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YBa2Cu4O8/La2/3Ca1/3MnO3/YBa2CU4O8 (Y-124/LCMO/Y-124) heterostructures were prepared by facing-target sputtering technique. The oscillatory behavior of superconducting transition temperature (T-C) with the thickness of LCMO (d(L)) has been observed. The strongest nonmonotonic information in T-C-d(L) curves appears clearly when d(L) is larger than the critical thickness d(L)(CR). The metal-insulator transition temperature (T-ml) can only be detected at d(L) > d(L)(CR). For LCMO/Y-124/LCMO trilayer films, the T-M1 has also shown an oscillation with decreasing spacer layer thickness. These kinds of symmetric dependence on the spacer layer in LCMO/Y- 124 systems suggest strongly the interplay of ferromagnetic and superconducting couplings. (C) 2004 Elsevier B.V. All rights reserved.