OPENSUBJECT AREAS:
EXPERIMENTAL PARTICLEPHYSICSSPINTRONICS

Received
5 March 2014
Accepted
10 April 2014
Published
29 April 2014

Correspondence andrequests for materials
should be addressed to
H.J.Z. (zhang.

Current-induced spin polarization on
metal surfaces probed by spin-polarized
positron beam
H. J. Zhang1, S. Yamamoto2, Y. Fukaya1, M. Maekawa1, H. Li1, A. Kawasuso1, T. Seki3, E. Saitoh3& K. Takanashi3
1

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
Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan.
2

Current-induced spin polarization (CISP) on the outermost surfaces of Au, Cu, Pt, Pd, Ta, and W
nanoscaled films were studied using a spin-polarized positron beam. The Au and Cu surfaces showed nosignificant CISP. In contrast, the Pt, Pd, Ta, and W films exhibited large CISP (3,15% per input charge
current of 105 A/cm2) and the CISP of Ta and W were opposite to those of Pt and Pd. The sign of the CISP
obeys the same rule in spin Hall effect suggesting that the spin-orbit coupling is mainly responsible for the
CISP. The magnitude of the CISP is explained by the Rashba-Edelstein mechanism rather than the diffusive
spin Hall effect. This settles a controversy, that which of these two mechanisms dominates the large CISP on
metal surfaces.

hongjun@jaea.go.jp)

Spintronics, which aims to produce, inject, transport, manipulate, and detect the electron spins, is promising
to go beyond the traditional charge-based electronics1. Current-induced spin polarization (CISP) plays acritical role in spintronics. The spin Hall effect (SHE) and the Rashba effect are the representative phenomena producing CISP.
In the SHE, a finite spin current appears due to the charge current and the spin-orbit coupling (SOC). The
efficiency of charge-to-spin conversion is defined as the ratio of spin to charge current densities (hSH 5 js/jc, called
spin Hall angle). At sample edges, opposite electron spins are accumulated. Large spin Hall effects have beenfound in metallic thin films of Pt2, Pd3, b-Ta4, and b-W5.
The Rashba effect induces in-plane spin polarization in a two-dimensional electron gas (2DEG) systemthrough the out-of-plane electric field and the SOC6. The strength of the Rashba effect is characterized by theenergy splitting of up and down spin bands. The Rashba splitting energy observed for semiconductor heterostructure is only a few meV. Recently, so-called giant Rashba effects (of the order of 100 meV) have been reported
for bulk Ir(111)7, Bi/Ag(111) surface alloy8, and Pb/Ag(111) surface alloy9.
It is still under debate that, which of the above two mechanisms is responsible for the large CISP on metal
surfaces and interfaces. To solve this issue, surface sensitive probes are needed. Magneto-optical Kerr effectmagnetometry is used for the observation of SHE in semiconductors10. However, this technique is not applicable
to metallic thin films with thickness ranging from several nm to a few tens of nm. Spin-polarized positron beam isa promising new tool for such a purpose. Positronium (Ps), which is a bound state of a positron and an electron, isformed at the outermost surface of a metal11. From the spin-dependence of Ps formation and annihilation, thespin polarization of metal surface can be determined12. Recently, we reported the observation of the CISP on Pt
surfaces13 by this technique. However, the origin of the observed CISP was not clarified. In the present study, we
systematically investigate CISP in some other 4d and 5d transition metals. Consequently, we found that the CISP
on these metal surfaces is explained in terms of the Rashba-Edelstein mechanism.

Results
Experimental setup and the principle. Figure 1 shows a schematic diagram of the experimental setup. The
transversely spin-polarized positron beam, which was generated by a 22Na source (370 MBq) and an electrostatic
apparatus, was implanted into the center of the sample14. The diameter and the spin polarization (P1) of thepositron beam were 1 mm and 0.3, respectively. The beam energy (E1) was adjusted from 50 eV to 12 keV. The
SCIENTIFIC REPORTS | 4 : 4844 | DOI: 10.1038/srep04844

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Figure 1 | Experimental setup. Transversely polarized positrons are
injected into the center of the sample under a direct current (6jc). The
beam energy of 12 keV is reduced to 50 eV by a deceleration tube. The c ray
detector is perpendicular to the beam axis.

