Fei-Ting Huang

Experimental demonstration of hybrid improper ferroelectricity and the presence of abundant charged walls in (Ca,Sr)3Ti2O7 crystals

On the basis of successful first-principles predictions of new functional ferroelectric materials, a number of new ferroelectrics have been experimentally discovered. Using trilinear coupling of two types of octahedron rotation, hybrid improper ferroelectricity has been theoretically predicted in ordered perovskites and the Ruddlesden-Popper compounds (Ca3Ti2O7, Ca3Mn2O7 and (Ca/Sr/Ba)3(Sn/Zr/Ge)2O7). However, the ferroelectricity of these compounds has never been experimentally confirmed and even their polar nature has been under debate. Here we provide the first experimental demonstration of room-temperature switchable polarization in bulk crystals of Ca3Ti2O7, as well as Sr-doped Ca3Ti2O7. Furthermore, (Ca, Sr)3Ti2O7 is found to exhibit an intriguing ferroelectric domain structure resulting from orthorhombic twins and (switchable) planar polarization. The planar domain structure accompanies abundant charged domain walls with conducting head-to-head and insulating tail-to-tail configurations, which exhibit a conduction difference of two orders of magnitude. These discoveries provide new research opportunities, not only for new stable ferroelectrics of Ruddlesden-Popper compounds, but also for meandering conducting domain walls formed by planar polarization.

Symmetry Switching of Negative Thermal Expansion by Chemical Control

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.

Self-poling with oxygen off-stoichiometry in ferroelectric hexagonal manganites

Topological vortices with complex domains and domain walls exist in hexagonal manganites, which undergo a structural transition accompanying ferroelectric polarization and trimerization. We have systematically studied the origin of two different types (type-I and type-II) of vortex domains with controlled oxygen stoichiometry and found that the evolution between type-I and type-II vortex domains and the presence of charged walls between two different trimerization antiphase domains result from an oxygen vacancy gradient. We have also discovered a rare phenomenon of vortex core split, which appears to stem from the instability of charged ferroelectric walls between domains with the same trimerization antiphase.

Aperiodic topological order in the domain configurations of functional materials

Domains and domain walls are relevant for the engineering of materials functionalities. In this Review, a new classification scheme for topological domain configurations is presented and applied to several materials, including multiferroics, ferroelectrics, transition metal dichalcogenides and magnetic superconductors.

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 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 irregula...

The First Room-Temperature Ferroelectric Sn Insulator and Its Polarization Switching Kinetics

Sr3 Sn2 O7 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.

Interlocked chiral/polar domain walls and large optical rotation in Ni3TeO6

Chirality, i.e., handedness, pervades much of modern science from elementary particles, DNA-based biology to molecular chemistry; however, most of the chirality-relevant materials have been based on complex molecules. Here, we report inorganic single-crystalline Ni3TeO6, forming in a corundum-related R3 structure with both chirality and polarity. These chiral Ni3TeO6 single crystals exhibit a large optical specific rotation (α)—1355° dm−1 cm3 g−1. We demonstrate, for the first time, that in Ni3TeO6, chiral and polar domains form an intriguing domain pattern, resembling a radiation warning sign, which stems from interlocked chiral and polar domain walls through lowering of the wall energy.

Ferroelectric and Structural Antiphase Domain and Domain Wall Structures in Y(Mn,Ti)O3

We have investigated ferroelectric (FE) and structural antiphase (SAP) domains and domain wall structures at the atomic scale in hexagonal YMn1-xTixO3-δ by using aberration-corrected high-angle annular-dark-field scanning transmission electron microscopy (HAADF-STEM) experiments, combining with the conventional TEM observation techniques. Atomic shifts of Y3+ and Mn3+ (Ti4+) in the FE and SAP domains and domain wall structures in YMn1-xTixO3-δ are successfully visualized at the atomic scale. When Ti4+ are substituted for Mn3+, the magnitude of the shifts of Y3+ is reduced and the regions without any displacement of Y3+ along the [001] direction dominated around x-0.40. Our experimental results clearly revealed that partial substitution of Ti4+ for Mn3+ suppress the magnitude of the shifts of Y3+ ions along the [001] direction and the FE properties tend to diminish with increasing the amount of Ti4+ in YMn1-xTixO3-δ.

Color Theorems, Chiral Domain Topology, and Magnetic Properties of FexTaS2

Common mathematical theories can have profound applications in understanding real materials. The intrinsic connection between aperiodic orders observed in the Fibonacci sequence, Penrose tiling, and quasicrystals is a well-known example. Another example is the self-similarity in fractals and dendrites. From transmission electron microscopy experiments, we found that FexTaS2 crystals with x = 1/4 and 1/3 exhibit complicated antiphase and chiral domain structures related to ordering of intercalated Fe ions with 2a × 2a and √3a × √3a superstructures, respectively. These complex domain patterns are found to be deeply related with the four color theorem, stating that four colors are sufficient to identify the countries on a planar map with proper coloring and its variations for two-step proper coloring. Furthermore, the domain topology is closely relevant to their magnetic properties. Our discovery unveils the importance of understanding the global topology of domain configurations in functional materials.

Vortex ferroelectric domains, large-loop weak ferromagnetic domains, and their decoupling in hexagonal (Lu, Sc)FeO 3

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 LuFeO3. Here, we report the first successful growth of highly-cleavable Sc-stabilized hexagonal Lu0.6Sc0.4FeO3 (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.Multiferroics: decoupled domainsExperiments reveal the decoupling between vortex ferroelectric domains and loop weak ferromagnetic domains in hexagonal Lu0.6Sc0.4FeO3. A team led by Sang-Wook Cheong from the Rutgers University in the USA grows cleavable Sc-stabilized hexagonal Lu0.6Sc0.4FeO3 (h-LSFO) single crystals and visualize cloverleaf patterns of vortex ferroelectric (FE) domains and large-loop weak ferromagnetic (WFM) domains. The WFM domains are much larger in size than FE domains. The distinct sizes and shapes demonstrate the decoupling between ferroelectricity and out-of-plane weak ferromagnetism, which is consistent with a former prediction on the absence of coupled magnetoelectric effects in h-LSFO. Furthermore, they indicate that the magnetic topological charge tends to be identical with the structural topological charge. The results provide insights into coupling between magnetism and ferroelectricity mediated through structural distortions, which might be useful for future applications.