78.70.Bj

In the last few years, increased attention has been paid tospintronics due to its potential industrial applications todata processing and information storage. The chargeto-spin conversion in nonmagnetic materials, a central
issue in spintronics, is usually realized via the spin Hall
effect (SHE), the Rashba-Edelstein effect (REE), andtopological insulators [1].
The REE is the energy splitting of spin bands induced byspin-orbit coupling and broken spatial symmetry. In a
two-dimensional (2D) electron gas system, the REE
Hamiltonian is usually expressed as H R ¼ αR ðk × ẑÞ · σ,
where αR is the Rashba parameter, k is the electron
momentum, ẑ is the unit vector of surface normal, and σ
is the vector of the Pauli matrix [2]. Giant REE has beenfound in Bi/Ag, Pd/Ag, and Sb/Ag surface alloy systems byusing angle-resolved photoemission spectroscopy [3–5].
Recently, Rojas Sánchez et al. reported the spin-to-charge
conversion due to the giant REE at the Bi/Ag interface [6].
They used microwave spin pumping to inject a spin current
from a NiFe layer into a Bi/Ag bilayer and detected theresulting charge current. They proposed that the spinto-charge conversion could be ascribed to the REE
coupling at the Bi/Ag interface but not the SHE. Theirfindings imply that the REE is more efficient than the SHE
to produce spin-to-charge conversion in spintronics. It is
anticipated that the charge-to-spin conversion is also
possible due to the giant REE at the Bi/Ag interface.
Positronium (Ps), which is the bound state of a positron
and an electron, can only be formed at a local region where
the electron density is low enough (typically, less than1013 cm−2 in 2D density) [7]. Therefore, in a metal,
formation of Ps is only possible at the outermost surface(vacuum side, a few Å away from the first surface layer[8]). There are two types of Ps: ortho-Ps (spin triplet,
0031-9007=15=114(16)=166602(5)jS; mi ¼ j1; 1i, j1; −1i, and j1; 0i) and para-Ps (spinsinglet, jS; mi ¼ j0; 0i). Para-Ps decays into two γ rays
with energy of ∼511 keV, and is difficult to distinguishfrom free positron-electron two-γ annihilation. In contrast,
ortho-Ps, which decays mostly into three γ rays (the decay
possibilities into other odd numbers of γ rays are negligibly
small) with energy ranging between 0 and 511 keV, isdistinguishable from two-γ events. Inspired by the exciting
progress of spintronics in the last decade, a spin-polarized
positron beam was developed in order to detect the spinpolarization of the outermost surface electrons [9,10].
The change in the ortho-Ps annihilation intensity is
obtained by integrating the intensity over part of the energy
spectrum that is below 511 keV: R ¼ AL =AP , where AL is
the area under the energy curve in the low energy region(from 383 to 468 keV), and AP is the area under the 2γ peak(from 494 to 528 keV). When the Ps formation probability
is low, the increment of R from R0 (subscript “0” means no
Ps formation) is proportional to the ortho-Ps intensity (F3γ
Ps ),ΔR ¼ R − R0 ∝ F3γ
Ps :

ð1Þ

In this study, R and R0 were measured using positron
implantation energies of Eþ ¼ 50 eV and 12 keV,
respectively.
The asymmetry of ΔR that is induced by the spin flip of
the outermost electrons (þP− ↔ − P− ) can be written
as [11]ΔRðþP− Þ − ΔRð−P− Þ 2ϵð1Þ − ϵð0Þ
P Py ;¼
ΔRðþP− Þ þ ΔRð−P− Þ 2ϵð1Þ þ ϵð0Þ þ −ð2Þwhere ϵð1Þ and ϵð0Þ are the detection efficiencies ofannihilation γ rays from j1; 1i plus j1; −1i and j1; 0i,

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respectively. From the known values of Pþ , ϵ, and theexperimental asymmetry, the transverse spin polarization(Py− ) can be determined. For our detector alignment(perpendicular to the positron beam), the factor½2ϵð1Þ − ϵð0Þ=½2ϵð1Þ þ ϵð0Þ equates to a constant of 0.6.
A schematic of the spin-polarized Ps annihilation experiment is shown in Fig. 1(a). A transversely spin-polarized
positron beam was generated by a 22 Na source(∼370 MBq) and an electrostatic beam apparatus. Thebase pressure of the positron beam apparatus was∼6 × 10−8 Pa. The final beam diameter was 1 mm. The
spin polarization of the positron beam, Pþ , was measured
to be 0.3 [12]. The beam was guided to inject into the centerof a sample. A reversible direct current (jc ), which wasperpendicular to Pþ , was applied to the two sample ends
through two electrodes. The beam energy Eþ was adjusted
to 50 eV by a deceleration tube from the initial value of
12 keV. The center of the sample was electrically grounded.
The Ps annihilation γ rays were detected by using a highpurity Ge detector.
The component of the surface spin polarization (P− )along the y axis was obtained fromPy− ¼ P− cos ϕ ¼

