Investigation of electrically driven transition in magnetite nanostructures
Hafner, Jason H.
Doctor of Philosophy
Magnetite, Fe3O4, is a strongly electronically correlated system and thus exhibits remarkable electrical and magnetic properties, including the Verwey transition at TV 122 K, which has attracted much attention since its 1939 discovery. Fe3O 4 has recently revealed a new effect. By performing experiments at the nanoscale, we have discovered a novel electric-field driven transition (EFD) in magnetite below TV, from high- to low-resistance states driven by application of high bias. The EFD transition is detected both in Fe3O4 nanoparticles and thin films, is hysteretic in voltage under continuous biasing, and is not caused by self-heating. In this thesis we report on a thorough investigation of this new EFD transition. First, we unveil the origin of hysteresis observed in I-V curves. By applying voltage in a pulsed manner with controlled parameters, we unambiguously demonstrate that while the transition is field-driven, hysteresis results from Joule heating in the low-resistance state. A simple relaxation-time thermal model captures the essentials of the hysteresis mechanism. Second, by doing multilead (four-terminal) electrical measurements, we quantitatively separate the contributions of the Fe3O4 channel and each metal/electrode interface, and explore the contact effects upon testing devices incorporating various contact metals We demonstrate that on the onset of the transition, contact resistances at both source and drain electrodes and the resistance of Fe3O4 channel decrease abruptly. Finally, we measured the distribution of switching voltages, V sw, its evolution with temperature, and its dependence on out-of-plane magnetic field. Based on the experimental facts collected in this work we suggest the possible mechanism of EFD transition in Fe 3O4 as a charge gap closure by electric field. This is one of the first experimental observation of a theoretically predicted EFD transition in correlated insulators. These studies demonstrate that nanoscale, nonequilibrium probes can reveal much about the underlying physics of strongly correlated materials.
Electromagnetics; Condensed matter physics