Ferromagnetic/Superconducting All Oxide Superlattices and Heterostructures

The physical properties of the perovskite-type oxide RuSr2GdCu2O8
have been recently discussed in the view of a simultaneous occurrence of
superconductivity and ferromagnetism. In order to explore some peculiarities of
these compounds we have prepared superlattices of oxides that are known to be
either ferromagnetic (La.67Ca.33MnO3) or superconducting (YBa2Cu3O7). These
superlattices serve as model systems to understanding the physics of
RuSr2GdCu2O8. The YBCO/LCMO superlattices have been grown by pulsed
laser deposition with individual layer thickness ranging from 4 to 200 unit cells
for the YBa2Cu3O7 and 10 to 500 unit cells for the La.67Ca.33MnO3. Measuring
dc-transport and magnetic properties some novel effects have been found due to
a coupling between the layers observed in the superlattices. Superlattices with
individual thickness of the constituent materials of 4 nm e.g. show a reduced
Curie temperature of 120K and a superconducting transition temperature of
52K. Lowering the temperature of reentrant normal state occurs at T = 25K.
Switching off the electronic interlayer coupling by the introduction of insulating
SrTiO3 spacer layers leads to the intrinsic critical temperatures. For the
explanation of the results several novel concepts have to be developed based on
a long range ferromagnetic interlayer coupling and a novel long range
superconducting proximity effect.