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Exchange springs and exchange biasstudied with nuclear methods
Francisco Miguel Cruz Simoes de Almeida
Instituut voor Kern-en Stralingsfysica
December 6, 2011
Promotors:Prof. Dr. Andre Vantomme
Dr. Johan Meersschaut
Francisco Almeida (IKS) Doctoral dissertation December 6, 2011 1 / 36
Outline
Magnetism and magnetic interactionsFerromagnetism and antiferromagnetismInterlayer couplingExchange bias
Researched systemsSurface spin-flop in an Fe/Cr superlatticeExchange bias in Fe-Pt heterostructuresExchange bias through ion beam synthesis
Conclusions
Francisco Almeida (IKS) Doctoral dissertation December 6, 2011 2 / 36
Magnetism and magnetic interactions
Magnetism and magnetic interactions
Francisco Almeida (IKS) Doctoral dissertation December 6, 2011 3 / 36
Magnetism and magnetic interactions Ferromagnetism and antiferromagnetism
FerromagnetsFerromagnetic hysteresis
-0.2 -0.1 0.0 0.1 0.2
-1.0
-0.5
0.0
0.5
1.0
M/M
S
0H (T)
MR
0HC
MS
0HS
I Total magnetization (MS).I Saturation field (µ0HS).I Remanent magnetization (MR).I Coercive field (µ0HC ).
Francisco Almeida (IKS) Doctoral dissertation December 6, 2011 4 / 36
Magnetism and magnetic interactions Ferromagnetism and antiferromagnetism
Antiferromagnets
Antiferromagnets have no net magnetization.
Francisco Almeida (IKS) Doctoral dissertation December 6, 2011 5 / 36
Magnetism and magnetic interactions Ferromagnetism and antiferromagnetism
Magnetic interactions
I Between ferromagnetic layersI Interlayer exchange coupling
I Between a ferromagnet and an antiferromagnetI Exchange bias
Francisco Almeida (IKS) Doctoral dissertation December 6, 2011 6 / 36
Magnetism and magnetic interactions Interlayer coupling
Interlayer coupling
Coupling between ferromagnetic layers separated by a spacer layer.
Bilinear Biquadratic
Francisco Almeida (IKS) Doctoral dissertation December 6, 2011 7 / 36
Magnetism and magnetic interactions Exchange bias
Exchange biasExample of an oxidized cobalt thin film
Effects observed on the hysteresis loop upon field cooling
I Loop shift and broadening
I Training
-0.3 -0.2 -0.1 0.0 0.1 0.2 0.3
-1.0
-0.5
0.0
0.5
1.0
M/M
S (a
.u.)
0H (T)
Francisco Almeida (IKS) Doctoral dissertation December 6, 2011 8 / 36
Magnetism and magnetic interactions Exchange bias
Exchange biasExample of an oxidized cobalt thin film
Effects observed on the hysteresis loop upon field cooling
I Loop shift and broadening
I Training
-0.3 -0.2 -0.1 0.0 0.1 0.2 0.3
-1.0
-0.5
0.0
0.5
1.0 room temperature 10 K
M/M
S (a
.u.)
0H (T)
Francisco Almeida (IKS) Doctoral dissertation December 6, 2011 8 / 36
Magnetism and magnetic interactions Exchange bias
Exchange biasExample of an oxidized cobalt thin film
Effects observed on the hysteresis loop upon field cooling
I Loop shift and broadening
I Training
-0.3 -0.2 -0.1 0.0 0.1 0.2 0.3
-1.0
-0.5
0.0
0.5
1.0 room temperature 10 K (first) 10 K (second)
M/M
S (a
.u.)
0H (T)
Francisco Almeida (IKS) Doctoral dissertation December 6, 2011 8 / 36
Researched systems Surface spin-flop in Fe/Cr
Surface spin-flop in an Fe/Cr superlattice
Francisco Almeida (IKS) Doctoral dissertation December 6, 2011 9 / 36
Researched systems Surface spin-flop in Fe/Cr
Fe/Cr superlattice
Multilayer consisting of 22 repetitions of an Fe/Cr bilayer structure
I Fe layers 40 A thick.
I Cr layers 11 A thick.
Fe
Cr
Fe
Cr
Fe
Cr
Fe
Cr
Francisco Almeida (IKS) Doctoral dissertation December 6, 2011 10 / 36
Researched systems Surface spin-flop in Fe/Cr
Fe/Cr superlattice
Multilayer consisting of 22 repetitions of an Fe/Cr bilayer structure
I Fe layers 40 A thick and antiferromagnetically coupled.
I Cr layers 11 A thick.
