Session T24: Focus Session: Dielectric, Ferroelectric, and Piezoelectric Oxides -- Domains

Advanced atomic force microscopy studies of ferroelectric domains and domain walls

The nanoscale resolution of atomic force microscopy (AFM) makes it a powerful tool for local studies of ferroelectric domain nucleation and growth. In particular, domain walls provide a useful model elastic disordered system: their behavior is governed by the competition between their elastic energy, which tends to minimize the domain wall surface, and the randomly varying potential landscape due to disorder present in the samples, which allows pinning. The domain walls present a characteristic static roughness, and a complex dynamic response when subjected to a driving force (electric field), with non-linear creep observed for small forces [1]. In addition, as a result of different symmetries and electronic structure, as well as possible defect migration, these intrinsically nanoscale interfaces often show additional properties, beyond those of their already multifunctional parent material, opening new perspectives for device applications. I will present results of our AFM studies of the static and dynamic behavior of domain walls in epitaxially grown thin films of Pb(Zr$_{0.2}$Ti$_{0.8})$O$_3$ focusing in particular on thermal effects, and on the observation of a lateral piezoresponse signal specifically due to the shear displacement of 180$^{\circ}$ domain walls in this purely out-of-plane-polarized material [2], potentially useful for surface acoustic wave devices. I will also show how this same response can be more generally observed, necessitating care in the interpretation of lateral piezoresponse imaging in materials such as BiFeO$_3$, where it is superimposed on signal due to the in-plane polarization components. Finally, I will present our studies of the switching mechanisms in this latter material under the influence of the electric field applied by the AFM tip. \\[4pt] [1] P. Paruch et al., Phys. Rev. Lett 94, 197601 (2005); J. Appl. Phys. 100, 051608 (2006); T. Tybell et al, Phys. Rev. Lett. 89, 097601 (2002)\\[0pt] [2] J. Guyonnet et al., Appl. Phys. Lett. 95 132902 (2009)   

Structure of ferroelectric polarization domains written by PFM

In ferroelectric materials, polarization and atomic structure are intimately coupled. PFM is commonly used to image and write polarization domains in ferroelectric thin films, but the local structure of the resulting domains is unclear due to the uncertainty in depth sensitivity of the PFM imaging process. X-ray nanoprobe diffraction was used to simultaneously probe the structure and image polarization domains patterned by PFM into an 80nm-thick Pb(Zr$_{0.45}$, Ti$_{0.55})$O$_{3}$ thin film. The Bragg reflections are broader within the written domains, indicating that regions within the film are strained by the writing process. In addition, atomic planes tilt near the domain walls. This means the PFM writing process creates a more complicated structure than predicted by existing electrostatic models.   

Nanoscale polarization switching at a single 180$^{\circ}$ ferroelectric domain wall

Domain wall motion in ferroelectric materials is strongly affected by lattice, surface and defect pinning effects. A variant of Piezoresponse Force Microscopy (PFM) called Switching Spectroscopy PFM (SSPFM) is ideally suited to probe the local domain switching near ferroelectric domain walls and study domain dynamics and polarization switching on the nanoscale. In the vicinity of the biased probe tip, the domain wall bends, attracts or repels from the probe apex, depending on the sign and value of the applied bias. The wall profoundly affects switching on length scales of the order of micrometers. Systematic SSPFM experiments with varying bias voltages are used to plot an \textbf{experimental phase diagram} summarizing the effect of bias voltage on nucleation in the vicinity of a 180\r{ } domain wall.   

Optically driven polarization modulation in lead titanate nanolayers probed by ultrafast x-ray diffraction

We perform time-resolved x-ray diffraction measurements on ferroelectric thin films of lead titanate (PTO) grown on strontium titanate (STO) and dysprosium scandate (DSO) with 100 ps resolution. Under 400 nm excitation, we observe a shift of the diffraction peak in theta-2theta scans to low Q that mostly recovers in a few nanoseconds, which may be partially associated with carrier screening effects. Surprisingly, rocking curve scans indicate that no domain wall movement accompanies this excitation. We systematically study the dynamics of this structural change as a function of pump fluence, sample temperature and thin film-substrate strain. Notably, we observe in the PTO/STO system that the diffraction peak in theta-2theta scans is dramatically reshaped at 550 degrees C, where the stripe phase dominates. In the PTO/DSO system, which minimizes thin film-substrate strain, weaker perturbations are seen. Controlled measurements with 800 nm excitation produce only step-function jumps in delay scans, which can be attributed to thermal heating effects.   

