Norbert M. Nemes

1. Seminario Alternativo, ICMM, 2006 Spin-diffusion, Tc suppression and proximity effect in ferromagnet/superconductor (LCMO/YBCO) bilayers and trilayers Norbert M. Nemes 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. Andreev reflection  NO with LCMO! http://www.csr.umd.edu/csrpage Proximity effect via exchange fields of ferromagnet suppressing superconductivity Seminario Alternativo, ICMM, 2006 What can happen when a ferromagnet and a superconductor get too close? 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) spin imbalance in superconductor due to Takahashi et al PRL82 (1999) injection or diffusion of spin-polarized carriers stray dipole fields of ferromagnet creating vortices in superconductor . .. ...

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

5. EF EF Eex=3 eV W W=4.5 eV LCMO Ni Half-metallic Ferromagnet Exchange splitting vs bandwidth Eex=0.6 eV Ferromagnet

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

7. Hc1 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

8. [La0.7Ca0.3MnO3 / YBa2Cu3O7 ] Interface LCMO Ba YBCO Y Ba La 2 nm LCMO terminates in MnO !!! M. Varela and S.Pennycoock

9. La0.7Ca0.3MnO3 /YBa2Cu3O7./ La0.7Ca0.3MnO3 F/S/F trilayers X-ray STEM YBCO LCMO Z-contrast M. Varela and S.Pennycoock

10. Grow F/S/F trilayers Seminario Alternativo, ICMM, 2006 Experiment • dFLa0.7Ca0.3MnO30.5453 nm/u.c.dSYBa2Cu3O71.1682 nm/u.c. • Measure transport: • current in plane (CIP) • Sweep magnetic field: in plane WhymanganiteandHighTc-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

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

12. Seminario Alternativo, ICMM, 2006 Neutron reflectometry: IPNS @ Argonne Ntl. Lab. There exists a region of AP alignment top and bottom saturation magnetizations are also different 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. YBCO LCMO STO LCMO Rmax Temperature Rmin Seminario Alternativo, ICMM, 2006 Magnetoresistance increases with lower temperature V. Peña et al. Phys Rev. Lett. 94 57002 (2005)

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

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

17. ΔTc Seminario Alternativo, ICMM, 2006

18. ~24nm ~9nm Spin diffusion length in YBCO: ~9nm >> ξGL Seminario Alternativo, ICMM, 2006 Quasi-particles with E>Δ may diffuse, reflect in AP alignment and pile up in superconductor they suppress Δ and Tc self-consistently

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

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

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

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

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

24. [La0.7Ca0.3MnO3 (15 u.c.)/ YBa2Cu3O7 (n u.c.)] superlattices n=4 n=5 n=8 resistance a.c. susceptibility n=1 n=2 n=3 n=5

25. [La0.7Ca0.3MnO3 (15 u.c.)/ YBa2Cu3O7 (n u.c.)] superlattices dScr = 25 nm V/FeV dScr = 70 nm Fe/Pb/Fe F/S interaction !!! ~ 2.4 nm = 4 S dScr dScr Z. Sefrioui et al APL 81, 4568 (2002)

26. [La0.7Ca0.3MnO3 (m u.c.)/ YBa2Cu3O7 (5 u.c.)] superlattices FixedYBa2Cu3O7 (5 u.c.) thickness Varying La0.7Ca0.3MnO3(3<m<90 u.c.) thickness m=60 m=30 m=3

27. [La0.7Ca0.3MnO3 (m u.c.)/ YBa2Cu3O7 (5 u.c.)] superlattices ~ 20 nm !!! dFcr Very long length scale into the ferromagnet!!! Z. Sefrioui et al cond-mat/0301235 (2003)

28. Seminario Alternativo, ICMM, 2006