Detecting depinning of polar nanoregions under dc external electric field in canonical relaxor ferroelectric Pb(Mg_1/3Ta_2/3)O₃ via acoustic emission

Canonical relaxor ferroelectric Pb(Mg_1/3Ta_2/3)O₃ single crystals were studied via acoustic emission in temperature range of 180–250 K covering their dielectric constant temperature maximum at T_m ≈ 202 K, and under dc external electric fields up to 1.2 kV cm⁻¹. The dependence of T_m on dc external electric field exhibits a nonlinear behavior, characteristic of relaxors: it initially decreases, attains a sharp minimum at the threshold field of 0.25 kV cm⁻¹ and then starts to increase, as the field enhances. The count rate of the accompanying acoustic emission exhibits two maxima: at 0.1 and 0.6 kV cm⁻¹. The latter obviously corresponds to the end critical point, also characteristic of relaxors, while the former is attributed to a polar nanoregions depinning field at which the polar nanoregions start to be switched off from the intrinsic random electric fields to the dc external electric field. These data are discussed from the viewpoint of competition between random electric fields and an external electric field when affecting the polar nanoregions.