Near-field second-harmonic imaging of ferromagnetic and ferroelectric materials

A novel scanning probe technique that is able to image surface magnetic and electric properties has been developed. It is based on the near-field microscopy of surface second-harmonic generation. We have demonstrated the capability of the technique by imaging the domain structure of a Ni single crystal and a piezoelectric ceramic.     

Crystallization kinetics in ferroelectric thin films: Viability of atomic force microscopy

The pyrochlore to perovskite phase transformation was studied in lead zirconate titanate (PZT) thin films. The films were fabricated on platinum electrodes and annealed by rapid thermal processing (RTP). The phases which formed and their location in the film were analyzed using glancing angle XRD and depth profiling was demonstrated. Grain size and structure, nucleation sites and surface morphology were determined with transmission electron microscop (TEM) and atomic force microscopy (AFM). The role of AFM in this type of transformation study was assessed.

The PZT films crystallized with a (100) orientation which was preferentially nucleated at the platinum/film interface. RTP at 650°C for 15 s was sufficient to complete the transformation. However, columnar grain growth and improvements in the ferroelectric properties were obtained with increased RTP time. A PZT film with RTP at 650°C for 1 min possessed a remanent polarization of 25 μC/cm² and a dielectric constant of ε = 650.     

Leakage currents in ferroelectric thin films

The main mechanisms of leakage currents in thin lead zirconate titanate (PZT) ferroelectric films prepared by the sol–gel method are discussed. Four specific regions are determined in I–V dependencies. At very weak fields (10–20 kV/cm), the current falls with the voltage increase as a result of depolarization. In the low fields region (about 70–100 kV/cm), the leakage current decreases with the decrease of voltage ramp speed and its components are the ohmic and displacement currents. In the high fields region (≥130 kV/cm), the leakage current increases with the decrease of step voltage ramp in contrast to the previous case. Possible conductivity mechanisms are the Poole– Frenkel emission and hopping conduction. In the transition region between above-mentioned ones (from 80–90 to ～130 kV/cm), an abrupt unstable increase of current is observed caused by breakdown of reverse bias Schottky barrier. Depolarization currents are studied for sol–gel PZT films prepared at different preparation conditions.     

Leakage currents in thin ferroelectric films

The basic mechanisms of leakage current components of thin lead zirconate titanate (PZT) ferroelectric films grown by the sol-gel method have been studied. Characteristic regions of current-voltage characteristics with different charge transport mechanisms have been determined. It has been shown that there is an intermediate region which separates such regions. In one of them, the leakage current depends on properties of the contact of electrodes with PZT film at low voltages; in the other, the leakage current is controlled by intrinsic properties of the PZT film bulk, and the basic mechanism of charge transport is Poole-Frenkel emission. In the intermediate region, a stepwise change in the current has been observed, which is caused by relaxing breakdown of the Schottky barrier. Time dependences of the leakage currents have been determined. It has been shown that the leakage current decreases with increasing delay time before the Schottky barrier breakdown, and the dependence becomes opposite in character after the breakdown.     

Mesoscopic microwave dispersion in ferroelectric thin films

The magnetic and transport properties of pyrochlore R_{2}Mo_{2}O_{7} have been studied with variation of the rare earth ( R). The change of the mean ionic radius of R, which induces change of the lattice structure, determines the magnetic ground state (ferromagnetic or spin glass), and the magnetic phase boundary is correlated with the metal-insulator crossover. Furthermore, we found enhanced magnetoresistance and unusual residual anomalous Hall effect at low temperatures near the phase boundary, which can be attributed to the coexistence of both phases.     

Atomic-force microscopy of polarization domains in ferroelectric films

Single-crystal (001)-oriented PbZr_0.47Ti_0.53O₃ and polycrystalline (111)-oriented PbZr_0.47Ti_0.53O₃ thin ferroelectric films were studied using contact electrostatic force microscopy. Local electromechanical response measurements permitted study of the polarization vector distribution in natural and intentionally created polarization nanodomains in films. The principal components of an electromechanical response signal encountered in studies of ferroelectric films (the piezoelectric response and the additional capacitive contribution) were isolated and analyzed. The effect of tip-surface contact stiffness on the capacitive contribution to the electromechanical response signal was demonstrated experimentally and in terms of a model. It was shown that more accurate information on the distribution of the polarization vector in ferroelectric films can be gained by monitoring local variations in the tip-surface contact stiffness.     

