Domain Wall Creep in Epitaxial FerroelectricPb(Zr0.2Ti0.

Abstract

Ferroelectric switching and nanoscale domain dynamics were investigated using atomic force microscopy on monocrystalline PbZr 0:2 Ti 0:8 O 3 thin films. Measurements of domain size versus writing time reveal a two-step domain growth mechanism, in which initial nucleation is followed by radial domain wall motion perpendicular to the polarization direction. The electric field dependence of the domain wall velocity demonstrates that domain wall motion in ferroelectric thin films is a creep process, with the critical exponent close to 1. The dimensionality of the films suggests that disorder is at the origin of the observed creep behavior. [6]. In particular, the response to a small external force is of special theoretical and practical interest. It was initially believed that thermal activation above the pinning barriers should lead to a linear response at finite temperature [7]. However, it was subsequently realized that a pinning potential, either periodic [1] or disordered, [1,[8][9][10], can lead to diverging barriers and thus to a nonlinear ''creep'' response where the velocity is of the form v / expÿRf c =f . is the inverse temperature, R a characteristic energy, and f c a critical force. The dynamical exponent reflects the nature of the system and of the pinning potential. Despite extensive studies of the creep process in periodic vortex systems [1], precise determination of the exponents has proven difficult, given the many scales present in this problem [11]. For interfaces, the creep law has been recently verified in ultrathin magnetic films [6], where the measured exponent 0:25 is in very good agreement with the expected theoretical value for this system. Quantitative studies of creep in other microscopic systems with other pinning potentials are clearly needed.In this respect, ferroelectric materials are of special interest. These systems possess two symmetrically equivalent ground states separated by an energy barrier U 0 , as illustrated in Fig. 1. Each state is characterized by a stable remanent polarization, reversible under an electric field. Regions of different polarization are separated by elastic domain walls. The application of an electric field favors one polarization state over the other, by reducing the energy necessary to create a nucleus with a polarization parallel to the field, and thus promotes domain wall motion. In addition to theoretical interest, understanding the basic mechanism of domain wall motion in ferroelectrics has practical implications for technological applications, such as high-density memories. In bulk ferroelectrics, switching and domain growth were inferred to occur by stochastic nucleation of new domains at the domain boundary, a behavior observed in BaTiO 3 and triglycine sulphate, using combined optical and etching techniques [12,13]. Domain wall propagation via such nucleation was also invoked in early analyses of bulk systems to explain the reported field dependence of domain wall speed, v expÿ1=E [14].In this Letter, we report on studies of ferroelectric domai...

Ferroelectrics at the nanoscale

“…Domains were written perpendicular to the axis of a series of nanowires, by applying a voltage through a scanning probe microscopy tip, as has been done in previous nanoscale polarisation studies [19][20][21]. Non-contact electric force microscopy (EFM) was then used to image the field emanating from the written domains.…”

High-Resolution Studies of Domain Switching Behavior in Nanostructured Ferroelectric Polymers

“…3,4 Domain wall creep has been observed in the disordered systems with longrange variations in pinning potential (so-called random field disorder) with the wall velocity obeying an exponential field dependence, while the short-range (random bond) disorder leads to a power law dependence. 5,6 Dissimilar distribution functions of switching times in ferroelectric films with different microstructure are the root cause for qualitatively different time-dependent switching behavior described either by the statistical Kolmogorov−Avrami−Ishibashi model 7,8 or nucleation limited kinetics. 9,10 In the case of ferroelectric nanostructures, the switching behavior is further complicated by a delicate balance between surface and bulk energy, e.g., interfacial strain, asymmetry of electrical boundary conditions, surface depolarization energy, and so forth.…”

“…This approach has been extensively used over the last several years for investigation of the nanoscale mechanism of polarization reversal by measuring the time-field dependence of the growing domain size. [33][34][35] 36 In contrast to the local PFM switching observed in crystalline inorganic ferroelectrics, 6, 33, 37 the growing domain in the PVDF-TrFE nanomesa acquires a profoundly irregular shape, which can be an indication of a high degree of nonuniformity in the local switching potential. Because of this feature, extraction of a precise value for the wall velocity from the obtained data is difficult.…”

Crossed-ratchet effects and domain wall geometrical pinning

“…Prominent examples are the propagation of reaction fronts or surface growth in material science, 1 cell motility and membrane dynamics in biology, 2 domain walls (DW) in ferromagnetic [3][4][5][6][7][8] or ferroelectric films, [9][10][11] fluid invasion in porous media, 12 contact lines of liquids menisci, 13 and crack propagation. 14,15 In all these cases, the presence of heterogeneities, which locally promote wandering, compete with the elasticity of the interface, giving rise to complex collective pinning effects.…”