| Transition-metal oxides have long been the subject of study, because they exhibit a wide range of exotic and still incompletely understood structural, magnetic and electronic properties. This behavior cannot be explained within the context of the usual one-electron band theory that accounts well for the properties of most other solids, indicating the importance of strong electron-electron and electron-lattice correlations.; Recently, attention has become focused on a certain class of manganese oxides, the manganite perovskites. The current burst of activity was stimulated by reports of spectacularly large—“colossal” magnetoresistance in this family of compounds. In these materials, the interaction between the electrons and lattice vibrations (phonons) is unusually strong, leading to a wide range of striking physical phenomena such as magnetic ordering, charge ordering and the metal-insulator transition. These materials therefore provide an unprecedented opportunity to study the physics of systems in which a high density of electrons is strongly coupled to phonons.; The primary concern of this work is an experimental study of a poorly understood metal-insulator transition in the charge-ordered Colossal Magnetoresistance manganites. It mainly comprises the electrical transport and magnetization measurements, l/f and the telegraph noise measurements and the polarized optical microscopy of CMR systems at low temperatures.; One of the important results of this work is demonstration of the percolative metal-insulator transition in CMR materials. In the course of this research, it was also found that inhomogeneous phases in manganites exhibit slow mesoscopic non-equilibrium fluctuations—the telegraph noise. This finding confirms the percolation scenario of the metal-insulator transition and shows that the percolation in CMR has more complicated character than the conventional percolation.; Finally, we demonstrate that the charge-ordered manganites are typical martensitic systems. Our studies revealed a remarkable grain size effect in these systems and, therefore, demonstrated a leading role of the martensitic accommodation strain in defining the electronic properties of charge-ordered manganites. |
