Domain wall dynamics in hexagonal manganites are revealed by impedance microscopy and first-principles calculations.

The hexagonal phase of LuFeO 3 is a rare example of a multiferroic material possessing a weak ferromagnetic moment, which is predicted to be switchable by an electric field. We stabilize this structure in bulk form though Mn and Sc doping, and determine the complete magnetic and crystallographic structures using neutron-scattering and magnetometry techniques. The ferroelectric P 6 3 cm space group is found to be stable over a wide concentration range, ordering antiferromagnetically with Néel temperatures that smoothly increase following the ratio of c to a (c/a) lattice parameters up to 172 K, the highest found in this class of materials to date. The magnetic structure for a range of temperatures and dopings is consistent with recent studies of high quality epitaxial films of pure hexagonal LuFeO 3 including a ferromagnetic moment parallel to the ferroelectric axis. We propose a mechanism by which room-temperature multiferroicity could be achieved in this class of materials.

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.

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Optical properties and lattice dynamics of hexagonal 2H-BaMnO 3 single crystals are studied experimentally in a wide temperature range by means of rotating analyzer ellipsometry and Raman scattering. The magnitude of the direct electronic band gap is found to be E g = 3.2 eV. At room temperature the far-infrared (IR) ellipsometry spectra reveal six IR-active phonons, two of them are polarized along the c-axis and four are polarized within the a−b plane. Seven phonon modes are identified in the Raman scattering experiments. Group theoretical mode analysis and complimentary density functional theory lattice dynamics calculations are consistent with the 2H-BaMnO 3 structure belonging to the polar P6 3 mc space group at room temperature. All observed vibrational modes are assigned to the specific eigenmodes of the lattice. The neutron diffraction measurements reveal a structural phase transition upon cooling below T C =130±5 K, which is accompanied by the lattice symmetry change from the P6 3 mc to the P6 3 cm. Simultaneously, at temperatures below T C several additional IR-and Raman-active modes are detected in experimental spectra. This confirms the occurrence of a structural transition, which is possibly associated with appearance of the electrical polarization along the c-axis and a previously known tripling of the primitive cell volume at low temperatures. PACS number(s): 63.20.dh, 75.47.Lx, 75.85.+t