Room-temperature multiferroic responses arising from 1D phase modulation in correlated Aurivillius-type layer structures

Structural instability induced by chemical substitution in multiferroic materials, especially in layer-structured oxides of the Bi4Ti3O12-BiFeO3 system, gives rise to intriguing phenomena and extraordinary coupling properties. In this work, we carefully studied an analogous morphotropic structural transformation in Co-substituted Bi7Ti3Fe3O21 oxides, similar to a morphotropic phase boundary. Large effects (including ferroelectric, dielectric and ferromagnetic) and abnormal magnetic behaviour were recorded at the analogous morphotropic transformation region (AMTR). 1D phase-modulated structures composed of two different layered phases and structural distortions inside the AMTR were visualized. Analyses of the correlations between oxygen octahedral distortions and multiferroic responses confirmed that the enhanced multiferroic responses inside the AMTR are intrinsic to the 1D correlated interface structures, mainly arising from such structural distortions due to the existence of different lattice strains and the change of crystal symmetry.