Date/Time
Date(s) - Apr 11 2013
11:00 AM - 12:30 PM
Location
Shasta Room, Bldg 40, Room 361
Category(ies)
Nanosecond dynamics of polarization domains and lattice structure in novel ferroelectric oxide thin films
Pice Chen
Materials Science Program, University of Wisconsin-Madison
Ferroelectric materials are characterized by a spontaneous electric polarization that can be distorted or reversed by applied electric fields. Novel ferroelectric materials including
multiferroics and ferroelectric/dielectric superlattices provide additional means to modify the electronic and structural properties. Insight into the coupling of the polarization to
other degrees of freedom can be inferred from the dynamics of ferroelectrics under electric-field and optical excitations.
We have studied the switching mechanism of polarization domains at a nanosecond timescale in ferroelectric/dielectric PbTiO3/SrTiO3 superlattices using time-resolved xray
microdiffraction. In a superlattice with weakly-coupled component layers, the competition between the energy associated with the depolarization field and the energy of
domain walls leads to the formation of striped domains. The dielectric layers are polarized with a weaker polarization than the ferroelectric layers. The striped domains
and the non-equal distribution of the polarization have important consequences in the response of the superlattice to applied electric fields. We found that the switching of the
striped domains occurs heterogeneously over the areas under applied electric fields, with a nanosecond timescale. Each component layer, however, responses different to applied
electric fields. The dielectric SrTiO3 layers are less stable and show larger distortion of domains than the ferroelectric PbTiO3 layers at the early-time regime of switching. A
larger piezoelectric expansion in the SrTiO3 layers is found at the late-time regime, commensurate with polarization change due to the elimination of striped domains.
We have also studied the carrier dynamics in multiferroic BiFeO3 thin films by probing its time-dependent structure under above-bandgap femtosecond optical excitation. A
photoinduced strain on the order of half of one percent develops within 100 ps after a 400-nm laser pulse. This lattice distortion is consistent with the piezoelectric effect in
response to the screening of the depolarization field in the presence of photoinduced carriers. The relaxation of the strain can be interpreted as a carrier recombination process,
which is on the order of one nanosecond depending on the film thickness.
