Deterministic control of morphotropic phase boundary by electric field

Abstract

A multiferroic refers to a compound that has both ferromagnetic (or antiferromagnetic) order and ferroelectric (or antiferroelectric) order. It has been studied extensively for understanding inter-coupled phenomena and realizing magnetoelectric applications in a way that a spontaneous magnetization is switched by an applied electric field and a spontaneous electric polarization is controlled by an applied magnetic field. Bismuth ferrite (BiFeO3; BFO) receives a lot of attentions because it has ferroelectric and antiferromagnetic orders at room temperature. Moreover, morphotropic phase boundary (MPB) in which tetragonal-like monoclinic phase and distorted rhombohedral phase coexist, has been found in BFO films grown on LaAlO3 substrates. Since the MPB regions in BFO films possess large piezoelectric response and non-zero spontaneous magnetic moment, they hold promises for electromechanical or/and magnetoelectric devices. For the goals, realization of well-aligned MPB and reliable electrical control of the MPB should be necessarily studied. In the thesis, we report that La 5% substitution provides a pathway to formation of well-aligned stripe MPB regions. Using an in-plane electric field produced by a tip of piezoresponse force microscopy (PFM), we can create and erase the MPBs with good continuity in large length scales reversibly. By changing the scanning direction of the electrically-biased tip, the direction of MPB alignment can be controlled deterministically.