Public Doctoral Defense in Nuclear Physics

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


Magnetism and magnetic interactions

Magnetism and magnetic interactions


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 ).



Antiferromagnets

Antiferromagnets have no net magnetization.



Magnetic interactions

I Between ferromagnetic layersI Interlayer exchange coupling

I Between a ferromagnet and an antiferromagnetI 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 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)






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)






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)


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

I Fe layers 40 A thick.

I Cr layers 11 A thick.

Fe

Cr

Fe

Cr

Fe

Cr

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.



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)



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?



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.



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



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


Researched systems Exchange bias in Fe-Pt alloys

Exchange bias in Fe-Pt heterostructures



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.



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



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)



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)



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



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



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



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.



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.


Researched systems Exchange bias through ion implantation

Exchange bias through ion beam synthesis


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.



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).



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).



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?



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.



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)



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)


Conclusions

Conclusions


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.


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.


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.


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).


Thank you


Public Doctoral Defense in Nuclear Physics

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