sample center was electrically grounded. Reversible currents (6jc)
were applied to the samples through the two edges. The direct
current was perpendicular to P1. A high purity Ge detector wasplaced perpendicular to the beam axis to record the annihilation c
ray spectra.
Spin-polarized slow (low energy) positrons injected into a metallic
thin film could lead to a remarkable formation of Ps by picking up theelectrons on the outermost surface. The formation probability of
3c), which is influenced by the spin polarization of theortho-Ps (FPsoutermost surface electrons (P2), could be derived from the positronannihilation c ray spectra as the Ratio between the intensity of thelow energy region and the 511 keV peak region (denoted as R). A
3c(details
function DR is defined to quantitatively characterize the FPs
shown in the section of methods):
ð1Þ

DR~R{R0 ,where R and R0 are derived from the c ray spectra measured at
E1550 eV and 12 keV, respectively. The component of surface spinpolarization along y axis (P2cosw) is calculated byP{ cosw~

DRzjc {DR{jc,
0:18 DRzjc zDR{jcð2Þ

where w is the relative angle of P2 to P1 (y axis), DRzjc and DR{jccorrespond to an input charge current density of 1jc and 2jc,
respectively.
Experiments. All films were deposited by magnetron sputtering on
different substrates (10 3 20 3 0.5 mm) at various growthtemperatures. The details of the films are listed in Table 1. The
thickness of Fe seed layer for Au film was 1 nm. The low resistivity(a phase: bcc structure) Ta and W films were grown on Al2O3(0001)
substrates, and the high resistivity (b phase: A15 structure) Ta and W

Figure 2 | XRD patterns of a-Ta/Al2O3(0001), b-Ta/SiO2/Si(001), a-W/
Al2O3(0001), and ba-W/SiO2/Si(001) samples.

films were grown on 100 nm thick SiO2 layers. The Au, Pt, Pd, a-Ta,
and a-W films were single crystals, which were confirmed byobserving the reflection high energy electron diffraction patterns.
The Cu, b-Ta, and ba-W (a mixture of b and a phases in which b
is dominant) films were polycrystals. The XRD patterns shown in
Fig. 2 confirmed the a-Ta, b-Ta, a-W, and ba-W films15–17. At least
two samples were subjected to the CISP measurement for each film.
Input charge current densities jc are also listed in Table 1. To
suppress the Joule heating, the applied electric powers were regulated
to be less than 3 watts and the temperature was measured to be lower
than 150uC. In this temperature range, fast Ps with the maximum
energy of its work function (WPs < 0.7 eV (Au), 2.5 eV (Cu), 2.9 eV(Pt), 0.4 eV (Pd), 4.0 eV (Ta(111)), 4.9 eV (W(111))) will be predominant over the thermal (, 100 meV) Ps18–21. Therefore, positrons will pick up surface electrons with the energy from EF (Fermi
level) to EF 2 WPs.
3c
Material dependence of CISP. Figure 3 shows DR ( / FPs) uponsuccessive current reversal (1jc « 2jc) of all the films. For the Au
and Cu films, no regular changes of DR upon current reversal could

Table 1 | Sample characteristics (film thickness (tN), substrate, growth temperature (Tg), resistivity (r)), input charge current density (jc) andobserved transverse spin polarization (P2cosw)
SampletN (nm)Substrate

Tg (uC)r (mVcm)

jc (A/cm2)

P2cosw (%)

Au(001)
Cu
Pt(111)
Pd(111)
a-Ta
b-Ta
a-W
ba-W

25
25
25
25
10
10
10
10

Fe(001)/MgO(001)MgO(001)
Al2O3(0001)
Al2O3(0001)
Al2O3(0001)
SiO2/Si(001)
Al2O3(0001)
SiO2/Si(001)

27
27
600
500
600
27
600
27

16
8
21
27
43
128
28
110

2.4 3 10
2.4 3 105
2.0 3 105
2.4 3 105
2.5 3 105
5.0 3 104
1.0 3 105
1.0 3 105

Null
Null
11 6 2862
212 6 327 6 226 6 229 6 3SCIENTIFIC REPORTS | 4 : 4844 | DOI: 10.1038/srep04844

