Application of Spin-Polarized Positron Spectroscopy to Current-Induced Surface Spin Polarization

A. Kawasuso, H. J. Zhang, H. Li, K. Zhou, M. Maekawa A. Miyashita, H. Abe, S. Sakai, S. Yamamoto, K. Wada

J. Ieda, G. Bo, S. Maekawa

National Institutes for Quantum and Radiological Science and Technology

T. Seki, E. Saitoh, K. Takanashi Tohoku University

Financial support : JSPS KAKENHI under Grant No. 24310072.

Japan Atomic Energy Agency

S. Iida, Y. Nagashima Science University of Tokyo

T. Hyodo, I. Mochizuki High Energy Acceleration Research Organization

Contents: 1. Introduction to Positron Spectroscopy General aspects Spin-Polarized Positron Spectroscopy

2. Current-Induced Surface Spin Polarization Spin-Hall systems (Pt, Pd, W, Ta, CuIr/Bi…) Rashba system (Bi-Ag bilayer)

3. Summary & Future Prospects Topological Insulators Further Research & Development

1. Introduction to Positron Spectroscopy Positron = Antiparticle of Electron（Dirac’s Relativistic QM）

Pair annihilation

Pair creation

Anderson1933

High energy photon

＋

p n ＋ ＋ n

proton neutron positron neutrino

β+ radioisotopes (RI)

E=2xmc2 Dirac1932

Many β+ RI’s are available

E=2xmc2

+

Vacancies are detected Annihilation lifetime

Photon energy spectrum

e+

-

Positrons in matter Trapped by atomic vacancies

Annihilation with electrons

Doppler Broadening of Annihilation Radiation

= Electron Momentum Distribution

＋

E=511+cp/2 keV E=511-cp/2 keV p

Photon Energy=Electron momentum

q =p, E=mc2=511keV

p

Core electrons

Valence electrons pF

If electron momentum p=0 ＋

If electron momentum p≠0

Doppler sift

-50 -40 -30 -20 -10 0 10 20 30 40 5010

-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

100

2p2s

Co

untin

g r

ate

(a

rb.

un

its)

Electron momentum (x10-3 m0c)

1s

core total

valence

Fe-polycrystal

Total spin S=0 : Two-photon emission

m0c2=511keV±DE

0～511keV continuous

Spin dependent annihilation

Total spin S=1 : Three-photon emission

Electron spins are detected via

Case A : 2γ-annihilation of e+ with unpaired electrons Case B : 3γ-annihilation of Positronium (Ps)

Na-22 Ge-68 Si-27

Method Intensity/Polarization

Source 106-1010e+/sec 70-100% Beam 103-106e+/sec 30-50% Commercial RI Nuclear Reaction

104-107e+/sec ~50% Beam Synchrotron

Polarized Laser

Pair Creation

Compton Scattering

Electron accelerator

Polarized electrons →Polarized gamma

e- 104-105e+/sec ~50% Beam

Pair Creation

Photo-cathode

β+ RIs Spin-Polarized positrons Parity non-conservation in the weak interaction

Ion accelerator/Reactor

In use

Planned

Planned

Ferromagnetic band structure

e+

Vacancy-induced magnetism

e+

Surface spin polarization

Ps

e+

Maybe More…

GaN, InN, SnO2, CeO2…

Half-Metals

EF

Magnetic semiconductor

Heusler alloys Metal oxide

Surface phenomena

Surface magnetism

Spin Hall effect, Rashba effect Topological Insulator

Several potential applications

-20 -10 0 10 20

-0.0005

0.0000

-0.0005

0.0000

-0.0005

0.0000

Electron momentum p (10-3 m0c)

Fe

N+ (

p)

-

N-

(p)

(a

rb.

un

its)

Co

Ni

●P+=70%, ●P+=27%

Spin-polarized DBAR spectra for Classical Ferromagnets

MS=0.6mB

MS=1.7mB

MS=2.2mB

0.00

0.01

0.00

0.01

0 50 100 150 200 250 300 3500.00

0.01

TC

=293K

Gd

Fie

ld-r

eve

rsa

l a

sym

me

try (

arb

. u

nits)

TC

=222K

Tb

TC

=90K

Temperature (K)

Dy

PRB83(2011)100406(R). PRB85（2012）024417.

