1. I NTERLAYER E XCHANGE C OUPLING, P AIR B REAKING & 2D V ORTEX D YNAMICS IN F ERROMAGNET - S UPERCONDUCTOR H ETEROSTRUCTURES R. C. B UDHANI Indian Institute of Technology, Kanpur Collaborator :- K. Senapati Funding :- D.R.D.O. Govt. of India Pair-breaking effects of ferromagnetic boundaries in cuprates. Influence of ferromagnetic boundaries on the vortex state of ultra- thin YBCO films. Exchange coupling between two ferromagnets separated by an exotic superconductor. Objectives :-

2. d FM --- 300 Å d Y --- 50 - 500 Å d NM --- 100 Å [100] STO LSMO YBCO (c-axis) [100] STO PBCO YBCO (c-axis) d NM dYdY d FM dYdY d NM T HIN F ILM S TRUCTURES

3. -: S TATE OF M AGNETIZATION IN LSMO :- Soft Magnetic Films with in-plane Magnetization M S (T)/M S (0) = 1 - AT 3/2  J ~ 2 meV

5. MOST OF THE DYNAMICS SCALES WITH THE EFFECTIVE THICKNESS Effective T c due to pair breaking (PB) Inhomogeneous stress field  Hole concentration  Reduced T c Exchange field Spin injection Interface transparency Enhanced PB in a reduced carrier system

6. Superconducting magnet Vacuum can Drive coil Sample Hall sensor G-10 base Brass rod Heating element -: VORTEX DYNAMICS :- Irreversibility temperature and field  T H3 Inverse sheet inductance (1/L)  T H ’ T H ’ = 1- (4  a 2 /c 2 d) (1/L) [d = thickness, a = effective radius] 1/L =  ln(1/Z) THERMALLY ACTIVETED INVERSE SHEET INDUCTANCE Senapati et al. Rev. Sci. Instrum. 75, 141 (2004)

7. T irr  T c (H)Critical temperature at which ac   Loss of pinning  Loss of phase stiffness Clean limit pair breaking : H PB (0) scales with d eff

8. Two regimes of thermal activation separated by a characteristic temperature T * LSMO-YBCO-LSMO Frequency : 121 Hz PBCO-YBCO-PBCO Frequency : 121 Hz

9. High T (> T*) --------- U 0 (T)  Energy of creation of dislocations Low T (< T*) --------- U 0 (T)  Energy required to move preexisting dislocations

11. Summary The dynamics of the vortex state in 2D films of YBCO sandwiched between FM-boundaries has been studied over a wide range of temperature, frequency and magnetic field strength. Flux motion is controlled by creation of dislocations near T m, and by the motion of preexisting dislocations at T<T m. The irreversibility temperature in the vortex state maps onto a clean-limit pair-breaking temperature T c (H), and the pair-breaking field scales with the thickness of the films.

12. LSMO – YBCO – LSMO (300 Å) – (100 Å) – (300 Å) M-H curves ; T c = 62 K K. Senapati and R. C. Budhani, Submitted to Phys. Rev. B.