My private defense for my PhD in Physics in the Catholic University of Leuven in Belgium.

1. Exchange springs and exchange bias studied with
nuclear methods
Francisco Miguel Cruz Sim˜es de Almeida
o
Instituut voor Kern-en Stralingsfysica
May 24, 2011
Francisco Almeida (IKS) Doctoral dissertation May 24, 2011 1 / 25

2. Outline
1 Surface spin-flop in an Fe/Cr superlattice
Interlayer coupling
Surface spin-flop
Polarized Neutron Reflectometry
2 Exchange bias in Fe-Pt heterostructures
Exchange bias
Fe-Pt alloys
FePt/FePt3 bilayers
Off-stoichiometric Fe-Pt film
3 Exchange bias through ion beam synthesis
Ion beam synthesis
Oxygen implanted cobalt
4 Conclusions
Francisco Almeida (IKS) Doctoral dissertation May 24, 2011 2 / 25

3. Surface spin-flop in an Fe/Cr superlattice
Francisco Almeida (IKS) Doctoral dissertation May 24, 2011 3 / 25

4. Fe/Cr superlattice
Multilayer consisting of 22 repetitions of an Fe/Cr bilayer structure
Fe layers 40 ˚ thick.
A
Cr layers 11 ˚ thick.
A
Fe
Cr
Fe
Cr
Fe
Cr
Fe
Cr
Francisco Almeida (IKS) Doctoral dissertation May 24, 2011 4 / 25

5. Fe/Cr superlattice
Multilayer consisting of 22 repetitions of an Fe/Cr bilayer structure
Fe layers 40 ˚ thick and antiferromagnetically coupled.
A
Cr layers 11 ˚ thick.
A
Francisco Almeida (IKS) Doctoral dissertation May 24, 2011 4 / 25

6. Interlayer coupling
Coupling between ferromagnetic layers separated by a spacer layer.
Bilinear Biquadratic
Francisco Almeida (IKS) Doctoral dissertation May 24, 2011 5 / 25

7. Magnetic response of an Fe/Cr superlattice
1.0 Easy axis a
0.8 25 mT
[010]
H
0.6
[001]
M/Ms
0.4 250 mT
0.2
0.0
1.0 Hard axis b
0.8 25 mT H
1]
[0
0.6
0
1
[0
0]
M/Ms
0.4 75 mT
0.2
0.0
0.0 0.1 0.2 0.3 0.4 0.5 0.6
µ0H (T)
J. Meersschaut et al., PRB 73, 144428 (2006)
Francisco Almeida (IKS) Doctoral dissertation May 24, 2011 6 / 25

8. Magnetic response of an Fe/Cr superlattice
1.0 Easy axis a
0.8 25 mT
[010]
H
0.6
[001]
M/Ms
0.4 250 mT
0.2
0.0
1.0 Hard axis b
0.8 25 mT H
1]
[0
0.6
0
1
[0
0]
M/Ms
0.4 75 mT
0.2
0.0
0.0 0.1 0.2 0.3 0.4 0.5 0.6
µ0H (T)
J. Meersschaut et al., PRB 73, 144428 (2006)
Francisco Almeida (IKS) Doctoral dissertation May 24, 2011 6 / 25

9. Surface spin-flop
(S)
Transition (HC ) that causes a top spin/layer to switch orientation before the
remaining spins/layers of an antiferromagnet. The irregularity propagates to the
(B)
middle of the spin chain, and leads to a bulk spin-flop (HC ).
(S)
HC : surface spin-flop field
(B)
HC : bulk spin-flop field
Francisco Almeida (IKS) Doctoral dissertation May 24, 2011 7 / 25

10. Surface spin-flop
(S)
Transition (HC ) that causes a top spin/layer to switch orientation before the
remaining spins/layers of an antiferromagnet. The irregularity propagates to the
(B)
middle of the spin chain, and leads to a bulk spin-flop (HC ).
(S)
HC : surface spin-flop field
(B)
HC : bulk spin-flop field
How to visualize a surface spin-flop?
Francisco Almeida (IKS) Doctoral dissertation May 24, 2011 7 / 25

