Depolarization in ferroelectric materials has been studied since the 1970s, albeit quasi-statically. The dynamics are described by the empirical Merz law, which gives the polarization switching time as a function of electric field, normalized to the so-called activation field. The Merz law has been used for decades; its origin as domain-wall depinning has recently been corroborated by molecular dynamics simulations. Here we experimentally investigate domain-wall depinning by measuring the dynamics of depolarization. We find that the boundary between thermodynamically stable and depolarizing regimes can be described by a single constant, P_r/ε₀ε_ferroE_c. Among different multidomain ferroelectric materials the values of coercive field, E_c, dielectric constant, ε_ferro, and remanent polarization, P_r, vary by orders of magnitude; the value for P_r/ε₀ε_ferroE_c however is comparable, about 15. Using this extracted universal value, we show that the depolarization field is similar to the activation field, which corresponds to the transition from creep to domain-wall flow.

Recently reported piezoresponse force microscopy (PFM) measurements have proposed that porcine aortic walls are ferroelectric. This finding may have great implications for understanding biophysical properties of cardiovascular diseases such as arteriosclerosis. However, the complex anatomical structure of the aortic wall with different extracellular matrices appears unlikely to be ferroelectric. The reason is that a prerequisite for ferroelectricity, which is the spontaneous switching of the polarization, is a polar crystal structure of the material. Although the PFM measurements were performed locally, the phase-voltage hysteresis loops could be reproduced at different positions on the tissue, suggesting that the whole aorta is ferroelectric. To corroborate this hypothesis, we analyzed entire pieces of porcine aorta globally, both with electrical and electromechanical measurements. We show that there is no hysteresis in the electric displacement as well as in the longitudinal strain as a function of applied electric field and that the strain depends on the electric field squared. By using the experimentally determined quasi-static permittivity and Young's modulus of the fixated aorta, we show that the strain can quantitatively be explained by Maxwell stress and electrostriction, meaning that the aortic wall is neither piezoelectric nor ferroelectric, but behaves as a regular dielectric material.

In this work, the macroscopic polarization of a ferroelectric capacitor is correlated with the local domain morphology. To this end, a ferroelectric capacitor of the random copolymer poly(vinylidenefluoride-trifluoroethylene) [P(VDF-TrFE)] is poled to a set polarization state in a Sawyer-Tower setup. After chemically removing the top electrode, the exposed ferroelectric is locally probed with piezoresponse force microscopy. The domains without the top electrode are thermodynamically stable for weeks in ambient environment, as proven by comparing the remanent polarization measured before etching away and after re-depositing the top electrode. Out-of-plane PFM phase images show a random distribution of domains with up and down polarity. We unambiguously demonstrate a linear correlation between the mean PFM phase and the macroscopic polarization. As a demonstration of the insights that the global excitation and local probing method can provide, we show how thermal and electrical depoling can result in identical macroscopic polarization yet completely different domain morphologies.