Origin of 90$^{\circ}$ Domain Wall Pinning in Pb(Zr$_{0.2}$Ti$_{0.8})$O$_{3}$ Heteroepitaxial Thin Films

Researchers have studied the effect of ferroelectric fields in controlling the spin state via electric fields in multiferroic composite structures. For instance, in a bilayer system composed of a ferroelectric perovskite (PbZr$_{0.2}$Ti$_{0.8}$O$_{3})$ and a colossal magnetoresistive (CMR) manganite (La$_{0.8}$Sr$_{0.2}$MnO$_{3}$, LSMO), the spin state in the CMR film can be controlled by switching the ferroelectric polarization state, thereby generating a large magnetoelectric coupling. For this system, the domain's structure and switchability is critically important to the device's performance. We describe transmission-electron-microscopy study of the ferroelectric domains in a epitaxial Pb(Zr$_{0.2}$Ti$_{0.8})$O$_{3}$(PZT) film grown on La$_{0.8}$Sr$_{0.2}$MnO$_{3}$/SrTiO$_{3}$(001). We directly observe the pinning of 90$^{\circ}$ domain walls by pairs of misfit dislocations with Burgers vectors \textbf{\textit{a}}[100] and \textbf{\textit{a}}[001]. Model calculations based on the elastic theory confirm our finding that, in addition to the depolarization field surrounding the dislocation, the strain field of misfit dislocation-pairs plays the primary role in the formation and pinning of \textbf{\textit{a}} domains.