Stacking Ferroic Oxide Membranes with Controlled Twist Angles

Stacking freestanding ferroic oxide membranes with controlled twist angles offer rich opportunities to achieve novel electronic and magnetic states with highly tunable order parameters. In this work, we discuss the fabrication and characterization of two types of twisted oxide membranes: the ferroelectric PbZr_0.2Ti_0.8O₃ (PZT) and ferrimagnetic NiCo₂O₄ (NCO), examining the moiré patterns at controlled twist angles. Epitaxial 5-28 nm PZT (001) and NCO (001) films were deposited on Sr₃Al₂O₆-buffered SrTiO₃ and LaAlO₃ substrates using off-axis RF magnetron sputtering followed by water etching to dissolve the Sr₃Al₂O₆ buffer layer. X-ray diffraction (XRD) and atomic force microscopy (AFM) studies confirmed the high crystallinity and smooth surface of the samples, respectively, before and after water etching. The suspended PZT and NCO membranes were lifted by a gel-film stamp and stacked into twisted bilayers. Electron diffraction reveals distinct patterns from the top and bottom oxide layers, with the spacing consistent with the designed twist angle. We also observed the additional diffraction patterns from the lattice distortion, with periods matching those of the moiré pattern. We discuss the study of in-plane shear strain using high-resolution transmission electron microscopy. Our study provide material information for engineering the ferroic order in twisted oxide membrane heterostructures.