Non-Volatile Memory Effects in Transition Metal Oxides*

We have recently reported on a non-volatile memory effect we discovered in correlated oxides with temperature-driven insulator-metal transitions (IMT), such as VO₂, V₂O₃ and NdNiO₃. The memory appears as a resistance increase at predefined temperatures that are set or erased by simple heating-cooling (i.e., ramp-reversal) protocols, thus it is coined the ramp reversal memory (RRM) [1]. The characteristics of this memory effect do not coincide with any previously reported history or memory effects in similar systems.

We will review the broad range of experimental features of the RRM, including the ability to write multiple memories of the device, the effects of writing and erasing speeds on the memory, and the role of epitaxial coupling to the substrate in limiting the magnitude of the memory [2-4]. From these we conclude that the main ingredients for the effect to arise are the spatial phase-separation of metallic and insulating regions during the IMT and the coupling of local dynamic changes (strains or ionic motion) to the critical temperature of the phase transition, resulting in localized increase of the transition temperature along phase boundaries. Similar memory effects are predicted to arise in other materials that possess these rather common ingredients. To this end, we have recently observed a RRM-like memory effect in the Mott transition of TaS₂, showing however somewhat different features than those reported in oxides.