Proximity and domain wall effects in superconductor ferromagnet hybrids

1
Proximity and domain wall effects in
superconductor / ferromagnet hybrids A. Rusanov,
M. Hesselberth, J. Aarts, Kamerlingh Onnes
Laboratory, Leiden. collaborations with A.
Golubov, A. Buzdin, M. Aprili.

• ?-junctions and spin switches
• effects of magnetic domains in Nb / Py
• on the depairing current Jdp
• Jdp of Pd89Ni11 / Nb for the 0 p transition

2
S / F hybrids two basic phenomena
1. ?- junctions Cooper pair in exchange field
hex Oscillating order prm

• 2??F
• ?F (hD/hex)1/2

Tc oscillations, Jos.- junctions, ?-SQUIDs
Use weak magnets in order to have large ?F
S / F / S , with CuNi
S / I / F / S , with PdNi
3

• spin switch Theory
• inhomogeneous exchange field hex
• for instance via F / S / F

infinite F-banks
Baladié, Buzdin
Tagirov
AP
P
Large ds
d / ds
using oscillations in F-banks
effective length
4

• spin switch Experiments
• Gu / Bader (PRL, dec 02)

sample dimensions unknown no hysteresis loop ?
RP,AP - 300 Oe small effect ( 5 mK) bilayer
check ?
5
One more experiment Nb / Co bilayer
Ic near Tc
Ic far below Tc
M(H) above Tc
Kinsey / Blamire IEEE Trans Appl. Sup. (2001)

• A domain wall is also a realization of
  inhomogeneous he
• in-plane spin switch ?

6
S / F bilayers Nb / Ni80Fe20 (Py)
sputter-deposited
Nb TcNb ? 9 K dS 15 nm - 30 nm Py P ?
45 ?sat ? 1 ?B / at dF 20 nm, Néel
walls (?) in-plane rotation or 50
nm, Bloch walls out-of-plane
rotation easy axis defined at growth
Sharp loops, (very low Hc)
7
Samples 0.5 mm x 4 mm - large 2 ?m
x 20 ?m - small mostly contacts Au, to
minimize stray fields
large sample s / Py(20) / Nb (25) AMR (two
samples slightly different R)
8
s / Py (20) / Nb (20) - large sample
(deep) dips in R(T) where the Py-layer
switches, width no more than 1 mT
9
s / Nb (20) / Py (20) - large sample
sharp transition 10 ? in 40 mK

• dips always present, slightly varying switching
  fields

10
Difference in the transition between 0-field and
dip-field

• R(T, 6,3 mT) below
• R(T, 0 mT)
• same order of magnitude
• as for spin switch

Tc
Due to domains ? Some checks
11
Compare a small sample
and a large one with AlOx barrier
s / Nb (20) / AlOx (8) / Py (20)
s / Nb (17) / Py (20)

• no dips domain state important, stray fields
  not effect is hex

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• N.B. domain wall width ?w ? 0.1 ? m
• domains in large samples very probable
• no domains in small samples ? Possible ?

• on 2 µ x 10 ? ?

Bitter patterns on 10 µ x 10 ? Py ( Blamire)
Simulate
no domains
13

• Basic explanation ( with A. Buzdin)
• In the transition ( T / Tc ? 6 ? 10-4 ) ?S(T)
  ? 0.15 ?m ? ?w
• (so large ?w helps)
• Cooper pair samples much of the rotation of M
  and hex .
• For smaller ?w , the effects should be found
  below Tc.
• published PRL 04

How to measure below Tc ? Not just a critical
current, but the depairing current Jdp - can be
quantified.
measures average D rather than highest D (as
Tc does).
14
Near Tc GL regime F/S/F trilayer Fe / Nb
/ Fe Jdp(t) / Jdpbulk (0) JdpGL(0)r1/2 /
Jdp,NbGL(0)rNb1/2
Geers et al. PRB 01
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Smaller samples, pulsed current ? full
T-dependence
Nb a-MoGe ? 7 ??cm ? 50 nm ? 160
??cm ? 0.5 ?m
Fully according to Kupriyanov Lukichev Jdp is
measurable
Rusanov et al PRB 04
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The 0 - ? transition Tc is not very sensitive
Cu1-xNix
Because of transparency
Fominov e.a, PRB 02
Note the position of the minimum about 0.75 ?/2
?F 0.75 ? / 4
Lower than for minimum Ic in SFIF 3?/4 ?F 3 ?
/ 8
N.B. No ?Tc - no spin switch
17
Is Jdp more sensitive ? Preliminary results on
Pd89Ni11 / Nb bilayers
Clear switching at all temperatures
18
Jdp and Tc vs. dF
I-V width vs. dF

• Clear peak at 3? / 8

• No minimum in Tc
• Jdp minimum near ? / 4

Jdp can find both the minimum and the 0 - ?
crossover
Next step will be spin switch by depairing current
19

• In conclusion
• F / S bilayers show a spin-switch-like effect
  the inhomogeneous hex of the domain state lowers
  R, c.q. enhances superconductivity.
• Jdp may turn out to be a useful quantity to
  measure the 0 - ? crossover
• and to measure superconducting spin switching