Compositional Complexity as a Design Principle for Stabilizing Magnetization Reversal in Corundum-Derived A₄Nb₂O₉ Phases

Abstract

Corundum and related compounds possess interesting properties for functional applications, such as magnetization reversal or linear magneto electricity, depending on their structures and magnetic ground states. In this system, materials corresponding to the A 4 Nb 2 O 9 formula crystallize in two different structures, i.e., Pbcn for Ni 4 Nb 2 O 9 and P3̅ c1 for A = Co, Fe, and Mn, leading to ferrimagnetism and antiferromagnetism at low temperature, respectively. In this family, the nickel-based compound is special because Ni 4 Nb 2 O 9 is the only one crystallizing in the orthorhombic structure. The Pbcn and P3̅ c1 structures are both able to adapt to various substitutions of the A-cation, and as soon as Ni is involved, the two structural types are stabilized depending on the Ni/A ratio and A. In this context, the differences reported in the structural and magnetic behaviors of the Ni 4−x Co x Nb 2 O 9 and Ni 4−x Zn x Nb 2 O 9 series motivated the (Ni/Co/Zn) 4 Nb 2 O 9 study by mapping the magnetization-structure-composition phase diagram. This first step toward compositionally complex materials allows us to identify an unexpected large area of compounds that exhibit ferrimagnetism and magnetization reversal. Moreover, inside this domain, interesting variations of Neél temperature and compensation temperature are observed, including an increase of T comp induced by nonmagnetic substituting cations. The originality of these structural and magnetic behaviors makes these materials promising for theoreticians to provide conceptual models, especially since the partial substitution of the magnetic cation by a d 0 one may enhance the magnetization reversal.