Glassy response to gate and magnetic fields in ultrathin manganite films

We have studied the low temperature behavior of thin films of La$_{0.8}$Ca$_{0.2}$MnO$_{3}$ in a field effect geometry. The films exhibit the usual insulator-metal transition near the Curie temperature, but below 30K there is a re-entrant insulating regime. In this regime, we observe glassy dynamics and a hierarchical response in the resistance to both electronic and magnetic perturbations. In addition, the magnetization of the film responds to electrostatic gating. We interpret these results in a framework where the system dynamics are governed by strain relaxation. This work was supported by the National Science Foundation under grant NSF/DMR-0138209 and the Univ. of Minnesota MRSEC (NSF/DMR-0212032).