Multiferroic Aurivillius Bi₄Ti_2-MnFe_0.5Nb_0.5O₁₂ (n = 3) compounds with tailored magnetic interactions

Aurivillius compounds with the general formula (Bi₂O₂)(ABO₃) are a highly topical family of functional layered oxides currently under investigation for room-temperature multiferroism. A chemical design strategy is the incorporation of magnetically active BiMO₃ units (M: Fe³⁺, Mn³⁺, Co³⁺ …) into the pseudo-perovskite layer of known ferroelectrics like Bi₄Ti₃O₁₂, introducing additional oxygen octahedra. Alternatively, one can try to directly substitute magnetic species for Ti⁴⁺ in the perovskite slab. Previous reports explored the introduction of the M³⁺ species, which required the simultaneous incorporation of a 5+ cation, as for the Bi₄Ti_3-2NbFeO₁₂ system. A larger magnetic fraction might be attained if Ti⁴⁺ is substituted with Mn⁴⁺, though it has been argued that the small ionic radius prevents its incorporation into the pseudo-perovskite layer. We report here the mechanosynthesis of Aurivillius Bi₄Ti_2-MnNb_0.5Fe_0.5O₁₂ (n = 3) compounds with increasing Mn⁴⁺ content up to x = 0.5, which corresponds to a magnetic fraction of 1/3 at the B-site surpassing the threshold for percolation, and equal amounts of Mn⁴⁺ and Fe³⁺. The appearance of ferromagnetic superexchange interactions and magnetic ordering was anticipated and is shown for phases with x ≥ 0.3. Ceramic processing was accomplished by spark plasma sintering, which enabled electrical measurements that demonstrated ferroelectricity for all Mn⁴⁺-containing Aurivillius compounds. This is a new family of layered oxides and a promising alternative single-phase approach for multiferroism.