We report on the observation of nanoscale conduction at ferroelectric domain walls in hexagonal HoMnO(3) protected by the topology of multiferroic vortices using in situ conductive atomic force microscopy, piezoresponse force microscopy, and Kelvin-probe force microscopy at low temperatures. In addition to previously observed Schottky-like rectification at low bias [Phys. Rev. Lett. 104, 217601 (2010)], conductance spectra reveal that negatively charged tail-to-tail walls exhibit enhanced conduction at high forward bias, while positively charged head-to-head walls exhibit suppressed conduction at high reverse bias. Our results pave the way for understanding the semiconducting properties of the domains and domain walls in small-gap ferroelectrics.
We discovered stripe patterns of trimerization-ferroelectric domains in hexagonal REMnO(3) (RE=Ho,···,Lu) crystals (grown below ferroelectric transition temperatures (T(c)), reaching up to 1435 °C), in contrast with the vortex patterns in YMnO(3). These stripe patterns roughen with the appearance of numerous loop domains through thermal annealing just below T(c), but the stripe domain patterns turn to vortex-antivortex domain patterns through a freezing process when crystals cross T(c) even though the phase transition appears to not be Kosterlitz-Thouless-type. The experimental systematics are compared with the results of our six-state clock model simulation and also the Kibble-Zurek mechanism for trapped topological defects.
Giant tunability of ferroelectric polarization (ΔP=5000 μC/m2) in the multiferroic GdMn2O5 with external magnetic fields is discovered. The detailed magnetic model from x-ray magnetic scattering results indicates that the Gd-Mn symmetric exchange striction plays a major role in the tunable ferroelectricity of GdMn2O5, which is in distinction from other compounds of the same family. Thus, the highly isotropic nature of Gd spins plays a key role in the giant magnetoelectric coupling in GdMn2O5. This finding provides a new handle in achieving enhanced magnetoelectric functionality.
The interaction among topological defects can induce novel phenomena such as disclination pairs in liquid crystals and superconducting vortex lattices. Nanoscale topological vortices with swirling ferroelectric, magnetic, and structural antiphase relationships were found in multiferroic h-YMnO 3 . Herein, we report the discovery of intriguing, but seemingly irregular configurations of a zoo of topological vortices and antivortices. These configurations can be neatly analyzed in terms of graph theory and reflect the nature of self-organized criticality in complexity phenomena. External stimuli such as chemistry-driven or electric poling can induce the condensation and eventual annihilation of topological vortexantivortex pairs.
Topological defects have been playgrounds for many emergent phenomena in complex matter such as superfluids, liquid crystals, and early universe. Recently, vortex-like topological defects with six interlocked structural antiphase and ferroelectric domains merging into a vortex core were revealed in multiferroic hexagonal manganites. Numerous vortices are found to form an intriguing self-organized network. Thus, it is imperative to find out the magnetic nature of these vortices. Using cryogenic magnetic force microscopy, we discovered unprecedented alternating net moments at domain walls around vortices that can correlate over the entire vortex network in hexagonal ErMnO 3 . The collective nature of domain wall magnetism originates from the uncompensated Er 3+ moments and the correlated organization of the vortex network. Furthermore, our proposed model indicates a fascinating phenomenon of field-controllable spin chirality. Our results demonstrate a new route to achieving magnetoelectric coupling at domain walls in single-phase multiferroics, which may be harnessed for nanoscale multifunctional devices. 2Multiferroics are materials with coexisting magnetism and ferroelectricity 1 . The cross-coupling between two ferroic orders can result in giant magnetoelectric coupling for potential applications [2][3][4][5] . Because formation of domains is the hallmark of any ferroic order 6 , it is of both fundamental and technological interests to visualize cross-coupled domains or walls in multiferroics. Hexagonal (h-) REMnO 3 (RE = Sc, Y, Ho, … Lu) are multiferroics with coexistence of ferroelectricity (T C ≈ 1200 -1500 K) 7 and antiferromagnetism (T N ≈ 70 -120 K) 8 . The ferroelectricity is induced by structural instability called trimerization 9,10 , which lifts presumably the frustration of antiferromagnetic interactions of Mn 3+ spins on triangular lattice. Indeed, a 120º antiferromagnetic order of Mn 3+ spin in the ab-plane sets in below T N . Recently, an intriguing 6-state vortex domain structure in YMnO 3 is revealed by transmission electron microscopy, conductive atomic force microscopy and piezoresponse force microscopy (PFM) at room temperature [11][12][13] . The formation of 6-state vortices originates from the cyclic arrangement of 6 interlocked structural antiphase (α, β, γ) and ferroelectric (+/−) ground states (i.e. α + , β -, γ + , α -, β + , γ -) 11,14 . The intriguing network of vortex-antivortex pairs has a profound connection to graph theory, where 6-valent planer graphs with even-gons are two-proper-colorable 15 . Using second harmonic generation optics, it has been claimed that ferroelectric domain walls (DWs) in millimeter-size YMnO 3 tend to pin antiferromagnetic DWs, but free antiferromagnetic DWs also exist 16 . Thus, it is of great interest to find out the magnetic nature of vortex domains and DWs. However, this has been an experimental challenge, particularly due to the lack of suitable high resolution imaging technique of antiferromagnetic domains or DWs for h-REMnO 3 at low temperatures (...
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