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Recent advances in the exploration of two-dimensional (2D) van der Waals (vdW) ferroelectrics revealed not only a wealth of fundamentally exciting properties but also a strong potential for nanoelectronic applications facilitated by their semiconducting nature and tunable polarization-coupled physical properties. Here, using scanning probe microscopy techniques, we investigate the effects of mechanical stress and optical illumination on the transport behavior of one of the most actively studied 2D ferroelectrics, α-In2Se3. Local I–V measurements reveal a strongly asymmetric polarization-dependent conductivity of α-In2Se3, which can be continuously tuned by the tip-induced mechanical pressure. While the local conductivity increases up to two orders of magnitude for both polarization states, the upward polarization displays a much sharper change. Further enhancement of conductivity by an order of magnitude is observed under optical illumination, resulting from a cumulative modulation of the junction barrier via polarization, strain, and optical excitation. The obtained results make α-In2Se3 a promising material for application in electronic devices with optomechanical functionality. 

Abstract Chromia (Cr₂O₃) is a magnetoelectric oxide that permits voltage‐control of the antiferromagnetic (AFM) order, but it suffers technological constraints due to its low Néel Temperature (T_N≈307 K) and the need of a symmetry‐breaking applied magnetic field to achieve reversal of the Néel vector. Recently, boron (B) doping of Cr₂O₃films led to an increaseT_N>400 K and allowed the realization of voltage magnetic‐field free controlled Néel vector rotation. Here, the impact of B doping is directly imaged on the formation of AFM domains in Cr₂O₃thin films and elucidates the mechanism of voltage‐controlled manipulation of the spin structure using nitrogen‐vacancy (NV) scanning probe magnetometry. A stark reduction and thickness dependence of domain size in B‐doped Cr₂O₃(B:Cr₂O₃) films is found, explained by the increased germ density, likely associated with the B doping. By reconstructing the surface magnetization from the NV stray‐field maps, a qualitative distinction between the undoped and B‐doped Cr₂O₃films is found, manifested by the histogram distribution of the AFM ordering, that is, 180°domains for pure films, and 90°domains for B:Cr₂O₃films. Additionally, NV imaging of voltage‐controlled B‐doped Cr₂O₃devices corroborates the 90°rotation of the AFM domains observed in magnetotransport measurement. 

Abstract One of the exceptional features of the van der Waals (vdW) ferroelectrics is the existence of stable polarization at a level of atomically thin monolayers. This ability to withstand a detrimental effect of the depolarization fields gives rise to complex domain configurations characterized, among others, by the presence of layered “antipolar” head‐to‐head (H‐H) or tail‐to‐tail (T‐T) dipole arrangements. In this study, tomographic piezoresponse force microscopy (TPFM) is employed to study the 3D polarization arrangement in vdW ferroelectricα‐In₂Se₃. Sequential removal of thin layers from the polar surface using the PFM tip reveals a complex 3D profile of the domain walls in theα‐In₂Se₃crystals. Antiparallel domain layers stacked along the polar direction are also observed by PFM imaging of the non‐polar surfaces showing that H‐H and T‐T domain boundaries are commonly present inα‐In₂Se₃. Application of TPFM to the electrically written domains allows evaluation of their geometrical lateral‐to‐vertical size aspect ratio, which shows a strong prevalence for the sidewise expansion in comparison to the forward growth. LocalI–Vmeasurements reveal a strong polarization direction dependence of conductivity due to the modulation of the energy barrier height as corroborated by theoretical modeling. 

Abstract Strong coupling between polarization (P) and strain (ɛ) in ferroelectric complex oxides offers unique opportunities to dramatically tune their properties. Here colossal strain tuning of ferroelectricity in epitaxial KNbO₃thin films grown by sub‐oxide molecular beam epitaxy is demonstrated. While bulk KNbO₃exhibits three ferroelectric transitions and a Curie temperature (T_c) of ≈676 K, phase‐field modeling predicts that a biaxial strain of as little as −0.6% pushes itsT_c> 975 K, its decomposition temperature in air, and for −1.4% strain, toT_c> 1325 K, its melting point. Furthermore, a strain of −1.5% can stabilize a single phase throughout the entire temperature range of its stability. A combination of temperature‐dependent second harmonic generation measurements, synchrotron‐based X‐ray reciprocal space mapping, ferroelectric measurements, and transmission electron microscopy reveal a single tetragonal phase from 10 K to 975 K, an enhancement of ≈46% in the tetragonal phase remanent polarization (P_r), and a ≈200% enhancement in its optical second harmonic generation coefficients over bulk values. These properties in a lead‐free system, but with properties comparable or superior to lead‐based systems, make it an attractive candidate for applications ranging from high‐temperature ferroelectric memory to cryogenic temperature quantum computing.