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 interfaceswhere 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
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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
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J. Stahn
4.6
PAUL SCHERRER INSTITUT
&
first findings
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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
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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
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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
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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
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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