ΔRþjc − ΔR−jc;
0.18ðΔRþjc þ ΔR−jc Þð3Þ

FIG. 1 (color online). The positron beam and the samples.
(a) Schematic of the spin-polarized positron beam. The sample
layer stack of (b) Bi=Agð25Þ=Al2 O3 and (c) Ag=Bið8Þ=Al2 O3 .
The yellow blocks represent the Mo electrodes.

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where ϕ is the relative angle of P− to Pþ (y axis), ΔRþjc
and ΔR−jc are the three-γ annihilation intensities that
correspond to an input charge current density of þjcand −jc , respectively. In this experiment, the charge current
was repeatedly reversed between þjc and −jc . To determine Py− , the averages of all ΔRþjc and ΔR−jc were
calculated. The positive (negative) sign of Py− corresponds
to the direction of surface spin polarization in the y axis (−yaxis) with an input charge current of þjc in the z direction.
Two types of Bi/Ag bilayer structures, Bið0–5Þ=Agð25Þ=Al2 O3 and Agð25–500Þ=Bið8Þ=Al2 O3 (numbers
in round parenthesis denote film thickness in nm), wereprepared on α-Al2 O3 ½0001 substrates. As shown in
Figs. 1(b) and 1(c), both samples have the Ag layer connectedto the two electrodes of the dc power supply. To determine
the resistivity of Bi films, three Bi films (100, 200, and
500 nm) were deposited on α-Al2 O3 ½0001 substrates [13].
Consequently, the resistivity of the present Bi films wasdetermined to be ∼300 μΩ cm, which was approximately
60 times larger than that of Ag films (∼5 μΩ cm). Thus, thecharge currents mainly flow in the Ag layers.
The square-shaped substrates with length of 20 mm andwidth of 5–7 mm, were cut from α-Al2 O3 ½0001 wafers(mean roughness <0.1 nm). All film depositions werecarried out at a substrate temperature of 300 K. The
substrates were annealed at 873 K for 30 min in a vacuum
chamber (with a base pressure of 3 × 10−7 Pa), which was
separated from the beam apparatus. The preparation of each
Ag=Bi=Al2 O3 sample was completed in this chamber. First,
the Bi layer was deposited onto the substrate by thermal
deposition with Bi granules (99.9999%). Subsequently,
using rf magnetron sputtering with an Ag target (99.99%),
the Ag layer was deposited onto the Bi layer in a pure Ar(99.999%) ambient at a pressure of 0.3 Pa. The growth ratesof Bi and Ag in this chamber were 0.1 and 1.9 nm=min,
respectively. During the Bi deposition process, the Bi
thickness was monitored by using a quartz crystal thickness
monitor (SQM-160, Sigma instruments, measurement errorof 0.1 nm) that was positioned close to the substrate. The
Ag=Bi=Al2 O3 sample was then transferred to anotherchamber that was connected to the beam apparatus. The
transfer took place through air and took approximately 20
minutes. To remove any oxide layer from the sample
surface, a 1 keV Arþ sputtering was utilized.
Each Bi=Ag=Al2 O3 sample was prepared as follows:
The Ag film was deposited onto the substrate in theabove separated chamber. Subsequently, the Ag=Al2 O3
sample was transferred to the chamber that wasconnected to the beam apparatus through air within 20
minutes. After cleaning the Ag surface with a 1 keV Arþ
sputtering, the Bi film was deposited at a growth rate of∼0.05 nm=min.
The crystallinity and surface roughness of samples werecharacterized by XRD patterns (SmartLab, Rigaku) andatomic force microscopy (AFM) observation (AFM5300E,

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FIG. 2 (color online). XRD patterns of (a) α-Al2 O3 substrate,(b) Agð25Þ=Al2 O3 , (c) Bið8Þ=Al2 O3 , and (d) Agð25Þ=Bið8Þ=
Al2 O3 . The “filled diamond” marks represent the imperfectionsin the α-Al2 O3 substrates.

