Bidirectional mechanical switching window in ferroelectric thin films predicted by first-principle-based simulations

Mechanical control of ferroelectric domain evolution has attracted much interest over the past decade.Nevertheless,bidirectional 180°mechanical switching,i.e.,a complete cycle of mechanical writing and then erasing of a ferroelectric nanodomain,has not yet been realized in tip-film architectures.Here,via first-principles-based molecular dynamics simulations,we demonstrate that bidirectional 180°mechanical switching is possible in tip-film architectures when screening condition of ferroelectric films and tip loading force are within an appropriate window.The switching utilizes a delicate competition between the flexoelectric field and an overlooked effective dipolar field.The effective dipolar field dominates at small tip force and trigger switching from a downward single-domain state to an upward poly-domain state,whereas the flexoelectric field dominates at relatively large tip force and enables a back-switching.Bidirectional mechanical switching is achieved by applying tip force pulses with alternatively varying strength.The dipole-dipole interaction dynamics play important roles in mechanical switching.