Post on 13-Jan-2016
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Magnetism in ultrathin films
W. Weber
IPCMS Strasbourg
Orbital and Spin moment
1 with
222
)(2
)(2
22
lBlorb
orb
glgm
lm
ql
m
qL
m
q
mrm
qrqFIm
Intuitive : Orbital moment Mysterious : Spin moment
2 with sBsspin gsgm
s : momentumspin Angular
FerromagnetismParamagnetic behavior: usually one has to apply an external magnetic field in order to align the magnetic moments
T
H
Ferromagnetic behavior: magnetization without an external magnetic field at non-zero temperatures possible
H
FerromagnetismHow can we explain magnetic order up to temperatures of 1000 K?
Curie temperature
FerromagnetismEarly explanation (1907): Weiss’ molecular field
A molecular field exists within the ferromagnet which orders the moments against the thermal motion. It is so large that the ferromagnet can be saturated even without an external magnetic field.
Order of magnitude:
mBcB HTk 0K 1000cT A/m 109mH
Ferromagnetism
What is the physical interaction responsible for it?
Dipole-dipole interaction?
rmrmr
mmr
Edip 215213
31
K 1 3
20 a
BStrength Too weak!
Ferromagnetism
Interplay of Pauli principle and Coulomb interaction
Two electrons of opposite spin can share the same orbital and come close
Two electrons of same spin cannot farther apart Lower Coulomb energy
This interaction does not act like a real magnetic field
J positive : parallel orientation (ferromagnetic)J negative : anti-parallel orientation (anti-ferromagnetic)
Exchange interaction in a solid
ji
ji SSJH
The strength of the interaction depends on the orbital overlap between neighbouring atoms decreases exponentially with distance
Indirect exchange coupling in multilayers
FM1
FM2
nonmagnetic metal
Indirect exchange coupling
Unguris et al., Phys.Rev. B 49, 14 (1994)
Indirect exchange coupling in multilayers
Amplitude of the coupling strength decreases with thickness
Parkin et al., Phys.Rev. B 44, 7131 (1991)
Co80Ni20 / Ru / Co80Ni20Co80Ni20 / Ru / Co80Ni20
Indirect exchange couplingRKKY interaction (Ruderman, Kittel,Kasuya, Yosida).
It explains for instance the coupling in rare earth systems
Virtually no overlap between magnetic 4f-orbitals
Indirect exchange through conduction electrons
RKKY interaction
distance
Spin density
RKKY interaction
distance
Spin density
RKKY interaction
distance
Spin density
RKKY interaction
distance
Spin density
RKKY interaction
distance
Spin density
RKKY interaction
RKKY interaction
RKKY interaction
RKKY interaction
)2sin(1
2Rk
RJ F
Giant magnetoresistance
Baibich et al., PRL 61,2472 (1988)
Magnetic
Magnetic
Nonmagnetic
Fe
Cr
Fe
Spinfilter effect
E
EF
)(ED)(ED
Paramagnet
Spinfilter effect
E
EF
)(ED)(ED
Ferromagnet
strong scattering
weak scattering
Giant magnetoresistance
22
RrRRtot
r
r
R
R
Giant magnetoresistance
R R
r r
rrR
RrRtot 2
2
GMR read head
voltage
voltage
voltage
voltage
voltage
voltage
voltage
voltage
Spin-resolved photoemission spectroscopy on MnPc/Co(001):
spin-polarized interface states
Insulating spacer layer : tunneling MR
min
min
NN
NNP
maj
maj
21
21
PP1
P2PTMR
Pi = polarisation
=
DOSDOS
DOS-DOS
Jullière’s model
De Teresa et al., Science 286 (1999)
The polarisation dependson the interface !!LSMO
LSMO
PP1
P2PTMR
Co
Co
STO LSMOCo / / ALO LSMOCo / STO //
Mn(II)-phthalocyanine : Mn-C32H16N8
MnPc
Cu(001)
Co(001)
Advantage:large spin diffusion length expecteddue to weak spin-orbit coupling inlow-Z materials.
Photoemission spectroscopy
Spin detector
nPfNL 1 nPfNR 1
nPfNN
NNA
RL
RL
Au foiln
Spin-resolved spectra
E
EF
)(ED)(ED
Spin-resolved spectra
Spin-resolved spectra
Spin-resolved spectra
Spin-resolved spectra
Interface state
Difference spectra
Difference spectra
Difference spectra
Difference spectra
first layer second layer third layer
contribution of the different Pc layers to the interface states
Polarization of difference spectra
Character of interface states
Determination of the character by exploiting the variation of the cross section with photon energy. By going from 20 to 100 eV the cross sections change by the following factors:
Co 3d: 1.4
Mn 3d: 0.7
C 2p: 1/40
N 2p: 1/20
0,0 0,5 1,0 1,5 2,0
0
100
200
2,6 ML Pc/Co(3 ML) - Co(3 ML)
h=100 eV
Inte
nsity
(ar
b. u
nits
)
Binding energy (eV)
spin up spin down
Character of interface states
EF
Co Interface Pc/Co
EF
C. Barraud et al., Nature Phys. (2010)
EF
Co Interface Pc/Co
EF
C. Barraud et al., Nature Phys. (2010)
EF
Co Interface Pc/Co
EF
Electron spin motion: a new tool to study ferromagnetic films
M
Spin up
0
1
Spin down
1
0
1
0
0
10
1
0
0
1 ii erer
?
x
y
z
P0
M
x
y
z
M
x
y
z
M
x
y
z
M
x
y
z
ε
M
M
Spin up
0
1
Spin down
1
0
1
0
0
10
1
0
0
1 ii erer
rr
rr
2arctan
22
?
