About the Competition of Superconductivity and Ferromagnetism in Multilayers

PAUL SCHERRER INSTITUT & Zurich Jochen Stahn Justin Hoppler Christof Niedermayer Christian Bernhard Laboratory for Neutron Scattering ETH Zurich &...
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PAUL SCHERRER INSTITUT

&

Zurich

Jochen Stahn Justin Hoppler Christof Niedermayer Christian Bernhard

Laboratory for Neutron Scattering ETH Zurich & Paul Scherrer Institut and University Fribourg, FriMat

About the Competition of Superconductivity and Ferromagnetism in Multilayers

Nature Materials, doi:10.1038/nmat2383 Phys. Rev. B 78, 134111 (2008) Phys. Rev. B 71, 140509(R) (2005) FOxE

25.-27. 03. 2009

J. Stahn

1.1

PAUL SCHERRER INSTITUT

&

motivation

Zurich

what happens at interfaceswhere electronic    chemical properties do not match? crystallographic   magnetic  SC and magnetism avoid each other — unless forced together on an atomic scale

⇒ how do they arrange?

used system: multilayers of the type [SC/FM]n /STO grown by pulsed laser doposition

TEM image FOxE

25.-27. 03. 2009

J. Stahn

2.1

PAUL SCHERRER INSTITUT

&

the samples

Zurich

multilayers of the type

[SC/FM]n /STO

FM: La2/3Ca1/3MnO3 TCurie ≈ 180 K

Cu Ba

B

O

B B

B

B B

B

TCurieB

J. Stahn

AFM

90 K (x = 0)

B B

B B

Mn crystal types: (close to) perovskite-like

c→t

B B

pseudogap B

25.-27. 03. 2009

TCurie

SC: Y1−x Prx Ba2Cu3O6 Tc ≈ 40 K (x = 0.4), T

FOxE

6

STO: SrTiO3 used as substrate T ≈ 105 K: cubic to tetragonal T ≈ 65 K: tetragonal to orthorombic ⇒ surface fragmentation

Y Pr

La Ca

T

B

BN



 





 





 

SC

 



t→o

Tc

Tc FL

0

carrier concentration in CuO planes 2.2

PAUL SCHERRER INSTITUT

&

the question

Zurich

T

how does the magnetisation in the film look like?

6

TCurie

depth profile of magnetic induction: B(z ) has SC an influence? ⇒ T -dependence of B(z )

⇒ need for a method to probe B(z ) and ρ(z ) – with B(z ) ρ (z )

z

– in the range – in a magnetic field

c→t

0 < z < 2000 ˚ A A ∆z ≈ 1 ˚ 10 K < T < 200 K H < 1000 Oe

t→o

Tc

→ polarised neutron reflectometry 0 FOxE

25.-27. 03. 2009

J. Stahn

3.1

PAUL SCHERRER INSTITUT

&

polarised neutron reflectometry

Zurich

index of refraction n (as for visible light: |n − 1| = |δ| < 10−5 δ = δnuclear ± δmagnetic δmagnetic ∝ µ n B⊥ neutron magnetic moment: µ n in-plane magnetic induction: B⊥

)

measured quantity: intensity vs. normal momentum transfer qz qz z

δ = δn ± δm period

for parallel interfaces: interference of (multiply) reflected beams

substrate

FOxE

25.-27. 03. 2009

J. Stahn

4.1

PAUL SCHERRER INSTITUT

&

polarised neutron reflectometry

Zurich

index of refraction n (as for visible light: |n − 1| = |δ| < 10−5 δ = δnuclear ± δmagnetic δmagnetic ∝ µ n B⊥ neutron magnetic moment: µ n in-plane magnetic induction: B⊥

)

measured quantity: intensity vs. normal momentum transfer qz qz z

angle dispersive mode

δ = δn ± δm period

substrate

FOxE

25.-27. 03. 2009

J. Stahn

4.2

PAUL SCHERRER INSTITUT

&

polarised neutron reflectometry

Zurich

index of refraction n (as for visible light: |n − 1| = |δ| < 10−5 δ = δnuclear ± δmagnetic δmagnetic ∝ µ n B⊥ neutron magnetic moment: µ n in-plane magnetic induction: B⊥

