The Coupling of Ferroelectric Polarization and Oxygen Vacancy Migration Enables Electrically Controlled Thermal Memories

Here we investigate epitaxial Hf_0.5Zr_0.5O₂ ferroelectric thin films as potential candidates to be used as non-volatile electric-field-modulated thermal memories. The electric-field dependence of the thermal conductivity of metal/Hf_0.5Zr_0.5O₂/Y₂O₃:ZrO₂ devices is found to be hysteretic-resembling a polarization vs. electric field hysteresis loop-, reaching a maximum (minimum) at large applied positive (negative) electric fields from the top metallic electrode. This dynamic thermal response is compatible with the effects of the coupling between the ferroelectric polarization and oxygen ion migration in the Hf_0.5Zr_0.5O₂ layer, in which the oxygen vacancies are the main phonon scattering centers and the polarization acts as an electrically active ion migration barrier that creates the hysteresis. This new mechanism enables two non-volatile states: high (ON) and low (OFF) thermal conductivity states when the electric field is removed, with an ON/OFF ratio of 1.6, which can be switched with applied voltages lower than -5 and +5 V, respectively. Both the ON and OFF states exhibit high stability over time, though the switching speed is limited by ion mobility in the Y₂O₃:ZrO₂ electrode.