Journal of Magnetism and Magnetic Materials 342 (2013) 139–143

Contents lists available at SciVerse ScienceDirect

Journal of Magnetism and Magnetic Materials
journal homepage: www.elsevier.com/locate/jmmm

Current-induced spin polarization on a Pt surface: A new approach
using spin-polarized positron annihilation spectroscopy
A. Kawasuso a,n, Y. Fukaya a, M. Maekawa a, H. Zhang a, T. Seki b, T. Yoshino b, E. Saitoh b,
K. Takanashi bab

Advanced Science Research Center, Japan Atomic Energy Agency, 1233 Watanuki, Takasaki, Gunma 370-1292, Japan
Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan

art ic l e i nf o

a b s t r a c t

Article history:
Received 16 December 2012
Available online 24 April 2013

Transversely spin-polarized positrons were injected near Pt and Au surfaces under an applied electric
current. The three-photon annihilation of spin-triplet positronium, which was emitted from the surfaces
into vacuum, was observed. When the positron spin polarization was perpendicular to the current
direction, the maximum asymmetry of the three-photon annihilation intensity was observed uponcurrent reversal for the Pt surfaces, whereas it was signiﬁcantly reduced for the Au surface.
The experimental results suggest that electrons near the Pt surfaces were in-plane and transversely
spin-polarized with respect to the direction of the electric current. The maximum electron spinpolarization was estimated to be more than 0.01 (1%).
& 2013 Published by Elsevier B.V.

Keywords:
Positron annihilation
Positronium
Surface magnetism
Surface spin accumulation

1. Introduction
Current-induced spin polarization (CISP) on non-magnetic
material surfaces has attracted much attention due to the rapid
development of spintronics devices and their underlying physics.
Spin current arises from the spin–orbit (SO) interaction between
the conduction electrons and nuclei, which then causes CISP [1–9].
The Rashba SO interaction, which originates from space-reversal
symmetry-breaking at a surface, gives rise to CISP near surfaces
and interfaces [7–9]. The surface boundary conditions, as well as
surface structure and electronic state, affect CISP.
Reducing the coordination number of the surface atoms resultsin band narrowing and strong electron localization. Surface lattice
distortion also modulates the electronic state of the surface;
therefore, the magnetic properties of surface may be different
from those of the bulk. Some theoretical studies have predicted
the appearance of a large magnetic moment on Pt surfaces, eventhough the bulk is paramagnetic [10,11]. Thus, it is particularlyinteresting to use newly developed surface-sensitive tools toinvestigate CISP on Pt [12–16] and Au [17] surfaces, where large
spin currents are observed.
Spin-polarized positron annihilation spectroscopy is a possible
probe for observing electron spin polarization on metal surfaces.
Spin-polarized electrons can be detected through either freeannihilation between positrons and electrons or self-annihilationnCorresponding author. Tel.: +81 273469331; fax: +81 273469432.
E-mail address: kawasuso.atsuo@jaea.go.jp (A. Kawasuso).

0304-8853/$ - see front matter & 2013 Published by Elsevier B.V.
http://dx.doi.org/10.1016/j.jmmm.2013.04.006

of positronium, which is a bound state of a positron and anelectron. The former has recently been used to study ferromagnetic band structures [18,19]. The latter positronium methodshould be well suited to the detection of spin-polarized conduction electrons at the outermost surface layer; when positroniums
are emitted into vacuum, positrons pick up surface electrons near
the Fermi level. A pioneering study of the surface magnetism of Ni[20] demonstrated that this method has an extremely high surface
selectivity, which can be applied to bare surfaces without device
fabrication. The detection limit of spin polarization is low(10−3 (0.1%)) for normal positron spin polarization (≈30%) using
conventional sources. These are major advantages for studying
surface spin phenomena.
In this paper, we describe CISP on Pt and Au surfaces, as
investigated by spin-polarized positron annihilation spectroscopy
based on the positronium method. For Pt surfaces, the spinpolarization of surface electrons near the Fermi level was a few
percent. We discuss the possible reasons for this large CISP on thePt surfaces in relation to the Rashba effect and the ferromagnetism
associated with the outermost surface state.
2. Method and materials
2.1. Method
We ﬁrst summarize the principles underpinning our method.
A portion of the positrons injected into a sample diffuse back tothe surface, pick up surface electrons near the Fermi level, and140