Francisco Almeida (IKS) Doctoral dissertation December 6, 2011 10 / 36
Researched systems Surface spin-flop in Fe/Cr
Fe/Cr superlatticeMagnetic response
Easy axis
M/M
s0.0
0.2
0.4
0.6
0.8
1.0
25 mT
250 mT
[001]
[010]
H
a
Hard axis
µ0H (T)
M/M
s
0.0 0.1 0.2 0.3 0.4 0.5 0.6
0.0
0.2
0.4
0.6
0.8
1.0
25 mT
75 mT
[001
][010]
H
b
J. Meersschaut et al., PRB 73, 144428 (2006)
Francisco Almeida (IKS) Doctoral dissertation December 6, 2011 11 / 36
Researched systems Surface spin-flop in Fe/Cr
Fe/Cr superlatticeMagnetic response
Easy axis
M/M
s0.0
0.2
0.4
0.6
0.8
1.0
25 mT
250 mT
[001]
[010]
H
a
Hard axis
µ0H (T)
M/M
s
0.0 0.1 0.2 0.3 0.4 0.5 0.6
0.0
0.2
0.4
0.6
0.8
1.0
25 mT
75 mT
[001
][010]
H
b
J. Meersschaut et al., PRB 73, 144428 (2006)
Francisco Almeida (IKS) Doctoral dissertation December 6, 2011 11 / 36
Researched systems Surface spin-flop in Fe/Cr
Surface spin-flopTransition (H
(S)C ) that causes a top spin/layer to switch orientation before the
remaining spins/layers of an antiferromagnet. The irregularity propagates to the
middle of the spin chain, and leads to a bulk spin-flop (H(B)C ).
H(S)C : surface spin-flop field
H(B)C : bulk spin-flop field
How to visualize a surface spin-flop?
Francisco Almeida (IKS) Doctoral dissertation December 6, 2011 12 / 36
Researched systems Surface spin-flop in Fe/Cr
Surface spin-flopTransition (H
(S)C ) that causes a top spin/layer to switch orientation before the
remaining spins/layers of an antiferromagnet. The irregularity propagates to the
middle of the spin chain, and leads to a bulk spin-flop (H(B)C ).
H(S)C : surface spin-flop field
H(B)C : bulk spin-flop field
How to visualize a surface spin-flop?
Francisco Almeida (IKS) Doctoral dissertation December 6, 2011 12 / 36
Researched systems Surface spin-flop in Fe/Cr
Polarized Neutron Reflectometry
Similar to X-ray reflectometry.
I Sensitive to structural periodicity.
I Also sensitive to magnetic periodicity.
Experiment performed in the neutron reflectometer at the HelmholtzZentrum, Berlin.
I Neutron wavelength λ = 4.66 A.
Francisco Almeida (IKS) Doctoral dissertation December 6, 2011 13 / 36
Researched systems Surface spin-flop in Fe/Cr
Polarized Neutron ReflectometryResults
R++
(a.u.)
10
10
10
10
10
-4
-3
-2
-1
0
5.0x10-5
1.0x10-4
1.5x10-4
Γ ( )-2
Å
0 200 400 600 800 1000Depth (Å)
0.00 0.04 0.08 0.12 0.16
QZ (
-1)Å
0 200 400 600 800 1000Depth (Å)
0.00 0.04 0.08 0.12 0.16
QZ (
-1)Å
0 200 400 600 800 1000
Depth (Å)
0.00 0.04 0.08 0.12 0.16
QZ (
-1)Å
0.00 0.04 0.08 0.12 0.16
QZ (
-1)Å
0 200 400 600 800 1000
Depth (Å)
μ H = 600 mT 0 μ H = 200 mT 0 μ H = 61.8 mT 0 μ H = 30 mT 0
Francisco Almeida (IKS) Doctoral dissertation December 6, 2011 14 / 36
Researched systems Surface spin-flop in Fe/Cr
Polarized Neutron ReflectometrySpins configuration: surface spin-flop transition
10-4
10-3
10-2
10-1
100
R+
+ (
a.u
.)
0.00 0.04 0.08 0.12 0.16
QZ (
-1)Å
10-4
10-3
10-2
10-1
100
R+
+ (
a.u
.)
10-4
10-3
10-2
10-1
100
R+
+ (
a.u
.)
10-4
10-3
10-2
10-1
100
R+
+ (
a.u
.)