11. Polarized Neutron Reflectometry
Similar to X-ray reflectometry.
Sensitive to structural periodicity.
Also sensitive to magnetic periodicity.
Experiment performed in the neutron reflectometer at the Helmholtz
Zentrum, Berlin.
˚
Neutron wavelength λ = 4.66 A.
Francisco Almeida (IKS) Doctoral dissertation May 24, 2011 8 / 25

12. Polarized Neutron Reflectometry
Results
μ0H = 600 mT μ0H = 200 mT μ0H = 61.8 mT μ0H = 30 mT
R++ (a.u.)
0
10
-1
10
-2
10
-3
10
-4
10
0.00 0.04 0.08 0.12 0.16 0.00 0.04 0.08 0.12 0.16 0.00 0.04 0.08 0.12 0.16 0.00 0.04 0.08 0.12 0.16
-2 QZ (Å-1) QZ (Å-1) QZ (Å-1) QZ (Å-1)
Γ (Å )
-4
1.5x10
-4
1.0x10
-5
5.0x10
0 200 400 600 800 1000 0 200 400 600 800 1000 0 200 400 600 800 1000 0 200 400 600 800 1000
Depth (Å) Depth (Å) Depth (Å) Depth (Å)
Francisco Almeida (IKS) Doctoral dissertation May 24, 2011 9 / 25

13. Polarized Neutron Reflectometry
Spins configuration: surface spin-flop transition
]
[0
01
H
10
μ0H = 600 mT
0
[0
10
]
-1
R++ (a.u.)
10
-2
10
-3
10
-4
10
μ0H = 200 mT
0
10
-1
R++ (a.u.)
10
-2
10
-3
10
-4
10
10
0
μ0H = 61.8 mT
-1
R++ (a.u.)
10
-2
10
-3
10
-4
10
μ0H = 30 mT
0
10
-1
R++ (a.u.)
10
-2
10
-3
10
-4
10
0.00 0.04 0.08 0.12 0.16
QZ (Å-1)
Francisco Almeida (IKS) Doctoral dissertation May 24, 2011 10 / 25

14. Exchange bias in Fe-Pt heterostructures
Francisco Almeida (IKS) Doctoral dissertation May 24, 2011 11 / 25

15. Exchange bias
Example of an oxidized cobalt thin film
1.0
0.5
(a.u.)
0.0
S
M/M
-0.5
-1.0
-0.3 -0.2 -0.1 0.0 0.1 0.2 0.3
H (T)
0
Francisco Almeida (IKS) Doctoral dissertation May 24, 2011 12 / 25

16. Exchange bias
Example of an oxidized cobalt thin film
Effects observed on the hysteresis loop upon field cooling
Loop shift and broadening
1.0 room temperature
10 K
0.5
(a.u.)
0.0
S
M/M
-0.5
-1.0
-0.3 -0.2 -0.1 0.0 0.1 0.2 0.3
H (T)
0
Francisco Almeida (IKS) Doctoral dissertation May 24, 2011 12 / 25

17. Exchange bias
Example of an oxidized cobalt thin film
Effects observed on the hysteresis loop upon field cooling
Loop shift and broadening
Training
1.0 room temperature
10 K (first)
10 K (second)
0.5
(a.u.)
0.0
S
M/M
-0.5
-1.0
-0.3 -0.2 -0.1 0.0 0.1 0.2 0.3
H (T)
0
Francisco Almeida (IKS) Doctoral dissertation May 24, 2011 12 / 25

18. Exchange bias
Motivation
Origin of exchange bias notoriously difficult to identify.
Behavior very varied from system to system.
Many models of limited applicability.
Most applied techniques focus on the ferromagnet.
Francisco Almeida (IKS) Doctoral dissertation May 24, 2011 13 / 25

19. Exchange bias
Motivation
Origin of exchange bias notoriously difficult to identify.
Behavior very varied from system to system.
Many models of limited applicability.
Most applied techniques focus on the ferromagnet.
Looking into the antiferromagnet:
Local information from the antiferromagnet.
M¨ssbauer spectrometry of 57 Fe.
o
Francisco Almeida (IKS) Doctoral dissertation May 24, 2011 13 / 25