A. Kawasuso et al. / Journal of Magnetism and Magnetic Materials 342 (2013) 139–143

positronium spin state are given by
F j00〉 ¼ ð1−P þ P − cos φÞ=4;

ð1Þ

F j10〉 ¼ ð1−P þ P − cos φÞ=4;

ð2Þ

F j11〉 ¼ ð1 þ P þ þ P − cos φ þ P þ P − cos φÞ=4;

ð3Þ

F j1−1〉 ¼ ð1−P þ −P − cos φ þ P þ P − cos φÞ=4;

ð4Þ

where P − is the electron polarization and φ is the angle between
the positron and the electron polarization directions. The twophoton annihilation fraction is given by F 2γ
Ps ¼ F j00〉 . The total threephoton annihilation fraction is given by
F 3γ
Ps ¼ ϵð1ÞðF j11〉 þ F j1−1〉 Þ þ ϵð0ÞF j10〉 ;ð5Þ

where ϵð1Þ and ϵð0Þ are the detection efﬁciencies of annihilation
photons from the m ¼ 71 and m ¼0 states, respectively. This
efﬁciency also depends on the angle between the direction of
the detector and the positron polarization [21]. When the positronand electron polarization directions are the same (Fig. 1(a), left),the j11〉 state is preferentially formed and the fractions of the otherstates are lower, resulting in the increase in F 3γ
Ps and the decrease in
F 2γ
Ps . Conversely, when the positron and electron polarization
directions are reversed (Fig. 1(a), right), the fractions of the j11〉
and j1−1〉 states are lower compared with the j10〉 and j00〉 states.
2γ
Hence, F 3γ
Ps decreases and F Ps increases.
In the annihilation photon energy spectrum, the two-photon
annihilation produces a peak at 511 keV ( ¼ m0 c2 ) with a ﬁnite
width, whereas the intensity of the three-photon annihilation
continuously increases from 0 to 511 keV (see Fig. 1(c)). The twophoton annihilation spectrum shows the free annihilation of
positrons and electrons inside the sample and the selfannihilation of positronium emitted into the vacuum. Because
the probability of the three-photon annihilation between positrons and electrons inside a sample is negligible (1/371), theobserved three-photon annihilation comes only from positroniums emitted into the vacuum. The positronium three-photon
annihilation is traditionally characterized by a quantity R [22],R ¼ ðT−UÞ=U ¼
Fig. 1. (a) Principles of the present spin-polarized positron annihilation spectroscopy. Transversely polarized positrons are injected into the sample under anelectric current applied along the z-axis. The implanted positrons diffuse back tothe surface and form positronium (Ps) with the surface conduction electrons. The
positron polarization direction (Pþ ) is ﬁxed and the electron polarization direction(P− ) is changed by current reversal (þj↔−j). The fractions of each spin state of
positronium are also changed by current reversal. The relationship between theelectron polarization and the current direction is not known a priori.
(b) Experimental setup. Transversely polarized positrons with energy of 15 keV
are injected into the sample under electric current. Positron energy is adjusted to
below 15 keV by a deceleration tube. An annihilation photon detector is set
perpendicular to the beam axis. The angle between the positron polarization andcurrent direction, θ, is changed by rotating the sample holder. (c) Energy spectra ofannihilation photons obtained for the Pt/Fe/MgO sample at Eþ ¼ 15 keV and 50 eV.
The intensity is normalized to the 511 keV area intensity. The gray area denotes ΔR.
Similar spectra were obtained for the other samples.