[001
][010] Hμ H = 600 mT 0
μ H = 200 mT 0
μ H = 61.8 mT 0
μ H = 30 mT 0
Francisco Almeida (IKS) Doctoral dissertation December 6, 2011 15 / 36
Researched systems Exchange bias in Fe-Pt alloys
Exchange bias in Fe-Pt heterostructures
Francisco Almeida (IKS) Doctoral dissertation December 6, 2011 16 / 36
Researched systems Exchange bias in Fe-Pt alloys
Fe-Pt heterostructures
I Origin of exchange bias notoriously difficult to identify.I Behavior very varied from system to system.I Many models of limited applicability.I Most applied techniques focus on the ferromagnet.
I Looking into the antiferromagnet:I Local information from the antiferromagnet.I Mossbauer spectrometry of 57Fe.
Let us take advantage of Mossbauer spectrometry.
Francisco Almeida (IKS) Doctoral dissertation December 6, 2011 17 / 36
Researched systems Exchange bias in Fe-Pt alloys
Fe-Pt heterostructures
I Origin of exchange bias notoriously difficult to identify.I Behavior very varied from system to system.I Many models of limited applicability.I Most applied techniques focus on the ferromagnet.
I Looking into the antiferromagnet:I Local information from the antiferromagnet.I Mossbauer spectrometry of 57Fe.
Let us take advantage of Mossbauer spectrometry.
Francisco Almeida (IKS) Doctoral dissertation December 6, 2011 17 / 36
Researched systems Exchange bias in Fe-Pt alloys
Fe-Pt heterostructures
I Origin of exchange bias notoriously difficult to identify.I Behavior very varied from system to system.I Many models of limited applicability.I Most applied techniques focus on the ferromagnet.
I Looking into the antiferromagnet:I Local information from the antiferromagnet.I Mossbauer spectrometry of 57Fe.
Let us take advantage of Mossbauer spectrometry.
Francisco Almeida (IKS) Doctoral dissertation December 6, 2011 17 / 36
Researched systems Exchange bias in Fe-Pt alloys
Mossbauer spectrometry
Source AbsorberDetector
γ
v
γ
γ
X-rayse-
I Resonant absorption of γ radiation through Doppler shift
I Very sensitive to small energy level shifts
Francisco Almeida (IKS) Doctoral dissertation December 6, 2011 18 / 36
Researched systems Exchange bias in Fe-Pt alloys
Fe-Pt alloys
I FePt3 ( Antiferromagnetic, L12 )
FePt (d=100 Å)
MgO (110) (substrate)
Au (capping)
FePt (h)357
I FePt ( Ferromagnetic, L10 )
(Fe: red spheres, Pt: white spheres)
Francisco Almeida (IKS) Doctoral dissertation December 6, 2011 19 / 36
Researched systems Exchange bias in Fe-Pt alloys
Fe-Pt alloys
I FePt3 ( Antiferromagnetic, L12 )
FePt (d=100 Å)
MgO (110) (substrate)
Au (capping)
FePt (h)357
I FePt ( Ferromagnetic, L10 )
(Fe: red spheres, Pt: white spheres)
Francisco Almeida (IKS) Doctoral dissertation December 6, 2011 19 / 36
Researched systems Exchange bias in Fe-Pt alloys
Magnetism of FePt3
a) T < 160 K: Q1 wave mode, uncompensated (110) surfaceb) T < 100 K: Q2 wave mode, compensated (110) surface
S. Maat et al., PRB 63, 134426 (2001)
Francisco Almeida (IKS) Doctoral dissertation December 6, 2011 20 / 36
Researched systems Exchange bias in Fe-Pt alloys
FePt/FePt3 (110) bilayersMagnetic response
FePt semi-easy axis FePt hard axis
FePt
H
FePt3
-2.0 -1.0 0.0 1.0 2.0
-1.0
-0.5
0.0
0.5
1.0
M/M
S (
a.u
.)
0H (T)
SSSSSSSSSSSSSSSSSSSSSSSSSSSSSSS
μ
FePt
// HFePt3
-1.0 0.0 1.0
-1.0
-0.5
0.0
0.5
1.0
M/M
S (
a.u
.)
0H (T)
SSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSS
μ
Francisco Almeida (IKS) Doctoral dissertation December 6, 2011 21 / 36
Researched systems Exchange bias in Fe-Pt alloys
FePt/FePt3 (110) bilayersExchange bias (100 A FePt/300 A FePt3)
Observed using a dual approach:
I Volume magnetization information (magnetometry).
I Local information (Mossbauer from the 57FePt3).
-100
0
100
200
300
400 HE
HC
-1.0 -0.5 0.0 0.5 1.0
-1.0
-0.5
0.0
0.5
1.0
M/M
S (
a.u
.)
0H (T)
μ H
,
μ H
(m
T)
00
CE
μ
0 50 100 150 200 250 300
T (K) v (mm/s)
yiel
d (a
.u.)
