Nanoscale ferromagnetism and doping effect in manganites

We investigated temperature variation of ferromagnetic (FM) microstructures in La$_{1-x}$Sr$_{x}$MnO$_{3}$ (x=0.175) by low-temperature Lorentz microscopy. Also, changes of the FM domain structures by substituting non-magnetic Al$^{3+}$ ion for Mn one were examined. We succeeded in observing changes of magnetic domain structures from large FM domains with the average width of 1$\mu $m to stripe-shaped FM ones with the 100nm width in the metallic phase. On the other hand, the insulator phase was found in La$_{1-x}$Sr$_{x}$Mn$_{1-y}$Al$_{y}$O$_{3}$ (x=0.175, y=0.02). We found that the insulating phase exhibits a characteristic bi-modal distribution of the FM domains, which is characterized as the coexisting state of distinct types of the FM domains; one is the stripe-shaped FM domains and the other is the plate-shaped one. This mixture of distinct types of the FM domains breaks the coherence of the conductivity and results in the insulating character in La$_{1-x}$Sr$_{x}$Mn$_{1-y}$Al$_{y}$O$_{3}$ (x=0.175, y=0.02). In addition, we found the nanoscale FM domains around the rhombohedral-to-orthorhombic structural transition temperature in La$_{1-x}$Sr$_{x}$Mn$_{1-y}$Al$_{y}$O$_{3}$ (x=0.175, y=0.04).