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A method for the analysis of temperature and field dependent magnetization data near the Curie temperature T C is devised, which allows the reliable determination of critical

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We report on the magnetic and superconducting properties of La 0.7 Ca 0.3 MnO 3 /YBa 2 Cu 3 O 7 ͑LCMO/ YBCO͒ superlattices. For a constant LCMO layer thickness of 6 unit cells ͑u.c.͒, resistance and susceptibility measurements show superconductivity for YBCO layer thickness in excess of 4 unit cells. The critical temperature increases with YBCO thickness, and a T c of 58 K is found for a YBCO thickness of 10 unit cells. Magnetization measurements show a ferromagnetic transition at 100 K in a (LCMO 6 u.c. /YBCO 5 u.c. ͒ 15 bilayer superlattice, and a depressed value of the saturation magnetization of 20 emu cm Ϫ3. These results are discussed in terms of interface disorder ͑analyzed by x-ray diffraction and transmission electron microscopy͒ and of the possible interaction between magnetism and superconductivity.

The magnetic properties of a series of YBa2Cu3O7−x/La2/3Ca1/3MnO3 (YBCO/LC1/3MO) superlattices grown by dc sputtering at high oxygen pressures (3.5 mbar) show the expected ferromagnetic behavior. However, field-cooled hysteresis loops at a low temperatures show the unexpected existence of exchange bias effect associated with the existence of ferromagnetic/antiferromagnetic (AF) interfaces. The blocking temperature (TB) is found to be thickness dependent and the exchange bias field (HEB) is found to be inversely proportional to the ferromagnetic layer thickness, as expected. The presence of an AF material is probably associated with interface disorder and Mn valence shift toward Mn4+.