x
y
z
ε
M
εx
y
z
M
εx
y
z
M
εx
y
z
M
εx
y
z
M
Experiment
Spin-dependent band gaps and their influence on the
electron-spin motion
Typical electronic band structure
Theory
Experimental results
Joly et al., PRL 96, 137206 (2006)
Spin-dependent band gaps and their influence on the
electron-spin motion
Fabry-Pérot experiments with spin-polarized electrons
Cu (001)
0P
Co
Quantum interference
0P
Cu (001)
Co
Quantum interference
Cu
0P
Cu (001)
Co
Quantum interference
Cu
0P
Cu (001)
Co
Quantum interference
Cu
0P
Cu (001)
Co
Quantum interference
Cu
Experimental results and simulations
Joly et al., PRL 97, 187404 (2006), Joly et al., PRB 76, 104415 (2007)
Joly et al., PRL 97, 187404 (2006), Joly et al., PRB 76, 104415 (2007)
Spin-dependent band gaps and their influence on the
electron-spin motion
Fabry-Pérot experiments with spin-polarized electrons
Morphology-induced oscillations of the electron-
spin precession
Tati Bismaths et al., PRB 77, 220405(R) (2008)
Fe/Ag(001)
A/B without relaxation at the islands edges
A/B with relaxation at the islands edges
coverage
para
met
er
coverage
para
met
er
coverage
para
met
er
coverage
para
met
er
coverage
para
met
er
coverage
para
met
er
coverage
para
met
er
coverage
para
met
er
coverage
para
met
er
coverage
para
met
er
Spin-dependent band gaps and their influence on the
electron-spin motion
Fabry-Pérot experiments with spin-polarized electrons
Morphology-induced oscillations of the electron-
spin precession
Influence of sub-monolayer MgO coverages on the spin-dependent
reflection properties of Fe
T. Berdot et al., PRB 82, 172407 (2010)
d (ML)
H.L. Meyerheim et al., Phys. Rev. B 65, 144433 (2002)
MgO-induced perpendicular relaxation of the Fe surface
MgO-induced normal relaxation of the Fe surface
H.L. Meyerheim et al., Phys. Rev. B 65, 144433 (2002)
MgO-induced normal relaxation of the Fe surface
0,0 0,5 1,0 1,5 2,0 2,5 3,00
5
10
15
20
Rel
axat
ion
(%)
MgO thickness (ML)
H.L. Meyerheim et al., Phys. Rev. B 65, 144433 (2002)
Ab initio calculations based on linear muffin-tin orbital method (LMTO) and the Korringa-Kohn-Rostoker (KKR) method.
Ab initio calculations
- 9 ML Fe
- First interlayer distance is relaxed without actually putting MgO on top of Fe
d=dFe bulk=1,43 Å
d = ?
Fe(001)
0
10
20
30
40
0,0 0,2 0,4 0,6 0,8 1,0-20
-10
0
10
20
30
40
50
60
10
15
20
25
30
35
40
-5
0
5
10
15
20
25
(de
g.)
Theo
Exp
MgO thickness (ML)
(de
g.)
Theo
(d
eg.)
Exp
(d
eg.)
T. Berdot et al., PRB 82, 172407 (2010)
0,0 0,5 1,0 1,5 2,0 2,5 3,00
5
10
15
20
Re
lax
ati
on
(%)
MgO thickness (ML)
Spin-dependent band gaps and their influence on the
electron-spin motion
Fabry-Pérot experiments with spin-polarized electrons
Morphology-induced oscillations of the electron-
spin precession
Influence of sub-monolayer MgO coverages on the spin-dependent
reflection properties of Fe
Influence of lattice relaxation on the spin precession in Fe/Ag(001)
prec
essi
on a
ngle
(de
gree
s)
A. Hallal et al., PRL 107, 087203 (2011)
prec
essi
on a
ngle
(de
gree
s)
A. Hallal et al., PRL 107, 087203 (2011)
prec
essi
on a
ngle
(de
gree
s)
A. Hallal et al., PRL 107, 087203 (2011)
prec
essi
on a
ngle
(de
gree
s)
A. Hallal et al., PRL 107, 087203 (2011)
prec
essi
on a
ngle
(de
gree
s)
A. Hallal et al., PRL 107, 087203 (2011)
prec
essi
on a
ngle
(de
gree
s)
A. Hallal et al., PRL 107, 087203 (2011)
prec
essi
on a
ngle
(de
gree
s)
A. Hallal et al., PRL 107, 087203 (2011)
MgO/Fe(001)
pseudomorphic growth
dislocations
Ramsauer-Townsend effect
Resonance condition weak scattering
on-resonance scattering phase is zero
off-resonance scattering phase is non-zero
Energy
Eex
A. Hallal et al., PRL 107, 087203 (2011)
Spin-dependent band gaps and their influence on the
electron-spin motion
Fabry-Pérot experiments with spin-polarized electrons
Morphology-induced oscillations of the electron-
spin precession
Influence of sub-monolayer MgO coverages on the spin-dependent
reflection properties of Fe
Influence of lattice relaxation on the spin precession in Fe/Ag(001)
Organic molecules on ferromagnetic surfaces
0,0 0,2 0,4 0,6 0,8 1,0
0
5
10
15
20
25
of H2Pc
of C60
(vert. shifted by 2o)
of Pentacontane (vert. shifted by 4o)
of carbon (vert. shifted by 12o)
prec
essi
on o
r ro
tatio
n an
gle
(deg
rees
)
thickness (ML)
Laser
Polarizer
Pockels cell
Deflector
GaAs
Coils
Electron optics
SampleCoils
Electron optics
Retarding field analyser
Spin detector
GaAs : Source of polarized electrons
% 50
NN
NNP