Nonvolatile Ferroelastic Strain from Flexoelectric Internal Bias Engineering

Internal bias in ferroelectric materials is a well-known phenomenon that offsets the ferroelectric polarization hysteresis loop along the electric-field axes. Control over this degree of freedom could lead to alternative classes of ferroelectric devices, as with control over the analogous concept of exchange bias in ferromagnetic devices. Currently, there lacks a systematic approach to engineering internal bias by design that allows for device-by-device control over this parameter, leading to difficulty in translating these concepts to the large-scale integration of ferroelectric electronics. In this work, the flexoelectric effect is used to engineer internal bias through the controlled deposition of stressed thin films onto ferroelectrics. Large strain gradients are generated near the surface of ferroelectric Pb(Mg1/3Nb2/3)(0.71) Ti0.29O3 single crystals through the deposition of stressed thin film strain gauges with increasing film force (film stress x film thickness). Using this technique, it is possible to continuously tune internal bias to control ferroelastic strain applied by the ferroelectric versus applied electric field, thereby achieving control of ferroelastic nonvolatility. Flexoelectric control of internal bias is verified by density functional theory and finite element analysis using a model with no free parameters, which matches both the expected magnitude and directionality of the flexoelectric field, and then further confirmed by piezoresponse force microscopy. This stress-induced flexoelectric effect utilizes popular strain engineering techniques already widely adopted by commercial complementary metal-oxide-semiconductor (CMOS) industrial fabrication processes, therefore sharing its advantages in scalability and reliability. Utilizing these techniques may lead to device-by-device level control of nonvolatility in ferroelectric field-effect or straintronic devices.