escape into the vacuum as positronium (Fig. 1(a)). The allowed
spin states of positronium are the spin-singlet state (S ¼0 andm ¼0; j00〉) and spin-triplet state (S¼1 and m¼ 0, 71; j10〉, j11〉 orj1−1〉), where S and m are the total spin and the magnetic quantum
number, respectively. These spin wave functions arej00〉 ¼ j⇑↓〉−j⇓↑〉, j10〉 ¼ j⇑↓〉 þ j⇓↑〉, j11〉 ¼ j⇑↑〉 and j1−1〉 ¼ j⇓↓〉,respectively, where the thick arrows denote positrons and thinarrows denote electrons. Spin-singlet positronium decays into two
photons, and spin-triplet positronium decays into three photons. It
is assumed that the positrons are always transversely polarized (ydirection) with a net polarization of P þ . Fractions of each3γ
ð1−F 3γPs ÞR0 þ F Ps R1 U 1 =U 03γ
1−F 3γPs þ F Ps U 1 =U 0;

ð6Þ

where T is the area under the entire intensity curve, U is the area
under the two-photon annihilation spectrum, and the subscripts
0 and 1 denote 0% and 100% positronium emission, respectively.
Although R is a complicated function of F 3γ
Ps , for its small value as in
the present case, ΔR ¼ R−R0 is approximately proportional to F 3γ
Ps .
Thus, Eqs. (2)–(6) can be used to express the asymmetry of ΔR
upon spin ﬂip (þP − ↔−P − ) as
A¼

ΔRðþP − Þ−ΔRð−P − Þ
2ϵð1Þ−ϵð0ÞP þ P − cos φ:¼
ΔRðþP − Þ þ ΔRð−P − Þ
2ϵð1Þ þ ϵð0Þð7ÞThe direction of the electron spin polarization can be changed by
the current reversal (þj↔−j). Accordingly, CISP can be detected by
the change of ΔR upon current reversal. The two-photon annihilation of positronium mirrors the change in ΔR, providing it is notmasked by the two-photon annihilation of positrons and electrons
inside the sample.
Fig. 1(b) shows a schematic of the experimental setup. The
samples are mounted on a plate holder, and an electric current isapplied through two electrodes on the edges of the samples in the
direction shown in the ﬁgure. The ﬁnite spin polarization of the
conduction electrons perpendicular to the current direction may
appear near the surface if CISP occurs. The relationship between
the current and the electron spin-polarization directions is notknown a priori. In the present experiment, a positron beam withan energy of Eþ ¼15 keV is electrostatically generated by using a

A. Kawasuso et al. / Journal of Magnetism and Magnetic Materials 342 (2013) 139–143

141

22

Na source (370 MBq) and a tungsten mesh moderator. The
longitudinal spin polarization of the positron beam is determined
as P þ ¼ 0:3 [23]. The beam is bent 901 using a plate deﬂector. Thistransversely (y direction) polarized positron beam is directed onto
the sample at normal incidence. The beam energy is adjusted to
below 15 keV by using a deceleration tube under high voltage. The
angle θ between the current and positron polarization directions ischanged by rotating the sample around the beam axis. The energy
spectrum of the annihilation photons is measured by a Ge detector
set perpendicular to the beam axis. The efﬁciency function
ð2ϵð1Þ−ϵð0ÞÞ=ð2ϵð1Þ þ ϵð0ÞÞ in Eq. (7) is approximately 0.6.
2.2. Materials
The samples used in this study were Pt(001) and Au(001)single-crystal ﬁlms (10  20 mm, 50 nm thick) grown epitaxiallyon Fe(001) seed layers (1 nm thick) on MgO(001) substrates bymagnetron sputtering in an ultrahigh vacuum chamber (2 
10−7 Pa) at room temperature, and are referred to as Pt/Fe/MgO
and Au/Fe/MgO, respectively. A polycrystalline Pt ﬁlm without an
Fe layer was also grown directly on a MgO(001) substrate, and isreferred to as Pt/MgO.