)

measured quantity: intensity vs. normal momentum transfer qz qz z

energy dispersive mode

δ = δn ± δm period

substrate

FOxE

25.-27. 03. 2009

J. Stahn

4.3

PAUL SCHERRER INSTITUT

&

polarised neutron reflectometry

Zurich

neutron reflectometer AMOR at SINQ, PSI time-of-flight spin polarisation

measured quantity: intensity vs. normal momentum transfer qz qz z

energy dispersive mode

δ = δn ± δm period

substrate

FOxE

25.-27. 03. 2009

J. Stahn

4.4

PAUL SCHERRER INSTITUT

&

polarised neutron reflectometry

Zurich

reflectivity R

1 0.8 0.6 0.4 0.2 0 0

0.01

0.02

0.03

0.04 −1

qz /˚ A

0.05

0.06

substrate

measured quantity: intensity vs. normal momentum transfer qz qz z

energy dispersive mode

δ = δn ± δm period

substrate

FOxE

25.-27. 03. 2009

J. Stahn

4.5

PAUL SCHERRER INSTITUT

&

polarised neutron reflectometry

Zurich

reflectivity R

1 0.8 0.6 0.4 0.2 0 0

0.01

0.02

0.03

0.04 −1

qz /˚ A

0.05

0.06

substrate

measured quantity: intensity vs. normal momentum transfer qz qz z

energy dispersive mode

δ = δn ± δm period

substrate

FOxE

25.-27. 03. 2009

J. Stahn

4.6

PAUL SCHERRER INSTITUT

&

first findings

Zurich

0

field cooled and measured in H = 100 Oe

reflectivity log10 R (ω)

T dependence of R(ω) for an ML with underdoped SC

185 K 165 K 145 K 125 K 105 K 85 K 65 K 55 K 45 K 35 K 25 K 15 K 8K

-1

-2

-3

T

6

TCurie

c→t

-4

-5 0

0.5

1

1.5 ω

2

2.5

3

3.5 t→o Tc

ω

0

FOxE

25.-27. 03. 2009

J. Stahn

5.1

PAUL SCHERRER INSTITUT

&

first findings

Zurich

0

new peak below Tc

reflectivity log10 R (ω)

T dependence of R(ω) for an ML with underdoped SC

1st Bragg peak displays increasing B

185 K 165 K 145 K 125 K 105 K 85 K 65 K 55 K 45 K 35 K 25 K 15 K 8K

-1

-2

-3

T

6

TCurie

c→t

-4

-5 0

0.5

1

1.5 ω

2

2.5

3

3.5 t→o Tc

appearance of 2nd Bragg peak magnetic screening in YPBCO below TCurie

ω

0

FOxE

25.-27. 03. 2009

J. Stahn

5.2

PAUL SCHERRER INSTITUT

&

first findings

Zurich

0

reflectivity log10 R (ω)

T dependence of R(ω) for an ML with underdoped SC

new peak below Tc 2.5

185 K 165 K 145 K 125 K 105 K 85 K 65 K 55 K 45 K 35 K 25 K 15 K 8K

-1

-2

-3

T

6

TCurie

c→t

-4

2

-5 Tc

1.5

0

0.5

1

1.5 ω

2

2.5

3

3.5 t→o Tc

?

1 0

20

40

T /K

FOxE

25.-27. 03. 2009

J. Stahn

60

80

0

5.3

PAUL SCHERRER INSTITUT

&

Zurich

interpretation: modulation of B T

magnetic peak comparable to a fractional Bragg peak in diffraction indication for a (magnetic) superstructure

model assumpton:

YPBCO z 6

TCurie

LCMO

Tc < T < TCurie all LCMO layers have the same B = B0

c→t

-

T < Tc B = B0 ± ∆B where sign changes each period ⇒ layerwise AFM on top of the FM

z

FOxE

25.-27. 03. 2009

J. Stahn

B ⊥ (z )

6

t→o

Tc

-

respective moments on Mn: 2.1 ± 1.9 µB

6

B ⊥ (z )

