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Interplay of Magnetic and Superconducting Proximity effect in F/S hybrid structures

Interplay of Magnetic and Superconducting Proximity effect in F/S hybrid structures. Thierry Champel. Institut f ü r Theoretische Festk ö rperphysik. Collaborators:. Tomas L ö fwander. Matthias Eschrig. Grenoble 16/03/2006. PLAN. S/F PROXIMITY EFFECT. Characteristic features. Motivation.

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Interplay of Magnetic and Superconducting Proximity effect in F/S hybrid structures

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  1. Interplay of Magnetic and Superconducting Proximity effect in F/S hybrid structures Thierry Champel Institut für Theoretische Festkörperphysik Collaborators: Tomas Löfwander Matthias Eschrig Grenoble 16/03/2006

  2. PLAN S/F PROXIMITY EFFECT Characteristic features Motivation RESULTS Study of triplet correlations within: 1/ S/F bilayer with a rotating exchange field 2/ F/S/F trilayer with noncollinear moments 3/ F/S/F/S/F pentalayer with noncollinear moments

  3. SUPERCONDUCTOR/FERROMAGNET HYBRID NANOSTRUCTURES CONTEXT: SPINTRONICS S F Systems under study: Singlet Cooper pairing Zeeman splitting Possibility for a new physics with interesting applications

  4. S/F PROXIMITY EFFECT Proximity effect: Penetration of Pairing correlations into non- superconducting areas nanoscale if J=0 In F: center of mass momentum of Cooper pair: 2q fS Spatial modulation of the pair amplitude in F change of sign = influence on jC in Josephson junctions leakage of pairs = influence on TC Dependence on impurities, interface quality, domain state of F, ...

  5. Some characteristic features

  6. S/F PROXIMITY EFFECT : SOME CHARACTERISTIC FEATURES Nonmonotonic behavior of Tc(df) S F Transparency - dF + Th: Buzdin et al. (1991) for recent th. developments see e.g. Fominov et al., PRB 2002 Superconducting switch Tagirov, PRL 1999 Buzdin et al., EPL 1999 Th: F F S or dS dF dF

  7. S/F PROXIMITY EFFECT : SOME CHARACTERISTIC FEATURES p-junctions S F S 2dF Th: Buzdin et al. (1982) • Ryazanov et al, PRL (2001) • Kontos et al, PRL (2002) • Guichard et al, PRL (2003) • Bauer et al, PRL (2004) Exp: From Ryazanov et al. (2005) Taken in Buzdin, RMP 77, 935 (2005) Critical current Ic of Cu0.47Ni0.53 junctions as a function of the F layer thickness

  8. MOTIVATION : ROLE OF INHOMOGENEITY OF EXCHANGE FIELD? Theoretical motivation • S/F wire with a local inhomogeneity of J Bergeret et al., PRL 2001 • F/S/F trilayer with noncollinear F moments Volkov et al., PRL 2003 As a result of inhomogeneity: Production of triplet components which penetrate far in F QUESTIONS: Why do we have triplet correlations? Under which conditions are long-range triplet components produced? Signatures and consequences on physical quantities?

  9. Triplet correlations

  10. S/F PROXIMITY EFFECT : THEORETICAL ASPECTS METHODS Quasi-classical equations of superconductivity for the Nambu-Gorkov Green functions Quasi-classical approximation + consideration of the diffusive limit main difficulties: System of coupled nonlinear differential eqs. Coupling with the self-consistent gap eq. Inhomogeneous superconductivity

  11. S/F PROXIMITY EFFECT : COEXISTENCE OF SINGLET/TRIPLET CORRELATIONS Pair states Order parameter: triplet triplet vector singlet Linearized Usadel equation within this basis • Coexistence of singlet/triplet correlations • Role of inhomogeneity of J T. C. and M. Eschrig, PRB 71, 220506(R) (2005) T. C. and M. Eschrig, PRB 72, 054523 (2005)

  12. S/F PROXIMITY EFFECT : COEXISTENCE OF SINGLET/TRIPLET CORRELATIONS Sup. Fer. Energy of pair states: Sz=-1 S=1 Picture for a fixed exchange field S=0, Sz=0 S=0 Sz=+1 S F

  13. S/F PROXIMITY EFFECT : COEXISTENCE OF SINGLET/TRIPLET CORRELATIONS PECULIARITY OF THE DIFFUSIVE LIMIT Symmetry Properties of triplet correlations Correlator: Pauli principle: Singlet component: Triplet vector: So, ft dropped out of the gap equation. But it isn’t zero… Order parameter:

  14. S/F PROXIMITY EFFECT : LOCAL DENSITY OF STATES AND SPIN MAGNETIZATION Spin structure of the Green function g Normalization condition localdensity of states spin magnetization T. C. and M. Eschrig, PRB 72, 054523 (2005)

  15. TRIPLET CORRELATIONS IN S/F HYBRID STRUCTURES To resume: Physical consequences of the presence of triplet correlations: • Spin-dependent local DOS • Spin magnetization • Superconductivity dependent on moment orientation Signatures at the quantitative level on physical quantities?

  16. S/F bilayer with spiral magnetic order

  17. EFFECT OF INHOMOGENEITY OF EXCHANGE FIELD: STUDY OF A S/F BILAYER Within this model, only short-range triplet correlations (SZ=0) are produced T. C. and M. Eschrig, PRB 71, 220506(R) (2005) Spatial dependence of pair correlations Effect of inhomogeneity of J on Tc(df) TC • Enhancement of Tc with Q

  18. EFFECT OF INHOMOGENEITY OF EXCHANGE FIELD : MAGNETIC SPIRAL ORDER TC superconducting switch operated by controlling the domain state in F jC S/F/S junctions Destruction of π phase by inhomogeneity unpublished (2006)

  19. F/S/F trilayer with noncollinear moments

  20. STUDY OF A F/S/F TRILAYER : LOCAL INDUCED MAGNETIZATION collinear moments Spatial dependence of the induced magnetization or S M F F x T. Löfwander, T. C., et al., PRL 95, 187003 (2005)

  21. STUDY OF A F/S/F TRILAYER : PAIR CORRELATIONS AND MAGNETIZATION perpendicular moments Spatial dependences of: F S F the pair correlations the induced local magnetization z x y M (a)-(c): projections (b)-(d): angles relative to z-axis T. Löfwander, T. C., et al., PRL (2005)

  22. STUDY OF A F/S/F TRILAYER : TOTAL INDUCED MAGNETIZATION AND TORQUE Integrated magnetization : screening effect F F S z x y B applied in yz plane sup. state Changes in torque between normal and superconducting states norm. state angle relative to z-axis for B T. Löfwander, T. C., et al., PRL (2005)

  23. F/S/F/S/F pentalayer

  24. SWITCHING BETWEEN THE 0 AND πSTATES IN F-S-F-S-F PENTALAYERS T. C. et al., unpublished (2006) TC Switching between the 0 and π states by controlling the moment orientations in F π state 0 state switching region

  25. CONCLUSION The coexistence near the S/F interface of singlet components with triplet components is the hallmark of the S/F proximity effect Consequences: • Sensitivity of superconductivity on moment direction • Spin-dependent density of states • Spin magnetization Concrete examples: • S/F bilayer within spiral order model Superconducting switch controlled by domain state • F/S/F trilayer Torque variation by cooling down the trilayer • F/S/F/S/F pentalayer Switching between the 0 and π states by acting on the moment orientation Future directions: Influence on transport properties?

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