The effect of structure on the optical, transport and magnetic properties of single-crystalline thin LaSr(Ca)MnO films with metallic clusters coherently contained within the basic crystallographic matrix was investigated. The films were grown by pulsed laser deposition. With growth temperature Ts in the 450 C < Ts <600 C range, the films are composed predominantly of a rhombohedral (dielectric) matrix and rhombohedral (metallic) clusters. Magnetic properties are similar to those of clusterized spin glasses and show a characteristic difference between Field Cooled (FC) and Zero Field Cooled (ZFC) magnetization M values in low (< 1kOe) magnetic fields. For growth temperature Ts >= 650 C, the film structure changes to rhombic matrix and rhombic clusters. This change is accompanied by a 3-9 orders of magnitude increase of film conductivity and the appearance of a, characteristic for manganites R(T) curve showing minima and maxima. magnetization at low temperatures is increased by an order of magnitude in comparison to the Ts <= 600C case.
We show that electronic, optical and magnetic properties of the films depend on the ordering of the cluster structure. There exists a threshold value of magnetic ordering, above which the magnetic subsystem starts to affect actively the film conductivity. In the case of correlated changes in the cluster structure, this leads to a percolative dielectric-metal transition. Due to the tunneling interaction mechanism between clusters, the electrical conductivity exponentially grows with the magnetization magnitude. The considerable differences in the gradients dR/dT observed for samples of nominally identical composition are caused by the changes in the interaction mechanism between atomic, electronic and magnetic subsystems.
This work was supported by Polish Government (KBN) Grant PBZ-KBN-013/T08/19.