Memory against temperature or electric field sweeps in potassium niobo-tantalate crystals

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Aging, memories after temperature sweeps (double ramp and double jump) and memories after electric field sweeps (double ramp and double jump) were studied as a function of frequency. The experiments were performed at low temperatures in the ferroelectric phase of two potassium niobo-tantalate crystals K Ta1-xNbxO3 with the niobium concentration x close to 0.02. Five complex quantities are defined, which respectively characterize these five phenomena. The main feature is that isothermal aging and memories after temperature sweeps have exactly the same frequency dependence while after electric field sweeps the frequency dependence is clearly different. Additionally, the role of the characteristics of the sweeps (amplitudes, rates of changes, durations) on these memories were measured. The observed behaviours are discussed in term of a model which attributes the time dependent effects to growth and reconformation of ferroelectric domains and takes into account that the domain wall motion is hindered by pinning sites. The difference in the frequency dependences against the nature of the swept parameters shows that the distribution of the reconformation time is sensitive to the biasing electric field.