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Spin resolved Inverse Photoemission
Franco Ciccacci
Dipartimento di Fisica – Politecnico di Milano
Franco Ciccacci - PESP-2002, MIT-Bates Linear Accelerator Center
1
PhotoEmission versus Inverse PhotoEmission
E
E
E
Electron energy
eEkin
hv
vacuum
level
Fermi level
hv
vacuum
level
F
Fermi level
EB
DOS
photon in
isochromat
spectrum
occupied
states
DOS
electron in
electron out
empty
states
photon out
sample
sample
(direct) PhotoEmission Spectroscopy
PES
Inverse PhotoEmission Spectroscopy
==>
IPES
time reversal
 same matrix element <i|A·p|f>
 same cross section behaviour s(E)
 Yph/Yel =(lel/lph)2 ~ 10-5 in the UV
Franco Ciccacci - PESP-2002, MIT-Bates Linear Accelerator Center
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Isochromat UV IPES
e-
(E,k)
Ein,2
Energy
band-pass photon
detector
hv = 9.4 ± 0.3 eV
Efin,2
Efin,1
 band structure
0
k ph << k BZ
hv
E – EF
Ein,1
q
EF
l ~ 5- 10 Å  surfaces, thin films
k
experimentally controlled parameters
Intensity
electronic structure: resolution in
monochromatic electron beam
energy
collimated beam + single crystal
k parallel
beam spin polarization
spin
Franco Ciccacci - PESP-2002, MIT-Bates Linear Accelerator Center
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Spin resolved IPES
E
e
Ei
Spin polarized electron gun:
GaAs polarized source
Fp
hv
GaAs
vacuum level
CsOx
photocathode
hv
F
Efin
EF
Ni(110)
INTENSITY (arb.units)
q = 20°
first measurements:
J. Unguris, A. Seiler, R.J. Celotta, D.T. Pierce, P.D.Johnson, N. Smith,
Phys. Rev. Lett. 49, 1047 (1982)
review: M. Donath, Surface Sci. Rep. 20, 251 (1994)
F. Ciccacci, Phys. Scrip. T66, 190 (1996)
 U. Kolac, M. Donath, K. Ertl, H. Liebl, V. Dose, Rev. Sci. Instrum. 59, 1933 (1988)
0
1
2
E - EF (eV)
Franco Ciccacci - PESP-2002, MIT-Bates Linear Accelerator Center
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Experimental apparatus
IPES
energy analyzer
(CMA)
LEED/Auger
magnetic coils
quartz
microbalance
H
Fe
P
UHV
GaAs
air
X-ray source (XPS)
ion gun
hv
effusion cells
(MBE)
sample preparation chamber
photocathode preparation chamber
measurement chamber
spin rotator
circularly polarized
light (810 nm)
SrF2 window
photon detector
(hv = 9.4 + 0.3 eV)
G. Chiaia, S. De Rossi, L. Mazzolari, F. Ciccacci,
Phys. Rev. B 48, 11298 (1993)
• F. Ciccacci, E. Vescovo, G. Chiaia, S. De Rossi, M. Tosca
Rev. Sci. Instrum. 63, 3333 (1992)
• M. Finazzi, A. Bastianon, G. Chiaia, F. Ciccacci
Meas. Sci. Technol (J. Phys. E) 4, 234 (1993)
Franco Ciccacci - PESP-2002, MIT-Bates Linear Accelerator Center
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Systems investigated
clean surfaces - adsorbates
“negative” exchange splitting of image states in Fe(001)
adsorbate-induced enhancement of spin dependent effects at Fe(001)-p(1x1)O
thin magnetic films
evolution of electronic and magnetic structure: Fe on Ag(001), Au(001), and Cu(001)
