Public Doctoral Defense in Nuclear Physics

Exchange springs and exchange bias
studied with nuclear methods

Francisco Miguel Cruz Sim˜es de Almeida
o

Instituut voor Kern-en Stralingsfysica

December 6, 2011

Promotors:
Prof. Dr. Andr´ Vantomme
e
Dr. Johan Meersschaut

Francisco Almeida (IKS) Doctoral dissertation December 6, 2011 1 / 36

Outline

Magnetism and magnetic interactions
Ferromagnetism and antiferromagnetism
Interlayer coupling
Exchange bias

Researched systems
Surface spin-ﬂop in an Fe/Cr superlattice
Exchange bias in Fe-Pt heterostructures
Exchange bias through ion beam synthesis

Conclusions


Magnetism and magnetic interactions Ferromagnetism and antiferromagnetism

Ferromagnets
Ferromagnetic hysteresis
M
S
1.0 M
R

H
0 S
0.5

S
M/M

0.0
H
0 C

-0.5

-1.0

-0.2 -0.1 0.0 0.1 0.2

H (T)
0

Total magnetization (MS ).
Saturation ﬁeld (µ0 HS ).
Remanent magnetization (MR ).
Coercive ﬁeld (µ0 HC ).


Antiferromagnets

Antiferromagnets have no net magnetization.



Magnetic interactions

Between ferromagnetic layers
Interlayer exchange coupling
Between a ferromagnet and an antiferromagnet
Exchange bias


Magnetism and magnetic interactions Interlayer coupling

Interlayer coupling
Coupling between ferromagnetic layers separated by a spacer layer.

Bilinear Biquadratic


Magnetism and magnetic interactions Exchange bias

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



Exchange bias

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



Exchange bias

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


Researched systems Surface spin-flop in Fe/Cr

Surface spin-flop in an Fe/Cr superlattice



Fe/Cr superlattice
Multilayer consisting of 22 repetitions of an Fe/Cr bilayer structure
˚
Fe layers 40 A thick.
Cr layers 11 ˚ thick.
A

Fe
Cr
Fe
Cr
Fe
Cr
Fe
Cr



Fe/Cr superlattice
Multilayer consisting of 22 repetitions of an Fe/Cr bilayer structure
˚
Fe layers 40 A thick and antiferromagnetically coupled.
Cr layers 11 ˚ thick.
A



Fe/Cr superlattice
Magnetic response

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)



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



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?



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.



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)



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)


Researched systems Exchange bias in Fe-Pt alloys




Fe-Pt heterostructures

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





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



¨
Mossbauer spectrometry

Source Absorber
Detector
γ

γ

v γ
e-
X-rays

Resonant absorption of γ radiation through Doppler shift
Very sensitive to small energy level shifts



Fe-Pt alloys
FePt3 ( Antiferromagnetic, L12 )

FePt ( Ferromagnetic, L10 )

(Fe: red spheres, Pt: white spheres)



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)



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)



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)



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)



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)



Off-stoichiometric Fe-Pt film

What happens if we ﬁeld cool a mixture of Fe0.33 Pt0.67 ?



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)

Two thirds of the sample ferromagnetically ordered at room
temperature.


Researched systems Exchange bias through ion implantation

Exchange bias through ion beam synthesis


Researched systems Ion implantation

Ion implantation

Ion source

Target

Mass separation

Isotope selective technique.
Control over implantation energy and amount of ions (ﬂuence).
Gaussian implantation proﬁles.



Ion beam synthesis

New method for obtaining exchange bias.



Ion beam synthesis

Reactive implantation (the antiferromagnet is formed inside the
ferromagnet).
Extremely granular structure.
Optimal contact surface.
Parametric control (e.g. ﬂuence, energy).



Ion beam synthesis

Reactive implantation (the antiferromagnet is formed 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



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



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

How to interprete this magnetic response data?



Loop shape analysis

μ0HE
μ(x) y = r-rμ
μ+3σ
rμ rmax
μ

μ-3σ rmin

μ-3σ μ μ+3σ μ0HC

x

Quantify the hysteresis loop shape based on statistics.
Use a gaussian distributions for the quantities as an approximation.
Mean and variance of exchange bias.
Mean and variance of coercivity.
This gaussian is associated to the implantation proﬁle.



Effect of implantation energy on the exchange bias

1.0 E = 30 keV

E = 45 keV

0.5
S

0.0
M/M

-0.5

-1.0

-0.30 -0.15 0.00 0.15 0.30

H (T)
0



Effect of implantation energy on the exchange bias

120 40
E = 30 keV E = 30 keV

E = 45 keV E = 45 keV

90 30

(mT)
(mT)

C
H
C

0
H

60 20
0

Variance
Mean

30 10

0 0
0 50 100 150 200 250 300 0 50 100 150 200 250 300
25 15
E = 30 keV E = 30 keV

E = 45 keV E = 45 keV

20

(mT)
(mT)

10
15

E
H
E

0
H
0

Variance
10
Mean

5

5

0 0
0 50 100 150 200 250 300 0 50 100 150 200 250 300
T (K) T (K)


Conclusions

Conclusions


Conclusions Surface spin-flop in Fe/Cr

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.


Conclusions Exchange bias in Fe-Pt

Conclusions

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.


Conclusions Exchange bias in Co:O thin films

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.


Conclusions Overall conclusion

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).


Thank you


Public Doctoral Defense in Nuclear Physics

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Public Doctoral Defense in Nuclear Physics