Two-dimensional multiferroics with large magnetoelectric coupling may open interesting avenues in the context of multifunctional systems. Here, we study a class of spin-chirality-driven van der Waals multiferroic monolayers, such as transition metal halides, by combining first-principles calculations with the generalized spin-current model. Our recent report of multiferroicity in NiI2 layers [Q.Song et al, Nature 602, 601 (2022)], obtained via a joint theory-experiments approach down to the single-layer limit, shows the potentiality of cross-coupling phenomena in van der Waals magnets. Another example is represented by vanadium dihalide-monolayers, featuring 120-degree frustrated magnetic structures which develop on the underlying triangular lattice and show competing spin-spiral planes spanning the (001) and (100) crystalline planes. The non-collinear spin configurations induce a ferroelectric polarization which is perpendicular to the spin-spiral plane, switched by the spin-chirality change and whose magnitude is not only determined by the strength of the atomic spin-orbit coupling of the halogen anion, but also affected by structural properties.

: Long before the advent of topological insulators, topology held an important place in the study of ferroic materials. It protects topological defects, such as domain walls, vortices and Bloch points, so that they can only be removed through annihilation with antidefects of opposite topological charge or at sample boundaries. But recent studies point to the reach of topology beyond topological defects: switching processes, which are at the heart of ferroic applications, can also enjoy topological protection, as in GdMn2O5 where the field-induced deformations of free energy surface pump the order parameter between different orientations [1]. In another context, the spin winding in a spiral multiferroics leads to unprecedented nonlocal domain wall motion, where the switching is facilitated by spin precession not only within domain walls but in the entire domains, leading to unusual equations of motion [2,3]

The search for materials with improved physical properties that can be explored to develop new sensors, transducers or actuators remains the subject of innumerous investigations. The coexistence and improvement of the coupling between some intrinsic physical properties (such thermal, ferroelectric, magnetic, photovoltaic…) constitutes promising approaches for developing new multifunctional materials and devices. Single phase complex ferroelectric oxides offer exceptional options to tune room temperature physical properties, such as photovoltaic, multiferroic or magnetoelectric, with high temperature stability.
In this work the influence of iso- and heterovalent doping in the structural, dielectric, ferroelectric, magnetic and magnetoelectric properties of PZT/PFN perovskites (Pb(Zr0,53Ti0,47)O3(1-x)/xPb(Fe0,5Nb0,50)O3) (x=0,10,0,20 and 0,30) and Aurivillius structures (bismuth titanate-BIT based ceramics of Bi3.25A0.75Ti3-x(Co, Fe)x/2O12, where A=La, Nd or Sm and x=0, 0.1, 0.2, 0.3, 0.4), have been investigated. Single phase bulk ceramics were synthesized through the conventional oxide mixture process. It was observed that heterovalent (Nb, Fe) and (Co, Fe) co-dopings promotes ferromagnetic ordering, still maintaining a ferroelectric spontaneous polarization. The room temperature multiferroic state (coexistence of ferroelectric and ferromagnetic states), consequently, promotes a magnetoelectric coupling. The origin of the magnetism and, consequently, the magnetoelectric coupling was explained with the evolution of complex defect and/or oxygen vacancies, resulting from the heterovalent doping. Experimental results of multiferroic state and magnetoelectric coupling, as well as optical absorption, are discussed and correlated with changes in the electronic structure, octahedral distortions (tilting angles and bond lengths), and oxygen vacancies formation. These materials constitute promisor candidates to develop new magnetoelectric devices.

Acknowledgments:
The authors gratefully acknowledge the Brazilian funding agencies CNPq and FAPESP.

Keywords: Multiferroics, Magnetoelectrics, Aurivillius structures, Ferroelectrics, Photoferroelectrics