Acoustic Detection of Phase Transitions at the Nanoscale

Materials near structural phase transitions fi nd applications in a wide range of devices. Typically, phase transitions are determined macroscopically through measurements of relevant order parameters and related property coeffi cients. Here, a method for understanding electric fi eld induced phase transitions in ferroelectrically active materials at the nanometer scale via acoustic detection with band-excitation piezoresponse force microscopy (BE-PFM) is introduced. Specifi cally, the fi eld-induced rhombohedral (R) to tetragonal (T) phase transition in single crystal 0.72PbMg_1/3Nb_2/3O₃-0.28PbTiO₃ (PMN-PT) is mapped. It is shown that due to sample heterogeneity, some regions are more prone to the R–T transition, and display signatures in the acquired piezoresponse loops, as well as pronounced softening in the elastic modulus (monitored via the resonant frequency and calibrated with models of cantilever dynamics) that occurs just prior to phase switching. Landau Devonshire thermodynamic theory confi rms the stability of the tetragonal phase under applied fi elds in PMN-PT, while phase-fi eld modeling suggests that the transition evolves smoothly in the probed volume of the tip, both in agreement with the BE-PFM results. These results confi rm the validity and utility of utilizing acoustic changes at phase transitions to detect their onset in nanoscale probed volumes, allowing spatial mapping of their onset with unprecedented resolution.