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.

We demonstrate that atomic force microscopy can be used to precisely manipulate individual sub-50 nm ferroelectric domains in ultrahigh density arrays on high-quality epitaxial Pb͑Zr 0.2 Ti 0.8 ͒O 3 thin films. Control of domain size was achieved by varying the strength and duration of the voltage pulses used to polarize the material. Domain size was found to depend logarithmically upon the writing time and linearly upon the writing voltage. All domains, including those written with ϳ100 ns pulses, remained completely stable for the 7 day duration of the experiment. © 2001 American Institute of Physics. ͓DOI: 10.1063/1.1388024͔Increasing demand for ultrahigh density ͑uhd͒ information storage has fueled significant interest in the use of atomic force microscopy ͑AFM͒ for nanoscopic read/write operations. General requirements of nonvolatile uhd memories are fast operating times, small bit size, and long-term data retention. Nonreversible AFM lithography by local oxidation and thermomechanical processes has been extensively researched. 1-3 Solutions incorporating parallel processing have also been explored, increasing the scan range and speed of possible applications. 4,5 A particularly appealing approach, allowing dynamic memory as well as data storage, is to locally modify the reversible and nonvolatile polarization of ferroelectric oxides with an AFM-generated electric field, 6-12 a technique recently extended to ferroelectric/silicon heterostructures. 13 Detailed studies of domain switching behavior in these materials, focusing on domain size and stability in relation to writing time, writing voltage, and the shape of the AFM tip, are therefore important for the development of memory applications. Such studies would also aid in understanding the fundamental physics of domain dynamics in thin films. The perovskite Pb͑Zr x Ti 1Ϫx )O 3 ͑PZT͒, a stable compound with high remanent polarization, has been widely recognized as an attractive candidate for memory applications. Epitaxially grown monocrystalline films of this material are particularly suitable for domain behavior studies due to the uniformity of their switching properties over the sample surface. 10 Although desirable long-term stability has been found for standard 100 nm sized capacitors in 1000-Å-thick films of related ferroelectric compounds, 11 studies of retention loss in sub-100 nm PZT domains using the AFM approach, with the tip itself serving as a mobile top electrode, present contradictory results. The extrapolation of temperature dependence data for epitaxial PZT films gives polarization retention estimates of decades at RT, 9 while other groups report spontaneous reversal of polarization after a few hours. 8 In this letter we demonstrate nanoscopic control of read/ write operations in uhd arrays, and report on the time dependence of domain switching behavior for domains as small as 40 nm, over eight orders of magnitude in writing time, down to ϳ100 ns. We also discuss how different applied voltages affect the size of the polarized domains. Final...

Atomic force microscopy was used to investigate ferroelectric switching and nanoscale domain dynamics in epitaxial Pb͑Zr 0.2 Ti 0.8 ͒O 3 thin films. Measurements of the writing time dependence of domain size reveal a two-step process in which nucleation is followed by radial domain growth. During this growth, the domain wall velocity exhibits a v ϰ exp− ͑1/E͒ dependence on the electric field, characteristic of a creep process. The domain wall motion was analyzed both in the context of stochastic nucleation in a periodic potential as well as the canonical creep motion of an elastic manifold in a disorder potential. The dimensionality of the films suggests that disorder is at the origin of the observed domain wall creep. To investigate the effects of changing the disorder in the films, defects were introduced during crystal growth ͑a-axis inclusions͒ or by heavy ion irradiation, producing films with planar or columnar defects, respectively. The presence of these defects was found to significantly decrease the creep exponent , from 0.62-0.69 to 0.38-0.5 in the irradiated films and 0.19-0.31 in the films containing a-axis inclusions.

The properties of ferroelectric domain walls can significantly differ from those of their parent material. Elucidating their internal structure is essential for the design of advanced devices exploiting nanoscale ferroicity and such localized functional properties. Here, we probe the internal structure of 180° ferroelectric domain walls in lead zirconate titanate (PZT) thin films and lithium tantalate bulk crystals by means of second-harmonic generation microscopy. In both systems, we detect a pronounced second-harmonic signal at the walls. Local polarimetry analysis of this signal combined with numerical modelling reveals the existence of a planar polarization within the walls, with Néel and Bloch-like configurations in PZT and lithium tantalate, respectively. Moreover, we find domain wall chirality reversal at line defects crossing lithium tantalate crystals. Our results demonstrate a clear deviation from the ideal Ising configuration that is traditionally expected in uniaxial ferroelectrics, corroborating recent theoretical predictions of a more complex, often chiral structure.