-1 0 1
630000
640000
650000
Francisco Almeida (IKS) Doctoral dissertation December 6, 2011 22 / 36
Researched systems Exchange bias in Fe-Pt alloys
FePt/FePt3 (110) bilayersExchange bias (100 A FePt/300 A FePt3)
What we observe:
I Very high roughness and/or intermixing.
I Spin-flop coupling between the FePt and the FePt3.
-100
0
100
200
300
400 HE
HC
-1.0 -0.5 0.0 0.5 1.0
-1.0
-0.5
0.0
0.5
1.0
M/M
S (
a.u
.)
0H (T)
μ H
,
μ H
(m
T)
00
CE
μ
0 50 100 150 200 250 300
T (K) v (mm/s)
yiel
d (a
.u.)
-1 0 1
630000
640000
650000
Francisco Almeida (IKS) Doctoral dissertation December 6, 2011 22 / 36
Researched systems Exchange bias in Fe-Pt alloys
Off-stoichiometric Fe-Pt film
What happens if we field cool a mixture of Fe0.33Pt0.67?
0 20 40 60 80 100 120 140
-30
-20
-10
0
0HE (m
T)
T (K)
SSSSSSSSSSSSSSS
v (mm/s)
Yie
ld (
a.u.
)
-5 0 5
615000
620000
625000
I Two thirds of the sample ferromagnetically ordered at roomtemperature.
Francisco Almeida (IKS) Doctoral dissertation December 6, 2011 23 / 36
Researched systems Exchange bias in Fe-Pt alloys
Off-stoichiometric Fe-Pt film
What happens if we field cool a mixture of Fe0.33Pt0.67?
0 20 40 60 80 100 120 140
-30
-20
-10
0
0HE (m
T)
T (K)
SSSSSSSSSSSSSSS
v (mm/s)
Y
ield
(a.
u.)
-5 0 5
615000
620000
625000
I Two thirds of the sample ferromagnetically ordered at roomtemperature.
Francisco Almeida (IKS) Doctoral dissertation December 6, 2011 23 / 36
Researched systems Exchange bias through ion implantation
Exchange bias through ion beam synthesis
Francisco Almeida (IKS) Doctoral dissertation December 6, 2011 24 / 36
Researched systems Ion implantation
Ion implantation
Target
Ion source
Mass separation
I Isotope selective technique.
I Control over implantation energy and amount of ions (fluence).
I Gaussian implantation profiles.
Francisco Almeida (IKS) Doctoral dissertation December 6, 2011 25 / 36
Researched systems Ion implantation
Ion beam synthesis
I New method for obtaining exchange bias.
I Reactive implantation (the antiferromagnet is formed inside theferromagnet).
I Extremely granular structure.I Optimal contact surface.I Parametric control (e.g. fluence, energy).
I Synthesis of CoxOy through implantation of 16O into Co.I MBE grown, capped cobalt thin films (thicknesses 600 A and 1000 A).
Francisco Almeida (IKS) Doctoral dissertation December 6, 2011 26 / 36
Researched systems Ion implantation
Ion beam synthesis
I New method for obtaining exchange bias.I Reactive implantation (the antiferromagnet is formed inside the
ferromagnet).I Extremely granular structure.I Optimal contact surface.I Parametric control (e.g. fluence, energy).
I Synthesis of CoxOy through implantation of 16O into Co.I MBE grown, capped cobalt thin films (thicknesses 600 A and 1000 A).
Francisco Almeida (IKS) Doctoral dissertation December 6, 2011 26 / 36
Researched systems Ion implantation
Ion beam synthesis
I New method for obtaining exchange bias.I Reactive implantation (the antiferromagnet is formed inside the
ferromagnet).I Extremely granular structure.I Optimal contact surface.I Parametric control (e.g. fluence, energy).
I Synthesis of CoxOy through implantation of 16O into Co.I MBE grown, capped cobalt thin films (thicknesses 600 A and 1000 A).
Francisco Almeida (IKS) Doctoral dissertation December 6, 2011 26 / 36
Researched systems Ion implantation
Exchange bias in oxygen implanted cobaltExample
Thin cobalt film implanted with 16O (E = 60 keV, Φ = 1016cm−2).
-0.4 -0.2 0.0 0.2 0.4
-1.0
-0.5
0.0
0.5
1.0 b)
M/M
S (a
.u.)
0H (T)
-1.0
-0.5
0.0
0.5
1.0 a)
M/M
How to interprete this magnetic response data?
