Sr Sn O is the first room-temperature ferroelectric Sn insulator with switchable electric polarization. The ferroelastic twin domains are observed using a polarized optical microscope. The polarization hysteresis loop clearly demonstrates the ferroelectric property. Intriguing polarization switching kinetics are observed through an in situ poling process using a dark-field transmission electron microscopy technique.

The layered perovskite Ca3-xSrxMn2O7 is shown to exhibit a switching from a material exhibiting uniaxial negative to positive thermal expansion as a function of x. The switching is shown to be related to two closely competing phases with different symmetries. The negative thermal expansion (NTE) effect is maximized when the solid solution is tuned closest to this region of phase space but is switched off suddenly on passing though the transition. Our results show for the first time that, by understanding the symmetry of the competing phases alone, one may achieve unprecedented chemical control of this unusual property.

We show that the spontaneous symmetry breaking in multiferroic hexagonal manganites can be chemically manipulated to yield two complementary ground states: the well-known ferroelectric P6(3)cm and an antipolar P3c phase. Both symmetry breakings yield topologically protected vortex defects, with the antipolar vortices dual to those of the ferroelectric. This duality stems from the existence of 12 possible angles of MnO5 tilting, and broad strain-free walls with low energy spontaneously emerge through an intermediate P3c1 state, providing a complete unified symmetry description.

The direct domain coupling of spontaneous ferroelectric polarization and net magnetic moment can result in giant magnetoelectric (ME) coupling, which is essential to achieve mutual control and practical applications of multiferroics. Recently, the possible bulk domain coupling, the mutual control of ferroelectricity (FE) and weak ferromagnetism (WFM) have been theoretically predicted in hexagonal LuFeO 3 . Here, we report the first successful growth of highly-cleavable Sc-stabilized hexagonal Lu 0.6 Sc 0.4 FeO 3 (h-LSFO) single crystals, as well as the first visualization of their intrinsic cloverleaf pattern of vortex FE domains and large-loop WFM domains. The vortex FE domains are on the order of 0.1-1 μm in size. On the other hand, the loop WFM domains are~100 μm in size, and there exists no interlocking of FE and WFM domain walls. These strongly manifest the decoupling between FE and WFM in h-LSFO. The domain decoupling can be explained as the consequence of the structure-mediated coupling between polarization and dominant in-plane antiferromagnetic spins according to the theoretical prediction, which reveals intriguing interplays between FE, WFM, and antiferromagnetic orders in h-LSFO. Our results also indicate that the magnetic topological charge tends to be identical to the structural topological charge. This could provide new insights into the induction of direct coupling between magnetism and ferroelectricity mediated by structural distortions, which will be useful for the future applications of multiferroics.

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 Ca3Mn2-xTixO7 through our systematic study of a series of single crystalline Ca3Mn2-xTixO7 (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.