A work done at IBM on thin epitaxial ferromagnetic metal films on GaAs(001) for spin injection and tunneling magnetoresistive junctions showing enhanced magnetic anisotropy by annealing.

1. Introduction
Experimental setup
Results
Magnetoelastic model
Conclusion
Acknowledgments
Thin epitaxial ferromagnetic metal films on
GaAs(001) for spin injection and tunneling
magnetoresistive junctions.
Enhancement of the uniaxial magnetic anisotropy
Fran¸cois Bianco
IBM - ETH Zurich
1st November 2008
Fran¸cois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin

2. Introduction
Experimental setup
Results
Magnetoelastic model
Conclusion
Acknowledgments
Outlook
1 Motivations and introduction
2 Experimental setup
3 Effect of post-growth annealing
4 Estimate of the magnetoelastic contribution
Fran¸cois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin

3. Introduction
Experimental setup
Results
Magnetoelastic model
Conclusion
Acknowledgments
Motivation
Orientation of the magnetization
Uniaxial magnetic anisotropy
Total magnetic energy density
Hysteresis curves
Motivation
Scientific motivations
The origin of the uniaxial magnetic anisotropy (UMA) of Fe
and FeCo thin films on GaAs(001) is, since its discovery by
Krebs et al. in 1987 (J. Appl. Phys. 61, 2596, 1987), still controversial.
Get a better understanding the origin of the UMA ...
... by studying the effect of post-growth annealing on the
magnetic properties of thin films.
Long-term goal/application
Spin-injection from Fe31Co69 into GaAs
Fran¸cois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin

4. Introduction
Experimental setup
Results
Magnetoelastic model
Conclusion
Acknowledgments
Motivation
Orientation of the magnetization
Uniaxial magnetic anisotropy
Total magnetic energy density
Hysteresis curves
Orientation of the magnetization
The preferred orientation of the magnetization is driven by
shape anisotropy (Gauss law) : for thin films favor in-plane
magnetization
magnetocrystalline anisotropy (Crystal symmetry)
magnetoelastic effect (Lattice strain)
uniaxial magnetic anisotropy (Interface)
Fran¸cois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin

5. Introduction
Experimental setup
Results
Magnetoelastic model
Conclusion
Acknowledgments
Motivation
Orientation of the magnetization
Uniaxial magnetic anisotropy
Total magnetic energy density
Hysteresis curves
Uniaxial magnetic anisotropy
The possible explanations are Krebs J. Appl. Phys. 61, 2596, 1987
Anisotropic bonding
The substrate atoms forms rows a the surface, then the symmetry
of the atomic orbitals favor bonds in a specific direction.
Anisotropic strain
Induced by a slight difference in the lattice constant along two
directions.
Fran¸cois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin

6. Introduction
Experimental setup
Results
Magnetoelastic model
Conclusion
Acknowledgments
Motivation
Orientation of the magnetization
Uniaxial magnetic anisotropy
Total magnetic energy density
Hysteresis curves
Total magnetic energy density
The magnetization goes in the direction of the energy minimum.
Utot(ϕ, θ) = −|Hext|Ms cos(ϕ − δ) sin θ
UZeeman
+ Ku sin2
(ϕ − ) sin2
θ
Uuniaxial,
+
1
2
µ0MsMeff cos2
θ
Ushape +Uuniaxial,⊥
+ K1(
1
4
sin2
θ sin2
(2ϕ) + cos2
θ) sin2
θ
UCubic
Depends on three parameters
Ku uniaxial magnetic anisotropy constant
K1 cubic magnetic anisotropy constant
Meff containing the perpendicular uniaxial anisotropy Ku⊥
Fran¸cois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin

7. Introduction
Experimental setup
Results
Magnetoelastic model
Conclusion
Acknowledgments
Motivation
Orientation of the magnetization
Uniaxial magnetic anisotropy
Total magnetic energy density
Hysteresis curves
Hysteresis curves
Ku and K1 are proportional to
the slope and the width of the
linear part along the HA.
Fran¸cois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin

8. Introduction
Experimental setup
Results
Magnetoelastic model
Conclusion
Acknowledgments
Sample fabrication
Sample characterization
Magneto-optical Kerr effect magnetometer
Sample fabrication
The samples were fabricated under UHV by MBE on GaAs(001)
cooled to -10‰. The Fe31Co69 thin films were protected with an Al
capping layer (2–3 nm).
Fran¸cois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin

