Multiferroic Aurivillius Bi 4 Ti 2- x Mn x Fe 0.5 Nb 0.5 O 12 ( n = 3) compounds with tailored magnetic interactions.

Aurivillius compounds with the general formula (Bi ₂ O ₂ )(A _{n -1} B _n O _{3 n +1} ) 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 BiMO ₃ units (M: Fe ³⁺ , Mn ³⁺ , Co ³⁺ …) into the pseudo-perovskite layer of known ferroelectrics like Bi ₄ Ti ₃ O ₁₂ , introducing additional oxygen octahedra. Alternatively, one can try to directly substitute magnetic species for Ti ⁴⁺ in the perovskite slab. Previous reports explored the introduction of the M ³⁺ species, which required the simultaneous incorporation of a 5+ cation, as for the Bi ₄ Ti _{3-2 x} Nb _x Fe _x O ₁₂ system. A larger magnetic fraction might be attained if Ti ⁴⁺ is substituted with Mn ⁴⁺ , 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 Bi ₄ Ti _{2- x} Mn _x Nb _0.5 Fe _0.5 O ₁₂ ( n = 3) compounds with increasing Mn ⁴⁺ 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 Mn ⁴⁺ and Fe ³⁺ . 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 Mn ⁴⁺ -containing Aurivillius compounds. This is a new family of layered oxides and a promising alternative single-phase approach for multiferroism.