My thesis presents a systematic study of magnetotransport and magnetic ordering in manganite-high Tc cuprate spin valve structures. YBCO - LSMO (Yttrium Barium Copper Oxide - Lanthanum Strontium Manganese Oxide) heterostructures of [110] orientation are grown to allow direct injection of spin polarised holes from the LSMO into the CuO2 superconducting planes of the YBCO. Galvanomagnetic studies on the LSMO-YBCO-LSMO trilayers reveal unusually high AMR (~72000%) on rotating the field in the plane of the heterostructure whose magnetic ground state is antiferromagnetic (AF). The coupling energy J1 of the AF state in these trilayers is much higher as compared to energy of [001] oriented hybrids. First the preparation and measurement of magnetic and galvanomagnetic properties of [110] and [001] oriented LSMO films are described. The magnetization vector (M) of the [001] and [110] type films is pinned along the [110] and [001] directions respectively at low fields. A magnetization orientation phase transition (MRPT) which manifests itself as a discontinuity and hysteresis in R(ψ), where ψ is the angle between H and the easy axis, for the H below a critical value H* has been established. The boundary of the pinned and depinned phase on the H-T plane has been established. The highly robust pinning of magnetisation seen in [110] films is related to their uniquely defined easy axis. The isothermal resistance Rppd and Rpar for I ppd H and I par H, respectively for both polytypes follows the inequality Rppd > Rpar for all ranges of fields (0 < H < 3500 Oe) and temperatures (10-300 K). Further, the relevance of pair-breaking by exchange and dipolar fields, and by injected spins in a low carrier density cuprate Y(1-x)P(x)BCO (Yttrium Praseodymium Barium Copper Oxide) sandwiched between two ferromagnetic LSMO layers is examined. At low external field (Hext), the system shows a giant magnetoresistance(MR), which diverges deep in the superconducting state. We establish a distinct dipolar contribution to MR near the switching field (Hc) of the magnetic layers. At Hext >> Hc, a large positive MR, resulting primarily from the motion of Josephson vortices and pair breaking by the in-plane field, is seen. (Link to the full thesis: http://arxiv.org/abs/0904.0373v1)

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Publications from the work in the thesis

1. J. Supercond. Nov. Magn. 24, 1501, July 2011

2. Phys. Rev. B 81, 014515, January 2010

3. J. Magn. Magn. Mater. 320, 3323- 3333, December 2008

4. Phys. Rev. B 78, 094502, September 2008

5. Appl. Phys. Lett. 89, 182508, November 2006