Impact of Manganese Substitution on the Crystal Structure and Magnetic Properties of Aurivillius Phases

Aurivillius phases are a family of layered ferroelectrics with the general formula Bi₂O₂(A_m-1B_mO_3m+1), where ‘m’ denotes the number of perovskite-type layers interleaved between [Bi₂O₂]²⁺ interface layers. This natural superlattice structure offers a versatile framework for designing multiferroic materials with potential applications in energy-efficient data storage. These phases can incorporate magnetic cations such as Fe, Mn, and Cr at the B-sites of their crystal lattice, facilitating magnetic super-exchange interactions. Our investigations of the m = 4 Aurivillius phase, Bi₅Ti_2.8Fe_1.1Mn_0.1O₁₅, with 2.5 % manganese substitution at the B-sites, demonstrated ferrimagnetic behavior below 200 K, with remanent magnetization (M_R) values of 3.51 emu/cm³ and 1.50 emu/cm³ at 20 K, on SrTiO₃ and LSAT (La_0.26Sr_0.76Al_0.61Ta_0.37O₃) substrates, respectively. The manganese substitution promotes ferrimagnetic ordering, in contrast to the negligible magnetization in the corresponding iron-only analogue (Bi₅Ti_2.9Fe_1.1O₁₅). Further increasing the B-site manganese concentration to 11 % in Bi₆Ti_2.99Fe_1.46Mn_0.55O₁₈ induces room temperature ferrimagnetism, with an M_R of 81.5 emu/cm³ at 300 K.

Motivated by the observed enhancement in magnetization and ferrimagnetic Curie temperature with increasing manganese content, we systematically examine the effects of higher manganese substitution (6 to 18% at the B-sites) on the crystal structure of m = 5 Aurivillius phase samples, synthesized on sapphire substrates using chemical solution deposition. Our study explores the solubility limits of manganese within this structure and its influence on the natural superlattice layering. Substituting Ti⁴⁺ with Fe³⁺ and Mn³⁺ necessitates charge compensation and accommodation of ionic radii differences. XRD and TEM analyses reveal that beyond 13% Mn, the m = 5 phase transitions into a mixed-phase material compromising m = 5 and m = 6 inter-growths, evolving fully into an m = 6 phase above 14% Mn. We propose that the formation of the m = 6 structure, with additional perovskite layers, facilitates the incorporation of more manganese cations by stabilizing a lower average oxidation state (+3.3) compared to the m = 5 stoichiometry (+4.0). Thus, structural reorganization into higher m phases becomes necessary to accommodate the increased magnetic content. While the minor out-of-plane ferroelectric response decreases as expected with increasing structural reorganization towards the m = 6 phase, the predominant in-plane piezoresponse remains largely unaffected by magnetic cation substitution.

This study demonstrates the structural evolution of higher-layered multiferroic Aurivillius phases through manganese substitution. It shows that aliovalent substitution with transition metal cations, using simple chemical solution deposition techniques, can circumvent thermodynamic barriers to synthesizing higher-layered Aurivillius homologues, without the need for epitaxial substrates or kinetically constrained growth methods.