Low ferroelectric hysteresis by strain and polarization gradients

Phase transitions are among the most interesting and ubiquitous phenomena in nature. In materials science, they are responsible for the technological impact of ferromagnets, ferroelectrics, shape-memory alloys or memristors. First order phase transitions are associated to desirably large, nonlinear changes in the order parameters (polarization, magnetization, resistance, etc) and susceptibilities, but at the same time they are often coupled to the presence of hysteresis, which is related to energy losses. Here local inhomogeneous strain and polarization gradients are found in BaTiO₃ thin films, connecting a tetragonal to an orthorhombic phase through a bridging monoclinic phase. The associated gradual polarization rotation gives bulk-like values for the coercive field and fully reversible low hysteresis ferroelectric switching. An essentially temperature-independent huge dielectric constant (~4000) is found, together with a rather large d₃₃ of 100 pm/V and the ability to create a strain diode with a five-fold increase of the piezoresponse by asymmetric electrodes. This low hysteresis direction enables energy-efficient electromechanical functionalities and can aid in the search for new ferroelectrics, piezoelectrics and ferromagnets.