Multiferroics are a family of materials in which two properties that do not usually go together coexist: magnetism and ferroelectricity. Magnetism occurs naturally in some materials (such as iron) which have a remanent magnetization, i.e. it remains after the magnetic field is removed. The direction of the remanent magnetization can be reversed by applying an opposite magnetic field, thus providing a memory effect. A ferroelectric compound has electric dipoles which, when aligned by an electric field, generate a polarization, also remanent, which can be reversed by applying an electric field. Normally, ferroelectricity – and thus multiferroic character – is only found in insulators, but a team from the Unité Mixte de Physique CNRS, Thales, Université Paris-Saclay (in collaboration with the CNR SPIN and the University Federico II of Naples, the Paul Scherrer Institute and the CRISMAT of Caen) has recently discovered a new type of material that is both ferroelectric and magnetic, and at the same time is an excellent electrical conductor.

This new material is a type of two-dimensional electron gas – a metallic conductor only a few atomic planes thick – formed at the interface between two compounds of the perovskite oxide family. The results of this study show for the first time that ferroelectricity, magnetism and metallicity are found in the same system. Moreover, these three properties are coupled, i.e. acting on one of them affects the others: for example, applying an electric field to reverse the ferroelectric polarization remanently modifies the electrical resistance of the electron gas. Its electronic transport properties are also modulated by its magnetic properties, all controlled by the direction of the ferroelectric polarization.

The coexistence of these properties and the possibility to control them easily with an electrical voltage opens the way to new devices for information storage and low energy computing.

This research received financial support from the ERC AdG FRESCO (#833973)

Coexistence and coupling of ferroelectricity and magnetism in an oxide two-dimensional electron gas, J. Bréhin et al, Nature Physics

Schematic cross-section of a sample containing a multiferroic electron gas. It is a multilayer of oxides with a perovskite structure ABO3. The upper layer (in blue) is made of LaAlO3, the intermediate layer (in orange) of EuTiO3 and the lower layer of Ca-SrTiO3. The electron gas extends into the first 2-3 layers of Ca-SrTiO3. The horizontal arrows represent the magnetic moment carried by the atoms. Ca-SrTiO3 is ferroelectric : the atoms are off-centered which generates an electric dipole. The amplitude and direction of the dipoles (different between the left and right parts of the figure) influence the size of the magnetic moments, corresponding to a magnetoelectric coupling in the electron gas.