Applications of Spin-Polarized Electrons in
Condensed Matter Physics
Dan Pierce
Electron Physics Group, NIST
http://physics.nist.gov/epg
Supported in part by the Office of Naval Research
Principle of GaAs Polarized e- Source
Optical excitation of spin polarized electrons
For positive helicity σ+ light, the theoretical polarization of the
electrons photoexcited by bandgap radiation is
N ↑ − N ↓ = 1-3
P= N
= -50%
↑ +N ↓
1+3
Polarization is easily and rapidly reversed by switching helicity of light
Pierce and Meier, PR B 13,5484-5500 (1976)
Negative Electron Affinity (NEA) GaAs
Surface treatment gives world’s best photoemitter
Source characteristics:
P ~ 30% for bulk wafer
I = 20 µA/mW
continuous or pulsed operation
low initial energy and energy spread
high figure of merit = P2I
Charlie’s Hippie Days
Spin Polarized Electron Gun
Spin polarized e- scattering apparatus
Measure
I −I
A = ↑↑ ↑↓ ∝ M
I +I
↑↑
↑↓
Pierce, Celotta, Wang, Unertl, Galejs, Kuyatt, Mielczarek, RSI 51, 478-499 (1980)
Polarized e- Gun for SLAC Parity Violation Experiment
Beam polarization, 35% to 45%
Beam current, 1 mA cw to 15 A peak
Complete Polarized e- Injection System
for SLAC Parity Violation Experiment
Technical Support in the Early Days
Development of Measurement Techniques
Using Spin-Polarized Electrons
Spin-polarized electron gun (NEA GaAs)
Spin polarized electron scattering
Spin polarized low energy electron diffraction (SPLEED)
Spin polarized inverse photoemission spectroscopy (SPIPES)
Spin polarized electron energy loss spectroscopy (SPEELS)
Spin polarized low energy electron microscopy (SPLEEM)
Electron spin polarization analyzer
Spin polarized photoemission
Spin polarized Auger electron spectroscopy
Spin polarized secondary electron emission
Scanning Electron Microscopy with Polarization Analysis (SEMPA)
Surface Sensitivity
Surface sensitive due to short electron mean free paths
Inelastic mean free path especially short in transition metals with many d holes
Spin dependent attenuation length in ferromagnets at low energy
Pappas et al, PRL 66, 504 (1991)
Spin Polarized Electron Gun
Spin polarized e- scattering apparatus
Measure
I −I
A = ↑↑ ↑↓ ∝ M
I +I
↑↑
↑↓
Pierce, Celotta, Wang, Unertl, Galejs, Kuyatt, Mielczarek, RSI 51, 478-499 (1980)
Spin Polarized Low Energy Electron
Diffraction (SPLEED)
First surface magnetization measurement with SPLEED
IAA, IAB
A(T)
A(H)
θ
A=A(E,θ,H,T)
Test for true magnetic effect by measuring field and temperature dependence
Celotta, Pierce, Wang, Bader, Felcher, PRL 43, 728 (1979)
Spin-Polarized Electron Scattering
Measure low temperature surface magnetization
Bs/Bb=3
Ni40Fe40B20
Thermal excitation of spin waves
M (T ) =1- BT 3/2 +....
M (0)
Bsurface
Theory:
=2
Bbulk
Experiment:
Bs >2
Bb
Results explained by extending theory to include
weaker exchange coupling of surface to bulk, JS⊥<JB
Pierce, Celotta, Unguris, Siegmann, PR B 26, 2566 (1982)
Spin-Polarized Electron Scattering
Determine surface critical exponent
A lot of theory but few experiments for
magnetic critical behavior at surfaces
Aex ∝ tβ, where t=[1-(T/TC)]. Find β≅0.78 for both
Ni(110) and Ni(100) surfaces, independent of E and θ
Neutron measurements give β=0.38 for bulk
Alvarado, Campagna, Hopster, PRL 48, 51 (1982)
Spin Polarized Inverse Photoemission Spectroscopy
Spin dependent bandstructure of unfilled states
Energy, angle (k), and spin-resolved inverse photoemission
Photon detector sensitivity
hw = 97
. ± 035
. eV
Measure
F 1 In - n
B
A
J
A=G
G
Jn + n
G
HP0 cosa J
KA B
Large minority spin peak just above
Fermi level from the d-holes
responsible for ferromagnetism in Ni
Unguris, Seiler, Celotta, Pierce, Johnson, Smith, PRL 49, 1047 (1982)
Running with Friends
Bay to Breakers
Stopping to read while others catch up
Tough
Happy on top
of the mountain
A Sinclair Grand Canyon
Expedition
Determined
You’re not tryin’ if
your not bleedin’!
