Effect of defect-induced internal field on the aging of relaxors

Abstract

The effect of a defect-induced internal field on the dielectric response of relaxor ferroelectrics is investigated using a Monte Carlo simulation. It was observed that only at a small temperature range near the temperature of the dielectric maximum does the susceptibility decrease markedly due to the internal field. This temperature range increases with enhancing internal field. We found that the susceptibility is almost independent of the internal field width at low internal field width, and then decreases linearly with enhancing internal field width. This dependence of the susceptibility on the internal field width is very similar to the relation of the dielectric constant with logarithmic aging time, which probably suggests a linear dependence of the internal field on the logarithm of the aging time. The frequency dependence of the susceptibility aging is sensitive to the temperature. With increasing temperature, the curve of the susceptibility change against logarithmic frequency varies from concave to approximately linear, and then to convex, which is in agreement with the recent aging rate measurement. Relaxor ferroelectrics ͑relaxors͒ have special dielectric characteristics compared with normal ferroelectrics. A typical relaxor ferroelectric displays a diffuse phase transition, a strong frequency dispersion of the dielectric properties, and an absence of macroscopic polarization at zero electric field. 1 Although the origin of these dielectric features is still controversial, it has been suggested that the presence of polar microregions in nanoscale is crucial to the relaxor behaviors. Various models, such as the dipolar glass model, the quenched random-field model, and the random-bandrandom-field model, have been proposed to account for the unusual physical properties of relaxor ferroelectrics. [2][3][4][5][6] The relatively high dielectric constant and lowtemperature coefficient has led to many successful applications of relaxors such as multilayered capacitors. Due to their influences on the actual applications and the comprehension of the dielectric mechanism, the dielectric aging behaviors of relaxor ferroelectrics have been extensively investigated in the last decade. 7-14 The experimental results have indicated the importance of the defect structure in the aging in this type of material. For example, very little dielectric aging was observed for a carefully prepared pure PMN. However, PMN with Mn and other acceptor dopants shows significant aging. 10 The main characteristic of acceptor dopants is the introduction of O 2Ϫ vacancies, which results in the formation of the oxygen-vacancy-metal-impurity defect dipoles. 7,8 The reorientation and alignment of the defect dipoles with the local nanoscale polar domains lead to the internal field, which provides a pinning to the polar microregion and results in a decrease of the switchable polarization. 15,16 It is widely accepted nowadays that the defect-induced internal field is responsible for the aging behavior in relaxor ferroelectrics, but how the inter...

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