IntroductionExperimental setup
ResultsMagnetoelastic model
ConclusionAcknowledgments
Thin epitaxial ferromagnetic metal films onGaAs(001) for spin injection and tunneling
magnetoresistive junctions.Enhancement of the uniaxial magnetic anisotropy
Francois Bianco
IBM - ETH Zurich
1st November 2008
Francois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin injection and tunneling magnetoresistive junctions.
IntroductionExperimental setup
ResultsMagnetoelastic model
ConclusionAcknowledgments
Outlook
1 Motivations and introduction
2 Experimental setup
3 Effect of post-growth annealing
4 Estimate of the magnetoelastic contribution
Francois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin injection and tunneling magnetoresistive junctions.
IntroductionExperimental setup
ResultsMagnetoelastic model
ConclusionAcknowledgments
MotivationOrientation of the magnetizationUniaxial magnetic anisotropyTotal magnetic energy densityHysteresis curves
Motivation
Scientific motivations
The origin of the uniaxial magnetic anisotropy (UMA) of Feand FeCo thin films on GaAs(001) is, since its discovery byKrebs 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 themagnetic properties of thin films.
Long-term goal/application
Spin-injection from Fe31Co69 into GaAs
Francois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin injection and tunneling magnetoresistive junctions.
IntroductionExperimental setup
ResultsMagnetoelastic model
ConclusionAcknowledgments
MotivationOrientation of the magnetizationUniaxial magnetic anisotropyTotal magnetic energy densityHysteresis curves
Orientation of the magnetization
The preferred orientation of the magnetization is driven by
shape anisotropy (Gauss law) : for thin films favor in-planemagnetization
magnetocrystalline anisotropy (Crystal symmetry)
magnetoelastic effect (Lattice strain)
uniaxial magnetic anisotropy (Interface)
Francois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin injection and tunneling magnetoresistive junctions.
IntroductionExperimental setup
ResultsMagnetoelastic model
ConclusionAcknowledgments
MotivationOrientation of the magnetizationUniaxial magnetic anisotropyTotal magnetic energy densityHysteresis 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 symmetryof the atomic orbitals favor bonds in a specific direction.
Anisotropic strain
Induced by a slight difference in the lattice constant along twodirections.
Francois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin injection and tunneling magnetoresistive junctions.
IntroductionExperimental setup
ResultsMagnetoelastic model
ConclusionAcknowledgments
MotivationOrientation of the magnetizationUniaxial magnetic anisotropyTotal magnetic energy densityHysteresis 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− ε) sin2 θ︸ ︷︷ ︸Uuniaxial,‖
+1
2µ0MsMeff cos2 θ︸ ︷︷ ︸Ushape +Uuniaxial,⊥
+ K1(1
4sin2 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⊥
Francois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin injection and tunneling magnetoresistive junctions.
IntroductionExperimental setup
ResultsMagnetoelastic model
ConclusionAcknowledgments
MotivationOrientation of the magnetizationUniaxial magnetic anisotropyTotal magnetic energy densityHysteresis curves
Hysteresis curves
Ku and K1 are proportional tothe slope and the width of thelinear part along the HA.
Francois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin injection and tunneling magnetoresistive junctions.
IntroductionExperimental setup
ResultsMagnetoelastic model
ConclusionAcknowledgments
Sample fabricationSample characterizationMagneto-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 Alcapping layer (2–3 nm).
Francois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin injection and tunneling magnetoresistive junctions.
IntroductionExperimental setup
ResultsMagnetoelastic model
ConclusionAcknowledgments
Sample fabricationSample characterizationMagneto-optical Kerr effect magnetometer
E-Beam deposition
The secondary current between the filament and the slug heat itup.
Francois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin injection and tunneling magnetoresistive junctions.
IntroductionExperimental setup
ResultsMagnetoelastic model
ConclusionAcknowledgments
Sample fabricationSample characterizationMagneto-optical Kerr effect magnetometer
Samples characterization
Ku and K1 determined from hysteresis curve measured withmagnetooptical Kerr effect magnetometer.
Ku,⊥ measured with ferromagnetic resonance (all-opticalsetup).
The in-plane film strain measured with grazing incidenceX-ray diffraction.
The samples were annealed in an Ar-filled glovebox for 10min. at each temperatures.
Francois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin injection and tunneling magnetoresistive junctions.
IntroductionExperimental setup
ResultsMagnetoelastic model
ConclusionAcknowledgments
Sample fabricationSample characterizationMagneto-optical Kerr effect magnetometer
Magneto-optical Kerr effect magnetometer
Kerr effect
Magnetized material isbirefrigent
The refractive indexesdepends on themagnetization direction
The rotation of the lightpolarization is therforedirectly related to themagnetization.
Francois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin injection and tunneling magnetoresistive junctions.
IntroductionExperimental setup
ResultsMagnetoelastic model
ConclusionAcknowledgments
Sample fabricationSample characterizationMagneto-optical Kerr effect magnetometer
Experimental setup
Francois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin injection and tunneling magnetoresistive junctions.