Principled
Two things I can’t
tolerate:
Incompetence and
Cold Coffee
Married in 1983!
How ‘bout them legs
Spin Polarized Secondary Emission
High spin polarization where have high secondary intensity
Fe81.5B14.5Si4
Valence electron polarization
n − n n # Bohr magnetons
P = n↑ + n↓ = nB =
V # Valence electrons
↑
↓
M =-m B( n - n )
A B
nB nV
P
Fe 2.2
8 0.28
Co 1.7
9 0.19
Ni 0.055 10 0.055
Unguris, Pierce, Galejs, Celotta, PRL 49, 72 (1982)
Scanning Electron Microscopy
with Polarization Analysis (SEMPA)
High resolution magnetization imaging
Incident Electrons From
Scanning Electron Microscope
Spin-Polarization Analyzer
P=
N↑ - N ↓
N↑ + N ↓
Spin-Polarized
Secondary Electrons
Magnetic
Specimen
Magnetization
M= - µ B (N ↑ - N ↓)
NIST SEMPA Apparatus
Electron
Source
In-Plane
Secondary &
Backscatter
Electron
Detectors
Polarization
Analyzers
Sample
Out-of-Plane
Metal
Evaporators
RHEED Screen
10 keV
20 keV
100
Old LaB6
10
1
New Field Emission
0.1
0.01
Auger
Electron
Energy
Analyzer
Ion Gun
Acquisition time (min)
1000
1
10
100
Beam diameter (nm)
1000
Low Energy Diffuse Scattering Analyzer
Y
Input Optics
Drift Tube
Anode
150 eV e-
A
D
B
C
End View
4 cm
MCP
G2
G1
E2
E1
Au & C Targets
Au evaporator
X
SEMPA Example – Iron Crystal
Measure the topography and two magnetization components
(simultaneously but separately):
Intensity
Mx
My
Then derive the
magnetization
direction and
magnitude:
Magnitude
Direction
θ = tan-1(Mx/My) M = (Mx2+ My2)1/2
Cobalt Vector Magnetization
Out-of-plane M
M contour
In-plane M
SEMPA – MFM Comparison
Hard disk test sample with Au patterned overlay
Intensity
SEMPA
350 nm
M
AFM
MFM
Interlayer Exchange Coupling
Cr wedge schematic
Fe Film
∼ 1-2 nm
0 - 20 nm
Cr Wedge
M
∼ 500 µm
Fe Whisker
Unguris, Celotta, Pierce, PRL 67, 140 (1991)
Fe/Cr/Fe Oscillatory Exchange Coupling
Precise measurement of coupling periods
M
P(Fe)
0.2
Interlayer
0.0
Cr
-0.2
0.144 nm/layer
Ag
0.204 nm/layer
RHEED
P(Cr)
0.04
Au
0.204 nm/layer
0.0
-0.04
0
10
20
30
40
50
60
Cr Thickness (layers)
70
80
Measured period
(layers)
2.105 ± 0.005
12 ± 1
2.37 ± 0.07
5.73 ± 0.05
2.48 ± 0.05
8.6 ± 0.3
Leadership
Vision
We’ll get there if
you can keep up
with me
Between a rock and a hard place
Magnetic Depth Profiling Co/Cu Multilayers
Magnetoresistance and domain structure
Current through magnetic multilayer depends on magnetization alignment,
i
M
but real domain structures can be complex:
Cu
Co
Ion milled crater
(2 keV Ar+)
Top Co layer
2nd Co layer
Top layer magnetization predominantly
antiparallel to 2nd layer
Borchers, Dura, Unguris, Tulchinsky, Kelley, Majkrzak, Hsu, Loloee, Pratt, Bass, PRL 82, 2796 (1999)
Interlayer Domain Correlations
Quantify layer-by-layer angle distributions
Angle images:
1st layer
2nd layer
Correlation distribution:
FM
AFM
P(∆φ)
1.0
0.8
0.6
0.4
0.2
0.0
0
90
∆φ (°)
180
Magnetic materials
Air Side
Transformer core ferromagnetic glass
(Allied Signal)
Wheel Side
Magnetic Nanostructures
Fe “nanowires”
M
Fe
Cr
213 nm
Fe evaporated at grazing incidence on Cr lines
fabricated by laser focused atom deposition
Tulchinsky, Kelley, McClelland, Gupta, Celotta, JVST A16, (1998)
Ready to retire???
Acknowledgements
Spin-polarized electron work at NBS/NIST
Many contributors,
especially Bob Celotta and John Unguris
Pictures of Charlie
Joel Falk
Jim Murray
Roger Route