Cuprate/manganite superlattices - A model system for a bulk ferromagnetic superconductor

YBa2Cu3O7/La0.67Ca0.33MnO3, superlattices (SLs) and heterostructures have been grown by pulsed laser deposition with individual layer thickness ranging from 4 to 200 unit cells for the YBa2CU3O7 and 10-500 unit cells for the La0.67Ca0.33MnO3. Whereas simple heterostructures (single layer La0.67Ca0.33MnO3 and single layer YBa2Cu3O7 50 nm thickness each) reproduce the intrinsic properties of the constituent material rather well with reduced critical temperatures for the phase transitions (Curie temperature 250 K, superconducting transition at T = 70 K) the critical temperatures systematically vary with the SL composition due to coupling between the layers observed in the SLs. This systematic rules out a simple decoupling of the individual layers. Tentatively we ascribe the composition dependent changes of the critical temperatures to interaction effects at the electronic level. (C) 2001 Elsevier Science B.V. All rights reserved.