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Table 2 | hSH found by different experimental methods. YIG, CFB,
SA, STT, SP, ISHE, ST-FMR, and SMR denote Y3Fe5O12,
Co40Fe40B20, spin absorption, spin transfer torque, spin pumping, inverse SHE, spin torque induced ferromagnetic resonance,and spin Hall magnetoresistance, respectively
Film (nm)Pt(4)/Cu(80)Pt(10)/Py(10)Pt(15)/Py(15)Pt(6)/Py(4)Pt(20)/Cu(150)Pt(2-9)/Py(2.7-10.5)
Pt(15)/YIG
Pt(1.1-22.7)/YIG
Pd(10)/Py(10)Pd(15)/Py(15)Pd(20)/Cu(150)
Pd(2-9)/Py(2.2-7.0)
Au(15)/Py(15)Ta(20)/Cu(150)b-Ta(4)/CFB(4)b-Ta(8)/CFB(4)Ta(1.5-15)/YIG
b-W(5.2)/CFB(4)(a 1 b)-W(6.2)/CFB(4)a-W(15)/CFB(4)Figure 3 | DR as a function of successive current reversals for theAu(001)/Fe(001)/MgO(001), Cu/MgO(001), Pt(111)/Al2O3(0001),
Pd(111)/Al2O3(0001), a-Ta/Al2O3(0001), b-Ta/SiO2/Si(001), a-W/
Al2O3(0001) and ba-W/SiO2/Si(001) samples.

be seen. In contrast, the Pt, Pd, Ta and W surfaces show clear
oscillations of DR. In addition to this, the DR oscillations of Ta andW films are opposite to those of Pt and Pd films. These results suggestthat the CISP on the Au and Cu surfaces are rather small(P{ cosw=1%), while significant CISP are induced on the Pt, Pd,
Ta, and W surfaces. Also, the CISP on Ta and W surfaces areopposite to those on Pt and Pd surfaces. The transverse spinpolarizations (P2cosw) estimated by Eq. (2) are listed in Table 1.
Figure 4 shows P2cosw per input charge current of jc 5 1.0 3
105 A/cm2. The absolute values of P2cosw for the b-Ta and ba-W
surfaces are 3 , 5 times greater than those for the Pt and Pd surfaces.

Figure 4 | Spin polarizations of surface electrons (P2cosw) per input
charge current of jc 5 1 3 105 A/cm2 for the Au(001)/Fe(001)/MgO(001),
Cu/MgO(001), Pt(111)/Al2O3(0001), Pd(111)/Al2O3(0001), a-Ta/
Al2O3(0001), b-Ta/SiO2/Si(001), a-W/Al2O3(0001), and ba-W/SiO2/
Si(001) samples.
SCIENTIFIC REPORTS | 4 : 4844 | DOI: 10.1038/srep04844

hSH (%)

Method

Ref.

0.37
8.0
1.3 6 0.2
7:6+8:4
2:6
2.1 6 0.5
2.2 6 0.4
3:0+4:0
1:5
11 6 81.0
0.64 6 0.10
1.2 6 0.4
0.8 6 0.2
0.35 6 0.03
20.37 6 0.11
212 6 3215 6 3
{2:0+0:8
1:5
233 6 6
218 6 2
. 27

SA
STT
SP/ISHEST-FMRSA
ST-FMR
SMR
SMR
SP
SP/ISHESA
ST-FMR
SP/ISHESA
ST-FMR
ST-FMR
SMR
ST-FMR
ST-FMR
ST-FMR

[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[3]
[24]
[26]
[27]
[24]
[26]
[4]
[4]
[28]
[5]
[5]
[5]

For both high resistivity Ta and W films, P2cosw are significantly
bigger than those of low resistivity Ta and W films.

Discussion
Table 2 lists the hSH of undoped metals obtained by different experimental methods. The values of hSH are rather scattered. Even for Pt,
which is the most commonly studied spin Hall material, hSH varies
between 0.37% and 11.0%. The Pt, Pd, and Au films have positive
hSH, while the Ta and W films have negative hSH. Furthermore,absolute values of hSH of Ta and W tend to be greater than those ofPt and Pd. The magnitudes of hSH of b phase Ta and W films havebeen reported to be much bigger than those in a phases4,5. Theseobservations of hSH are mostly supported by theoretical studies ofhSH in which the sign is positive (negative) if the outermost d-shell ismore (less) than half filling30,31.
The sign and relative magnitude of the CISP observed for the Pt,
Pd, Ta, and W surfaces are in good agreement with those of hSH listed
in Table 2. This reveals that the observed CISP for these surfaces are
due to the SOC that is similar to SHE. According to the spin diffusion
theory32, the energy width of polarized electrons in the density ofstates is given by the shift of chemical potential: Dm 5 2hSHlSjcr,
where lS is the spin diffusion length. For hSH510%, lS510 nm,
r550 mVcm and jc 5 1.0 3 105 A/cm2, one finds Dm51 meV. The
typical density of states at EF is 1023 cm23eV21, and hence the accumulated spin density will be 1017 cm23. Assuming that positrons pickup electrons located from EF to EF21 eV, the observable electron
spin polarization will be ,1024%. Therefore, the huge CISP observed
above is hardly explained in terms of the diffusive SHE. More specificaspects of the surfaces should be considered.
Recently, the so-called giant Rashba effect has been reported forheavy metal surfaces7–9. The largest Rashba effects are five orders ofmagnitude greater than that estimated from the free electron model.
Such a giant Rashba effect is explained by considering both strong
SOC and steep gradient of electric potential near the surface. The
spin density Ædsyæ induced by the Rashba effect is given byh,dsy ~4peD2D EtaR =ð3Þwhere e is the elementary charge, D2D is the two-dimensional density
3