300 350 400 450 50010

-4

10-3 E

+=50eV with Ps (3)

Co

un

tin

g r

ate

(a

rb.

units)

Photon energy (keV)

E+=15keV without Ps

Area ∝ I3

511keV (2) spectrum

Asymmetry of 3-gamma

PP

II

II

PsPs

PsPs

)()(

)()(33

33

Surface spin polarization can be determined

Spin-Polarized Positron Spectroscopy –Surface–

+ More S=1

More 3- decay

vacuum e+

ー Less S=1

Less 3- decay

e+

Sample

0 20 40 60 80 100

To

tal P

s f

orm

atio

n p

rob

ab

ility

Electron density parameter, rs

Total Ps formation probability

Can. J. Phys. 42(1964)1908.

3)(3

41sBra

np

nPs : Electron density allowing Ps formation nBulk : Bulk electron density

nPs / nBulk = 0.03~0.17%

Typical nBulk=5x1022 cm-3

nPs=(1.5-8.5)x1019 cm-3

nPs2D=(0.6-1.9)x1013 cm-2

Ps is formed at the vacuum side of the surface

Fe(001)/MgO(001) 500nm thick BCC Co(001)/MgO(001) 500nm thick FCC Ni(001)/MgO(001) 500nm thick FCC 1.0ｋV Ar+ sputtering＋700℃×1min. Magnetic field ±150 Gauss @ 15 A DC

1 3 5 7 9

-0.005

0.000

0.005

0 2 4 6 8 10

1 3 5 7 9

2 4 6 8

1 3 5 7 9

2 4 6 8

Co

I3

- <

I3 >

Fe

Positive field

Repetation number

Negative field

Ni

Fe Co Ni

3.7% 2.6% 0.5%

Surface Spin-Polarization on Ferromagnets

Fe(001) surface, Wang&Freeman,PRB24(1981)4364.

2. Current-Induced Surface Spin Polarization

312310 cmeVDOS

%10)1/( 4

eVDOSnP spin

eV610mD 31710 cmnspin

θSHE : Spin Hall angle (10%) λS : Spin diffusion length (10nm)

jC3D : Current density (105 A/cm2) ρ : Resistivity (50μΩcm)

qmDD

CSSHE j3

2Chemical potential

Δμ

1eV DOS

Maj. Min.

In “Surface Dimension”

211102 cmnspin

%3~2/ nnP spin

1μeV 100meV

DOS Maj. Min.

211410 cmeVDOS

Spin diffusion theory (Bulk)

Spin Hall Angle and Spin-Polarization

21310 cmn

eVcmstates 1.0//10 213

SSHEP q @a DC voltage

)/sinh(/)/cosh(1 SSSSHE ddP q

for a finite sample width

Characteristics of low electron density region(n=1013cm-2)

*

2

2

)(

e

FF

m

kE

Fermi energy:

Fermi wave length:

Fermi wave number: 2/1)2( nkF p

FF k/2p

0.07 Å-1

0.09 eV

89 Å

1 Å-1

19 eV

6 Å

Bulk

m* ~0.2m0 assumed

Ps surface

Elongated wavelength Enhanced interaction with low q phonons q~0 acoustic phonon: Less states q~0 optical phonon : Excitation gap

Elongated phonon relaxation time

(Ballistic conduction)

211

2 106/4 cmEeDs Dy pD2D : 2D-DOS (~1014 eV-1cm-2) E: Electric field (~1kV/m) : Relaxation time (10-12 s), bulk 10-15 s

: Rashba constant (3x1010eVm)

Polarization

%10/106 1311 severalP

3/12 )3( nkF p

For Rashba system

D

C

F

ey j

e

ms

2*

P. M. Edelstein, Sol. Stat. Commun., 73(1990)233-235.

P. Gambardella & M. Miron, Phil. Trans. R. Soc. A369(2011)3157-3197.

pp2

2

*

2

em

nE

e

F

Variation of Spin Hall Angle

CuBi Positive SHA +8.1% by theory (Gradhand) PRL104(2010)186403, PRB81(2010)245109.

Negative SHA -24% by experiment (Niimi) PRL109(2012)156602.

Positive SHA by theory (Fedorov) PRB 88(2013)085116.

Negative SHA by theory (Gu). JAP117(2015)17D503.

CuIr Positive SH +2.1% by experiment (Niimi) PRL106(2011)126601.

Positive SHA by +3.5 to +2.9% by theory (Fedorov) PRB 88(2013)085116.

Sign conversion of SHA Xu, PRL114(2015)017202.