Simulation of thickness dependence in ferroelectric thin films

The thickness dependence of coercive field (E_C) and remanent polarization (P_r) in ferroelectric thin films has been numerically simulated using a two-dimensional four-state Potts model. In this model, each of the dipoles in the film is assigned to one of the four states corresponding to the four different mutually perpendicular orientations. Neighboring dipoles with the same orientation are then grouped together to form a domain. Four different kinds of domains exist. In the presence of the surface layer near the electrode/film interface, the thickness dependence of both coercive field and remanent polarization are simulated.     

Near-field scanning optical microscopy studies of thin film surfaces and interfaces

In this article, the results of the modeling of topography related artifacts appearing in near-field scanning optical microscopy measurements are presented. The results obtained for near-field scanning optical microscope operation in reflection mode with off-axis far field detector position are compared with experimental results. It is shown that the chosen numerical method - Finite Difference in Time Domain method (FDTD) - can be used for efficient modeling of main topography related artifact. It is also seen that the far field detector position can have large influence on the resulting reflection mode optical images.     

Giant flexoelectric effect in ferroelectric epitaxial thin films

We report on nanoscale strain gradients in ferroelectric HoMnO(3) epitaxial thin films, resulting in a giant flexoelectric effect. Using grazing-incidence in-plane x-ray diffraction, we measured strain gradients in the films, which were 6 or 7 orders of magnitude larger than typical values reported for bulk oxides. The combination of transmission electron microscopy, electrical measurements, and electrostatic calculations showed that flexoelectricity provides a means of tuning the physical properties of ferroelectric epitaxial thin films, such as domain configurations and hysteresis curves.     

Domain switching kinetics in disordered ferroelectric thin films

We investigated domain kinetics by measuring the polarization switching behaviors of (111)-preferred polycrystalline Pb(Zr,Ti)O3 films, which are widely used in ferroelectric memories. Their switching behaviors at various electric fields and temperatures could be explained by assuming the Lorentzian distribution of logarithmic domain-switching times. We suggested that the local field variation due to dipole defects at domain pinning sites could explain the Lorentzian distribution.     

Nonlinear repolarization processes in ferroelectric liquid-crystal thin films

The dynamics of spontaneous polarization switching of the ferroelectric smectic C* in a variable electric field are examined theoretically and experimentally with the help of polarized light scattering. The observed effect of quasiresonant scattering both in freely suspended smectic films and in ordinary electro-optical cells is interpreted within the framework of the nonlinear model of isolated movable kinks in the director orientation distribution. It is shown that the maximum of the scattering intensity at the characteristic frequency of the applied electric field disappears at low temperatures and for small thicknesses of the smectic film. The dependence of the “resonant” frequency on the electric field amplitude, the proximity to the phase transition temperature, the film thickness and thickness of the ferroelectric domains, and also various material parameters is found. Estimates are made of such important characteristics as the dielectric anisotropy, viscosity, and elasticity of the smectic films. The effect of film thickness on the density distribution of the polar anisotropy energy in the film and on the corresponding shape of the moving orientation front within the film are discussed. Zh. éksp. Teor. Fiz. 111, 919–937 (March 1997)     

Enhanced electrical properties in bilayered ferroelectric thin films

Sr2Bi4Ti5O18(SBTi) single layered and Sr2Bi4Ti5O18 /Pb(Zr0.53Ti0.47)O3(SBTi/PZT) bilayered thin films have been prepared on Pt/TiO2/SiO2/Si substrates by pulsed-laser deposition(PLD).The related structural characterizations and electrical properties have been comparatively investigated.X-ray diffraction reveals that both films have crystallized into perovskite phases and scanning electron microscopy shows the sharp interfaces.Both films show well-saturated ferroelectric hysteresis loops,however,compared with the single layered SBTi films,the SBTi/PZT bilayered films have significantly increased remnant polarization(Pr) and decreased coercive field(Ec),with the applied field of 260 kV/cm.The measured Pr and Ec of SBTi and SBTi/PZT films were 7.9 C/cm 2,88.1 kV/cm and 13.0 C/cm 2,51.2 kV/cm,respectively.In addition,both films showed good fatigue-free characteristics,the switchable polarization decreased by 9% and 11% of the initial values after 2.2 10 9 switching cycles for the SBTi single layered films and the SBTi/PZT bilayered films,respectively.Our results may provide some guidelines for further optimization of multilayered ferroelectric thin films.     

