Local electromechanical properties and grain size effects in ferroelectric relaxors studied by scanning piezoelectric microscopy

The local electromechanical properties of relaxor 0.9Pb(Mg_1/3Nb_2/3)O₃-0.1PbTiO₃ (PMN-PT) films are investigated by Scanning Force Microscopy (SFM) in a piezoelectric contact mode. The domain contrast is observed only in some grains (∼20 % of the entire surface), which showed clear ferroelectric behavior. Thus on the microscopic level the material behaves as a composite with ferroelectric regions embedded in the non-polar matrix. This was attributed to the relaxor-to-ferroelectric phase transition induced by the internal bias field. The local hysteresis loops are found to depend on the size of the grains. A distinct correlation between the values of the effective piezoelectric coefficients, d_eff, and the size of the respective grains is observed. Small grains exhibit slim piezoelectric hysteresis loops with low remanent d_eff, while relatively strong piezoelectric activity is characteristic of larger grains. In addition, large grains exhibit longer relaxation time after poling with an effective time constant increasing with the grain size. The nature of size effect is discussed taking in terms of dynamics of nanopolar clusters and SFM instrumentation.