Salia Cherifi

Predictive domain wall (DW) nucleation, propagation, and pinning is achieved by He ⁺ ion irradiation of spin–orbit torque thin film devices. Engineering the magnetic anisotropy landscape with nm precision, we demonstrate that non-irradiated gaps as small as 10 nm can act as reliable pinning sites, highlighting the potential of this method for device miniaturization. We also create artificial nucleation centers, enabling deterministic control of the switching process and subsequent reversal propagation. This study presents a simple and effective strategy for advancing nanoscale design of energy-efficient DW memory and logic devices.

Ferroelectric instabilities in perovskites are known to be suppressed by a moderate hydrostatic pressure. The prediction of their re-entrance in a much higher pressure regime is well accepted theoretically, but a conclusive experimental confirmation is still missing. Here, we show its occurrence in a classical but comparatively underlooked ferroelectric perovskite KNbO_{3}. We use single crystal x-ray diffraction, infrared and Raman spectroscopy, and second-harmonic generation to explore the phase transition sequence at high pressures up to 63 GPa. We show that polar cation displacements emerge locally but have to combine with tilts of the oxygen octahedra that are also developing under pressure. This results in a macroscopically centrosymmetric phase with an incommensurate modulation that reflects the tight competition between the polar and tilt instabilities. Soft modes associated with the tilts and the modulation, along with persistent order-disorder signatures, are clearly observed, suggesting that local disorder plays an important role in mediating this competition.

A significant progress toward next-generation electronic devices that combine memory and processing functions could involve the electrical manipulation of charge carrier spin textures in semiconductors. In this context, GeTe has recently emerged as a promising ferroelectric Rashba semiconductor, exhibiting a giant spin splitting in its band structure. This remarkable property stems from the inversion symmetry breaking induced by its ferroelectric polarization. Here, we address the control of the domain structure of GeTe thin films grown on miscut silicon substrates. We show that the domain structure of the GeTe thin films is strongly influenced by the miscut direction with respect to the nominal Si(111) substrate. Considering miscut in the [1¯1¯2] direction, highly crystallized GeTe films are grown without twins, and the domain structure exhibits a reduction in domain size parallel to the step edges. In the case of [112¯] miscut direction, we evidence that thin films exhibit a predominance of twins and a complex ferroelectric structure that is affected by a large density of interfacial defects. Our results also show that the miscut direction plays a key role in the growth morphology of the GeTe thin film. All these results support the view that atomic steps on silicon substrates have a profound effect on the structure and growth morphology of GeTe thin films as well as on the domain structure via local stress relaxation mechanisms.

This theoretical review examines the conditions that give rise to the formation of flexo-sensitive chiral polar structures in thin films and core–shell ferroelectric nanoparticles. It also analyzes possible mechanisms by which the flexoelectric effect impacts the polarization structure in core–shell ferroelectric nanoparticles. Special attention is given to the role of the anisotropic flexoelectric effect in forming a unique type of polarization states with distinct chiral properties, referred to as “flexons.” In the first half of the review, we study the influence of the flexoelectric coupling on the polarity, chirality, and branching of metastable labyrinthine domain structures in uniaxial ferroelectric core–shell nanoparticles. We reveal that the transition from sinuous branched domain stripes to spiral-like domains occurs gradually as the flexoelectric coupling strength is increased. Our findings indicate that the joint action of the flexoelectric effect and chemical strains, termed as “flexo-chemical” coupling, can significantly influence the effective Curie temperature, polarization distribution, domain morphology, and chirality in multiaxial ferroelectric core–shell nanoparticles. Furthermore, we demonstrate that the combination of flexo-chemical coupling and screening effects leads to the appearance and stabilization of a chiral polarization morphology in nanoflakes of van der Waals ferrielectrics covered by a shell of ionic-electronic screening charge. In the second half of the review, we discuss several advanced applications of flexo-sensitive chiral polar structures in core–shell ferroelectric nanoparticles for nanoelectronics elements and cryptography. We underline the possibilities of the flexoelectric control of multiple-degenerated labyrinthine states, which may correspond to a differential negative capacitance (NC) state stabilized in the uniaxial ferroelectric core by the presence of a screening shell. We show that the paraelectric-like state of van der Waals ferrielectric nanoflakes covered by a shell of ionic-electronic screening charge exhibits a pronounced NC effect over a relatively wide range of nanoflake thicknesses, flexo-chemical strains, and surface charge densities.

We evidence the ferroelectricity of two isostructural magnetic coordination networks obtained from a chiral imidazolium dicarboxylate salt ((S, S)‐1,3‐bis(1‐carboxylethyl)imidazolium or [HL*]) and Gd³⁺ or Dy³⁺ ions in presence of oxalic acid (H₂ox). The two compounds exhibit a chiral structure P1 and ferroelectric properties. The crystallographic study shows that the Dy based‐compound is isostructural to the Gd analog. The non‐centrosymmetry of the two samples was further checked by means of nonlinear optical spectroscopy. The room temperature ferroelectric character of the compounds has been investigated through by an original method, derived from that used for thin films, made possible by preparing the sample as thin slices of resin containing inclusions of the coordination compounds.

Ferroelectric domain boundaries are quasi-two-dimensional functional interfaces with high prospects for nanoelectronic applications. Despite their reduced dimensionality, they can exhibit complex non-Ising polarization configurations and unexpected physical properties. Here, the impact of the three-dimensional (3D) curvature on the polarization profile of nominally uncharged 180 degrees domain walls in LiNbO3 is studied using second-harmonic generation microscopy and 3D polarimetry analysis. Correlations between the domain-wall curvature and the variation of its internal polarization unfold in the form of modulations of the Neel-like character, which we attribute to the flexoelectric effect. While the Neel-like character originates mainly from the tilting of the domain wall, the internal polarization adjusts its orientation due to the synergetic upshot of dipolar and monopolar bound charges and their variation with the 3D curvature. Our results show that curved interfaces in solid crystals may offer a rich playground for tailoring nanoscale polar states.

He⁺ ion irradiation is used to pattern multiple areas of Pt/Co/W films with different irradiation doses in Hall bars. The resulting perpendicular magnetic anisotropy landscape enables selective multilevel current-induced switching with full deterministic control of the position and order of the individual switching elements. Key pattern design parameters are specified, opening a way to scalable multi-level switching devices.

Second‐harmonic generation (SHG) is a nonlinear optical method allowing the study of the local structure, symmetry, and ferroic order in noncentrosymmetric materials such as ferroelectrics. The combination of SHG microscopy with local polarization analysis is particularly efficient for deriving the local polarization orientation. This, however, entails the use of tedious and time‐consuming modeling methods of nonlinear optical emission. Moreover, extracting the complex domain structures often observed in thin films requires a pixel‐by‐pixel analysis and the fitting of numerous polar plots to ascribe a polarization angle to each pixel. Here, the domain structure of GeTe films is studied using SHG polarimetry assisted by machine learning. The method is applied to two film thicknesses: A thick film containing large domains visible in SHG images, and a thin film in which the domains' size is below the SHG resolution limit. Machine learning‐assisted methods show that both samples exhibit four domain variants of the same type. This result is confirmed in the case of the thick film, both by the manual pixel‐by‐pixel analysis and by using piezoresponse force microscopy. The proposed approach foreshows new prospects for optical studies by enabling enhanced sensitivity and high throughput analysis.