Internal-strain mediated coupling between polar Bi and magnetic Mn ions in the defect-free quadruple-perovskite BiMn3Mn4O12

By means of neutron powder diffraction measurements in the 1.5-300 K range, we investigated the effect of the polar Bi3+ ion on the magnetic ordering of the Mn3+ ions in BiMn3Mn4O12, the counterpart with quadruple perovskite structure of the simple perovskite BiMnO3. At all temperatures, the data are consistent with a noncentrosymmetric spacegroup Im which contrasts the centrosym- metric one I2/m previously reported for the isovalent and isomorphic compound LaMn3Mn4O12. This difference gives evidence of a Bi3+-induced polarization of the lattice in BiMn3Mn4O12. At low temperature, the two Mn3+ sublattices of the A? and B sites are found to order antiferromagnetically (AFM) in an independent fashion at 25 and 55 K, respectively, similarly to the case of LaMn3Mn4O12. However, both magnetic structures of BiMn3Mn4O12 radically differ from those of LaMn3Mn4O12. Specifically, in BiMn3Mn4O12 the moments MA? of the A? sites form an anti-body AFM structure, whilst the moments MB of the B sites result from a large and uniform modulation ±MB,b along the b-axis of the moments MB,ac in the ac-plane of an E-type structure. The modulation is found to be strikingly correlated with the displacements of the Mn3+ ions induced by the Bi3+ ions. Our symmetry analysis of this correlation unveils a strong magnetoelastic coupling between the internal strain field created by the Bi3+ ions and the moment of the Mn3+ ions in the B sites. We ascribe this phenomenon to the high symmetry of the oxygen sites and to the absence of oxygen defects, two characteristics of quadruple perovskites not found in simple ones, which prevent the release of the Bi3+-induced strain through distortions or disorder. This result demonstrates the possibility of achieving a large magnetoelectric coupling mediated by internal strain in proper ferroelectrics and suggests a novel concept of internal strain engineering for multiferroics design.