1. Spin-diffusion, T c suppression and proximity effect in ferromagnet/superconductor (LCMO/YBCO) bilayers and trilayers Norbert M. Nemes Seminario Alternativo, ICMM, 2006 ICMM, POMT contratado postdoctoral “Juan de la Cierva” con Mar Garcia-Hernandez

2. Seminario Alternativo, ICMM, 2006 project ideas, sample-growth, characterization (x-ray reflectivity, TEM etc.), magnetization (SQUID): Vanessa Pena, Zouhair Sefrioui, D. Arias, Carlos Leon, and Jacobo Santamaria GFMC, Departamento Fisica Aplicada III, Universidad Complutense de Madrid magnetoresistace: Jose Luis Martinez, Mar Garcia-Hernandez Instituto de Ciencia de Materiales de Madrid polarized neutron reflectivity Susanne G. E. te Velthuis and Axel Hoffmann Materials Science Division, Argonne National Laboratory high-res. TEM, STEM, EELS Maria Varela Oak Ridge National Laboratory

3. What can happen when a ferromagnet and a superconductor get too close? Seminario Alternativo, ICMM, 2006 http://www.csr.umd.edu/csrpage Andreev reflection  NO with LCMO! Proximity effect via exchange fields of ferromagnet suppressing superconductivity stray dipole fields of ferromagnet creating vortices in superconductor spin imbalance in superconductor due to Takahashi et al PRL82 (1999) injection or diffusion of spin-polarized carriers Reflection (triplet correlations) M. Eschrig et al PRL 90, (2003); Volkov et al PRL 90, (2003); Inverse proximity effect Bergeret et al PRB69 (2004) (induced surface spin polarization in superconductor) Magnetic exchange coupling (as in GMR devices) Sa de Melo PRL79 (1997)...

4. Spin transport between 2 ferromagnets through a superconductor P Alignment TcTc S. Takahashi, I. Imamura and S. Maekawa, Phys. Rev. Lett 82, 3911 (1999) AP Alignment TcTc Current in plane: Thus far the same is true with normal metal spin-switch devices What is extra with a superconductor? Increased density of quasi-particles!!!

5.  E ex =0.6 eV Exchange splitting vs bandwidth EFEF W=4.5 eV Ni Ferromagnet  E ex =3 eV EFEF W LCMO Half-metallic Ferromagnet

6. Seminario Alternativo, ICMM, 2006 Ferromagnets Coercive Field Saturation Magnetisation Saturation Field

7. Seminario Alternativo, ICMM, 2006 Superconductors Perfect conductor R=0: Perfect diamagnet, expunges field χ=-4π: Type I: coherence length > penetration depth  no vortices Type II: coherence length < penetration depth  vortices YBCO is extreme type II as coherence length is very short H c1

8. LCMO terminates in MnO !!! 2 nm LCMO YBCO [La 0.7 Ca 0.3 MnO 3 / YBa 2 Cu 3 O 7 ] Interface La Y Ba M. Varela and S.Pennycoock

9. X-ray STEM La 0.7 Ca 0.3 MnO 3 /YBa 2 Cu 3 O 7. / La 0.7 Ca 0.3 MnO 3 F/S/F trilayers Z-contrast M. Varela and S.Pennycoock YBCO LCMO

10. d F La 0.7 Ca 0.3 MnO 3 0.5453 nm/u.c. d S YBa 2 Cu 3 O 7 1.1682 nm/u.c. Grow F/S/F trilayers Experiment Why manganite and HighTc-YBCO? LCMO:fully spin polarised, half-metal small exchange fields YBCO: short coherence length high Tc Short range pair-breaking, high quality, smooth interfaces, good lattice matching Measure transport: current in plane (CIP) Sweep magnetic field: in plane Seminario Alternativo, ICMM, 2006

11. Polarised neutron reflectivity: AP alignment in F/S/F trilayers Peak in magnetoresistance occurs for AP alignment of F layers V. Peña et al. Phys Rev. Lett. 94 57002 (2005)

12. Neutron reflectometry: IPNS @ Argonne Ntl. Lab. There exists a region of AP alignment top and bottom saturation magnetizations are also different Seminario Alternativo, ICMM, 2006 V. Peña et al. Phys Rev. Lett. 94 57002 (2005)

13. Seminario Alternativo, ICMM, 2006 With increasing YBCO thickness coercive fields change and region of AP alignment diminishes

14. R max R min LCMO YBCO STO Temperature Magnetoresistance increases with lower temperature V. Peña et al. Phys Rev. Lett. 94 57002 (2005) Seminario Alternativo, ICMM, 2006

15. The little issues experimentalists have: How can we compare GMR of various samples? Seminario Alternativo, ICMM, 2006 YBCO 15uc

16. LCMO YBCO STO LCMO YBCO STO LCMO YBCO STO Pair-breaking effect is larger with thin YBCO! Seminario Alternativo, ICMM, 2006

17. ΔTcΔTc

18. Quasi-particles with E>Δ may diffuse, reflect in AP alignment and pile up in superconductor they suppress Δ and T c self-consistently Spin diffusion length in YBCO: ~9nm >> ξ GL ~9nm ~24nm

19. Seminario Alternativo, ICMM, 2006 With increasing YBCO thickness coercive fields change and region of AP alignment diminishes 24nm: - electron mean free path in YBCO - Change of linear behavior of ΔT c - Length-scale of decay for thicker films Antiferromagnetic exchange coupling of LCMO layers is mediated by spin polarized carrier?

20. Seminario Alternativo, ICMM, 2006 Peaks can not originate from vortices

21. MR@R min =10 -4 Ω 25%, very narrow Effect small in bilayers So, dipole-field effect is ruled out MR@R min =10 - 4 Ω 100%, broad What is wrong with this sample? MR@R min =10 -4 Ω Very small Bilayers

22. In a particular bilayer: evidence of proximity effect resistivity drops when magnetisation of ferromagnet is non-uniform

23. Conclusions Large magnetoresistance in FM/SC/FM trilayers due to polarised spin diffusion when FM are aligned AntiParalel T c is suppressed by AP alignment, this pair-breaking increases for thinner YBCO Spin diffusion length can be estimated as 9 nm >> coherence length Seminario Alternativo, ICMM, 2006

24. [La 0.7 Ca 0.3 MnO 3 (15 u.c.)/ YBa 2 Cu 3 O 7 (n u.c.)] superlattices resistance n=1 n=5 n=3 n=2 a.c. susceptibility n=4 n=5 n=8

25. [La 0.7 Ca 0.3 MnO 3 (15 u.c.)/ YBa 2 Cu 3 O 7 (n u.c.)] superlattices d S cr ~ 2.4 nm = 4  S d S cr = 25 nm V/FeV d S cr = 70 nm Fe/Pb/Fe F/S interaction !!! Z. Sefrioui et al APL 81, 4568 (2002)

26. m=60 m=30 m=3 Fixed YBa 2 Cu 3 O 7 (5 u.c.) thickness Varying La 0.7 Ca 0.3 MnO 3 (3<m<90 u.c.) thickness [La 0.7 Ca 0.3 MnO 3 (m u.c.)/ YBa 2 Cu 3 O 7 (5 u.c.)] superlattices

27. d F cr ~ 20 nm !!! Very long length scale into the ferromagnet!!! Z. Sefrioui et al cond-mat/0301235 (2003) [La 0.7 Ca 0.3 MnO 3 (m u.c.)/ YBa 2 Cu 3 O 7 (5 u.c.)] superlattices

28. Seminario Alternativo, ICMM, 2006