Session K30: Complexity in Ferroic Systems

Artificial chemical and magnetic structure at the domain walls of an epitaxial oxide.

Progress in nanotechnology requires new paradigms for materials synthesis that allow controlling their functionality down to the smallest scales. Here we report a novel two-dimensional ferromagnetic phase that is synthesized at the domain walls (DWs) of the antiferromagnetic insulator TbMnO$_{\mathrm{3}}$ when grown in thin layers under epitaxial strain. This Mn oxide phase presents an atomic arrangement that does not exist in bulk and cannot be synthesized by standard chemical routes. The number of 2D ferromagnetic sheets can be controlled by tuning the thickness of the thin films, giving rise to volume fractions that go up to 25{\%} of the total film volume. Such novel phases are driven by a unique environment induced by the symmetry breaking and large stresses present at domain walls, which function as nanoreactors. This new class of nanoscale materials may find innovative applications in nanoelectronics and spintronics. The work is published as S. Farokhipoor, C. Mag\'{e}n, S. Venkatesan, J. \'{I}\~{n}iguez, C. J. M. Daumont, D. Rubi, E. Snoeck, M. Mostovoy, C. de Graaf, A. M\"{u}ller, M. D\"{o}blinger, C. Scheu, B. Noheda, Nature 515, 379 (2014)   

Microscopic order parameters coupling at domain walls and its effect on macroscopic properties.

Domain and domain wall (DW) engineering provides an alternative model to tune the physical properties of materials, typically done via conventional materials chemistry. The interplay of coexisting non-ferroelectric structural order parameters, ferroelectric and magnetic order parameters at the DWs in hexagonal manganites provides a new pathway to determine macroscopic properties by tuning the DW characteristics [1]. Here, we report different types of domain structures and DW types associated with the crystal growth conditions in hexagonal manganites. We show that differences in the DW polar state manifest themselves as variations in the conductivity measured macroscopically. Piezo force microscopy and X-ray diffraction enables us to determine the plane of the DWs and hence, their strain state. The latter findings corroborate with the topographical study. Finally, these results show that DWs under strain lower the critical field of the magnetic phase transition compared to samples with strain-free DWs. This work represents the first example of non-local physical properties being determined by the presence of topographically protected DWs. [1] S. Artyukhin et al., Nat Mater \textbf{13} (2014)   

Visualization of weak ferromagnetic domains in multiferroic hexagonal ferrite thin film

Hexagonal $\textit{h}$-LuFeO$_3$ thin film has been reported to be a room-temperature multiferroic [1]. Extensive studies on high quality MBE thin films revealed a magnetoelectric phase with weak ferromagnetism emerges below $T_N\sim 147$\,K [2]. However, the direct observation of weak ferromagnetic domain structures is still lacking. Here we report cryogenic magnetic force microscopy (MFM) results on 200 nm thick $\textit{h}$-LuFeO$_3$ film grown by molecular-beam epitaxy (MBE) on (111)-oriented yttria-stablized cubic zirconia (YSZ) substrates. Labyrinth-like weak ferromagnetic domain structures were observed with a domain size $\sim 1\,\mu m$ and domain wall width $\sim 0.4\,\mu m$. Field-dependent MFM data indicates the coercive field is $\sim 2.66$\,T at 50\,K and $\sim 3.15$\,T at 6\,K. [1] W. Wang et al., Phys. Rev. Lett. 110, 237601(2013). [2] J. A. Moyer et al., APL MATER. 2, 012106 (2014).   

Spin-phonon interaction of mixed-phase BiFeO$_3$ films studied by Raman spectroscopy

Multiferroic BiFeO$_3$ (BFO) has ferroelectricity and antiferromagnetism at room temperature, and so was motivated for novel magnetoelectric applications. When a BFO film is epitaxially grown on the LaAlO$_3$ (LAO) substrate, the strong compressive strain will transform BFO to a mixed-phase state, including rhombohedrally (R-) and tetragonally (T-) distorted monoclinics. The stripe-shape R-BFO is embedded in T-BFO matrix, forming periodic domains which possess enhanced piezoelectric response and spontaneous magnetization. Moreover, the magnetic R-BFO phases can be switched by external electric fields. Here, in order to study the mechanism of strain induced magnetic properties, we report a detailed Raman study of the mixed-phase BFO. The phonons of each phase are distinguished by an in-situ testing system combining an atomic force microscope (AFM) and a Raman spectroscope. When external magnetic fields are applied, a low-frequency phonon mode of T-BFO phase changes due to the magnetostrictive effect. Variations of mixed-phase phonons versus temperature are also studied, which show that phonon shifts of high-frequency modes are contributed from phonon-phonon anharmonic interaction; by contrast, the shifts of low-frequency modes are due to spin-phonon interactions.   

