Electric field dependences of Curie and Néel phase transition temperatures in magnetoelectric relaxor multiferroic Pb(Fe_0.5Ta_0.5)O₃ crystals seen via acoustic emission

Pb(Fe_0.5Ta_0.5)O₃ magnetic and ferroelectric relaxor multiferroic crystals have been studied under both temperature and dc external electric field conditions by means of acoustic emission technique. All the characteristic relaxor points as Burns T_d≈610K and intermediate T*≈500K temperatures, as well as both Curie ferroelectric cubic-tetragonal T_c1≈259K and tetragonal-monoclinic T_c2≈201K structural and Néel antiferromagnetic-paramagnetic T_N≈187K phase transitions have been successfully detected. It is shown that, when an external dc electric field is applied, both T_c1 and T_c2 exhibit a nonlinear or V-shape behavior and attain the sharp minima at threshold temperatures T_th1≈247K and T_th2≈200K with the threshold field E_th≈0.1kVcm^-1, as the field enhances, while their acoustic emission count rate monotonically increases. It is shown, too, that the T_N behaves linearly in dependence on dc electric field with a negative slope of about -20KcmkV^-1, while its acoustic emission count rate monotonically decreases. All these data are discussed from a viewpoint of presence of polar nanoregions and magnetoelectric coupling effect in Pb(Fe_0.5Ta_0.5)O₃ crystals.