0

6.1

PAUL SCHERRER INSTITUT

&

Zurich

influence of the substrate T

STO undergoes phase transitions

6

TCurie

⇒ twinning, buckling of the surface ⇒ surface is fragmented into facets ⇒ varying angle of incidence over the sample ⇒ lots of specularly reflected beams

c→t

t→o

Tc

0

FOxE

25.-27. 03. 2009

J. Stahn

7.1

PAUL SCHERRER INSTITUT

&

Zurich

influence of the substrate T

6

scattering angle 2θ

TCurie

c→t wavelength (2 to 10 ˚ A)

area detector to cover large angular range t→o

Tc

2θ 0

FOxE

25.-27. 03. 2009

J. Stahn

7.2

PAUL SCHERRER INSTITUT

&

Zurich

influence of the substrate T

6

scattering angle 2θ

TCurie

c→t wavelength (2 to 10 ˚ A)

magnetic superlattice peak appears only • below Tc • on some of the surface facets • when uniaxial in-plane pressure is applied to the substrate ⇒ alignment of domains?

FOxE

25.-27. 03. 2009

J. Stahn

t→o

Tc

0

7.3

PAUL SCHERRER INSTITUT

&

interpretation

Zurich

• LCMO has a complicated phase diagram and shows phase separation of structural and magnetic properties   strain  finite dimension in z coupling to neighboring FM layers 

might change the energies of competing magnetic states

• the changed coupling through YPBCO in the (energetically weak) SC state can then switch the ground state in the FM

• the SC gains surface energy

FOxE

25.-27. 03. 2009

J. Stahn





if he is strained



he can win!

8.1

PAUL SCHERRER INSTITUT

&

use E for p

Zurich

STO shows eletrostriction

⇒ strain is induced by E

(lattice is distorted by an external E field)

(and not by uniaxial pressure)

first result with E = 160 V/mm: 0

T = 120 K T = 50 K

log10 [R (ω)]

T = 15 K -1

-2

-3

0.4

0.6

0.8

1

1.2

ω

can we switch ∆B with E?

FOxE

25.-27. 03. 2009

J. Stahn

9.1

PAUL SCHERRER INSTITUT

&

acknowledgments

Zurich

sample preparation:

Hanns-Ulrich Habermeier (MPI Stuttgart) Georg Cristiani (MPI Stuttgart)

experiments: Justin Hoppler Max Wolff Helmut Fritsche Rob Dalgliesh Vivek Malik Alan Drew . . . with E -field: analysis:

audience:

FOxE

25.-27. 03. 2009

J. Stahn

(PSI, Fribourg) (ADAM, ILL) (Chalk River, Canada) (ISIS) (Fribourg) (Fribourg)

Cecile Garcia

(ETHZ, PSI)

Christian Bernhard Christof Niedermayer Alexandre Buzdin

(Fribourg) (PSI) (Amiens, France)

YOU 10.1

PAUL SCHERRER INSTITUT

&

Zurich

conclusion

• PNR can probe ρ(z ) and B⊥(z ) with almost atomic resolution • samples: [Y1−x Prx Ba2Cu3O6/La2/3Ca1/3MnO3]10/SrTiO3 • FM layers are aligned parallel • exception: in strained films below Tc a modulation is initated by SC spacer • hypothetical explanation: – strain lowers energy of modulated FM states – gain in surface energy in SC is enough to switch the ground state in FM • ”normal” case: energy scale of FM is much larger than of SC ⇒ competition normally below 1K • here: 40K FOxE

25.-27. 03. 2009

J. Stahn

11.1

PAUL SCHERRER INSTITUT

phase diagram of the entire sample

Zurich

T

6

STO

LCMO PM

TCurie

modulated FM in LCMO only with strained STO

c PM

FM

c→t t

PM FM SC c t o

paramagnetic ferromagnetic superconducting cubic tetragonal orthorhombic

YPBCO

t→o

Tc

o

metal

FM screening

&

FM with modulation

SC

0

FOxE

25.-27. 03. 2009

J. Stahn

12.1

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