antiferromagnetic films: Cr/Ag(001) and Cr/Fe(001)
metastable films: hcp Co on Fe(001)
thin oxide films: NiO on Ag(001) and Fe(001)
half-metals: LaMnSrO and LaMnSrO/SnTiO interfaces
Fe homoepitaxial growth
non-magnetic films on ferromagentic substrates
spin dependent quantum well states: Ag on Fe(001)
interface states: Pt on Fe(001)
electron mean free path: V on Fe(001)
localized f-states: Ce on Fe(001)
magnetic coupling in multilayers
oscillatory exchange coupling: Fe/Cr/Fe(001)
exchange bias: Fe/NiO/Fe(001)
magnetic-metal / semiconductor interfaces
Fe/GaAs(001) and Fe/ZnSe(001)
Franco Ciccacci - PESP-2002, MIT-Bates Linear Accelerator Center
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Systems investigated (examples)
clean surfaces - adsorbates
“negative” exchange splitting of image states in Fe(001)
adsorbate-induced enhancement of spin dependent effects at Fe(001)-p(1x1)O
thin magnetic films
evolution of electronic and magnetic structure: Fe on Ag(001), Au(001), and Cu(001)
antiferromagnetic films: Cr/Ag(001) and Cr/Fe(001)
metastable films: hcp Co on Fe(001)
thin oxide films: NiO on Ag(001) and Fe(001)
half-metals: LaMnSrO and LaMnSrO/SnTiO interfaces
Fe homoepitaxial growth
non-magnetic films on ferromagentic substrates
spin dependent quantum well states: Ag on Fe(001)
interface states: Pt on Fe(001)
electron mean free path: V on Fe(001)
localized f-states: Ce on Fe(001)
magnetic coupling in multilayers
oscillatory exchange coupling: Fe/Cr/Fe(001)
exchange bias: Fe/NiO/Fe(001)
magnetic-metal / semiconductor interfaces
Fe/GaAs(001) and Fe/ZnSe(001)
Franco Ciccacci - PESP-2002, MIT-Bates Linear Accelerator Center
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Spin-resolved IPES from Fe(001)
DE
B1
B2
12
D5
H15
INTENSITY [arb.units]
spin DOWN
E [eV]
8
spin UP
4
S
S
D1
G12
Fe(001)
q = 0  G-H line
2
4
H'25
D5
G
0
DE
EF
G'25
G12
H'25
D
H
6
A. Santoni, F.J. Himpsel, Phys. Rev. B 43, 1305 (1991)
E - EF [eV]
J. Kirschner,
J M. Glöbl, V. Dose, H. Scheidt: Phys. Rev. Lett. 53 (1984) , 612
S. De Rossi, F. Ciccacci, J. Electron Spectrosc. Relat. Phenom. 76 (1995) , 177
Franco Ciccacci - PESP-2002, MIT-Bates Linear Accelerator Center
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New polarized electron sources
extrapolartion to Po = 100%
measured spectra: I↑ , I↓
I↑ - I↓
asymmetry: A =
I↑ + I↓
I↑ + I↓
spin resolved spectra: S ± =
2
(1 ± A / Po)
highly polarized sources very useful
(but…)
F. Ciccacci, S. De Rossi, E. Pelucchi, A.Tagliaferri, Rev. Sci. Instrum 68, 1841 (1997)
Franco Ciccacci - PESP-2002, MIT-Bates Linear Accelerator Center
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Oxygen-induced enhancement of spin dependent effects:
Fe(001)-p(1x1)O
Fe bandstructure (*)
kz
maj.
min.
k
H12
Fe(001)
Fe(001)
H12
H15
H
H
(b)
C
C
B1 B2
20
S
S
RC (x10)
AC
1.0
0.5
0.0
Fe(001)-p(1x1)O
X
(x10)
0
-20
C
B1
D
C
B1
(a)
Fe(001)-p(1x1)O
B2
Asymmetry (%)
B1 B2