www.nature.com/scientificreportsof states, E is the applied electric field, t is the electron relaxation
time, and aR is the Rashba parameter (Rashba-Edelstein model)33,34.
Assuming aR 5 3 3 10210 eVm, D2D 5 1014 cm22eV21, t510 ps,
E51 kV/m, one finds the spin polarization of the order of 5%. Thus,
if the relaxation time is long enough, the above-observed huge CISP
can be explained.
A recent study reported the spin-to-charge conversion at Bi/Ag
interface, which is a well-known giant Rashba system35. The spin
density and the two-dimensional chargecurrent density j2D
c at anðeaR Þ, which is essentiallyinterface are related through dsy ~
hj2Dcthe same as Eq. (3). In the above study, excess spins of Ædsyæ 5 2 3107 cm22 supplied to the Bi/Ag interface by the spin pumping{5
A=cm. In the Ag layer, the spin-to-charge coninduced j2D
c ~10
version was negligible and independent of its thicknesses (5 to20 nm). This would manifest that the spin-to-charge conversion
was induced by an inverse Rashba effect but not inverse SHE. If we
adopt this conversion efficiency in the present experiments, the twodimensional charge current density j2D
c (0.05 to 0.5 A/cm) will generate excess surface spins of Ædsyæ 5 1012 cm22 at maximum. Thus,
assuming again D2D51014 cm22eV21, one finds the spin polarizationof 1%. This is comparable orders of magnitude as the above estimation using Eq. (3) in spite of many differences in experimental conditions. The aR and hSH are related via jhSH j~ma2R t36. This may be
the reason why the sign and the relative magnitude of the CISP
observed here are in good agreement with those of hSH.
Furthermore, besides the Rashba effect at the outermost surface,
one may naturally expect that the metal/substrate interface could also
contribute to the spin polarization on the outermost surface. The
thicknesses of the metallic films (10 and 25 nm) are close to the spin
diffusion lengths of the electron in these transition metals. A potential gradient also exists at the metal/substrate interface due to thedifference of the metal and the substrate. In consideration of the
Rashba effect at the metal/substrate interface, the transverse spinpolarization calculated from Eq. (3) will increase and be more consistent with the experimental result from spin-polarized positronbeam. To check this assumption in a future research, a metal/substrate interface with a strong Rashba effect is needed for theexperiment.
It is known that Pt and Pd nano-structures nearly satisfy the
Stoner criterion and hence ferromagnetic behavior appears37,38.
This implies that ferromagnetic order will easily be induced in Pt
and Pd surfaces. A recent anomalous Hall effect study of a
Pt=Al2 O3 ð0010Þ sample suggests that a magnetic moment of,10 mB is induced by an applied electric field39. The Rashba field
induced by the charge current may also contributes to the development of ferromagnetic order on the surface.
To summarize, we have observed huge CISP on the outermost
surfaces of Pt, Pd, Ta, and W thin films by using a spin-polarized
positron beam. The sign and magnitude of the CISP on these metal
surfaces are explained by the Rashba-Edelstein mechanism. This
work demonstrates that the spin-polarized positron beam is a usefultechnique for observing the outermost surface spin polarization of
spintronics materials.

