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Multiferroic materials composed of ferromagnetic and ferroelectric components are interesting for technological applications due to sizable magnetoelectric coupling allowing the control of magnetic properties by electric fields. Due to being compatible with the silicon-based technology, HfO₂-based ferroelectrics could serve as a promising component in the composite multiferroics. Recently, a strong charge-mediated magnetoelectric coupling has been predicted for a Ni/HfO₂multiferroic heterostructure. Here, using density functional theory calculations, we systematically study the effects of the interfacial oxygen stoichiometry relevant to experiments on the magnetoelectric effect at the Ni/HfO₂interface. We demonstrate that the magnetoelectric effect is very sensitive to the interface stoichiometry and is reversed if an oxidized Ni monolayer is formed at the interface. The reversal of the magnetoelectric effect is driven by a strong Ni−O bonding producing exchange-split polarization-sensitive antibonding states at the Fermi energy. We argue that the predicted reversal of the magnetoelectric effect is typical for other 3dferromagnetic metals, such as Co and Fe, where the metal-oxide antibonding states have an opposite spin polarization compared to that in the pristine ferromagnetic metals. Our results provide an important insight into the mechanism of the interfacial magnetoelectric coupling, which is essential for the physics and application of multiferroic heterostructures.

 

The Rashba effect has recently attracted great attention owing to emerging physical properties associated with it. The interplay between the Rashba effect and the Zeeman effect, being produced by the exchange field, is expected to broaden the range of these properties and even result in novel phenomena. Here we predict an insulator-to-conductor transition driven by the Rashba–Zeeman effect. We first illustrate this effect using a general Hamiltonian model and show that the insulator-to-conductor transition can be triggered under certain Rashba and exchange-field strengths. Then, we exemplify this phenomenon by considering an Ag₂Te/Cr₂O₃heterostructure, where the electronic structure of the Ag₂Te monolayer is affected across the interface by the proximity effect of the Cr₂O₃antiferromagnetic layer with well-defined surface magnetization. Based on first-principles calculations, we predict that such a system can be driven into either insulating or conducting phase, depending on the surface magnetization orientation of the Cr₂O₃layer. Our results enrich the Rashba–Zeeman physics and provide useful guidelines for the realization of the insulator-to-conductor transition, which may be interesting for experimental verification.

 

Abstract Dielectrics have long been considered as unsuitable for pure electrical switches; under weak electric fields, they show extremely low conductivity, whereas under strong fields, they suffer from irreversible damage. Here, we show that flexoelectricity enables damage-free exposure of dielectrics to strong electric fields, leading to reversible switching between electrical states—insulating and conducting. Applying strain gradients with an atomic force microscope tip polarizes an ultrathin film of an archetypal dielectric SrTiO 3 via flexoelectricity, which in turn generates non-destructive, strong electrostatic fields. When the applied strain gradient exceeds a certain value, SrTiO 3 suddenly becomes highly conductive, yielding at least around a 10 8 -fold decrease in room-temperature resistivity. We explain this phenomenon, which we call the colossal flexoresistance, based on the abrupt increase in the tunneling conductance of ultrathin SrTiO 3 under strain gradients. Our work extends the scope of electrical control in solids, and inspires further exploration of dielectric responses to strong electromechanical fields. 

Resonant tunneling is a quantum‐mechanical effect in which electron transport is controlled by the discrete energy levels within a quantum‐well (QW) structure. A ferroelectric resonant tunneling diode (RTD) exploits the switchable electric polarization state of the QW barrier to tune the device resistance. Here, the discovery of robust room‐temperature ferroelectric‐modulated resonant tunneling and negative differential resistance (NDR) behaviors in all‐perovskite‐oxide BaTiO₃/SrRuO₃/BaTiO₃QW structures is reported. The resonant current amplitude and voltage are tunable by the switchable polarization of the BaTiO₃ferroelectric with the NDR ratio modulated by ≈3 orders of magnitude and an OFF/ON resistance ratio exceeding a factor of 2 × 10⁴. The observed NDR effect is explained an energy bandgap between Ru‐t_2gand Ru‐e_gorbitals driven by electron–electron correlations, as follows from density functional theory calculations. This study paves the way for ferroelectric‐based quantum‐tunneling devices in future oxide electronics.