Session H37a: Dielectric and Ferroelectric Oxides III

Manifestations of Bloch walls in perovskite ferroelectrics

Recently, concerted efforts of several research laboratories have been employed to clarify theoretical conditions for the appearance of the ferroelectric analogues of the ferromagnetic Bloch walls. These ferroelectric Bloch walls can be distinguished from the usual ferroelectric domain boundaries by their bistability, their chiral nature, and the existence of an extraneous, symmetry breaking polarization component, restricted to the interior of the domain boundaries themselves. It is time to collect the experimental proofs for the existence of these peculiar nanoscale objects.   

Aharanov-Bohm quantum interference in a reconfigurable electron system

Aharanov-Bohm (AB) interference can arise in transport experiments when magnetic flux threads through two or more transport channels. The existence of this behavior requires long-range ballistic transport and is typically observed only in exceptionally clean materials. We observe AB interference in wide ($w \sim 100$ nm) channels created at the LaAlO$_3$/SrTiO$_3$ interface using conductive AFM lithography. Interference occurs above a critical field $B \sim 4$ T and increases in magnitude with increasing magnetic field. The period of oscillation implies a ballistic length that greatly exceeds the micron-scale length of the channel, consistent with Fabry-Perot interference in 1D channels. The conditions under which AB oscillations are observed will be discussed in the context of the electron pairing mechanism in LaAlO$_3$/SrTiO$_3$.   

Polar metals as electrodes to suppress the critical-thickness limit in ferroelectric nanocapacitors

Ferroelectric perovskites oxides have potential applications in nonvolatile random access memories and high-density data storage devices. Following consumer demands for higher density data storage technologies, a continuing miniaturization of electronic components has been the major focus of researchers in academia and industry. To realize such scaling for devices which integrate ferroelectric oxides, one of the most important issues is how to maintain ferroelectricity when the component size reduces to the nanometer scale. Using first-principles calculations we investigate the critical thickness for ferroelectricity in a symmetric capacitor structure consisting of ultrathin ferroelectric NaNbO$_3$ confined between the polar-noncentrosymmetric metal LiOsO$_3$. We find that the polar displacements in the metallic electrodes induce an interfacial ferroelectricity, which strongly supports the polar instability in the ferroelectric film even in subnanometer thick dielectric layers. Our results support the use of noncentrosymmetric metals as electrodes in ferroelectric nanocapacitors and related device structures as they maintain the functionality of the ferroelectric layer independently of the level of miniaturization.   

Thermodynamic Stability of Ferroelectric Oxide Surface in Pulsed Laser Deposition: Novel Way to Atomic-scale Interface Control.

We demonstrated the selective fabrication of BaO-RuO$_{\mathrm{2}}$ and TiO$_{\mathrm{2}}$-SrO atomic sequence at the interface of epitaxial SrRuO$_{\mathrm{3}}$/BaTiO$_{\mathrm{3}}$/SrRuO$_{\mathrm{3}}$ (SRO/BTO/SRO) heterostructure using pulsed laser deposition (PLD). The growth conditions for stabilizing each interface structure are strongly related to the phase diagram of BTO surface structure expected from thermodynamic equilibria. The approach allows us precise control of desired sequence of atomic layers with abrupt interfaces. By simply adjusting the oxygen partial pressure, we obtained an abrupt BTO top interface with a TiO$_{\mathrm{2}}$ termination. The interface-controlled BTO exhibits a robust ferroelectricity down to a thickness of 3.5 unit cells (\textasciitilde 1.4 nm)--- a theoretical limit that has to date remained elusive in experiment. Despite PLD has been considered to have non-equilibrium nature, these results highlight that thermodynamic considerations are useful to identify the correct stoichiometric surface/interface terminations.   

Ferroelectric domain alignment in PbTiO$_{\mathrm{3}}$/SrTiO$_{\mathrm{3}}$ superlattice nanostructures

Alignment of spontaneously formed 180\textordmasculine ferroelectric stripe domains is observed in PbTiO3/SrTiO3 superlattice (SL) ridge nanostructures created using focused-ion beam patterning. Synchrotron x-ray nanodiffraction studies exhibit angular shifts of diffraction patterns and enhanced domain scattering intensity, which are associated with lattice tilts and degree of domain alignment, respectively. The domain diffuse scattering intensity is enhanced in the region of reciprocal space corresponding to domains arranged parallel to the boundaries of the ridge. The domain diffuse scattering is approximately an order of magnitude brighter in nanostructures than unpatterned regions, with a Maier-Saupe distribution function fitting with an order parameter of 0.92. A Landau-Ginzburg-Devonshire theory simulation demonstrates that different magnitudes of elastic relaxation, caused by domain configurations with domain walls either antiparallel or parallel to the patterned edges, result in the parallel domain wall configuration in nanostructures more energetically favorable than the antiparallel configuration, which agrees with the experimental observation.   