Hitachi). Figure 2 shows the XRD θ − 2θ curves: The Ag
film in the Agð25Þ=Al2 O3 sample is a polycrystal with the(111), (200), (220), and (311) planes. The Bi layer of theAgð25Þ=Bið8Þ=Al2 O3 is also polycrystalline with the (012)
and (003) planes. The Ag layer of the Agð25Þ=Bið8Þ=
Al2 O3 exists mainly in the (220) orientation [14]. From
AFM images, the mean roughnesses of Agð25Þ=Al2 O3 andBið8Þ=Al2 O3 were determined to be approximately 1.1 and
2.5 nm, respectively. Additionally, the mean grain diameters of Ag in Agð25Þ=Al2 O3 and Bi in Bið8Þ=Al2 O3 were
found to be ∼40 nm from AFM measurements.
Various thicknesses of Bi layers (dBi ¼ 0.1, 0.2, 0.3, 0.6,
1, 2, 3, and 5 nm) were deposited on Agð25Þ=Al2 O3 . The
same charge current of 0.1 A (corresponding to the 2D
current density of jc ¼ 14–19 A=m) was applied. For theAgð25Þ=Al2 O3 sample, the difference of ΔRjc was rather
small at jc ∼ 15 A=m and could only be observed at muchhigher jc. Figure 3(a) shows ΔRjc for Agð25Þ=Al2 O3 at
jc ¼ 89.3 A=m. The surface spin polarization of 3.2% thatis estimated from Fig. 3(a), which corresponds to 0.5% at
jc ¼ 15 A=m, is probably induced by the spin Hall effectin Ag film. Figure 3(b) shows that the difference of ΔRjc
of Bið0.3Þ=Agð25Þ=Al2 O3 is larger than that of theAgð25Þ=Al2 O3 sample, even though jc is much lower.
Figure 4 shows the Bi thickness dependence of thesurface spin polarizations that normalized to the values atweek ending
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FIG. 3. Variation of ΔR as a function of input charge currentof þjc and −jc for (a) Agð25Þ=Al2 O3 at jc ¼ 89.3 A=m,(b) Bið0.3Þ=Agð25Þ=Al2 O3 at jc ¼ 18.9 A=m, and (c) Agð25Þ=
Bið8Þ=Al2 O3 at jc ¼ 17.5 A=m.

jc ¼ 15 A=m. The surface spin polarization increases from
0.5 to 0.9% with increasing dBi from 0 to 0.2 nm, reaches
4.1% at dBi ¼ 0.3 nm, and subsequently decreases gradually for dBi > 0.3 nm. Considering the Bi atomic radius(0.15 nm), dBi ¼ 0.3 nm is approximately one monolayer.
As shown by the solid line in Fig. 4, the above Bi thickness
dependence of the surface spin polarization can be fitted by
an exponential function:

FIG. 4. The surface spin polarization of Bi=Agð25Þ=Al2 O3
samples as a function of Bi thickness. The six data points ofBið≥0.3Þ=Agð25Þ=Al2 O3 were fitted to an exponential functionof Eq. (4).

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Py− ðdBi Þ ¼ Py− ð0.3Þ exp½−0.48ðdBi − 0.3Þ:ð4Þ

Similarly to the Bi=Agð25Þ=Al2 O3 samples, Ag layers ofdifferent thicknesses (dAg ¼ 25, 100, 200, 300, 400,
500 nm) were deposited on Bið8Þ=Al2 O3 . The input charge
current was also regulated to 0.1 A for each sample, and thesurface spin polarization was normalized to the value at
jc ¼ 15 A=m. As shown in Fig. 3(c), the difference ofΔRjc is observed at dAg ¼ 25 nm. More importantly, its
magnitude and sign are comparable and opposite, respectively, to those of Agð25Þ=Al2 O3 and Bi=Agð25Þ=Al2 O3 .
The opposite sign indicates an opposite surface spinpolarization. Figure 5 shows that the surface spin polarization decreases with increasing dAg . Again, this can be
fitted by an exponential function:Py− ðdAg Þ ¼ Py− ð25Þ exp½−0.0028ðdAg − 25Þ:ð5Þ