Intensity (arb.un.)
Intensity (arb. un.)
maj.
min.
Z
H15
(c)
Fe bandstructure (*)
B2
60
S
S
RC (x10)
AC
40
1.0
0.5
0.0
20
0
-20
0.0
4.0
8.0
12.0
16.0
0.0
20.0
4.0
12.0
16.0
20.0
E-EF (eV)
E-EF (eV)
AC: absorbed current
8.0
RC: reflected current
(*) E. Tamura and R. Feder, Phys. Rev. Lett. 57, 759 (1986)
R. Bertacco and F. Ciccacci, Phys. Rev. B 59, 4207 (1999)
Franco Ciccacci - PESP-2002, MIT-Bates Linear Accelerator Center
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Oxygen assisted Fe homoepitaxial growth
14
Fe(001)-p(1x1)O
Asymmetry (%)
Fe atom
12
| Asymmetry | (%)
Fe(001)-p(1x1)O
Fe-O-Fe-O-Fe-O-Fe-O-Fe
Fe substrate
10
4
0
-4
-8
-12
2
6
10
14
Kinetic Energy (eV)
8
6
•oxygen surfactant action
•layer-by-layer growth
4
Kinetic Energy = 6 eV
0
structure  magnetic properties
1
2
3
4
Fe coverage (ML)
F. Bisio, R. Moroni, M. Canepa, L. Mattera, R. Bertacco, F. Ciccacci, Phys. Rev. Lett. 83, 4868 (1999)
Franco Ciccacci - PESP-2002, MIT-Bates Linear Accelerator Center
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Ag(001)
Fe(001)
Cr4ML/Fe(001)
Cr4ML/Ag(001)
4 ML
3
3
2
1
1
Cr1ML/Fe(001)
Cr1ML/Ag(001)
4 ML
2
Intensity (arb. units)
Intensity (arb.
units)
Ultrathin Cr(001) films
Cr film
Cr film
Cr1ML/Ag(001)
Cr1ML/Fe(001)
E – EF (eV)
Fe(001)
Ag(001)
Cr 1ML
0
2
4
Cr 1ML
0
2
4
E – EF (eV)
theory:
S. Blügel, B. Drittler, R. Zeller , and P.H. Dederichs, Appl. Phys. A 49 (1989), 547
Franco Ciccacci - PESP-2002, MIT-Bates Linear Accelerator Center
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Cr films: spin resolution
Cr/Ag(001): no polarization effect
Intensity (arb. units)
Cr/Fe(001)
Cr1ML/Fe(001)
Cr1ML/Fe(001)
Cr 4 ML
Cr 4 ML
Cr 3 ML
Cr 3 ML
Fe(001)
Fe(001)
Cr 2 ML
Cr 2 ML
Cr1ML
Cr1ML
Majority
0
2
4
Cr 1 ML
Minority
0
2
4
Cr 1 ML
Majority
Minority
E – EF (eV)
G. Isella, R. Bertacco, M. Zani, L. Duò, F. Ciccacci
Solid State Commun. 116 (2000), 283
0
2
4
0
2
4
E – EF (eV)
Franco Ciccacci - PESP-2002, MIT-Bates Linear Accelerator Center
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Spintronics - Magnetoelectronics
new degree of freedom (electron spin)  realisation of new electronic devices
controlling the carrier spin rather than its charge
integrating magnetism and semiconductor physics
adding spin-up spin-down magnetic dualism to electron hole dualism
spin polarized electron injection into semiconductors
spin-dependent field effect transistor
coupling semiconductor logic with non volatile magnetic memory
nanometric layered magnetic structures
- metallic magnetic multilayers – metal oxide junctions – half-metallic magnets - magnetic semiconductors - ferromagnetic metal / semiconductor interfaces - small scale magnetic elements combined with conventional semiconductor electronics
Franco Ciccacci - PESP-2002, MIT-Bates Linear Accelerator Center
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Exchange coupling in Fe/Cr/Fe multilayers
Cr  AFM




