thin epitaxial ferromagnetic metal films on gaas(001) for spin injection and tunneling...

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.




Outlook

1 Motivations and introduction

2 Experimental setup

3 Effect of post-growth annealing

4 Estimate of the magnetoelastic contribution





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






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)






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.






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⊥






Hysteresis curves

Ku and K1 are proportional tothe slope and the width of thelinear part along the HA.





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).






E-Beam deposition

The secondary current between the filament and the slug heat itup.






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.






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.






Experimental setup





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






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)






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






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






Perpendicular anisotropy

(Sample thickness 7.2 nm)

Increase like the in-planeanisotropy

with a 2-3 times steeperslope






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






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






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






Coercive field

No noticeable trend up toTa = 300�

Huge jump for Ta > 300�

Above 300� changes incrystal structure because ofGa diffusion






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





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






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






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






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






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






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.





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





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.





Thanks for your attention

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thin epitaxial ferromagnetic metal films on gaas(001) for spin injection and tunneling...

Technology

magnetic properties

lms favor

gaas francois bianco

spin injection

ku sin2 sin2 uuniaxial

shape anisotropy gauss

energy minimum

preferred orientation