Electrodynamics of superconductor-ferromagnet hybrid structures

Transcription

1 Electrodynamics of superconductor-ferromagnet hybrid structures A.S.Mel nikov Institute for Physics of Microstructures RAS

2 IPM RAS S.V.Mironov A.V.Samokhvalov D.Vodolazov Yu.Erin A.Bespalov V.Vadimov M.Sapozhnikov Universite Bordeaux I, France A.I.Buzdin

3 Outline Interplay of magnetism and superconductivity and elecrodynamic phenomena Orbital effect of stray magnetic fields on superconductivity nucleation and inhomogeneous states Effect of superconductivity on the domain structure Proximity effect and its influence on the electrodynamic response Spontaneous currents LOFF instabilities

4 Mechanisms of interaction of superconducting and magnetic order parameters Stray magnetic field affects the electron trajectories Exchange field affects spin orientation n s Cooper pair density V.L.Ginzburg (1956) S F x Something is forgotten? 1. Interference effects (oscillations of the Cooper pair wavefunction) S n s Josephson junction F S. Generation of triplet Cooper pairs

5 Structures and materials Ferromagnetic superconductors UGe, URhGe ErRh4B4, ErNiBC EuFe (As 1 x P x ) Superconductor-ferromagnet hybrid structures S h S F S h F

6 Ginzburg-Landau theory 4m ie c A a b Boundary conditions j rotb ie m 4 j c * e * A mc n I-S ie c A e e j m c j e n mc s A A London equation N-S

7 Localized superconducting channels. Domain wall superconductivity Cooper pair 1D superconducting channels

8 Electromagnetic (orbital) mechanism. Phenomenological Ginzburg-Landau theory External field H z + i A( r ) 1 ( T B Inhomogeneous magnetic field induced by magnetic moments ) H H c c Thin superconducting films: Only B z field component is important Assumption: Domain walls are pinned

9 Superconductivity nucleation in S/F bilayers Narrow domains w<<d D H /B 1 S T c of a nucleus localized at a single domain wall Nb/F 4M ~ 1 1 koe T c ~ 9 K Domain Wall S N 1.6 orb (T T )/ΔT c c c dh c ~.5 koe / dt T c ~ 1 3K K

10 Superconducting nucleus in a periodic domain structure in an external field f k (x), [arb. units] B z (x)/b H /B 3 1 H B w B w 1 Domain wall superconductivity 5 Bw 1 Bw f (x), [arb. units] k x/(w) B z (x)/b orb (T T )/ΔT 1 c c c x/(w) 4 8

11 Upper critical field in the presence of domain structure Phys.Rev.Lett (5), Phys.Rev.B (7-8) CoPt / Nb / CoPt: CoPt / Al:

12 Domain superconductivity measured by scanning laser microscopy Pb BaFe119 R. Werner, A.Yu. Aladyshkin et al., Phys. Rev. B 84, 55(R) (11).

13 Vortex states B z

14 Superconducting films with arrays of ferromagnetic dots Y.Otani, B.Pannetier, J.P.Noziers, D.Givord (1993) Unusual behavior of T c (H): W.Gillijns, A. Silhanek, V.Moshchalkov (6) H Tc(H) oscillations Nb-Gd/Co T c? H -G G

15 Little-Parks effect and multiquanta vortices in a hybrid S/F system Axially symmetric field profile Example: magnetic dot (dipole) above S film Superconducting ring with area S Winding number=m M b B z h 4M N f 3 3h r H T c (H) oscillations are caused by the quantization of flux through the area S

16 r Magnetic dot assisted superconductivity. T c oscillations for a nucleus at the ring B z ( r ) Local approximation: l r S nucleus in a linear B profile (1 T c /T c )h /(N f ξ ) m= S 1 N H/b (1 T c /T c )h /(N f ξ ) S m= N H/b N 4 1 f l 3 B r z ( N f ) F particle 3 x 3 x 3 nm, magnetization ~ 1 3 G, h~3 nm 11 3 M ~ 31 G cm 3 b ~ 1 G N f ~ 1 Nb film: T c ~ 9 K ~ 4 nm H ~ 1 G T c ~. 1K

17 Interaction of domain structures with superconducting phase far below the superconducting transition (SF bilayers) 1. Meissner state. Penetration of the first vortex 3. Vortex-antivortex structures

18 Interaction of domain structures with superconducting phase far below the superconducting transition (ferromagnetic superconductors)

19 Skyrmions (cylindrical magnetic domains) in SF bilayers. Metastable states at antidots

20 Vortices in superconductor-ferromagnet systems Vortex attraction: modulated vortex concentration Moving vortex generates magnons V