3. Results and discussion
Fig. 1(c) shows the typical annihilation photon energy spectra
obtained for the Pt/Fe/MgO sample at Eþ ¼ 50 eV and 15 keV.
Similar spectra were obtained for the other samples. The total
intensity was normalized to the area intensities of the two-photon
annihilation spectra. At Eþ ¼ 15 keV, the positronium fraction was
neglected because the positrons were injected deep into thesample. The non-zero intensity below 511 keV was mainly causedby the Compton scattering of 511 keV photons. When the incident
positron energy is low enough, most positrons diffuse back to thesurface. Four processes occur at the surface: (i) positron reemission, (ii) positron surface trapping, (iii) positronium emission,and (iv) positronium surface trapping. The increased intensity
below 511 keV in the energy spectrum for Eþ ¼ 50 eV indicates the
positronium emission and the three-photon annihilation. The
difference in the low-energy tail intensities around 480 keV forthe 100% and 0% positronium emissions has been reported to be
0.033 for an Al(110) surface [24]. The difference in intensities forEþ ¼ 50 eV and 15 keV was approximately 0.003 for our samples;
therefore the maximum positronium emission was 10%(0:003=0:033∼0:1). To evaluate the positronium emission more
precisely, a reference spectrum with a known positronium emission would be required. The positronium emission observed here
is consistent with the negative work functions for Pt and Au. The
sample temperatures were measured as approximately 250 1C due
to Joule heating. This was not high enough to generate thermal
vacancies which alter the detection of CISP. The sample heating
enhanced the positronium emission, because of the effect of
surface cleaning and the thermal activation of positronium [22].
Fig. 2 shows the ΔR and 511 keV peak intensity normalized toits area intensity for the Pt/Fe/MgO sample in successive measurements upon current reversal at Eþ ¼ 50 eV and θ ¼ 901. The valueof ΔR oscillated, which shows that surface electrons were in-planespin-polarized and the polarization direction was reversed byswitching the current direction. The higher ΔR corresponds to ⇑ þ↑ (left), and the lower corresponds to ⇑ þ ↓ (right) in Fig. 1(a). The
oscillation of ΔR decreased as Eþ increased above 200 eV, whichsuggests that it was caused by the positronium emission from thesurface. The oscillation of the 511 keV peak intensity appeared tobe very small. The maximum positronium emission was 10%. In
addition, F 2γ
Ps was only ≈1=4 of the total positronium fraction, when

Fig. 2. ΔR and 511 keV peak intensity normalized to its area intensity as a functionof the current reversal cycle number for the Pt/Fe/MgO sample with current
densities of j¼ 1.2  105 A/cm2, 1.8  105 A/cm2 and 2.4  105 A/cm2 at Eþ ¼ 50 eV
and at θ ¼ 901.

Fig. 3. ΔR as a function of the current reversal cycle number for the Pt/Fe/MgO,
Pt/MgO, and Au/Fe/MgO samples with current densities of j¼2.4  105 A/cm2,
1.8  105 A/cm2 and 3.0  105 A/cm2, respectively, at Eþ ¼ 50 eV and θ ¼ 901.

P − was not sufﬁciently high. The change in the 511 keV peakintensity caused by positronium was overcompensated by abundant free positron annihilation.
Fig. 3 shows the successive current reversal of ΔR obtained for
the Pt/MgO, Au/Fe/MgO, and Pt/Fe/MgO samples. For the Pt/MgO
sample, the oscillation of ΔR was similar to the Pt/Fe/MgO sample.
This suggests that the Fe layer was not essential for the appearanceof ΔR oscillation. The oscillation of ΔR for the Au/Fe/MgO sample

142

A. Kawasuso et al. / Journal of Magnetism and Magnetic Materials 342 (2013) 139–143

given as the shift of the chemical potential; Δμ≈2αSH λS j=s, where
αSH , λS , and s are the spin Hall angle, the spin diffusion length(≈10 nm), and the electrical conductivity (≈2:5  104 Ω−1 cm−1 for
Pt), respectively. The spin Hall angle of Pt is 0:05 o αSH o 0:16 [16].
Consequently, for the present current densities the value of P −predicted by spin diffusion theory is of the order of 10−5 , which isthree orders of magnitude smaller than the experimental value of
P − . There are no established theories to explain the large CISP for
the Pt surfaces. One possible explanation may be the surface
Rashba effect, which induces a much stronger spin accumulation
than that expected from spin diffusion theory. In relation to themagnetism, the Stoner parameter of bulk Pt is relatively high [28]and it may readily be enhanced at the ﬁrst surface layer by the
reduction of the coordination number and the lattice distortion[11,10]. In contrast, the Stoner parameter of bulk Au is small.
Therefore, the current-induced spin accumulation on the Pt surface may be enhanced further by inducing ferromagnetic order.