Methods
Figure 5(a) shows the principle of Ps formation and annihilation. Spin-polarized
positrons implanted into the sub-surface region are emitted into vacuum as Ps.
Two types of Ps exist: spin-triplet ortho-Ps (jS, mæ5j1, 0æ, j1, 1æ, j1, 21æ) andspin-singlet para-Ps (jS, mæ5j0, 0æ), where S and m are the total spin and themagnetic quantum number, respectively. Ortho-Ps decays into three c rays,
giving rise to a continuous energy distribution from 0 to 511 keV. Para-Ps decays
into two c rays of , 511 keV, that overlaps with direct annihilation of positrons
with electrons inside the sample. In the deep region of the metal, the probabilityof Ps formation is negligible. Shown as the shaded area in Fig. 5(b), the 3cannihilation of ortho-Ps (below 511 keV peak) near the surface is clearlyobservable.

SCIENTIFIC REPORTS | 4 : 4844 | DOI: 10.1038/srep04844

Figure 5 | (a) Principle of Ps formation and annihilation. Positrons (e1)implanted into the subsurface region are emitted into vacuum as Ps by
picking up electrons (e2) from the outermost surface. When thepolarizations of positrons and electrons are parallel (anti-parallel), more(less) ortho-Ps (S51) is formed. (b) Typical energy spectra ofannihilation c rays obtained at positron energies of E1512 keV and
50 eV. The total intensity is normalized to the 511 keV area intensity.
The increment DR represents the 3c annihilation of ortho-Ps.
The fraction of each spin state of Ps is given by13:
Fj0,0i ~ð1{Pz P{ coswÞ=4,

ð4Þ

Fj1,0i ~ð1{Pz P{ coswÞ=4,

ð5Þ

Fj1,1i ~ð1zPz zP{ coswzPz P{ coswÞ=4,

ð6Þ

Fj1,{1i ~ð1{Pz {P{ coswzPz P{ coswÞ=4,

ð7Þ

where P1 and P2 are spin polarizations of the positrons and the electrons, respectively, and w is the relative angle of P2 to P1. The formation probability of para-Ps is2c
FPs~Fj0,0i , and that of ortho-Ps is3c~ ð1Þ Fj1,1i zFj1,{1i z ð0ÞFj1,0i ,ð8ÞFPswhere ð1Þ and ð0Þ are detection efficiencies of annihilation c rays from j1, 1æ plus j1,
21æ, and j1, 0æ, respectively. The values of ð1Þ and ð0Þ depend on the angle between
the c ray detector and P1.
3c 40:
The intensity of the annihilation energy spectrum below 511 keV is a function of FPs

3c 
3c
1{FPs R0 zFPs R1 U1 U0
T{U

,
ð9Þ
R~
~
3c
3c
U
1{FPs
zFPs
U1 U0
where T is the total area under the intensity curve, U is the area under the 511 keV peak,
and the subscripts 0 and 1 of R and U denote 0% and 100% Ps emission, respectively. For3c3csmall FPs
, DR~R{R0 !FPs
. Thus, the asymmetry of DR upon spin flip (1P2 « 2P2)
can be written as12DRðzP{ Þ{DRð{P{ Þ 2 ð1Þ{ ð0Þ~Pz P{ cosw:DRðzP{ ÞzDRð{P{ Þ 2 ð1Þz ð0Þð10Þ

From the known values of P1, , and the experimental asymmetry, the transverse spinpolarization (P2cosw) is determined. For the detector alignment in the present study4www.nature.com/scientificreports(perpendicular to the positron beam), the factor ½2 ð1Þ{ ð0Þ=½2 ð1Þz ð0Þ in Eq. (10)
is 0.6.
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Acknowledgments
We are grateful to B. Gu, J. Ieda, and S. Maekawa of Japan Atomic Energy Agency (JAEA)
for their valuable suggestions and discussions. This work was financially supported by JSPS
KAKENHI Grant No. 24310072 and a Grant for Basic Science Research Projects from theSumitomo Foundation.

Author contributions
A.K. constructed the spin-polarized positron beam, A.K., Y.F., M.M., and H.J.Z.
constructed the CISP measurement system. H.J.Z. proposed the project. E.S., K.T., T.S.,
H.J.Z. and S.Y. prepared the samples, H.J.Z. and H.L. performed the CISP experiments, S.Y.
performed the XRD experiments. H.J.Z. performed the data analysis and wrote themanuscript, all authors discussed the results and revised the manuscript.

Additional information
Competing financial interests: The authors declare no competing financial interests.
How to cite this article: Zhang, H.J. et al. Current-induced spin polarization on metal
surfaces probed by spin-polarized positron beam. Sci. Rep. 4, 4844; DOI:10.1038/srep04844(2014).
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