Current-driven resistance change in $La_{0.7}Ca_{0.3}MnO_3$ exfoliated thin films

Abstract

The mixed-valent manganites exhibit various quantum phases due to the coupling among lattice, orbital, charge and spin degrees of freedom. In particular, much attention has been attracted on the colossal magnetoresistance (CMR) due to a large resistance change under application of a magnetic field. The studies about Ca-substituted lanthanum manganites have been conducted due to the large magnetoresistance. In the manganites, the ferromagnetic metallic phase and the charge-ordered insulating phase are spatially coexistent. The spatial distribution of the phases are changed by external perturbations, thereby leading to the metal-insulator transition. Many researches have been conducted to find the origin of the CMR and apply the sensitive resistance change to create new conceptual devices. As the thickness of manganite thin film becomes thinner, the Curie temperature is lowered and the magnetic ordering as well as the CMR effect is suppressed. The magnetic dead layer effect in such thin thickness regime is an obstacle to develop a miniaturized device. Static strain by the substrate, quantum confinement, and non-uniformity of the composition near the interface can be an origin of the magnetic dead layer, but the fundamental reason is not yet clarified. In this thesis, the result of dynamic clamping is investigated while excluding the substrate and the interfacial effects by exfoliating the thin film from the substrate. In order to exfoliate the thin film, the strontium aluminate ($Sr_3Al_2O_6$) which dissolves in water is used as a buffer layer. The Ca-substituted lanthanum manganite ($La_{0.7}Ca_{0.3}MnO_3$

LCMO) thin film deposited by pulsed laser deposition was exfoliated. The exfoliated thin film was transferred on silicon wafer so that it was free from the static and dynamic stresses. The current-voltage characteristic curve was measured at selected temperatures using the four probes method. The exfoliated thin film showed a non-Ohmic current-voltage curve and the relation between current and voltage could be described by a power law. The resistance quickly decreased as current increased, and the resistance measured at $1 \muA$ was approximately 1000 times lower than the resistance acquired at 1 nA. The transition from an insulating phase to a metallic phase was also confirmed in the temperature-dependent resistance curves under bias voltages. These non-Ohmic behavior was not found in the as-grown films. It is presumed that the removal of dynamic clamping lowers the thermodynamic barrier between the competing phases. The disappearance of the obstacle enables the phase transformation by significantly reduced small current. Our finding of the colossal electro-resistance with an ultra-low power consumption provides a promising pathway into new conceptual devices such as varistor, switch, and transistor based on a single compound material.