Inducing phase transitions of T-like BiFeO$_3$ films by low-energy He implantation

Ferroelectric phase transitions of BiFeO$_3$ are found to be controllable through the application of single axis, out-of-plane strain. Low-energy He implantation has been deployed to induce out-of-plane strain in T-like BFO films, while the compressive in-plane strain due to the coherent growth on LaAlO$_3$ substrates remains fixed. Our data shows that He implantation triggers a $M_C-M_A-T$ phase sequence of the T polymorph that is identical to structural changes that are induced with increasing temperature. Mixed phases nanodomains phases are gradually suppressed and disappear above a certain He doping level. Our data shows that the ferroelectric and optical properties of BiFeO3 films critically depend on the He doping level. Thus, the results demonstrates that He implantation can be used as an intriguing approach to study lines in the rich phase space of BFO films that can’t be accessed by simple heteroepitaxy. This effort was wholly supported by the US Department of Energy (DOE), Office of Basic Energy Sciences (BES), Materials Sciences and Engineering Division, with user projects supported at ORNL’s Center for Nanophase Materials Research (CNMS) which is also sponsored by DOE-BES.   

Effect of manganese substitution in barium titanate ($Mn-BaTiO_{3}$)

Barium titanate single crystals exhibit a very large recoverable nonlinear strain with aging, which is an order higher than the highly strained PZN-PT single crystals.[1] Transition metal dopants improve the electromechanical properties of these classic ferroelectrics and third generation relaxor ferroelectrics.[2] To understand the source of these effects, we systematically investigate the effect of Mn substitution in $BaTiO_{3}$ using theory [3] and experiments. Mn substituted $BaTiO_{3}$ ceramics were synthesized by solid state reaction. Mn substitution of up to 4 atomic \% showed tetragonal phase and further substitution leads to evolution of hexagonal phase. Raman spectra show increasing Mn substitution reduce the intensity of A1(TO) and B1, E(TO+LO) modes, broaden the E(TO), A1(TO) modes and a new peak evolution at ~ 629 $cm^{-1}$. Enhanced strain double hysteresis was observed with increase in Mn substitution on electric field. Influence of Mn substitution on dielectric properties will be presented. The enhanced strain properties with aging observed on Mn substituted PIN-PMN-PT crystals is also discussed. References: [1] X. Ren, Nature Mater. 3, 91 (2004). [2] Zhang et al., IEEE Trans. Ultrason.,60, 1572 (2013). [3] J. F. Nossa et al., Phys. Rev. B 91, 214105 (2015).   

Enhanced ferroelectric polarization and potential morphotrophic phase boundary in PZT-based alloys

We present a combined theoretical and experimental study of alloys of the high performance piezoelectric PZT (PbZr$_{0.5}$Ti$_{0.5}$O$_{3}$) with BZnT (BiZn$_{0.5}$Ti$_{0.5}$O$_{3}$) and BZnZr (BiZn$_{0.5}$Zr$_{0.5}$O$_{3}$), focussing on lattice instabilities, atomic displacements and ferroelectric polarization. From theory we find that the 75 - 25 PZT - BZnT alloy has substantially larger cation displacements, and hence ferroelectric polarization than the PZT base material, on the tetragonal side of the phase diagram. We also find a possible morphotrophic phase boundary in this system by comparing displacement patterns and optimized c/a ratios. Experiments indicate the feasibility of sample synthesis within this alloy system.   

Diffuse Scattering from Relaxor PMN-xPT

Relaxor ferroelectrics possess intriguing electromechanical and dielectric properties, the microscopic physics of which is widely regarded to be related to local, correlated atomic displacements from long-range symmetry. However, despite numerous studies over the last few decades, the details of how short range correlations and disorder drive the relaxor behavior remain unresolved. Single crystal diffuse scattering provides a powerful probe of such deviations from an average structure correlated over varying length scales, and over the last few years, techniques and instruments for measuring diffuse scattering with both x-rays and neutrons have seen a dramatic improvement, allowing for large volumes of reciprocal space to be measured in little time. We present our recent complementary neutron and x-ray measurements on solid solutions of PMN-xPT which revealed new structure to the diffuse scattering of relaxors close to the morphotropic phase boundary.   

Structural and Dynamical Properties of Barium Stannate

Barium stannate based perovskites have recently drawn attention due to their potential as transparent conducting oxides and reports of high carrier mobility in La-doped single crystals. Published DFT calculations and experimental results have suggested phonon instabilities at high symmetry zone boundary positions and local octahedral rotations, respectively, for BaSnO$_3$. Here, we discuss recent structural measurements of BaSnO$_3$, in which we have searched for such distortions by employing a combination of single crystal neutron diffraction and total scattering analysis of powder neutron diffraction. Moreover, we discuss lattice dynamical measurements, comparing phonon dispersion measurements to DFT calculations.   

Real-space observation of metal-insulator transition at complex oxide heterointerface with cross-sectional STM

We report the direct observation of tunable electronic property through visible light at $LaAlO_{3}/SrTiO_{3}$ (LAO/STO) complex oxide heterointerface using cross-sectional scanning tunneling microscopy and spectroscopy (XSTM/S). Many researches have shown that for the interface to be conducting, the thickness of LAO should be equal to or greater than the critical value 4 unit cells (u.c.). With LAO surface modification by Au clusters, interfacial two-dimensional electron gas presents a giant optical switching effect under visible light illuminated. In this study, through the interaction between photons and electrons system, a direct observation of the evolution of electronic structures from insulating to conducting has been revealed in the LAO (3u.c.)/STO model using the technique of cross-sectional scanning tunneling microscopy and spectroscopy. Results clearly reveal the changes in the built-in electric field in LAO and the band bending in the STO adjacent to the interface after light illumination.