IntroductionExperimental setup
ResultsMagnetoelastic model
ConclusionAcknowledgments
Thickness dependence of anisotropyEffect of the post-growth annealingPerpendicular anisotropyInterpretationCubic magnetic anisotropyCoercive fieldInterpretation
Thickness dependence of anisotropies
(As-grown anisotropies)Interface contribution
K = K vol +K int
t
K vol1 in excellent
agreement with bulkvalue of Fe31Co69
Assumption : Ku
arises only frominterface K vol
u = 0
Francois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin injection and tunneling magnetoresistive junctions.
IntroductionExperimental setup
ResultsMagnetoelastic model
ConclusionAcknowledgments
Thickness dependence of anisotropyEffect of the post-growth annealingPerpendicular anisotropyInterpretationCubic magnetic anisotropyCoercive fieldInterpretation
Huge enhancement of UMA
Post-growth annealingtemperature Ta induces
a huge increase of theUMA
Ku follows a lineardependence up toTa ≈ 300�
opposite behaviourobserved on Fe thinfilmsShaw et al. J. Appl. Phys., 2007
(Sample thickness 1.9 nm)
Francois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin injection and tunneling magnetoresistive junctions.
IntroductionExperimental setup
ResultsMagnetoelastic model
ConclusionAcknowledgments
Thickness dependence of anisotropyEffect of the post-growth annealingPerpendicular anisotropyInterpretationCubic magnetic anisotropyCoercive fieldInterpretation
Thickness dependence of the enhancement
The effect is strongly dependent on the thickness t, and starts at athreshold temperature Tth of about 75�.
Ku = KTthu + κ
t ∆T∆T := Ta − Tth
∆Ku∆T = κ1
t
Francois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin injection and tunneling magnetoresistive junctions.
IntroductionExperimental setup
ResultsMagnetoelastic model
ConclusionAcknowledgments
Thickness dependence of anisotropyEffect of the post-growth annealingPerpendicular anisotropyInterpretationCubic magnetic anisotropyCoercive fieldInterpretation
As-grown anisotropies vs. anisotropies at 200�
Effect on the film
K vol1 and K int
1 are notchanged
K intu is 3 times bigger
than the as-grownvalue
Francois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin injection and tunneling magnetoresistive junctions.
IntroductionExperimental setup
ResultsMagnetoelastic model
ConclusionAcknowledgments
Thickness dependence of anisotropyEffect of the post-growth annealingPerpendicular anisotropyInterpretationCubic magnetic anisotropyCoercive fieldInterpretation
Perpendicular anisotropy
(Sample thickness 7.2 nm)
Increase like the in-planeanisotropy
with a 2-3 times steeperslope
Francois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin injection and tunneling magnetoresistive junctions.
IntroductionExperimental setup
ResultsMagnetoelastic model
ConclusionAcknowledgments
Thickness dependence of anisotropyEffect of the post-growth annealingPerpendicular anisotropyInterpretationCubic magnetic anisotropyCoercive fieldInterpretation
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 = K intu (∆T )
t = K intu (Tth)+κ∆T
t = KTthu + κ
t ∆T∆T := Ta − Tth
Francois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin injection and tunneling magnetoresistive junctions.
IntroductionExperimental setup
ResultsMagnetoelastic model
ConclusionAcknowledgments
Thickness dependence of anisotropyEffect of the post-growth annealingPerpendicular anisotropyInterpretationCubic magnetic anisotropyCoercive fieldInterpretation
Interpretation 2/2
Post-growth annealing
Affects mainly the interface
Probably creates a coherent interface
Zega et al. showed forFe/GaAs(001) that annealing at200� produces a monolayer ofalternating Fe and As atoms.
Zega, Phys. Rev. Lett. 96(196101) 2006
Francois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin injection and tunneling magnetoresistive junctions.
IntroductionExperimental setup
ResultsMagnetoelastic model
ConclusionAcknowledgments
Thickness dependence of anisotropyEffect of the post-growth annealingPerpendicular anisotropyInterpretationCubic magnetic anisotropyCoercive fieldInterpretation
Cubic magnetic anisotropy
No noticeable trend upto Ta ≈ 250�
Decreases for Ta > 250�
Ga atoms begin todiffuse from substrateinto Fe for Ta > 220�Sano and Miyagawa Jpn. J. Appl. Phys.,
30(7) :1434–1441, 1991
Francois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin injection and tunneling magnetoresistive junctions.
IntroductionExperimental setup
ResultsMagnetoelastic model
ConclusionAcknowledgments
Thickness dependence of anisotropyEffect of the post-growth annealingPerpendicular anisotropyInterpretationCubic magnetic anisotropyCoercive fieldInterpretation
Coercive field
No noticeable trend up toTa = 300�
Huge jump for Ta > 300�
Above 300� changes incrystal structure because ofGa diffusion
Francois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin injection and tunneling magnetoresistive junctions.