Barrier photovoltaic effects in PZT ferroelectric thin films

The photoelectric characteristics (independent of ferroelectric polarization) of metal-ferroelectric-metal thin film structures upon exposure to radiation in different ranges of mercury arc lamp spectrum are studied for the Pb(Zr_0.52Ti_0.48)O₃ (PZT) ferroelectrics. The PZT films on platinized silicon substrates were prepared by the sol-gel technique. The relaxations of the short-circuit current and the open-circuit voltage are investigated at different intensities of light with wavelengths in the range 300–1200 nm. It is found that the open-circuit voltage returns to its original value after the cessation of light exposure and a short-term holding of structures in the short-circuited state. The factors responsible for the photocurrent and the photoemf are analyzed, and the conclusion is made that they are predominantly contributed by the barrier photovoltaic effects associated with the presence of the p-n junction in the bulk of films and the Schottky barrier in the film region adjacent to the lower platinum electrode.     

Spreading resistance microscopy of polycrystalline and single-crystal ferroelectric films

Thin films based on lead zirconate titanate with stoichiometric composition near the morphotropic boundary have been studied using atomic-force microscopy methods. The dependence of the local conductivity on the local polarization direction has been observed for all samples, independently of substrate type, deposition method, and film thickness. It has been shown that the current response to the applied voltage exhibits a long current relaxation, about several tens of seconds, which is two to three orders of magnitude greater than the current relaxation time in an external circuit, associated with the ferroelectric domain switching. The conductivity features have been explained by recharging of traps localized at ferroelectric grain boundaries near electrodes and involved in polarization charge screening.     

Enhancement of the magnetoelectric effect in thin ferroelectric films

The enhancement of the nonlinear magnetoelectric effect in a magnetic field is predicted for a ferroelectric film in a system consisting of ferroelectric and paraelectric layers in the vicinity of the size-induced ferroelectric phase transition. This effect is assumed to be maximum in semiconductor ferroelectrics.     

Pulsed laser deposition of perovskite relaxor ferroelectric thin films

Structural, dielectric and ferroelectric properties of thin films of La-doped lead zirconate titanate (PLZT) and sodium bismuth titanate-barium titanate (NBT-BT) perovskite relaxor ferroelectric have been investigated. PLZT films were deposited on Pt/Si substrates in oxygen atmosphere by pulsed laser deposition (PLD) and radio frequency (RF) discharge-assisted PLD, using sintered targets with different La content and Zr/Ti ratio, near or at the boundary relaxor ferroelectric. The films are polycrystalline with perovskite cubic or slightly rhombohedral structure. A slim ferroelectric hysteresis loop, typical for relaxors, has been measured for all film sets. Dielectric characterization shows a large value of capacitance tunability and low dielectric loss. However, common problems related to lead diffusion into the metallic electrode layer do not allow one to obtain high capacitance values, due to the formation of an interface layer with low dielectric constant. Lead-free NBT-BT thin films have been deposited on single crystal (1 0 0)-MgO substrates starting from targets with composition at the morphotropic phase boundary between rhombohedral and tetragonal phase. Films deposited by PLD are polycrystalline perovskite with a slight (1 0 0) orientation. Capacitance measurements were performed using interdigital metallic electrodes deposited on the film's top surface and showed high relative dielectric constant, on the order of 1300.     

Dynamical dielectric susceptibility of ferroelectric thin films and multilayers

The thickness dependence of the real and imaginary parts of the dynamical dielectric susceptibility is investigated phenomenologically for a multilayer structure consisting of alternating ferroelectric and paraelectric layers. It is shown that the frequency dependence of the linear dielectric response can be closely approximated by that of a damped harmonic oscillator, with the static susceptibility, relaxation time, and soft-mode frequency depending on the layer thickness and temperature. When the layer thickness and temperature are equal to their critical values corresponding to the onset of a size-driven ferroelectric phase transition, the static susceptibility and the relaxation time become anomalously large and then decrease with further increasing layer thickness. A spectrum of natural polarization oscillations is predicted to exist with thickness-dependent frequencies. This spectrum includes a soft-mode frequency which vanishes at the critical thickness and at the critical temperature. The frequency spectrum lies below the soft-mode frequency of a thick film (in which the gradient of polarization is negligible). The calculations are compared with experimentally measured dispersion of the dielectric response of a PbTiO₃-Pb_0.72La_0.28TiO₃ multilayer structure. The agreement between the theory and experiment is found to be good.     

Stochastic ferroelectric switching of lead zirconate titanate thin films

We investigate the repolarization phenomenon in a ferroelectric film. Our ferroelectric sample was lead zirconate titanate (PZT) obtained by sol-gel synthesis and deposited by spin coating on ITO/glass substrate. A series of repolarizations were induced in the ferroelectric film by applying a triangular wave and the current peaks related to the switchings of the ferroelectric domains were acquired for statistical analyses. It is shown that the dynamics and statistics of polarization switchings are well simulated by a simple mean-field model in which a double-well, asymmetric potential is included to describe the asymmetry at the PZT-ITO interface.