The complex differential susceptibility of a YBCO film and YBCO-STO-LCMO structures with different thicknesses of the STO layer (0, 2, and 7 nm) is investigated. It is shown that for a YBCO-LCMO structure the temperature of the superconducting transition T-onset and the critical current density j(c) are observed to decrease in comparison with a pure YBCO film. In a YBCO-STO-LCMO structure, on the contrary, T-onset increases, while j(c) increases for the structure with the 2 nm thickness of STO and decreases for that with 7 nm of STO. It is conjectured that there is a change of the vortex pinning mechanism as the temperature is lowered. (C) 2003 American Institute of Physics.