Study of retention loss in BTO heterostructure using optical second-harmonic generation microscopy

Barium titanate (BTO) is a promising material for ferroelectric non-volatile memory due to its compatibility with semiconductor substrate and low operating power. Recently, it has been demonstrated that BTO film grown on Ge substrate with STO buffer layer can have out-of-plane polarization and good ferroelectricity[1]. To use it in an actual application, one of the biggest challenges is minimizing retention loss, so that the memory can retain the information for a long time without consuming additional power. Second Harmonic Generation (SHG) microscopy is a non-destructive and fast way to study polarization change over time. In this work, we studied polarization retention loss in BTO heterostructure in a long time scale using SHG, and compared it with Microwave Impedance Microscopy (MIM) and Piezo Force Microscopy (PFM). We also varied the thickness of BTO and STO film to find the optimum thickness to minimize writing voltage and still maintain good polarization. We will present this experiment data, and compare it with the theory model[2]. [1] P. Ponath \textit{et al.}, \textit{Nature Comm.}, \textbf{6}, 6067 (2015) [2] B. S. Kang \textit{et al.}, \textit{Jpn. J. Appl. Phys.} \textbf{41}, 5281 (2002)   

Role of ferroelectric polarization during the growth of BaTiO$_{\mathrm{3}}$ /PbTiO$_{\mathrm{3}}$ ferroelectric superlattices

In epitaxially strained ferroelectric thin films and superlattices, the ferroelectric transition temperature can lie above or below the growth temperature. Ferroelectric polarization and domains can evolve during the growth of a sample, and electrostatic boundary conditions may play an important role. Short-period (1/1 to 5/5) BaTiO$_{\mathrm{3}}$ /PbTiO$_{\mathrm{3}}$ (BTO/PTO) ferroelectric superlattices with 50{\%} BTO composition were grown and we see a steady interface driven enhancement of piezoelectricity as the layer thickness is reduced. By contrast, the dielectric constant we measured in experiment has two characteristic regimes. Samples with layer thickness of 3 unit cells or more have higher dielectric constants than those with 2 or 1 unit cell thick layers. The ferroelectric nature was investigated by Piezoforce Microscopy and we found a domain transition from monodomain (1/1 and 2/2) to polydomain (3/3, 4/4 and 5/5) as grown. This appears to be associated with the ferroelectric state of the PTO layer during growth, i.e. if it is ferroelectric during the growth the samples are polydomain, and if not they are monodomain. Associated with this, we also found the BTO layer growth rates are strongly associated with the ferroelectric state of the PTO layers during the growth.   

Precise tuning of the built-in bias in PbTiO$_{3}$ based superlattice thin films.

The artificial layering in ferroelectric superlattices is a potential source of polarization asymmetry that results in a built-in bias which shifts polarization hysteresis loops. Here, we studied the origin of polarization asymmetry for ferroelectric PbTiO$_{3}$ based superlattice films. Using off-axis RF sputtering, we prepared several compositions of PbTiO$_{3}$/SrTiO$_{3}$ (PTO/STO) superlattice thin films; and for comparison PbTiO$_{3}$/SrRuO$_{3}$ (PTO/SRO) superlattices, which have an additional intrinsic compositional asymmetry at the interface. From theoretical modeling and experiments such as heat treatment and repetitive electrical cycling on the samples, we were able to identify and study the impact of Pb-O vacancy defect dipoles on the built-in bias. In addition, we were able to detect the presence of vacancies at the interfaces in the superlattice with STEM-EELS. Finally, we show the ability to tune this built-in bias by depositing a hybrid superlattice that combines PTO/STO and PTO/SRO superlattices. By tuning the composition of the PTO/STO/PTO/SRO$_{\, }$superlattice, the built-in bias in this combo-superlattice can be reduced to zero.   

