Coexisting magnetic orders and concomitant Morin-like transition and relaxor behavior in multiferroic Aurivillius Bi4Ti3 − 2xNbxFexO12 compounds

Abstract

Aurivillius layered oxides with general formula (Bi2O2)(Am−1BmO3m+1) stand out among room-temperature magnetoelectric multiferroics for their large magnetization. However, despite extensive research, there is an obvious lack of understanding of their magnetism. The chemical design strategy for obtaining multiferroism involves the incorporation of M3+ magnetic cations at the B-site of well-known ferroelectric compounds such as Bi4Ti3O12 (m = 3). We report here a study of the magnetism and dielectric properties of Aurivillius Bi4Ti3 − 2xNbxFexO12 phases with x ≥ 1 up to 1.2, which correspond to magnetic cation fractions at the B site between 0.33 and 0.4 above the threshold for percolation. This is a three-layer system, less prone to the formation of intergrowths, which nominally contains Fe3+ as single magnetic species. Despite that, a rich phenomenology is uncovered. Coexisting magnetic orders are present in the Aurivillius compounds, and a Morin-like transition takes place at low temperatures. The dielectric characterization does not show any associated anomaly that could indicate a polymorphic phase transition but the appearance of relaxor-like characteristics. Possible scenarios are discussed, which involve the presence of Fe2+, cation partitioning between nonequivalent B-sites, and the development of polar nanodomains within a ferroelectric phase at a spin reorientation transition.