Sample Deceleration tube Detector

GND HV 0 – 12 kV

R R R

DC voltage

e+ beam 50 eV

+jc

-jc

Experimental

-3

-2

-1

0

1

2

3

Sp

in p

ola

rizatio

n (

%)

per

j c=

10

5 A

/cm

2 a

nd

=

10

m

cm

-W

Pt

-W -Ta -Ta

Cu AuPd

Spin

-orb

it inte

raction

Tanaka PRB 77(2008)165117.

Spin Hall systems

0.11

0.12

0.13

0.14

0.10

0.11

0.13

0.14

0.10

0.11

0.10

0.11

0.09

0.10

0.10

0.11

Pt/Al2O3@0.5A

Pd/Al2O3@0.6A

+jc-jc.............+jc-jc +jc-jc.............+jc-jc

+jc-jc.............+jc-jc

Po

sitro

niu

mu

in

ten

sity,

I3 (a

rb.

un

its)

Au/Fe/MgO@0.6A

+jc-jc.............+jc-jc

Cu/MgO@0.6A

-Ta@0.05A

-Ta@0.25A

-W@0.1A

Current direction

-W@0.1A

Spin Hall systems -3

-2

-1

0

1

20.0 0.1 0.2 0.3 0.4

pure Cu

CuIr(3.9%)/MgO(001)

CuBi(0.3%)/MgO(001)

Applied Current (A)

Sp

in P

ola

riza

tio

n (

%)

Tickness: 25 nm

Cu and CuBi, polycrystal CuIr well-oriented

SHA is enhanced by additive Ir and Bi

SHA both Negative

Bi/Ag —Rashba system—

Efficiency of spin-to-charge conversion

Ag(10) Bi(8) Bi(8) /Ag(5)

Bi(8) /Ag(10)

Bi(8) /Ag(20)

0 1 5 5 6.5

Bi/Ag interface Giant Rashba splitting

J. C. Rojas Sanchez et al., Nat. Commun. 4, 2944 (2013)

Bi/Ag > Bi > > Ag

Ast et al., PRL 98(2007)186807.

ARPES spectroscopy

Inverse Rashba-Edelstein effect

Bi/Ag —Rashba system—

Film n (cm-3) ρ(μΩcm)

Bi 2.9x1017 ~ 300

Ag 5.8x1022 ~ 5

Ag: Magnetron sputtering Bi : Thermal deposition (K-Cell)

0 1 2 3 4 5 6

0

1

2

3

4

5

6

P- p

er

j c=

15

A/m

(%

)

Bi thickness (nm)

Bi/Ag —Rashba system—

Bi surface

Bi

Al2O3

Ag e+

)]3.0(48.0exp[ Bid

0 100 200 300 400 500 600-6

-5

-4

-3

-2

-1

0

P- per

j c=

15 A

/m (

%)

Ag thickness (nm)

Ag surface

Al2O3

Ag Bi

e+

Bi/Ag —Rashba system—

)]25(0028.0exp[ Agd

Bi/Ag —Rashba system—

λBi = 1.2nm D. Hou et al., APL101(2012)042403.

λAg = 132-700nm PRL96(2006)136601.

PRL99(2007)196604. Nat. Mater. 10((2011)527.

Spin diffusion length

Bi ~2 nm Ag ~357 nm

Bi Ag

Interface

Opposite spin polarization Exponential decay

Summary & Future Prospects: Introduction to Spin-Polarized Positron Spectroscopy Positrons convey information of polarized electrons. For surfaces, electrons at the vacuum side, low density region are detected.

Some remarks on Current-Induced Spin Polarization Spin Hall and Rashba effects may be detected by Positrons. Polarization will be a product of SHA and SDL.

Giant spin Hall and Rashba systems Sign of Pt, Pd, Ta, W agrees with other experiments & theory. Polarization of Pt is highest, Ta and W are moderated. Sign of CuIr and CuBi are both negative. Bi/Ag charge-to-spin conversion are observed via spin diffusion to outermost surfaces.

Future Prospects: Other systems Topological insulators Graphene and other layered materials

Further Positron Development Energy-resolved measurements

EF

|ΦPs|

EPs=-ΦPs

EPs=-ΦPs-EF+E

EPs=0 -ΦPs EPs

Ps energy spectrum

Energy

Up spin

Down spin

Positron Electron

DO

S

Psitronium

0 -ΦPs-EF+E E EF EF+ΦPs E

FPs3

(E)

EPs:Ps kinetic energy E :Electron energy level