4. Conclusions

Fig. 4. (a) Asymmetry (A) of ΔR as a function of the current density for the
Pt/Fe/MgO, Pt/MgO, and Au/Fe/MgO samples at Eþ ¼ 50 eV and θ ¼ 901.
(b) Asymmetry (A) of ΔR as a function of angle θ between the positron polarizationand current direction for the Pt/Fe/MgO sample at j¼2.4  105 A/cm2 andEþ ¼ 50 eV. The solid line is the sin θ curve.

In conclusion, spin-polarized positron annihilation spectroscopy
was used to observe the CISP on Pt and Au surfaces. A large CISP was
observed for the Pt surfaces, whereas it was suppressed for the Au
surface. Although this was related to the SO interaction of Pt and Au,
further experimental and theoretical studies are required to fully
explain the magnitude of CISP. Our study demonstrates that spinpolarized positron annihilation spectroscopy provides a unique toolfor investigating spin polarization on metal surfaces.

Acknowledgments
had a signiﬁcantly smaller amplitude compared with the Pt samples.
Fig. 4(a) shows the asymmetry of ΔR as a function of current density
for all the samples. Fig. 4(b) shows that the asymmetry for the Pt/Fe/
MgO sample increased from θ ¼ 01, reached a maximum at θ ¼ 901,
and then decreased until θ ¼ 1801. Because the maximum asymmetry occurred at θ ¼ 901, φ ¼ θ−901. The asymmetry of ΔR was
proportional to the sin θ curve.
These results suggest that electrons near the Pt surfaces are inplane and transversely spin-polarized with respect to the electric
current direction. The current directions þj and −j corresponded tothe spin directions ⇑ þ ↑ (left) and ⇑ þ ↓ (right), respectively, inFig. 1(a). The magnetic ﬁeld generated by the electric current wascalculated as less than 1 mT using the Biot-Savart principle, which
was too small to affect the positron beam and the positronium
annihilation characteristics. The spin polarization of the surface
electrons for the Pt/Fe/MgO sample was estimated to be P − ≈0:04
from Eq. (7) using the positron polarization P þ ¼ 0:3 and theasymmetry of ΔR in Fig. 4(a) (j¼2.4  105 A/cm2) and P − ≈0:06 for
the Pt/MgO sample (j¼1.8  105 A/cm2).
The relationship between the accumulated spin and the current
directions was the same as that observed in a previously reported
electrotransport experiment [25] and that predicted from theory[26]. This was explained by the negative LS coupling of the sd bandin Pt because of the occupation of the d-orbital with 9 electrons.
The sign change of the SO coupling for transition metals withdifferent numbers of d electrons was also observed in theseexperimental and theoretical studies. The spin Hall conductivityof Au is predicted to be much smaller than that of Pt, which
reﬂects the closed (Au)- and open (Pt)-d-shell structures [26].
Therefore, the CISP observed here could have originated from thespin Hall effect.
However, the magnitude of the CISP was not explicitly interpreted using the framework of the spin Hall effect. According tospin diffusion theory [27], the electron polarization on a surface isWe thank S. Maekawa and J. Ieda of the Japan Atomic Energy