Francisco Almeida (IKS) Doctoral dissertation December 6, 2011 27 / 36
Researched systems Ion implantation
Exchange bias in oxygen implanted cobaltExample
Thin cobalt film implanted with 16O (E = 60 keV, Φ = 1016cm−2).
-0.4 -0.2 0.0 0.2 0.4
-1.0
-0.5
0.0
0.5
1.0 b)
M/M
S (a
.u.)
0H (T)
-1.0
-0.5
0.0
0.5
1.0 a)
M/M
How to interprete this magnetic response data?
Francisco Almeida (IKS) Doctoral dissertation December 6, 2011 27 / 36
Researched systems Ion implantation
Exchange bias in oxygen implanted cobaltLoop shape analysis
μ μ+3σμ-3σ
μ-3σ
μ
μ+3σ
μ H0 C
μ H0 E
rmin
rmaxrμ
y = r-rμ
x
μ(x)
Quantify the hysteresis loop shape based on statistics.I Use a gaussian distributions for the quantities as an approximation.
I Mean and variance of exchange bias.I Mean and variance of coercivity.
I This gaussian is associated to the implantation profile.
Francisco Almeida (IKS) Doctoral dissertation December 6, 2011 28 / 36
Researched systems Ion implantation
Exchange bias in oxygen implanted cobaltEffect of implantation energy on the exchange bias
-0.30 -0.15 0.00 0.15 0.30
-1.0
-0.5
0.0
0.5
1.0 E = 30 keV E = 45 keV
M/M
S
0H (T)
Francisco Almeida (IKS) Doctoral dissertation December 6, 2011 29 / 36
Researched systems Ion implantation
Exchange bias in oxygen implanted cobaltEffect of implantation energy on the exchange bias
0 50 100 150 200 250 3000
30
60
90
120 E = 30 keV E = 45 keV
Mea
n 0H
C (m
T)
0 50 100 150 200 250 3000
10
20
30
40 E = 30 keV E = 45 keV
Var
ianc
e 0H
C (m
T)
0 50 100 150 200 250 3000
5
10
15
20
25 E = 30 keV E = 45 keV
Mea
n 0H
E (m
T)
T (K)0 50 100 150 200 250 300
0
5
10
15 E = 30 keV E = 45 keV
Var
ianc
e 0H
E (m
T)
T (K)
Francisco Almeida (IKS) Doctoral dissertation December 6, 2011 30 / 36
Conclusions
Conclusions
Francisco Almeida (IKS) Doctoral dissertation December 6, 2011 31 / 36
Conclusions Surface spin-flop in Fe/Cr
ConclusionsSurface spin-flop in an Fe/Cr superlattice
I Surface spin flop like transition detected in a biaxial Fe/Crsuperlattice, along the hard axis.
I Demonstrated through Polarized Neutron Reflectometry.
Francisco Almeida (IKS) Doctoral dissertation December 6, 2011 32 / 36
Conclusions Exchange bias in Fe-Pt
ConclusionsExchange bias in Fe-Pt heterostructures
I Exchange bias found in FePt/FePt3 bilayers.I FePt easy axis orthogonal to FePt3 easy axis (spin-flop coupling).I Local information extracted from the antiferromagnet via Mossbauer
spectrometry.
I Exchange bias detected in a single layer of disorderedoff-stoichiometric Fe-Pt.
Francisco Almeida (IKS) Doctoral dissertation December 6, 2011 33 / 36
Conclusions Exchange bias in Co:O thin films
ConclusionsExchange bias in O implanted Co thin films
I Exchange bias detected in oxygen implanted Co thin films.I New kind of exchange biased system.
I Antiferromagnet is embedded in the ferromagnet.I Heterogeneous effect.
I Requires a fluence of the order of at least Φ = 1016 cm−2.I The effect is sensitive to the fluence as well as the implantation energy.I Post implantation annealing enhances and homogenizes the obtained
effect.
Francisco Almeida (IKS) Doctoral dissertation December 6, 2011 34 / 36
Conclusions Overall conclusion
Conclusion
I Many new research opportunities regarding these new systems:
I Exchange bias between Fe-Pt alloys presents new challenges.I Evident direction for further research.I Mossbauer techniques give us more flexibility.
I Exchange bias obtained via ion implantation opens a new field ofresearch:
I Principle may be applied to many other systems besides Co/CoO.I Wealth of technological applications (e.g. patterned implantation).I Theoretical challenge: transition from 2D systems (bilayers) to 3D
systems (implanted films, fully granular structures).
Francisco Almeida (IKS) Doctoral dissertation December 6, 2011 35 / 36
Thank you
Francisco Almeida (IKS) Doctoral dissertation December 6, 2011 36 / 36