Observation of ferroelectricity at room temperature in 1 nm thick conducting BaTiO$_{3−\delta}$

Requirements of multi-functionalities in thin-film systems have led important findings of unique physical character and degree of freedom which only exist in film forms. As growth technique gets advanced, one can decrease the film thickness even $\sim$ nm scale where its unique physical character still appears. Among those intriguing film systems, ferroelectric has been of interest. As a prototype ferroelectric, electrical properties of ultra-thin BaTiO$_3$ (BTO) films have extensively studied, which is found that ferroelectricity sustains down to $\sim$ nm-thick films as theoretically predicted. However, efforts on determination of the minimum thickness in ferroelectric films was hindered by large leakage current. In this study, we used ~ nm-thick BTO films showing metallic-like behaviour around room temperature (RT). Surprisingly, even the 2 unit-cells-thick metallic-like BTO film shows ferroelectric switching behaviour at RT! Observation of such ultra-thin conducting ferroelectric will enlarge its applicable fields leading realization of new functional devices and investigations of further physical phenomena.   

Two-dimensional magneto-optical light modulation in EuTiO3

EuTiO$_{3}$ is antiferromagnetic at low temperature, namely below T$_{N}=$5.7K. In the high temperature paramagnetic phase the strongly nonlinear coupling between the lattice and the nomnially silent Eu 4f$^{7}$ spins induces magnetic correlations which become apparent in muon spin rotation experiments [1] and more recently in birefringence measurments [2] in an external magnetic field. It is shown here, that high quality films of insulating EuTiO$_{3}$ deposited on a thin SrTiO$_{3}$ substrate are versatile tools for light modulation. The operating temperature is close to room temperature and admits multiple device engineering. By using small magnetic fields the birefringence of the samples can be switched off and on. Similarly, rotation of the sample in the field can modify its birefringence $\Delta $n. In addition, $\Delta $n can be increased by a factor of 4 in very modest fields with simultaneously enhancing the operating temperature by almost 100K. The results can be understood in terms of paramagnon phonon interaction where spin activity is achieved via the local spin-phonon double-well potential. \newline [1] Z. Guguchia, et al.,Phys. Rev. B \textbf{90}, 064413 (2014). \newline [2] B. Stuhlhofer, et al., Phase Trans. \textbf{89}, 740 (2016).   

Intrinsic size effects and topological phase transformations in ferroelectric nanoparticles embedded in dielectric media

Self-assembled composite materials comprised of ferroelectric (FE) nanoinclusions dispersed in a dielectric matrix are being actively investigated for a variety of tunable functional properties attractive for a wide range of novel electronic and energy harvesting devices. However, the dependence of these functionalities on shapes, sizes, orientation and mutual arrangement of FE particles is currently poorly understood. In this study, we utilize a time-dependent thermodynamic Landau-Ginzburg-Devonshire approach combined with coupled-physics finite-element-method based simulations to elucidate the behavior of polarization in isolated spherical PbTiO$_3$ or BaTiO$_3$ nanoparticles embedded in the dielectric medium. The equilibrium polarization topology is strongly affected by particle diameter, as well as the choice of inclusion and matrix materials, with monodomain, vortex-like and multidomain patterns emerging for various combinations of size and materials parameters. In turn, this leads to radically different responses under hysteretic field switching, resulting in highly tunable size-dependent FE properties that should be easily observed experimentally.   

Ferroelastic switching in a layered-perovskite thin film.

A controllable ferroelastic switching in ferroelectric/multiferroic oxides is highly desirable due to the non-volatile strain and possible coupling between lattice and other order parameter in heterostructures. However, a substrate clamping usually inhibits their elastic deformation in thin films without micro/nano-patterned structure so that the integration of the non-volatile strain with thin film devices is challenging. Here, we report that reversible in-plane elastic switching with a non-volatile strain of approximately 0.4{\%} can be achieved in layered-perovskite Bi2WO6 thin films, where the ferroelectric polarization rotates by 90° within four in-plane preferred orientations. Phase-field simulation indicates that the energy barrier of ferroelastic switching in orthorhombic Bi2WO6 film is ten times lower than the one in PbTiO3 films, revealing the origin of the switching with negligible substrate constraint. The reversible control of the in-plane strain in this layered-perovskite thin film demonstrates a new pathway to integrate mechanical deformation with nanoscale electronic and/or magnetoelectronic applications.