9. Introduction
Experimental setup
Results
Magnetoelastic model
Conclusion
Acknowledgments
Sample fabrication
Sample characterization
Magneto-optical Kerr effect magnetometer
E-Beam deposition
The secondary current between the filament and the slug heat it
up.
Fran¸cois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin

10. Introduction
Experimental setup
Results
Magnetoelastic model
Conclusion
Acknowledgments
Sample fabrication
Sample characterization
Magneto-optical Kerr effect magnetometer
Samples characterization
Ku and K1 determined from hysteresis curve measured with
magnetooptical Kerr effect magnetometer.
Ku,⊥ measured with ferromagnetic resonance (all-optical
setup).
The in-plane film strain measured with grazing incidence
X-ray diffraction.
The samples were annealed in an Ar-filled glovebox for 10
min. at each temperatures.
Fran¸cois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin

11. Introduction
Experimental setup
Results
Magnetoelastic model
Conclusion
Acknowledgments
Sample fabrication
Sample characterization
Magneto-optical Kerr effect magnetometer
Magneto-optical Kerr effect magnetometer
Kerr effect
Magnetized material is
birefrigent
The refractive indexes
depends on the
magnetization direction
The rotation of the light
polarization is therfore
directly related to the
magnetization.
Fran¸cois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin

12. Introduction
Experimental setup
Results
Magnetoelastic model
Conclusion
Acknowledgments
Sample fabrication
Sample characterization
Magneto-optical Kerr effect magnetometer
Experimental setup
Fran¸cois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin

13. Introduction
Experimental setup
Results
Magnetoelastic model
Conclusion
Acknowledgments
Thickness dependence of anisotropy
Effect of the post-growth annealing
Perpendicular anisotropy
Interpretation
Cubic magnetic anisotropy
Coercive field
Interpretation
Thickness dependence of anisotropies
(As-grown anisotropies)
Interface contribution
K = Kvol
+
Kint
t
Kvol
1 in excellent
agreement with bulk
value of Fe31Co69
Assumption : Ku
arises only from
interface Kvol
u = 0
Fran¸cois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin

14. Introduction
Experimental setup
Results
Magnetoelastic model
Conclusion
Acknowledgments
Thickness dependence of anisotropy
Effect of the post-growth annealing
Perpendicular anisotropy
Interpretation
Cubic magnetic anisotropy
Coercive field
Interpretation
Huge enhancement of UMA
Post-growth annealing
temperature Ta induces
a huge increase of the
UMA
Ku follows a linear
dependence up to
Ta ≈ 300‰
opposite behaviour
observed on Fe thin
films
Shaw et al. J. Appl. Phys., 2007
(Sample thickness 1.9 nm)
Fran¸cois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin

15. Introduction
Experimental setup
Results
Magnetoelastic model
Conclusion
Acknowledgments
Thickness dependence of anisotropy
Effect of the post-growth annealing
Perpendicular anisotropy
Interpretation
Cubic magnetic anisotropy
Coercive field
Interpretation
Thickness dependence of the enhancement
The effect is strongly dependent on the thickness t, and starts at a
threshold temperature Tth of about 75‰.
Ku = KTth
u + κ
t ∆T
∆T := Ta − Tth
∆Ku
∆T = κ1
t
Fran¸cois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin

16. Introduction
Experimental setup
Results
Magnetoelastic model
Conclusion
Acknowledgments
Thickness dependence of anisotropy
Effect of the post-growth annealing
Perpendicular anisotropy
Interpretation
Cubic magnetic anisotropy
Coercive field
Interpretation
As-grown anisotropies vs. anisotropies at 200‰
Effect on the film
Kvol
1 and Kint
1 are not
changed
Kint
u is 3 times bigger
than the as-grown
value
Fran¸cois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin

17. Introduction
Experimental setup
Results
Magnetoelastic model
Conclusion
Acknowledgments
Thickness dependence of anisotropy
Effect of the post-growth annealing
Perpendicular anisotropy
Interpretation
Cubic magnetic anisotropy
Coercive field
Interpretation
Perpendicular anisotropy
(Sample thickness 7.2 nm)
Increase like the in-plane
anisotropy
with a 2-3 times steeper
slope
Fran¸cois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin

18. Introduction
Experimental setup
Results
Magnetoelastic model
Conclusion
Acknowledgments
Thickness dependence of anisotropy
Effect of the post-growth annealing
Perpendicular anisotropy
Interpretation
Cubic magnetic anisotropy
Coercive field
Interpretation
Interpretation 1/2
In- and out-of-plane uniaxial anisotropy
Linear increase with post-growth annealing temperature
In-plane effect starts at Tth ≈ 75‰
Model for the in-plane increase with Ta
Ku = Kint
u (∆T)
t = Kint
u (Tth)+κ∆T
t = KTth
u + κ
t ∆T
∆T := Ta − Tth
Fran¸cois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin

19. Introduction
Experimental setup
Results
Magnetoelastic model
Conclusion
Acknowledgments
Thickness dependence of anisotropy
Effect of the post-growth annealing
Perpendicular anisotropy
Interpretation
Cubic magnetic anisotropy
Coercive field
Interpretation
Interpretation 2/2
Post-growth annealing
Affects mainly the interface
Probably creates a coherent interface
Zega et al. showed for
Fe/GaAs(001) that annealing at
200‰produces a monolayer of
alternating Fe and As atoms.
Zega, Phys. Rev. Lett. 96(196101) 2006
Fran¸cois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin

20. Introduction
Experimental setup
Results
Magnetoelastic model
Conclusion
Acknowledgments
Thickness dependence of anisotropy
Effect of the post-growth annealing
Perpendicular anisotropy
Interpretation
Cubic magnetic anisotropy
Coercive field
Interpretation
Cubic magnetic anisotropy
No noticeable trend up
to Ta ≈ 250‰
Decreases for Ta > 250‰
Ga atoms begin to
diffuse from substrate
into Fe for Ta > 220‰
Sano and Miyagawa Jpn. J. Appl. Phys.,
30(7) :1434–1441, 1991
Fran¸cois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin

21. Introduction
Experimental setup
Results
Magnetoelastic model
Conclusion
Acknowledgments
Thickness dependence of anisotropy
Effect of the post-growth annealing
Perpendicular anisotropy
Interpretation
Cubic magnetic anisotropy
Coercive field
Interpretation
Coercive field
No noticeable trend up to
Ta = 300‰
Huge jump for Ta > 300‰
Above 300‰changes in
crystal structure because of
Ga diffusion
Fran¸cois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin

22. Introduction
Experimental setup
Results
Magnetoelastic model
Conclusion
Acknowledgments
Thickness dependence of anisotropy
Effect of the post-growth annealing
Perpendicular anisotropy
Interpretation
Cubic magnetic anisotropy
Coercive field
Interpretation
Interpretation
Cubic anisotropy K1 & Coercive field
Not much affected below Ta < 250–300‰
For Ta > 300‰changes are correlated to diffusion of GaAs
components
Post-growth annealing
If Ga atoms replace Fe or Co atoms in the film ⇒ reduction of
the crystal symmetry and therefore of K1
Induces probably a change in the crystalline structure of the
film
Fran¸cois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin

23. Introduction
Experimental setup
Results
Magnetoelastic model
Conclusion
Acknowledgments
In-plane expectations from MEL
Estimate of the in-plane MEL effect
Out-of-plane expectations from MEL effect
Estimate of the out-of-plane MEL effect
Discussion of the estimate
Magnetoelastic model
Magnetoelastic model (MEL)
We will show with GID measurements that MEL effect cannot
explain our results.
X-ray grazing incidence
diffraction (GID)
To determine the in-plane lattice
strain
Fran¸cois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin

24. Introduction
Experimental setup
Results
Magnetoelastic model
Conclusion
Acknowledgments
In-plane expectations from MEL
Estimate of the in-plane MEL effect
Out-of-plane expectations from MEL effect
Estimate of the out-of-plane MEL effect
Discussion of the estimate
In-plane expectations from MEL
In-plane uniaxial magnetic anisotropy
Umel, = B2(e[110] − e[110])
=KuAssumption
sin(2ϕ) = B2 sin(2ϕ)e12
If Ku changes with Ta we should find a change of the shear strain
e12
∆Ku = B2∆e12
Fran¸cois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin

25. Introduction
Experimental setup
Results
Magnetoelastic model
Conclusion
Acknowledgments
In-plane expectations from MEL
Estimate of the in-plane MEL effect
Out-of-plane expectations from MEL effect
Estimate of the out-of-plane MEL effect
Discussion of the estimate
Estimate of the in-plane MEL effect
There is no clear trend within ±0.8‡.
Model
∆Ku = B2∆e12
Assuming a change of
strain of 0.8 ‡we found
a B2 constant several
times bigger than Fe thin
films
Fran¸cois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin

26. Introduction
Experimental setup
Results
Magnetoelastic model
Conclusion
Acknowledgments
In-plane expectations from MEL
Estimate of the in-plane MEL effect
Out-of-plane expectations from MEL effect
Estimate of the out-of-plane MEL effect
Discussion of the estimate
Out-of-plane expectations from MEL effect
Out-of-plane uniaxial magnetic anisotropy
Umel,⊥ = B1(e⊥ − e0)
=K⊥Assumption
cos2
(θ)
If Ku,⊥ changes with Ta we should find a change of the average
in-plane strain e0
∆Ku,⊥ ∝ B1∆e0
Fran¸cois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin

27. Introduction
Experimental setup
Results
Magnetoelastic model
Conclusion
Acknowledgments
In-plane expectations from MEL
Estimate of the in-plane MEL effect
Out-of-plane expectations from MEL effect
Estimate of the out-of-plane MEL effect
Discussion of the estimate
Estimate of the out-of-plane MEL effect
B1, estimated from as-grown values of K⊥ and e0 assuming only
MEL effect, is 44 times larger than B1 for Fe films.
Contradictions
MEL effect requires
more compressive
strain
But we observe no
noticeable trend or
only a slight decrease
with Ta
Fran¸cois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin

28. Introduction
Experimental setup
Results
Magnetoelastic model
Conclusion
Acknowledgments
In-plane expectations from MEL
Estimate of the in-plane MEL effect
Out-of-plane expectations from MEL effect
Estimate of the out-of-plane MEL effect
Discussion of the estimate
Discussion of the estimate
Points against MEL
Strain measured changes in the wrong direction for MEL
The estimated B1 is one order of magnitude bigger than Fe
thin film
B2 is as well several times bigger than Fe thin films value
MEL vs. interface bonding
The results favor an interpretation of the change of Ku and Ku,⊥ in
terms of a magnetocrystalline anisotropy due to modifications of
the bonding at the Fe31Co69/GaAs(001) interface.
Fran¸cois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin

29. Introduction
Experimental setup
Results
Magnetoelastic model
Conclusion
Acknowledgments
Conclusion
Results
We observed a huge enhancement of the in- and out-of-plane
UMA with Ta.
This is related to changes at the FM-SC interface
The changes of K1 and of the coercive field indicates to the
diffusion of Ga into the film.
The MEL cannot explain the effect of Ta
Fran¸cois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin

30. Introduction
Experimental setup
Results
Magnetoelastic model
Conclusion
Acknowledgments
Acknowledgments
Thanks to all the members of the Physics of Nanoscale System
group at IBM, especially
Gian Salis
Andreas Bischof
Marilyne Sousa (Adv. Func. Mat.)
S. F¨alt, A. Badolato,
and. S. Sch¨on (FIRST lab., ETHZ)
Antoine Vanhaverbeke
Martin Witzig
Axelle Tapponnier (Adv. Func. Mat.)
Patrick Bouchon (E. Polytech. Palaisau)
Santos F. Alvarado
And I am very grateful to
Prof. D. Pescia, ETH Zurich
Rolf Allenspach, IBM Research Lab.
Fran¸cois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin

31. Introduction
Experimental setup
Results
Magnetoelastic model
Conclusion
Acknowledgments
Thanks for your attention
“Science has explained nothing ; the more we know the more
fantastic the world becomes and the profounder the surrounding
darkness.”
Aldous Leonard Huxley
“The most exciting phrase to hear in science, the one that heralds
new discoveries, is not ’Eureka !’ but ’That’s funny...’ ”
Isaac Asimov
Fran¸cois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin