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...

Domain wall conduction in insulating Pb(Zr(0.2) Ti(0.8))O(3) thin films is demonstrated. The observed electrical conduction currents can be clearly differentiated from displacement currents associated with ferroelectric polarization switching. The domain wall conduction, nonlinear and highly asymmetric due to the specific local probe measurement geometry, shows thermal activation at high temperatures, and high stability over time.

Domains in ferroelectric films are usually smooth, stripelike, very thin compared with magnetic ones, and satisfy the Landau-Lifshitz-Kittel scaling law (width proportional to square root of film thickness). However, the ferroelectric domains in very thin films of multiferroic BiFeO 3 have irregular domain walls characterized by a roughness exponent 0.5-0.6 and in-plane fractal Hausdorff dimension H jj 1:4 0:1, and the domain size scales with an exponent 0:59 0:08 rather than 1 2 . The domains are significantly larger than those of other ferroelectrics of the same thickness, and closer in size to those of magnetic materials, which is consistent with a strong magnetoelectric coupling at the walls. A general model is proposed for ferroelectrics, ferroelastics or ferromagnetic domains which relates the fractal dimension of the walls to domain size scaling.

The static configuration of ferroelectric domain walls was investigated using atomic force microscopy on epitaxial PbZr 0:2 Ti 0:8 O 3 thin films. Measurements of domain wall roughness reveal a power-law growth of the correlation function of relative displacements BL / L 2 with 0:26 at short length scales L, followed by an apparent saturation at large L. In the same films, the dynamic exponent was found to be 0:6 from independent measurements of domain wall creep. These results give an effective domain wall dimensionality of d 2:5, in good agreement with theoretical calculations for a twodimensional elastic interface in the presence of random-bond disorder and long-range dipolar interactions. DOI: 10.1103/PhysRevLett.94.197601 PACS numbers: 77.80.Dj, 68.37.Ps, 77.80.Fm, 77.84.Dy Understanding the behavior of elastic objects pinned by periodic or disorder potentials is of crucial importance for a large number of physical systems ranging from vortex lattices in type II superconductors [1], charge density waves [2], and Wigner crystals [3] to interfaces during growth [4] and fluid invasion [5] processes, and magnetic domain walls [6]. Ferroelectric materials, whose switchable polarization and piezoelectric and pyroelectric properties make them particularly promising for applications such as nonvolatile memories [7,8], actuators, and sensors [9], are another such system. In these materials, regions with different symmetry-equivalent ground states characterized by a stable remanent polarization are separated by elastic domain walls. The application of an electric field favors one polarization state by reducing the energy necessary to create a nucleus with polarization parallel to the field, and thereby promotes domain wall motion. Since most of the proposed applications use multidomain configurations, understanding the mechanisms that control domain wall propagation and pinning in ferroelectrics is an important issue.A phenomenological model derived from measurements of domain growth in bulk ferroelectrics [10 -12] initially suggested that the domain walls were pinned by the periodic potential of the crystal lattice itself. Such pinning was deemed possible because of the extreme thinness of ferroelectric domain walls (different from the case of magnetic systems). However, measurements of the piezoelectric effect [13], dielectric permittivity [14], and dielectric dispersion [15] in ferroelectric ceramics and sol-gel films have shown some features that cannot be described by the existing phenomenological theories. A microscopic study of ferroelectric domain walls could resolve these issues. Recently, we have measured domain wall velocity in epitaxial PbZr 0:2 Ti 0:8 O 3 thin films, showing that in this case commensurate lattice pinning is in fact not the dominant mechanism [16,17]. Rather, a creeplike velocity (v) response to an externally applied electric field E was observed with v expÿC=E , where C is a constant. The exponent characterizing the dynamic behavior of the system is a function of the domain wall dim...

The polarization field of the ferroelectric oxide lead zirconate titanate [Pb(ZrxTi1-x)O3] was used to tune the critical temperature of the hightemperature superconducting cuprate gadolinium barium copper oxide (GdBa2Cu3O7-x) in a reversible, nonvolatile fashion. For slightly underdoped samples, a uniform shift of several Kelvin in the critical temperature was observed, whereas for more underdoped samples, an insulating state was induced. This transition from superconducting to insulating behavior does not involve chemical or crystalline modification of the material.