Cr
Cr
Cr


Cr/Fe interface  AFM
Fe (sub)






Cr
substrate at 30 °C
Fe/Cr/Fe system  FM/AFM oscillations





Fe (sub)
even  AFM
substrate at 350 °C

period: 2 ML






Cr
Cr
Cr
Cr


Fe (top) 
period: 12 ML
Unguris, R. J. Celotta, D. T. Pierce,
Phys. Rev. Lett. 67 (1991), 140; ibid. 69 (1992), 1125
odd  FM
naïve model
experiment
Franco Ciccacci - PESP-2002, MIT-Bates Linear Accelerator Center
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Fe/Cr/Fe trilayers – empty states
7 ML
Cr
Spin resolved
IPES

Fe (sub)
Spin resolved
IPES


Fe (top) 
Cr


Fe (sub)
Fe
2) Fe deposition on Cr/Fe(001)


Cr
AFM
//
//


1) Cr deposition on Fe(001)
FM
Absorbed
current
G. Isella, R. Bertacco, L. Duò, F. Ciccacci, Surface Sci. 454, 909 (2000)
Franco Ciccacci - PESP-2002, MIT-Bates Linear Accelerator Center
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Asymmetry (%)
FM/AFM coupling in Fe/Cr/Fe(001)-p(1x1)O
G. Isella, R. Bertacco, L. Duò, F. Ciccacci, Mat. Sci. Forum 373, 169 (2001)
Franco Ciccacci - PESP-2002, MIT-Bates Linear Accelerator Center
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Magnetic Tunnelling Junctions (MTJ)
Insulator
FM1
FM2
M2
M1
Rp
eV
M2
M1
<
Rap
eV
M. Viret, M. Drouet, J. Nassar, J.P. Contour, C. Fermon, A. Fert, Europhys. Lett. 39, 545 (1997)
J.M. De Teresa, A. Barthélémy, A. Fert, J.P. Contour, F. Montaigne, P. Seneor, Science 286, 507 (1999)
Franco Ciccacci - PESP-2002, MIT-Bates Linear Accelerator Center
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Materials with high spin polarization at Fermi level:
Half-metals
Mn density of states in LSMO
LSMO crystal structure
Perovskite structure
La, Sr
Mn
O
?
Jex
3.87 Å
Jex = 2.5 – 4 eV
J.-H. Park, E. Vescovo, H.-J. Kim, C. Kwon, R. Ramesh, and T. Venkatesan, Nature, 392, 794 (1998)
A. Chattopadhyay, A. J. Mills, S. Das Sarma, Phys. Rev. B, 61, 10738 (2000)
Franco Ciccacci - PESP-2002, MIT-Bates Linear Accelerator Center
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Spin Resolved IPES from LSMO(001)
LSMO(001)
LSMO(001)
majority spin
minority spin
T = 300 K
 ~ 400 meV
T = 100 K
zoom
-2
0
2
4
6
8
10
12
intensity (arb. units)
intensity (arb. units)
majority spin
minority spin
T = 100 K
14
E-EF (eV)
T = 300 K
sample: La0.7Sr0.3MnO3(001)
350 Å thick film grown on SrTiO3(001) by
Pulsed Laser Deposition (Orsay)
-2
-1
0
1
2
3
E-EF (eV)
R. Bertacco, M. Portalupi, M.Marcon, L. Duò, F. Ciccacci, M. Bowen, J.P. Contour, A. Berthélèmy,
J. Magn. Magn. Mater. 242, 710 (2002)
Franco Ciccacci - PESP-2002, MIT-Bates Linear Accelerator Center
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LSMO/STO/LSMO MTJ
Spin resolved IPES
STO(2ML)/LSMO(001)
TEM: good epitaxy
intensity (arb. units)
majority spin
minority spin
La2/3Sr1/3MnO3
SrTiO3
La2/3Sr1/3MnO3
 ~ 350 meV
T = 100 K
-1,0
Understanding the bias dependence of TMR
-0,5
0,0
0,5
1,0
1,5
E - EF (eV)
eV
EF
EF+ eV
Bias-dependent tunneling reflects
the unoccupied DOS of the
collecting Insulator/FM interface
EF
Franco Ciccacci - PESP-2002, MIT-Bates Linear Accelerator Center
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Fe/ZnSe(001) interface
Se protecting layer
ZnSe – 200 Ǻ
ZnSe – 200 Ǻ
GaAs(001)
GaAs(001)
heating in UHV  clean ZnSe(001) surface
( 1x1 or 2x2 reconstruction )
sample: ZnSe(001) thin film grown by
Molecular Beam Epitaxy (Paris)
Fe
Fe epitaxy
D
rectl
i
atti
ce
Z
nS
e0
(01)
R
eci
p
ro
cal
atti
ce
F
e(001)
ZnSe(001)
GaAs(001)
Fe deposition
a=
56
. 67Å
2
1

1.
567Å
a2
b=
28
. 66Å
2
1
.
219
Å
b
Franco Ciccacci - PESP-2002, MIT-Bates Linear Accelerator Center
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Fe/ZnSe(001)
A, D: ZnSe bulk states
Spin polarised IPS from Fe/ZnSe(001)
Spin resolved IPES
CB: conduction band minimum (1.3 eV above Femi level)
Intensity (arb. units)
C
: band bending upon interface formation (0.5 eV)
B1
B2
S
CB
Fe(001)
 = 0.5 eV
Egap = 2.8 eV
D
Fe

VB
15 ML
A
CB
8 ML
EF
ZnSe(001)
1 ML Fe
2 ML Fe
Fe film - magnetic properties:
-4
-2
0
2
4
6
E - EF (eV)
8
10
12
14
ultrathin Fe films: no polarization effect (RT and LNT)
onset of polarization dependent effect(magentism): ~ 6-8 ML
Franco Ciccacci - PESP-2002, MIT-Bates Linear Accelerator Center
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J.P. Contour
A. Fert
M. Bowen
A. Barthèlèmy
Orsay - CNRS/Thales
Riccardo Bertacco
Lamberto Duò
Marco Marcon
Marco Portalupi
Milano Politecnico
V.H. Etgens
V.H. Etgens
M. Eddrief
M. Eddrief
Paris - Université VI-VII
Yu.A. Mamaev
Yu.A. Mamaev
Yu. Yashin
Yu. Yashin
St. Petersburg – Techn. Univ.
Franco Ciccacci - PESP-2002, MIT-Bates Linear Accelerator Center
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