Agency for their valuable comments. This work was supported by
the JSPS KAKENHI Grant no. 24310072.
References[1] M.I. Dyakonov, V.I. Perel, Physics Letters 35A (1971) 459.
[2] J.E. Hirsch, Physical Review Letters 83 (1999) 1834.
[3] S. Zhang, Physical Review Letters 85 (2000) 393.
[4] S.O. Valenzuela, M. Tinkham, Nature 422 (2006) 176.
[5] Y.K. Kato, R.C. Myers, A.C. Gossard, D.D. Awschalom, Science 306 (2004) 1910.
[6] J. Wunderlich, J. Sinova, T. Jungwirth, Physical Review Letters 047204 (2005)
94.
[7] J. Sinova, D. Culcer, Q. Niu, N.A. Sinitsyn, T. Jungwirth, A.H. MacDonald,
Physical Review Letters 26 (2004) 126603.
[8] Y.K. Kato, R.C. Myers, A.C. Gossard, D.D. Awschalom, Physical Review Letters 93(2004) 176601.
[9] I.M. Miron, G. Gaudin, S. Auffret, B. Rodmacq, A. Schuhl, S. Pizzini, J. Vogel,
P. Gambardella, Nature Materials 9 (2010) 230.
[10] L.H. Yang, J. Park, B.D. Yu, Journal of the Korean Physical Society 56 (2010) 791.
[11] S. Blügel, Physical Review B 51 (1995) 2025.
[12] T. Kimura, Y. Otani, T. Sato, S. Takahasi, S. Maekawa, Physical Review Letters 98(2007) 156601.
[13] E. Saitoh, M. Ueda, H. Miyajima, G. Tatara, Applied Physics Letters 88 (2006)
182509.
[14] K. Ando, S. Takahashi, K. Harii, K. Sasage, J. Ieda, S. Maekawa, E. Saitoh, PhysicalReview Letters 101 (2008) 036601.
[15] O. Mosendz, J.E. Pearson, F.Y. Fradin, G.E. Bauer, S.D. Bader, A. Hoffmann,
Physical Review Letters 104 (2010) 046601.
[16] L. Liu, T. Moriyama, D.C. Ralph, R.A. Buhrman, Physical Review Letters 106(2011) 036601.
[17] T. Seki, Y. Hasegawa, S. Mitani, S. Takahashi, H. Imamura, S. Maekawa, J. Nitta,
K. Takanashi, Nature Materials 7 (2008) 125.
[18] A. Kawasuso, M. Maekawa, Y. Fukaya, A. Yabuuchi, I. Mochizuki, PhysicalReview B 83 (2011) 100406(R).
[19] A. Kawasuso, M. Maekawa, Y. Fukaya, A. Yabuuchi, I. Mochizuki, PhysicalReview B 85 (2012) 024417.
[20] D.W. Gidley, A.R. Köymen, T.W. Capehart, Physical Review Letters 49 (1982)
1779.
[21] R.M. Drisko, Physical Review 102 (1956) 1542.
[22] K.G. Lynn, D.O. Welch, Physical Review B 22 (1980) 99.

A. Kawasuso et al. / Journal of Magnetism and Magnetic Materials 342 (2013) 139–143[23] A. Kawasuso, M. Maekawa, Applied Surface Science 255 (2008) 108.
[24] J. Lahtinen, A. Vehanen, H. Huomo, J. Mäkinen, P. Huttunen, K. Rytsölä,
M. Bentzon, P. Hautojärvi, Nuclear Instruments and Methods in Physics
Research B 17 (1986) 73.
[25] M. Morota, Y. Niimi, K. Ohnishi, D.H. Wei, T. Tanaka, H. Kotani, T. Kimura,Y. Otani, Physical Review B 83 (2011) 174405.

143[26] T. Tanaka, H. Kotani, M. Naito, T. Naito, D.S. Hirashima, K. Yamada, J. Inoue,
Physical Review B 77 (2008) 165117.
[27] S. Takahashi, H. Imamura, S. Maekawa, Spin injection and spin transport inhybrid nanostructures, in: S. Maekawa (Ed.), Concepts in Spin Electronics,
Oxford University Press, Oxford, 2006, p. 343.
[28] M.M. Sigalas, D.A. Papaconstantopoulos, Physical Review B 50 (1994) 7255.