A role for vacancies: Hafnia-based materials are of interest because of their potential use in microelectronic components. Hafnia-oxide is a ferroelectric material, but whether the polarization switching comes from the polar crystal phases or the migration of oxygen vacancies has remained an open question. Nukala et al. attempted to resolve this controversy by conducting electron microscopy during the operation of a hafnium zirconium oxide capacitor. The authors found that vacancy migration is intertwined with the ferroelectric switching, which has implications for the use of these materials in a range of microelectronic applications. Science , this issue p. 630

Ferroelectric tunnel barriers in between two ferromagnetic electrodes (multiferroic tunnel junctions, or MFTJs) hold great promise for future microelectronic devices. Here, we utilize ${\mathrm{Hf}}_{0.5}{\mathrm{Zr}}_{0.5}{\mathrm{O}}_2$ (HZO) tunnel barriers with an ultralow thickness of only 2 nm, epitaxially grown on ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{Mn}\mathrm{O}}_3$ ferromagnetic bottom electrodes and with cobalt top electrodes. Both tunneling electroresistance and tunneling magnetoresistance effects are observed, demonstrating four nonvolatile resistance states in HZO-based junctions. The large band gap and excellent homogeneity of the HZO tunnel barriers enable a high yield of working devices, as well as devices with sizes of tens of micrometers. This allows working with fixed electrodes, as opposed to the use of scanning probes, bringing MFTJs closer to applications.