Direct Magnetization-Polarization Coupling in BaCuF_{4}

Unveiling the mechanisms behind the ferroelectric response in the Sr(Nb,Ta)O2N oxynitrides

Oxynitride perovskites of the type ABO₂N have attracted considerable attention thanks to their potential ferroelectric behavior and tunable bandgap energy, making them ideal candidates for photocatalysis processes. Therefore, in order to shed light on the origin of their ferroelectric response, here we report a complete analysis of the structural and vibrational properties of SrNbO₂N and SrTaO₂N oxynitrides. By employing first-principles calculations, we analyzed the symmetry in-equivalent structures considering the experimentally reported parent I4/mcm space group (with a phase a⁰a⁰c^- in Glazer's notation). Based on the I4/mcm reference within the 20-atoms unit-cell, we found and studied the ensemble of structures where different octahedral anionic orderings are allowed by symmetry. Thus, by exploring the vibrational landscape of the cis- and trans-type configuration structures and supported by the ionic eigendisplacements and the Born effective charges, we explained the mechanism responsible for the appearance of stable ferroelectric phases in both anionic orderings. The latter goes from covalent-driven in the trans-type ordering to the geometrically-driven in the cis-type configuration. Finally, we found in both cases that the biaxial xy epitaxial strain considerably enhances such ferroelectric response.

Design of Two-Dimensional Multiferroics with Direct Polarization-Magnetization Coupling

The recent discovery of two-dimensional (2D) ferromagnetism in van der Waals materials has opened the door to the control of 2D magnetism by means of electric field. Here we demonstrate the magnetization reversal through switching polarization in a designed 2D multiferroic oxide by combining group theory analysis and first-principles calculation. We show that ferroelectricity can be induced by a specific octahedral rotation in a perovskite bilayer. Ferromagnetism can be introduced simultaneously by extending the guideline to the B-site ordered double-perovskite bilayer. We have found two coupling mechanisms between polarization and magnetization that enable the reversal of the in-plane magnetization by ferroelectric switching. Our work provides guidelines for the design of 2D multiferroics with intrinsic magnetoelectric coupling and helps to control the 2D magnetism by electric field.

Magneto-electric multiferroics: designing new materials from first-principles calculations

In parallel with the revival of interest for magneto-electric multiferroic materials in the beginning of the century, first-principles simulations have grown incredibly in efficiency during the last two decades. Density functional theory calculations, in particular, have so become a must-have tool for physicists and chemists in the multiferroic community. While these calculations were originally used to support and explain experimental behaviour, their interest has progressively moved to the design of novel magneto-electric multiferroic materials. In this article, we mainly focus on oxide perovskites, an important class of multifunctional material, and review some significant advances to which contributed first-principles calculations. We also briefly introduce the various theoretical developments that were at the core of all these advances.

Single-phase multiferroics: new materials, phenomena, and physics

Multiferroics, where multiple ferroic orders coexist and are intimately coupled, promise novel applications in conceptually new devices on one hand, and on the other hand provide fascinating physics that is distinctly different from the physics of high-T_C superconductors and colossal magnetoresistance manganites. In this mini-review, we highlight the recent progress of single-phase multiferroics in the exploration of new materials, efficient roadmaps for functionality enhancement, new phenomena beyond magnetoelectric coupling, and underlying novel physics. In the meantime, a slightly more detailed description is given of several multiferroics with ferrimagnetic orders and double-layered perovskite structure and also of recently emerging 2D multiferroics. Some emergent phenomena such as topological vortex domain structure, non-reciprocal response, and hybrid mechanisms for multiferroicity engineering and magnetoelectric coupling in various types of multiferroics will be briefly reviewed.