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Ferroelectrics with electrically switchable spontaneous polarizations can be used for information storage, where a low switching barrier is favorable to reduce the energy cost and enhance the speed for data writing. Meanwhile their robustness at working temperature should be ensured, which is a challenge for the designs of low‐barrier ferroelectrics. Here we review several types of ferroelectric mechanisms that may render both low switching barriers and room‐temperature robustness, which have been theoretically proposed in previous studies. (1) The prediction of sliding ferroelectricity with ultralow switching barriers has been experimentally confirmed in a series of van der Waals layers, which may enable convenient electrical control of various physical properties in 2D materials, like magnetic, photovoltaic, valleytronic and topological properties. (2) Hydrogen‐bonded ferroelectricity spontaneously formed by head‐to‐tail chains can be switched by proton‐transfer crossing a low barrier, and a mechanism of ultra‐high piezoelectricity utilizing the specific features of hydrogen bonding has been proposed. (3) High‐ionicity ferroelectricity induced by covalent‐like ionic bondings may entail high polarizations and low barriers during switching, which is attributed to the features of long‐range Coulomb interaction, and the long ion‐displacements crossing unitcell may give rise to unconventional ferroelectricity with quantized polarizations even in crystals of non‐ferroelectric point groups. Those low‐barrier ferroelectric mechanisms may bring in both new physics and technological advances, which are to be further explored. This article is categorized under:Structure and Mechanism > Computational Materials Science [ABSTRACT FROM AUTHOR]

We propose a new approach to obtain ultra-high piezoelectric coefficients that can be infinitely large theoretically, where ferroelectrics with strain-sensitive Curie temperature are necessary. We show the first-principles plus Monte Carlo simulation evidence that many hydrogen-bonded ferroelectrics (e.g. organic PhMDA) can be ideal candidates, which are also flexible and lead-free. Owing to the specific features of hydrogen bonding, their proton hopping barrier will drastically increase with prolonged proton transfer distance, while their hydrogen-bonded network can be easily compressed or stretched due to softness of hydrogen bonds. Their barriers as well as the Curie temperature can be approximately doubled upon a tensile strain as low as 2%. Their Curie temperature can be tuned exactly to room temperature by fixing a strain in one direction, and in another direction, an unprecedented ultra-high piezoelectric coefficient of 2058 pC/N can be obtained. This value is even underestimated and can be greatly enhanced when applying a smaller strain. Aside from sensors, they can also be utilized for converting either mechanical or thermal energies into electrical energies due to high pyroelectric coefficients. [ABSTRACT FROM AUTHOR]

[Display omitted] • We show that prevalent 2D ferromagnetic materials like CrI 3 , Cr 2 Ge 2 Te 6 and Fe 3 GeTe 2 , can all be entailed with sliding ferroelectricity in their bilayers and are thus multiferroic. • Fe 3 GeTe 2 bilayer exhibits magnetoelectric coupling effect with a net magnetization moment switchable upon ferroelectric switching. • Fe 3 GeTe 2 bilayer is shown to be both piezoelectric and piezomagnetic as both its vertical electric polarization and the net magnetic moment increase upon vertical compression, giving rise to "piezo-multiferroic" effect. It has been predicted that for a series of two-dimensional (2D) materials, their van der Waals bilayers may possess sliding ferroelectricity with vertical polarizations switchable via interlayer sliding. Such intriguing ferroelectricity has been recently experimentally confirmed in a series of nonmagnetic bilayer systems like BN, MoS 2 , etc., while magnetic bilayers remain unexplored in those reports. In this paper we show first-principles evidence that three most prevalent 2D ferromagnetic materials, CrI 3 , Cr 2 Ge 2 Te 6 and Fe 3 GeTe 2 , can all be entailed with sliding ferroelectricity (FE) with vertical polarizations over 0.1 pC/m in their bilayers and are thus multiferroic. In particular, Fe 3 GeTe 2 bilayer may exhibit magnetoelectric coupling effect with a net magnetization moment of 0.01 μ B per unit cell switchable upon ferroelectric switching. Moreover, it is shown to be both piezoelectric and piezomagnetic as both its vertical electric polarization and the net magnetic moment can be approximately doubled when the interlayer distance is compressed by 10%, giving rise to unprecedented "piezo-multiferroic" effect where the magnetoelectric coupling effect is intensified with reduced interlayer distance. [ABSTRACT FROM AUTHOR]