20. Exchange bias
Motivation
Origin of exchange bias notoriously difficult to identify.
Behavior very varied from system to system.
Many models of limited applicability.
Most applied techniques focus on the ferromagnet.
Looking into the antiferromagnet:
Local information from the antiferromagnet.
M¨ssbauer spectrometry of 57 Fe.
o
Let us take advantage of M¨ssbauer spectrometry.
o
Francisco Almeida (IKS) Doctoral dissertation May 24, 2011 13 / 25

21. Fe-Pt alloys
FePt3 ( Antiferromagnetic, L12 )
FePt ( Ferromagnetic, L10 )
(Fe: red spheres, Pt: white spheres)
Francisco Almeida (IKS) Doctoral dissertation May 24, 2011 14 / 25

22. Fe-Pt alloys
FePt3 ( Antiferromagnetic, L12 )
Au (capping)
57
FePt 3(h)
FePt (d=100 Å)
MgO (110) (substrate)
FePt ( Ferromagnetic, L10 )
(Fe: red spheres, Pt: white spheres)
Francisco Almeida (IKS) Doctoral dissertation May 24, 2011 14 / 25

23. Magnetism of FePt3
a) T < 160 K: Q1 wave mode, uncompensated (110) surface
b) T < 100 K: Q2 wave mode, compensated (110) surface
S. Maat et al., PRB 63, 134426 (2001)
Francisco Almeida (IKS) Doctoral dissertation May 24, 2011 15 / 25

24. FePt/FePt3 (110) bilayers
Magnetic response
FePt semi-easy axis FePt hard axis
1.0 SSSSSSSSSSSSSS 1.0 SSSSSSSSSSSSSSSSS
SSSSSSSSS SSSSSSSSSSSSSS
SSSSSSSS SSSSSSSSSSSSS
SSSSSSSSSS
0.5 0.5 SSSSSSSS
M/MS (a.u.)
M/MS (a.u.)
SS
0.0 0.0
-0.5 FePt -0.5 FePt
FePt 3
-1.0 H -1.0 FePt 3 // H
-2.0 -1.0 0.0 1.0 2.0 -1.0 0.0 1.0
μ0H (T) μ0H (T)
Francisco Almeida (IKS) Doctoral dissertation May 24, 2011 16 / 25

25. FePt/FePt3 (110) bilayers
Exchange bias (100 ˚ FePt/300 ˚ FePt3 )
A A
Observed using a dual approach:
Volume magnetization information (magnetometry).
Local information (M¨ssbauer from the
o 57 FePt
3 ).
400 HE 1.0
HC 0.5
M/MS (a.u.)
650000
300 0.0
μ 0HC , μ 0HE (mT)
-0.5
yield (a.u.)
200 -1.0
640000
-1.0 -0.5 0.0 0.5 1.0
μ0H (T)
100
630000
0
-100
0 50 100 150 200 250 300 -1 0 1
T (K) v (mm/s)
Francisco Almeida (IKS) Doctoral dissertation May 24, 2011 17 / 25

26. FePt/FePt3 (110) bilayers
Exchange bias (100 ˚ FePt/300 ˚ FePt3 )
A A
What we observe:
Very high roughness and/or intermixing.
Spin-flop coupling between the FePt and the FePt3 .
400 HE 1.0
HC 0.5
M/MS (a.u.)
650000
300 0.0
μ 0HC , μ 0HE (mT)
-0.5
yield (a.u.)
200 -1.0
640000
-1.0 -0.5 0.0 0.5 1.0
μ0H (T)
100
630000
0
-100
0 50 100 150 200 250 300 -1 0 1
T (K) v (mm/s)
Francisco Almeida (IKS) Doctoral dissertation May 24, 2011 17 / 25

27. Off-stoichiometric Fe-Pt film
What happens if we field cool a mixture of Fe0.33 Pt0.67 ?
Francisco Almeida (IKS) Doctoral dissertation May 24, 2011 18 / 25