21 Josephson transport in the field of magnetic particles No proximity effect M Λ AV d a AV JJ H = H a (n + d/a) Φ a = Φ (n + d/a)

22 What is the electromagnetic response of superconductor and ferromagnet? Microscopic consideration. London equation j e n mc s A

23 Exchange mechanism. Proximity effect in FS structures. Question: Is it possible to affect vortex states by the exchange field? Inhomogeneous superconductivity induced by the exchange field: ˆ H h ˆ 1. FFLO state. Interference effects for Cooper pairs in FS layered structures Damped oscillatory S N dependence of pair wave S F function in ferromagnets x x ξ n ~(D n /T) 1/ h= exchange energy ξ f in dirty limit:

24 Spontaneous currents in S/F bilayers. Inverse proximity effect.

25 Manifestation of proximity effect in electrodynamic response S F h H London penetration depth Experiment : Lemberger et al., J. Appl. Phys. 8. Measurable quantity: integrated over the system thickness screening length. Theory : Houzet, Meyer, PRB 9. Slightly non monotonous dependence of vs d F

26 Screening properties of Nb/PdNi/Nb H = Nonmonotoneous behavior of Transition between and π - states? N.Pompeo, et al., PRB 9, 6451 (14)

27 Screening properties of SFS structures N.Pompeo, et al., PRB 9, 6451 (14) Nb/Pd.84 Ni.16 /Nb -π переход I рода: A.I. Buzdin PRB 5 (диффузионная SFS, жесткие гран. условия) P.H. Barsic, O.T. Valls and K. Halterman, PRB6 ( баллистическая SFS система) Экранирующие свойства SF(S) структур T.R.Lemberger, et al JAP 8 M. Houzet, J.S. Meyer, PRB 9 S.Mironov, A.Melnikov, A.Buzdin, PRL 1

28 Nb/Cu.35 Ni.65 /Nb [ С.Н.Вдовичев, Е.Е.Пестов, Ю.Н.Ноздрин, П.А.Юнин ]

29 Paramagnetic Meissner effect in dirty S/F bilayers H S F h 16 T j 1 4 A f f dx c s t d n S f s F f f t s ( x) d S f t fˆ f f ˆ iˆ s t y d F x local paramagnetic Meissner effect Can the superfluid density change its sign?

30 Paramagnetic Meissner effect and the FFLO instability j 1 4 A F A A FA 1 8 AdV F A A A dv F A dv en 4 s m The uniform ground state can be unstable!

31 FFLO states in thin-film S/F systems z y x A d F A fˆ e ikr S dx A k 8 A hallmark of the instability: vanishing Meissner effect 1 en s dx d m FFLO state

32 FFLO state in S/F bilayers d 1. z d S S ˆ ˆ ˆ ˆ f h f f hˆ ˆ x D f ˆ D i n k F d F x F F d s f D D s f : Stability of the FFLO state: h T c d F ~ D f f d S f Ds f D D f s T h c T T c c 16 dss 1 Tc 1Re 1 ecddstc ( k ) k

33 Other systems with in plane FFLO instability d=.5ξ s ; σ s /σ f =.5; ξ s /ξ f =.65 T c A.V. Samokhvalov, A.S. Mel nikov and A.I. Buzdin, Phys. Rev. B 76, (7) I.V. Bobkova and A.M. Bobkov, Phys. Rev. B 88, 1745 (13)

34 What happens below T c? T c S F FFLO d f

35 The idea about the low temperature FFLO phase c c T AT T BT T... A FFLO instability at T = T c A No FFLO at all? S S F B T c T FFLO T c T

36 Analytical results 4 a T b T... a b FFLO instability at T a b Nonlinear Usadel f ( x ) f ( x) s t A. V. Samokhvalov, A. I. Buzdin, PRB 9, (15) 16 T f f dx c s t d n

37 FFLO states in S/F/N structures 16 T f f dx c s t d n

38 FFLO states in S/F/N structures 16 T f f dx c s t d n N

39 Phase diagrams of the superconducting films

40 Phase diagrams of the S/F/N sandwiches

41 Towards the experimental observation of the FFLO phase S: NbN, TaN, WSi [ ~ 1 nm] F: CuNi, PdFe [ ~ 5 nm] N: Ag, Au, Al [ ~ nm]

42 Dependence of supercurrent on supervelocity S S F J v? s

43 Dependence of supercurrent on supervelocity J v v ~ s s a v v b v v 4 s s s s v s Change of the sign of the third harmonic?

44 Summary S: NbN, TaN, WSi F: CuNi, PdFe N: Ag, Au, Al v s

45 Key points domain superconductivity LOFF instability spontaneous currents