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

H. J. Zhang1), S. Yamamoto2), B. Gu1), H. Li1),
M. Maekawa1), Y. Fukaya1), and A. Kawasuso1)1)
ASRC, JAEA 2) QuBS, JAEA.

Spintronics is promising to go beyond traditional electronics.
Current-induced spin polarization (CISP) at surface andinterface plays a critical role in spintronics. The spin Hall effect(SHE) and the Rashba effect are the representative phenomena
producing CISP due to spin-orbit coupling (SOC). In the last
fiscal year, using a spin-polarized positron beam [1], wesucceeded in observing the CISP in the Bi-Ag bilayer system,
which is known as a giant Rashba system, and spin diffusion in
Bi and Ag layers for the first time [2].
Positrons implanted into a sample diffuse back to the surface
and emitted into vacuum as positronium which is the positronelectron bound state like hydrogen. Two spin states, singlet(S=0) and triplet (S=1), are possible. These decay into two andthree gamma quanta, respectively. The annihilation probability
shows asymmetry upon flipping the mutual polarization vectorof positrons and electrons. From this, the electron spinpolarization at the outermost surface can be determined. Since
two-gamma annihilation of positronium overlaps the free
positron-electron annihilation, three-gamma annihilation event
is used for observing the surface electron spin polarization.
Figure Fig. 1 shows the observed spin polarizations of Bi-Ag
and Ag-Bi systems as functions of outer Bi and Ag thicknesses(dBi and dAg), respectively, at the input electric current density of
jc=15 A/m. The electric current flows mostly in the Ag layer
since the resistivity of Bi layer (300 cm) is much larger thanthat of Ag layer (~5 cm). In the upper panel of Fig. 1, atdBi=0 nm corresponding to the Ag surface, a small but finite
positive spin polarization appears. This may be due to the CISPof Ag itself. At dBi=0.3 nm, the spin polarization suddenly
jumps up to +4 % and thereafter, decreases with increasing dBi.
The lower panel of Fig. 1 shows that at dAg=25 nm, the spinpolarization reaches -5% and decreases with increasing dAg. AtdAg=0 nm corresponding to the Bi surface, the spin polarization
was not observable because of very high resistivity of the Bilayer and hence difficulty in applying electric current. Thus, on
Bi and Ag surfaces, an opposite sign of spin polarization
appears in the same direction as electric current.
The above results suggest that the excess spins generated at theBi/Ag interface due to the Rashba effect diffuse into both Bi andAg layers and eventually reach their outermost surfaces.
Positive spin polarization at dBi=0 nm in the Bi-Ag system isprobably over compensated by larger negative spin polarizationin the Ag-Bi system. Assuming a simple exponential form ofexp(-dBi(Ag)/Bi(Ag)), where Bi(Ag) is the spin diffusion length inBi(Ag) layer, the observed decays of spin polarizations are well
explained as shown by the solid curves in Fig. 1. Here, weobtained Bi=2.1 nm and Ag=357 nm. These values are inagreement with the previous reports.
The spin density induced by the Rashba effect is given by<sy>=me*Rjc/(eħEF), where e is the elementary charge, EF is
the Fermi energy, me* is the effective electron mass, and R is
the Rashba parameter. For the Bi/Ag[111] system, me* =0.35 m0(m0 is the electron rest mass), R=3.05×10−10 eVm, and EF=0.18
eV is calculated from the Fermi wavelength kF=0.13 Å−1 andme*. On a metal surface, positronium is formed at the vacuumside where the electron density (n2D) is low enough, typically,
less than 1013 cm−2. For the Bi surface, n2D=0.5~4×1013 cm−2 atthe first surface layer, which may nearly fulfill the above
positronium formation condition. For the Ag surface, such a low
electron density is available at a vacuum region, a few Å awayfrom the first surface layer. Therefore, an observable spinpolarization is estimated to be of the order of percent.
Thus, the spin polarization and its sign change for Bi to Ag
surfaces observed here could be explained by the Rashba effect.
Especially, the sign change is hardly explained in terms of thespin Hall effect and hence is the hallmark of the Rashba
mechanism for the observed CISP.

Fig. 1 Surface spin polarizations of Bi-Ag (upper panel) andAg-Bi (lower panel) systems as functions of outer Bi and Ag
thicknesses, respectively, at jc=15 A/m. Solid lines are thefitting of an exponential function exp(-dBi(Ag)/Bi(Ag)), wheredBi(Ag) and Bi(Ag) are the thickness and spin diffusion lengthof Bi(Ag) layer, respectively.
References[1] M. Maekawa et al., Nucl. Inst. Meth. Phys. Res. B308
(2013) 9-14.
[2] H. J. Zhang, S. Yamamoto, B. Gu, H. Li, M. Maekawa, Y.
Fukaya and A. Kawasuso, Phys. Rev. Lett. 114 (2015)
166602-1-5.