28. Off-stoichiometric Fe-Pt film
What happens if we field cool a mixture of Fe0.33 Pt0.67 ?
0
625000
(mT)
-10
Yield (a.u.)
E
H
620000
0
-20
SSSSSSSSSSSSSSS
615000
-30
0 20 40 60 80 100 120 140 -5 0 5
T (K) v (mm/s)
Francisco Almeida (IKS) Doctoral dissertation May 24, 2011 18 / 25

29. Exchange bias through ion beam synthesis
Francisco Almeida (IKS) Doctoral dissertation May 24, 2011 19 / 25

30. Ion beam synthesis
New method for obtaining exchange bias.
Francisco Almeida (IKS) Doctoral dissertation May 24, 2011 20 / 25

31. Ion beam synthesis
New method for obtaining exchange bias.
Reactive implantation grows the antiferromagnet inside the
ferromagnet.
Extremely granular structure.
Optimal contact surface.
Parametric control (e.g. fluence, energy).
Francisco Almeida (IKS) Doctoral dissertation May 24, 2011 20 / 25

32. Ion beam synthesis
New method for obtaining exchange bias.
Reactive implantation grows the antiferromagnet inside the
ferromagnet.
Extremely granular structure.
Optimal contact surface.
Parametric control (e.g. fluence, energy).
Synthesis of Cox Oy through implantation of 16 O into Co.
MBE grown, capped cobalt thin films (thicknesses 600 ˚ and 1000 ˚).
A A
Francisco Almeida (IKS) Doctoral dissertation May 24, 2011 20 / 25

33. Exchange bias in oxygen implanted cobalt
Example
Thin cobalt film implanted with 16 O (E = 60 keV, Φ = 1016 cm−2 ).
1.0 a)
0.5
0.0
M/M
-0.5
-1.0
1.0 b)
(a.u.)
0.5
0.0
S
M/M
-0.5
-1.0
-0.4 -0.2 0.0 0.2 0.4
H (T)
0
Francisco Almeida (IKS) Doctoral dissertation May 24, 2011 21 / 25

34. Conclusions
Surface spin-flop in an Fe/Cr superlattice
Surface spin flop like transition detected in a biaxial Fe/Cr
superlattice, along the hard axis.
Demonstrated through Polarized Neutron Reflectometry.
Francisco Almeida (IKS) Doctoral dissertation May 24, 2011 22 / 25

35. Conclusions
Exchange bias in Fe-Pt heterostructures
Exchange bias found in FePt/FePt3 bilayers.
FePt easy axis orthogonal to FePt3 easy axis (spin-flop coupling).
Local information extracted from the antiferromagnet via M¨ssbauer
o
spectrometry.
Exchange bias detected in a single layer of disordered
off-stoichiometric Fe-Pt.
Francisco Almeida (IKS) Doctoral dissertation May 24, 2011 23 / 25

36. Conclusions
Exchange bias in O implanted Co thin films
Exchange bias detected in oxygen implanted Co thin films.
New kind of exchange biased system.
Antiferromagnet is embedded in the ferromagnet.
Heterogeneous effect.
Requires a fluence of the order of at least Φ = 1016 cm−2 .
The effect is sensitive to the fluence as well as the implantation energy.
Post implantation annealing enhances and homogenizes the obtained
effect.
Francisco Almeida (IKS) Doctoral dissertation May 24, 2011 24 / 25

37. Conclusion
Many new research opportunities regarding these new systems:
Exchange bias between Fe-Pt alloys presents new challenges.
Evident direction for further research.
M¨ssbauer techniques give us more flexibility.
o
Exchange bias obtained via ion implantation opens a new field of
research:
Principle may be applied to many other systems besides Co/CoO.
Wealth of technological applications (e.g. patterned implantation).
Theoretical challenge: transition from 2D systems (bilayers) to 3D
systems (implanted films, fully granular structures).
Francisco Almeida (IKS) Doctoral dissertation May 24, 2011 25 / 25