Polarization switching in ferroelectrics has been thought to occur only through the nucleation and growth of new domains. Here we use in situ synchrotron x-ray scattering to monitor switching controlled by applied chemical potential. In sufficiently thin PbTiO₃ films, nucleation is suppressed and switching occurs by a continuous mechanism, i.e., by uniform decrease and inversion of the polarization without domain formation. The observed lattice parameter shows that the electric field in the film during switching reaches the theoretical intrinsic coercive field.
"What we are interested in is whether it has a continuous polarization switching without domain formation (continuous mechanism). Continuous mechanism has been studied in PbTiO 3  and this mechanism will not cause fatigue because according to domain wall pinning mechanism, electrostatic coupling between a non-neutral domain wall and mobile carriers forms a pined electro-neutral structure . "
[Show abstract][Hide abstract] ABSTRACT: Rhombohedral BiFeO 3 which has a large spontaneous polarization is a candidate material for ferroelectric random access memory (FeRAM). And the polarization reversal is the key factor for the application of FeRAM. Here, the electronic, linear and nonlinear optical properties, structure and its stability of BiFeO 3 during the continuous polarization reversal from reference cubic phase to ferroelectric R3c phase were systematically investigated by LSDA + U (local spin density approximation plus Hubbard U) and DFPT (density functional perturbation theory) method. We proposed that the nonlinear optical second order coefficients could be indicators to check the polarization reversal of BiFeO 3 and our result showed that the coefficient reach its maximum between 20% distortion and 40% distortion during the half path of the reversal. The relationship between the nonlinear optical second order coefficient and the Bader charge was also compared and we found the coefficient is also dependent on the Bader charge. In addi-tion, the linear optical absorption coefficient was calculated; the obvious change in absorption peaks was observed during the transition. By comparing the crystal orbital of the reference cubic phase and dis-torted R3c phase we found that the hybridized Bi 6s and O 2p is the main reason that causes the structure distortion. The local phonon density of states proves that the Bi and O interaction is the origin of the instability of cubic phase BiFeO 3 . The factor group analysis showed the change of Raman and IR modes during the transition.
Journal of Alloys and Compounds 12/2014; 623:393. DOI:10.1016/j.jallcom.2014.11.062 · 3.00 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Thermodynamic theory is developed for the ferroelectric phase transition of
an ultrathin film in equilibrium with a chemical environment that supplies
ionic species to compensate its surface. Equations of state and free energy
expressions are developed based on Landau-Ginzburg-Devonshire theory, using
electrochemical equilibria to provide ionic compensation boundary conditions.
Calculations are presented for a monodomain PbTiO$_3$ (001) film coherently
strained to SrTiO$_3$ with its exposed surface and its electronically
conducting bottom electrode in equilibrium with a controlled oxygen partial
pressure. The stability and metastability boundaries of phases of different
polarization are determined as a function of temperature, oxygen partial
pressure, and film thickness. Phase diagrams showing polarization and internal
electric field are presented. At temperatures below a thickness-dependent Curie
point, high or low oxygen partial pressure stabilizes positive or negative
polarization, respectively. Results are compared to the standard cases of
electronic compensation controlled by either an applied voltage or charge
across two electrodes. Ionic surface compensation through chemical equilibrium
with an environment introduces new features into the phase diagram. In
ultrathin films, a stable non-polar phase can occur between the positive and
negative polar phases when varying the external chemical potential at fixed
temperature, under conditions where charged surface species are not present in
sufficient concentration to stabilize a polar phase.
[Show abstract][Hide abstract] ABSTRACT: Recent developments in ferroelectric (FE) domain imaging techniques have established an understanding of intriguing polarization switching dynamics. In particular, nanoscale studies of FE domain switching phenomena using piezoresponse force microscopy (PFM) can provide important microscopic details on nucleation and subsequent growth of domains, complementing conventional electrical measurements that only give macroscopic information. This review covers recent nanoscale PFM studies of domain switching dynamics in FE thin films. Recent nanoscale PFM-based studies have demonstrated that quenched defects inside the FE thin films play important roles in domain switching processes, including defect-mediated inhomogeneous nucleation, pinning-dominated nonlinear dynamics of domain walls, and many other intriguing phenomena.Highlights►We review recent piezoresponse force microscopy studies of ferroelectric domains. ►Piezoresponse force microscopy has provided nanoscale details of domain switching. ►Quenched defects in ferroelectric thin films act as nucleation centers. ►Defects also act as pinning sites for propagating ferroelectric domain walls.
Current Applied Physics 09/2011; 11(5):1111-1125. DOI:10.1016/j.cap.2011.05.017 · 2.21 Impact Factor
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