Upload
phamthu
View
226
Download
0
Embed Size (px)
Citation preview
Applications of Spin-Polarized Electrons inCondensed 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- SourceOptical excitation of spin polarized electrons
For positive helicity σ+ light, the theoretical polarization of the electrons photoexcited by bandgap radiation is
N NN NP ↑ ↓
↑ ↓
−+= 1-3
1+3= -50%
Polarization is easily and rapidly reversed by switching helicity of light
=
Pierce and Meier, PR B 13,5484-5500 (1976)
Negative Electron Affinity (NEA) GaAsSurface treatment gives world’s best photoemitter
Source characteristics:P ~ 30% for bulk waferI = 20 µA/mWcontinuous or pulsed operationlow initial energy and energy spreadhigh figure of merit = P2I
Charlie’s Hippie Days
Spin Polarized Electron Gun
Spin polarized e- scattering apparatus
I IA I I
↑↑ ↑↓
↑↑ ↑↓
−= + M∝Measure
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 Systemfor 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 analyzerSpin 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
I IA I I
↑↑ ↑↓
↑↑ ↑↓
−= + M∝Measure
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
A=A(E,θ,H,T)
Test for true magnetic effect by measuring field and temperature dependence
A(H
) A(T
)
I IAAAB,
θ
Celotta, Pierce, Wang, Bader, Felcher, PRL 43, 728 (1979)
Spin-Polarized Electron Scattering
Pierce, Celotta, Unguris, Siegmann, PR B 26, 2566 (1982)
M TM BT( )( ) ..../0 1 3 2= - + Theory:
BBsurface
bulk=2 Experiment: BB
sb>2
Results explained by extending theory to include weaker exchange coupling of surface to bulk, JS⊥<JB
Thermal excitation of spin waves
Measure low temperature surface magnetization
Ni40Fe40B20
Bs/Bb=3
Spin-Polarized Electron ScatteringDetermine surface critical exponent
A lot of theory but few experiments formagnetic 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 SpectroscopySpin dependent bandstructure of unfilled states
hw = ±97 035. . eV
A Pn nn n=-+
F
HGGG
I
KJJJA BA B
10cosa Large minority spin peak just above
Fermi level from the d-holes responsible for ferromagnetism in Ni
Energy, angle (k), and spin-resolved inverse photoemission
Photon detector sensitivity
Measure
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 EmissionHigh spin polarization where have high secondary intensity
Fe81.5B14.5Si4
nB nV PFe 2.2 8 0.28
Co 1.7 9 0.19
Ni 0.055 10 0.055
M n nB=- -A Bm ( )
Valence electron polarization
# Bohr magnetons# Valence electrons
BV
n n nP nn n↑ ↓
↑ ↓
−= = =+
Unguris, Pierce, Galejs, Celotta, PRL 49, 72 (1982)
Scanning Electron Microscopywith Polarization Analysis (SEMPA)
High resolution magnetization imaging
Magnetic Specimen
Incident Electrons FromScanning Electron Microscope
MagnetizationM= -µB (N↑ - N↓)
Spin-PolarizedSecondary Electrons
Spin-Polarization Analyzer
P =N↑ - N↓
N↑ + N↓
PolarizationAnalyzers
In-Plane
Out-of-Plane
MetalEvaporators
RHEED Screen
Secondary &Backscatter
ElectronDetectors
AugerElectronEnergy
Analyzer
Ion GunSample
ElectronSource
1 10 100 10000.01
0.1
1
10
100
1000
Beam diameter (nm)
Acqu
isiti
on ti
me
(min
) 10 keV20 keV
New Field Emission
Old LaB6
NIST SEMPA Apparatus
Input Optics
Drift TubeAnode
MCPG2
G1
E1
E2
Au evaporatorAu & C Targets
150
eV e
-
Low Energy Diffuse Scattering Analyzer
A
C
D B X
Y
End View
4 cm
SEMPA Example – Iron CrystalMeasure the topography and two magnetization components(simultaneously but separately):
MyMxIntensity
Directionθ = tan-1(Mx/My)
MagnitudeM = (Mx
2+ My2)1/2
Then derive the magnetization direction and magnitude:
Cobalt Vector Magnetization
Out-of-plane M
In-plane M
M contour
SEMPA – MFM ComparisonHard disk test sample with Au patterned overlay
SEMPAIntensity M AFM MFM
350 nm
Interlayer Exchange CouplingCr wedge schematic
Cr Wedge
Fe Whisker
Fe Film
0 - 20 nm
∼ 1-2 nm
∼ 500 µm
M
Unguris, Celotta, Pierce, PRL 67, 140 (1991)
Fe/Cr/Fe Oscillatory Exchange CouplingPrecise measurement of coupling periods
0 10 20 30 40 50 60 70 80
-0.2
0.0
0.2
Cr Thickness (layers)
P(Fe
)
-0.04
0.0
0.04
P(C
r)
RHEED
M
5.73 ±0.05
2.48 ±0.05
Interlayer Measured period(layers)
Cr0.144 nm/layer
Ag0.204 nm/layer
Au0.204 nm/layer
2.105 ±0.00512 ±1
2.37 ±0.07
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 MultilayersMagnetoresistance and domain structure
Current through magnetic multilayer depends on magnetization alignment,
Mi
but real domain structures can be complex:
Co
Ion milled crater(2 keV Ar+)
Cu Top Co layer
2nd Co layer
Top layer magnetization predominantlyantiparallel to 2nd layer
Borchers, Dura, Unguris, Tulchinsky, Kelley, Majkrzak, Hsu, Loloee, Pratt, Bass, PRL 82, 2796 (1999)
Interlayer Domain CorrelationsQuantify layer-by-layer angle distributions
Angle images:
1st layer 2nd layer
0 90 1800.00.20.40.60.81.0
FM AFM
P(∆φ
)
∆φ (°)
Correlation distribution:
Magnetic materials
Air Side Wheel Side
Transformer core ferromagnetic glass(Allied Signal)
Magnetic NanostructuresFe “nanowires”
213 nm
M
FeCr
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/NISTMany contributors,
especially Bob Celotta and John Unguris
Pictures of CharlieJoel Falk
Jim Murray
Roger Route