The observed opposite sign in the surface spin polarization and the thickness dependencies for Bi=Agð25Þ=Al2 O3 and Ag=Bið8Þ=Al2 O3 suggest that excess electron
spins generated at the Bi/Ag interface diffuse into both Bi
and Ag layers and eventually appear at the outermost
surfaces. Also, the current-induced spin polarization within
the Ag layers of Ag=Bið8Þ=Al2 O3 samples is overcompensated by excess and opposite spins supplied from the Bi/Ag
interface.
We assume a simple exponential form of expð−d=λsd Þfor spin diffusion, where λsd is the spin diffusion length andthe prefactors in the exponentials of Eqs. (4) and (5)correspond to 1=λsd . Thus, we determine a spin diffusion
length of λsd ðBiÞ ¼ 1=0.48 ≅ 2.1 nm for the Bi layer andλsd ðAgÞ ¼ 1=0.0028 ≅ 357 nm for the Ag layer. The spin
diffusion length in Bi is comparable to a recent value of
λsd ðBiÞ ¼ 1.2 nm that was obtained from the inverse SHEof a Py/Bi bilayer [15]. Also, the above λsd ðAgÞ of 357 nm
does not conflict with the previous reports of 132, 152, 700,
and 300 nm [16–18].

The spin diffusion length is a critical parameter inspintronics. The present study demonstrates that it has
the potential to quantitatively characterize the spin diffusion length by detecting the surface spin polarization ofsamples with different thicknesses of material upon thesame film with a known value of spin polarization [suchas Agð25Þ=Bið8Þ=Al2 O3 ].
In the study of spin-to-charge conversion in Bi/Ag
bilayers, the authors attributed the spin-to-charge conversion to the inverse REE but not the inverse SHE since thespin Hall angle of a BiAg alloy (−2.3%) has the opposite
sign to their observation [19]. Considering the fact that theabove-obtained spin diffusion lengths agree with thosereported so far, the spin polarizations on the outermost
surfaces of the Bi/Ag system observed here may be aconsequence of the REE, which is the inverse mechanismof the one observed by Rojas Sánchez et al. with the spin
pumping method.
The charge-to-spin conversion in a sample that contains
a magnetic layer has been observed before. In 2010, Mironet al. detected a current-driven spin torque induced by the
REE in Ptð3Þ=Coð0.6Þ=Alð1.6Þ=SiO2 [20]. In 2011, theyobserved the perpendicular switching of a single Co
ferromagnetic layer in the same sample [21]. Our previous
report on current-induced spin polarization of six transition
metals (Pt, Pd, Au, Cu, Ta, and W) was tentatively
explained as the surface spin accumulation due to the
REE [10]. These samples were also associated with themagnetic layer due to the ferromagnetic property of
the nanoscaled Pt and Pd. The validity of the explanation
still remains a problem that needs to be experimentally
addressed. In this sense, the present observation of opposite
spin polarizations at opposite surfaces in Bi/Ag bilayers, as
far as we know, is the first direct observation of the REE ina sample with no magnetic layer inside.
The spin density hδsi resulting from the REE and acharge current is given by [22]hδsi ¼

FIG. 5. The surface spin polarization of Ag=Bið8Þ=Al2 O3
samples as a function of Ag thickness. The data were fitted toan exponential function of Eq. (5).

me αRj;eℏE F cð6Þ

where me is the effective electron mass, e is the elementarycharge, and E F is the Fermi energy. For the Bi/Ag[111]system, me ¼ 0.35 m0 (m0 is the electron rest mass),
αR ¼ 3.05 × 10−10 eVm, and E F ¼ 0.18 eV is calculated
from the Fermi wavelength kF ¼ 0.13 Å−1 and me [3].
Thus, at the Bi/Ag interface, hδsi ≈ 5 × 1010 cm−2 for
jc ¼ 15 A=m. On a metal surface, Ps is formed at thevacuum side where the electron density (n2D ) is low
enough, typically, less than 1013 cm−2 . For the Bi surface,n2D ¼ ð0.5 ∼ 4Þ × 1013 cm−2 at the first surface layer[23–25], which may nearly fulfill the above Ps formation
condition. For the Ag surface, such a low electron density isavailable at a vacuum region, a few Å away from the firstsurface layer [8]. Therefore, an observable spin polarization

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Py− is estimated to be at least ð0.1–1Þ%. Thus, the order ofmagnitude of the spin polarization observed here,Py− ¼ ð4–5Þ%, could be explained by the REE.
In conclusion, we demonstrate charge-to-spin conversion
in Bi/Ag bilayers by using spin-polarized Ps annihilation
spectroscopy. Direct evidence of spin diffusion is found byanalyzing the outermost layer thickness dependence of
surface spin polarization of Bi=Ag=Al2 O3 and Ag=Bi=
Al2 O3 samples.
We are grateful to J. Ieda and S. Maekawa of JAEA, T.
Seki, K. Takanashi, and E. Saitoh of Tohoku University for
their valuable suggestions and discussions. This work was
financially supported by JSPS KAKENHI under Grant
No. 24310072 and the NSFC under Grant No. 11475130.

*

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