Tunable Multi-Level Memory States in Compositionally Graded Lead-Free Ferroelectric Thin Films

Multistate non-volatile ferroelectric memories are promising for in-memory and neuromorphic computing owing to their high speed and low power operation. Yet, overcoming the intrinsic bi-stability of ferroelectric switching to reliably achieve multiple polarization states remains a major challenge. Here, we demonstrate robust multi-level polarization states-exhibiting antiferroelectric-like hysteresis loops-through selective domain switching in compositionally graded BiFeO₃-BaTiO₃ epitaxial thin films. These films display well-separated switching fields and large polarization contrast between adjacent states (ΔP > 40 µC/cm²). Our systematic studies further revealed that the multistate behaviour is attributed to the pinning of ferroelectric domains by oppositely aligned defect dipoles, whose configuration is strongly correlated with the compositional gradient. By engineering this gradient, we effectively tune the internal field and reshape the ferroelectric hysteresis, enabling deterministic control over multiple stable states. This study introduces a new strategy for tailoring ferroelectric energy landscapes, paving the way for high-density, low-power, and adaptive ferroelectric memory and neuromorphic architectures.