Interrelation between domain structures and polarization switching in hybrid improper ferroelectric Ca3(Mn,Ti)2O7.

Ca₃Mn₂O₇ and Ca₃Ti₂O₇ have been proposed as the prototypical hybrid improper ferroelectrics (HIFs), and a significant magnetoelectric (ME) coupling in magnetic Ca₃Mn₂O₇ is, in fact, reported theoretically and experimentally. Although the switchability of polarization is confirmed in Ca₃Ti₂O₇ and other non-magnetic HIFs, there is no report of switchable polarization in the isostructural Ca₃Mn₂O₇. We constructed the phase diagram of Ca₃Mn_2-xTi_xO₇ through our systematic study of a series of single crystalline Ca₃Mn_2-xTi_xO₇ (x=0, 0.1, 1, 1.5, and 2). Using transmission electron microscopy, we have unveiled the unique domain structure of Ca₃Mn₂O₇: the high-density 90° stacking of a- and b-domains along the c-axis due to the phase transition through an intermediate Acca phase and the in-plane irregular wavy ferroelastic twin domains. The interrelation between domain structures and physical properties is unprecedented: the stacking along the c-axis prevents the switching of polarization and causes the irregular in-plane ferroelastic domain pattern. In addition, we have determined the magnetic phase diagram and found complex magnetism of Ca₃Mn₂O₇ with isotropic canted moments. These results lead to negligible observable ME coupling in Ca₃Mn₂O₇ and guide us to explore multiferroics with large ME coupling. [ABSTRACT FROM AUTHOR]