Correlation between structure and magnetic properties of manganite-based multilayers

We have studied the structure, magnetic, and transport properties of manganite-based multilayers $A/Bi,$ with $A=La0.55Sr0.45MnO3$ (metallic ferromagnet), and spacers $(Bi)$ of different nature: $B1=SrTiO3$ (nonmagnetic insulator), $B2=La0.9Sr0.1MnO3$ (insulator ferromagnet), and $B3=La0.67Ca0.33MnO3$ (paramagnetic-insulator/ferromagnet-metal). The samples are strongly textured in the direction perpendicular to the sample surface, and present a good interface quality with small roughness and interdiffusion. We have found that in the $A/B1/A$ trilayers, the ferromagnetic electrodes are ferromagnetically coupled for thin spacer layers and becomes decoupled for spacer thickness larger than 3 nm. Instead, the others multilayers are ferromagnetic for all spacer thicknesses and temperature range. This result was expected for temperatures well below the spacer Curie temperature. We attributed the ferromagnetic behavior of the system, found for temperatures above the ordering temperature of the spacer, to direct exchange coupling through short-range ordered zones in spacer layer. As expected from magnetization results, in fact, no extrinsic magnetoresistance was measured in these systems in the temperature range between 4.2 and 300 K.