A material that holds an electric polarisation – one side slightly positive, the other slightly negative – with no voltage applied, and that can be flipped by a field and stay flipped. That memory of the last field applied is what makes it useful for storing data.
Going deeper
A ferroelectric has two stable polarisation states separated by an energy barrier. A strong enough field tilts the wells and switches it, and after the field is removed the polarisation stays – the hysteresis loop that makes ferroelectrics useful as memory.
A polarisation that remembers
In a ferroelectric, ions shift slightly from symmetric positions so that every carries a small electric dipole, and the dipoles line up. The two opposite arrangements have the same energy, separated by a barrier. An electric field larger than the coercive field pushes the crystal over the barrier and reverses the polarisation, which then stays after the field is removed: the loop. Above its the distortion disappears and the material becomes paraelectric.
Ferroelectricity was first identified in Rochelle salt around 1920. Today ferroelectrics serve in capacitors, , actuators and non-volatile memories, and hafnium-oxide-based ferroelectrics have been built into silicon chips.
Keeping a polarisation in a thin film
An out-of-plane polarisation leaves bound charges on the two surfaces, and their depolarising field opposes the polarisation. As a film thins, that field wins, and conventional oxide ferroelectrics lose their order below a critical thickness of a few unit cells unless electrodes screen the charges well.
Layered materials offer ways around this. In-plane polarisation produces no depolarising field across the film, as in atomically thin SnTe and in SnS and SnSe. CuInP2S6 keeps an out-of-plane polarisation to room temperature, with a transition near 315 K and switching demonstrated in about 4 nm thick. α-In2Se3 was predicted to have coupled in-plane and out-of-plane polarisation down to a single layer, and ferroelectric switching has since been reported down to monolayer flakes about 1.2 nm thick. , in which the polarisation lives at the interface between two layers, is a third route.
Proving it is ferroelectric
A hysteresis loop in force microscopy is not proof on its own. Surface charging, ion migration and electrochemical reactions under the tip produce loops in materials that are not ferroelectric. Stronger evidence combines domains that can be written and read back, stable over time; polarisation–field loops measured with pulse sequences that subtract leakage and capacitive currents; a confirmed by or structural measurements; and consistent device behaviour.
Leakage is the main practical difficulty in very thin layers, because it hides switching currents and can cause the loops being measured. Devices combine 2D ferroelectrics with 2D in ferroelectric , whose depends on the stored polarisation, and in ferroelectric .
For specialists
A spontaneous, switchable electric polarisation produced by a structural distortion that breaks . In 2D the usual obstacle – depolarising fields destroying the polarisation as a film thins – is sidestepped in three ways: in-plane polarisation, layered compounds whose dipoles survive to the monolayer such as CuInP2S6 and In2Se3, and sliding ferroelectricity, where the polarisation belongs to the interface between layers rather than to the layers themselves.