IntroductionExperimental setup
ResultsMagnetoelastic model
ConclusionAcknowledgments
Thickness dependence of anisotropyEffect of the post-growth annealingPerpendicular anisotropyInterpretationCubic magnetic anisotropyCoercive fieldInterpretation
Interpretation
Cubic anisotropy K1 & Coercive field
Not much affected below Ta < 250–300�
For Ta > 300� changes are correlated to diffusion of GaAscomponents
Post-growth annealing
If Ga atoms replace Fe or Co atoms in the film ⇒ reduction ofthe crystal symmetry and therefore of K1
Induces probably a change in the crystalline structure of thefilm
Francois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin injection and tunneling magnetoresistive junctions.
IntroductionExperimental setup
ResultsMagnetoelastic model
ConclusionAcknowledgments
In-plane expectations from MELEstimate of the in-plane MEL effectOut-of-plane expectations from MEL effectEstimate of the out-of-plane MEL effectDiscussion of the estimate
Magnetoelastic model
Magnetoelastic model (MEL)
We will show with GID measurements that MEL effect cannotexplain our results.
X-ray grazing incidencediffraction (GID)
To determine the in-plane latticestrain
Francois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin injection and tunneling magnetoresistive junctions.
IntroductionExperimental setup
ResultsMagnetoelastic model
ConclusionAcknowledgments
In-plane expectations from MELEstimate of the in-plane MEL effectOut-of-plane expectations from MEL effectEstimate of the out-of-plane MEL effectDiscussion 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 straine12
∆Ku = B2∆e12
Francois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin injection and tunneling magnetoresistive junctions.
IntroductionExperimental setup
ResultsMagnetoelastic model
ConclusionAcknowledgments
In-plane expectations from MELEstimate of the in-plane MEL effectOut-of-plane expectations from MEL effectEstimate of the out-of-plane MEL effectDiscussion 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 ofstrain of 0.8 � we founda B2 constant severaltimes bigger than Fe thinfilms
Francois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin injection and tunneling magnetoresistive junctions.
IntroductionExperimental setup
ResultsMagnetoelastic model
ConclusionAcknowledgments
In-plane expectations from MELEstimate of the in-plane MEL effectOut-of-plane expectations from MEL effectEstimate of the out-of-plane MEL effectDiscussion 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 averagein-plane strain e0
∆Ku,⊥ ∝ B1∆e0
Francois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin injection and tunneling magnetoresistive junctions.
IntroductionExperimental setup
ResultsMagnetoelastic model
ConclusionAcknowledgments
In-plane expectations from MELEstimate of the in-plane MEL effectOut-of-plane expectations from MEL effectEstimate of the out-of-plane MEL effectDiscussion of the estimate
Estimate of the out-of-plane MEL effect
B1, estimated from as-grown values of K⊥ and e0 assuming onlyMEL effect, is 44 times larger than B1 for Fe films.
Contradictions
MEL effect requiresmore compressivestrain
But we observe nonoticeable trend oronly a slight decreasewith Ta
Francois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin injection and tunneling magnetoresistive junctions.
IntroductionExperimental setup
ResultsMagnetoelastic model
ConclusionAcknowledgments
In-plane expectations from MELEstimate of the in-plane MEL effectOut-of-plane expectations from MEL effectEstimate of the out-of-plane MEL effectDiscussion 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 Fethin 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,⊥ interms of a magnetocrystalline anisotropy due to modifications ofthe bonding at the Fe31Co69/GaAs(001) interface.
Francois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin injection and tunneling magnetoresistive junctions.
IntroductionExperimental setup
ResultsMagnetoelastic model
ConclusionAcknowledgments
Conclusion
Results
We observed a huge enhancement of the in- and out-of-planeUMA with Ta.
This is related to changes at the FM-SC interface
The changes of K1 and of the coercive field indicates to thediffusion of Ga into the film.
The MEL cannot explain the effect of Ta
Francois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin injection and tunneling magnetoresistive junctions.
IntroductionExperimental setup
ResultsMagnetoelastic model
ConclusionAcknowledgments
Acknowledgments
Thanks to all the members of the Physics of Nanoscale Systemgroup at IBM, especially
Gian Salis
Andreas Bischof
Marilyne Sousa (Adv. Func. Mat.)
S. Falt, A. Badolato,and. S. Schon (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.
Francois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin injection and tunneling magnetoresistive junctions.
IntroductionExperimental setup
ResultsMagnetoelastic model
ConclusionAcknowledgments
Thanks for your attention
“Science has explained nothing ; the more we know the morefantastic the world becomes and the profounder the surroundingdarkness.”
Aldous Leonard Huxley
“The most exciting phrase to hear in science, the one that heraldsnew discoveries, is not ’Eureka !’ but ’That’s funny...’ ”
Isaac Asimov
Francois Bianco Thin epitaxial ferromagnetic metal films on GaAs(001) for spin injection and tunneling magnetoresistive junctions.
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