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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
2
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
3
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
4
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
5
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
6
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
7
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
8
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
9
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
10
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
11
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
12
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)
0
G. Isella, R. Bertacco, M. Zani, L. Duò, F. Ciccacci
Solid State Commun. 116 (2000), 283
2
4
0
2
4
E – EF (eV)
Franco Ciccacci - PESP-2002, MIT-Bates Linear Accelerator Center
13
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
14
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
15
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
16
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
17
Magnetic Tunnelling Junctions (MTJ)
In su la to r
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
18
Materials with high spin polarization at Fermi level:
Half-metals
Mn density of states in LSMO
LSMO crystal structure
P e ro vs kite stru ctu re
La, Sr
Mn
O
?
Jex
3.8 7 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
19
Spin Resolved IPES from LSMO(001)
L S M O (0 0 1 )
L S M O (0 0 1 )
m a jo rity s p in
m in o rity s p in
T = 300 K
 ~ 400 m eV
T = 100 K
zoom
-2
0
2
4
6
8
10
12
in te n s ity (a rb . u n its )
in te n s ity (a rb . u n its )
m a jo rity sp in
m in o rity sp in
T = 100 K
14
E -E F (e V )
T = 300 K
sample: La0.7Sr0.3MnO3(001)
350 Å thick film grown on SrTiO3(001) by
-2
Pulsed Laser Deposition (Orsay)
-1
0
1
2
3
E -E F (e V )
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
20
LSMO/STO/LSMO MTJ
Spin resolved IPES
S T O (2 M L )/L S M O (0 0 1 )
TEM: good epitaxy
in te n s ity (a rb . u n its )
m a jo rity s p in
m in o rity s p in
La2/3Sr1/3MnO3
SrTiO3
La2/3Sr1/3MnO3
 ~ 350 m eV
T = 100 K
-1 ,0
Understanding the bias dependence of TMR
-0 ,5
0 ,0
0 ,5
1 ,0
1 ,5
E - E F (e V )
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
21
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
e
r
i
c
a
l
t
c
i
t
e
Z
n
S
e
(
0
0
1
)
R
e
c
p
i
o
r
c
a
a
l
c
i
t
e
F
e
(
0
0
1
)
ZnSe(001)
GaAs(001)
Fe deposition
a
=
5
.
6
6
7
Å

2
1
1.

56Å
7
a 2
b
=
2
.
8
6
6
Å
2

1
2.
1Å
9
b
Franco Ciccacci - PESP-2002, MIT-Bates Linear Accelerator Center
22
Fe/ZnSe(001)
A, D: ZnSe bulk states
S pin polarised IP S from Fe/ZnS e(001)
CB: conduction band minimum (1.3 eV above Femi level)
Spin resolved IPES
C
In te n s ity (a rb . u n its )
: band bending upon interface formation (0.5 eV)
B2
B1
S
CB
F e(001)
 = 0.5 eV
Egap = 2.8 eV
D
EF
Fe

VB
15 M L
A
CB
8 ML
Z nS e(001)
1 M L Fe
2 M L Fe
Fe film - magnetic properties:
-4
-2
0
2
4
6
E - E F (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
23
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
24

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