Electric field effect on transition metal oxide thin films

The main objective of this dissertation is to study the electric field effect in complex transition metal oxides (TMO) with perovskite structure, primarily the mixed-valent manganites and also the high-temperature superconducting cuprates. Both manganites and cuprates show non-Fermi liquid behaviors with strong interplay of charge, spin, orbital and lattice. Additionally, carriers in some TMO's are distributed inhomogeneously, forming separated phases with different electric and magnetic properties. Particularly, in some manganites, ferromagnetic metallic (FMM) phase and charge ordered insulating (COI) phase coexist in certain temperature regimes. In such phase-separated systems, the electric field is expected to change the material properties in interesting ways.; Since strain exists in all thin film devices, we have studied the strain effect in pulsed laser deposited La_0.67−xPr_xCa _0.33MnO₃ thin films. Biaxial strain has been applied by changing the film thickness and selecting substrates with different lattice mismatch. Strain is found to play an important role in controlling the FMM/COI ratio. As a result, strong correlation was observed between the strain and the electrical transport and magnetic properties of these films.; Devices have been fabricated with channels of La_0.7Ca _0.3MnO₃, Nd_0.7Sr_0.3MnO₃, La_0.7Ba_0.3MnO₃, La_0.5Ca_0.5 MnO₃ and La_0.54Pr_0.13Ca _0.33MnO₃. On the devices with La_0.7Ca_0.3 MnO₃ channel, an electro-resistance (ER) effect of 76% was observed under an electric field of 4 × 105 V/cm. However, the capacitance vs. gate voltage measurement shows that the carrier density modulation within the Thomas-Fermi screening length is smaller than 1%. Therefore, such a large ER is possible only if the manganite channel is phase separated, i.e. the percolative process near the insulator-to-metal transition amplifies the effect of a small carrier density modulation. The observed complimentary nature of ER and MR is also consistent with the phase separation picture. This large electric field effect is absent in less phase separated Nd _0.7Sr_0.3MnO₃, La_0.7Ba_0.3MnO ₃, and La_0.5Ca_0.5MnO₃, while the effect is strong also in the phase-separated La_0.54Pr_0.13Ca _0.33MnO₃.; Another system we have studied is oxygen deficient YBa₂Cu ₃O_7−δ. The oxygen content and the consequent carrier density are controlled by low temperature annealing in oxygen and argon. The temperature dependence of ER shows consistent and interesting features. The two characteristic crossover temperatures T₁ and T₂ might have connections with the temperature T₁* and T₂* in the so-called stripe phase scenario.; In this dissertation, field effect devices has been shown to be a powerful tool to probe the distribution and dynamics of carriers in the TMO systems. Our experiments open a new avenue for using the electronically phase-separated TMO as the channel materials in field effect devices. Along these lines more materials could be explored and the device structures could be further improved.