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The recent observation of ferroelectricity in the metastable phases of binary metal oxides, such as HfO2, ZrO2, Hf0.5Zr0.5O2, and Ga2O3, has garnered a lot of attention. These metastable ferroelectric phases are typically stabilized using epitaxial strain, alloying, or defect engineering. Here, we propose that hole doping plays a key role in the stabilization of polar phases in binary metal oxides. Using first-principles density-functional-theory calculations, we show that holes in these oxides mainly occupy one of the two oxygen sublattices. This hole localization, which is more pronounced in the polar phase than in the nonpolar phase, lowers the electrostatic energy of the system, and makes the polar phase more stable at sufficiently large concentrations. We demonstrate that this electrostatic mechanism is responsible for stabilization of the ferroelectric phase of HfO2 aliovalently doped with elements that introduce holes to the system, such as La and N. Finally, we show that spontaneous polarization in HfO2 is robust to hole doping, and a large polarization persists even under a high concentration of holes. 

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 discovery of ferroelectricity marks its 100th anniversary this year ( 1 ), and this phenomenon continues to enrich our understanding of many fields of physics and material science, as well as creating subfields on its own. All of the ferroelectrics discovered have been limited to those exhibiting a polar space group of the bulk crystal that supports two or more topologically equivalent variants with different orientations of electric polarization. On pages 1458 and 1462 of this issue, Yasuda et al. ( 2 ) and Vizner Stern et al. ( 3 ), respectively, show that ferroelectricity can be engineered by artificially stacking a nonpolar in bulk, two-dimensional (2D) material, boron nitride (BN). A relatively weak van der Waals (vdW) coupling between the adjacent BN monolayers allows their parallel alignment in a metastable non-centrosymmetric coordination supporting 2D ferroelectricity with an out-of-plane electric polarization. These findings open opportunities to design 2D ferroelectrics out of parent nonpolar compounds.