Nonlinear dynamics of domain wall propagation in epitaxial ferroelectric thin films

The dynamics of an elastic interface in random media is of crucial importance to understand numerous intriguing natural phenomena, including domain walls in ferromagnetics, surfaces of epitaxially grown films, contact lines in wetting, and so on. In such media, velocity of an interface should have a nonlinear behavior, classified with various dynamic phases such as creep, depinning, and flow [1]. Despite several significant theoretical progresses, there are few experimental works on it. Here, we present our recent studies on ferroelectric (FE) domain wall dynamics in the epitaxial PbZr$_{0.2}$Ti$_{0.8}$O$_{3}$ (PZT) thin films. We demonstrated that the data of domain wall velocity $v$ driven by dc electric field $E$ in FE film could be classified with the creep, depinning, and flow regimes [1]. First, we measured the data of $v$ at room temperature $T$ directly using the modified-piezoresponse force microscopy (PFM) [2]. To widen the accessible region of $T$ and $E$, we used switching current measurements, combined with direct $v$ data from PFM images. The measured values of velocity exponent \textit{$\theta $} $\sim $ 0.7 and dynamical exponent \textit{$\mu $} $\sim $ 1.0 indicate that the FE domain walls in the epitaxial films are fractal and pinned by a disorder-induced local field. In addition, we investigated domain wall dynamics driven by ac field in the epitaxial PZT films, using frequency $f$-dependent hysteresis loops under various $T$. We observed the novel $f$-dependence of coercive field $E_{C}$ such that the slopes for log $f$ vs. log $E_{C}$ changed at all measured $T$. We found that this indicated the dynamic phase crossover from creep regime to flow regime. Based on these experimental results, we determined the dynamic phase diagram for ferroelectric domain walls driven by ac field. \\[4pt] [1] J. Y. Jo\textit{ et al}., Phys. Rev. Lett. \textbf{102}, 045701 (2009). \\[0pt] [2] S. M. Yang \textit{et al}., Appl. Phys. Lett. \textbf{92}, 252901 (2008).   

Ferroelectric behavior of ultra-thin BiFeO$_{3}$ films

BiFeO$_{3}$ (BFO) is a potential oxide-barrier material for spintronics devices like magnetic tunnel junctions. Also, recent theoretical predictions have opened up the possibility of realizing multi-level devices with ferroelectric (FE) barriers. But understanding the FE properties of ultra-thin BFO films is at its early stages. Control over FE domains with robust polarization switching is a challenge and crucial for achieving any device-related objectives. In this work we have investigated the FE domains of BFO films of thickness between 5 - 100 nm using peizo-force microscopy (PFM) technique, and local properties using switching spectroscopy PFM (SS-PFM). Our films show significant polarization switching loops down to 5nm. We find domains to be irregular-shaped, in sharp contrast to thicker films. Detailed analysis shall be presented.   

Scanning tunneling microscopy investigation of local electronic properties at domain walls in multiferroic BiFeO$_{3}$

Magnetoelectric coupling in multiferroic materials has attracted much attention because of the intriguing science underpinning this phenomenon and the exciting potential for applications and devices that take advantage of these materials with multiple order parameters. BiFeO$_{3}$ (BFO) is the room temperature, single-phase magnetoelectric multiferroic. Recently, the room-temperature electronic conductivity at ferroelectric domain walls in the oxide BFO multiferroics has been successfully demonstrated.\footnote{J. Seidel and et al., \textit{Nature Mater.} \textbf{8}, 229 (2009).} The fundamental mechanism responsible for the striking result mainly occurred with structurally driven changes in locally electronic structures. As motivated by the significance of this discovery, in the work, by using cross-sectional scanning tunneling microscopy, precise structural and electronic information on these epitaxial films are investigated. A combination of scanning tunneling spectroscopy and analysis of the ferroelectric domain walls on electronic structures suggests that domain walls in the oxide BFO multiferroics reveals a significant decrease in the band gap.   

\textit{In Situ} Studies of Domain Dynamics and Wall Pinning Using Scanning Transmission Electron Microscopy

The mechanism of ferroelectric domain nucleation and growth is studied using in-situ Scanning Tunneling Microscopy (STM) -- Scanning Transmission Electron Microscopy (STEM). A 300 nm multiferroic BiFeO$_{3}$ thin film is grown on DyScO$_{3}$ and has a large density of 71$^{\circ}$ domain walls. A local electrical field is applied using a W tip inside the STEM. Domain formation can be detected from the strain contrast associated with the newly formed ferroelastic domain wall. A step-wise increase of probe bias reveals the critical voltage for the formation of a new domain as 800 mV. This critical domain nucleation bias is much lower than the value observed by Piezoresponse Force Microscopy, which is of the order of 2-5 V. Notably, it also depends on the sample thickness along the beam direction. Repeated switching experiments in the vicinity of a pre-existing 71 $^{\circ}$ domain wall reveal that the acute angle region between the domain wall and the surface is a preferential nucleation site. A strong asymmetry of domain wall pinning is observed during domain growth. The dependence of domain nucleation and growth kinetics on applied bias will also be discussed.   

Probing Octahedral Rotations and Ferroelectric Domain Structures by Nonlinear Optics

Oxygen octahedron is a basic structural unit in perovskite and related complex oxides. Octahedral rotations, involving rotations of the octahedral units are by far the most common phase transitions in complex oxides. Their symmetry is typically represented by conventional space groups and Glazer notation. We will first show that structures with octahedral rotations possess two-color symmetry that is completely isomorphic with magnetic point groups and space groups. (This is irrespective of whether the material is itself magnetic or not.) More broadly, we will discuss a range of ``roto'' properties in analogy with ``magneto'' properties, such as rotoelectric, piezorotation, rotooptic, and rotomagnetic effects. We will also address questions such as: (1) How do we define a composite symmetry in structures with two or more rotational and/or spin order parameters? (2) How are the rotation reversal symmetry operation and time reversal symmetry operation related in such systems? (3) What are the transformation rules for property tensors in such systems? (4) How can we probe magnetic color symmetries?   

Molecular dynamics study of ferroelectric 90 degree domain walls in lead titanate

Molecular dynamics (MD) simulations were carried out to study the 90$^{\circ}$ domain wall dynamics of PbTiO$_{3}$ under mechanical strain. By using a well-parameterized interatomic potential, the rate of formation of new 90$^{\circ}$ domains at different temperatures and strain states were extracted. Due to faster stress relaxation, the nucleation rate is slower and the critical nucleus larger at higher temperature. Furthermore, alternative stress relief mechanisms are studied. The critical nucleation size is found to be small. A simple mathematical model describing the relationship between rate and strain is formulated.   

Relaxation dynamics of ferroelectric domains in epitaxial BiFeO3 (111) films

Advances in thin-film growth techniques by strain engineering had improved various physical properties in advanced functional materials. The epitaxial strain in multiferroic BiFeO3 (BFO) films induced relatively large ferroelectric polarization at room temperature, which makes BFO materials appealing for applications in non-volatile devices. Moreover, single-domain like environment was obtained in strained epitaxial BFO (111) films and provided an ideal system for the observation of 180- degree domain wall motion. In this study, the dominant mechanism of domain relaxation dynamics in BFO (111) films were investigated. The domains were initially written by different voltage pulses and then relaxed thorough time. A two-step depolarization process was observed, which varied with the initial domain sizes. The transitions of relaxation curves agreed with the meta-stable sizes observed during domain growth, indicating that the electrostatic boundary condition can be an important factor. The results of surface potential measurement implied switching voltages may inject asymmetry charges to the domain surfaces and increase the stable domain size. The dissipation of surface charges affected the 1st step relaxation significantly.