  The stability of the disordered glassy phase in the relaxors PbMg1/3Nb2/3O3
and (PbMg1/3Nb2/3O3)0.88(PbTiO3)0.12, called PMN and PMN-PT, was investigated
by preparing partially polarized samples and allowing them to age at zero field
in the temperature range for which the phase is history-dependent. The PMN-PT
polarization would spontaneously increase until long-range order formed, first
appearing as giant polarization noise. Thus the thermodynamically stable phase
in PMN-PT appears to be ferroelectric. In contrast, a PMN sample lacking the
sharp first-order field-driven transition found in some other samples
spontaneously depolarized, consistent with its glassy state being
thermodynamically stable. Detailed thermal depolarization results in PMN show
two distinct broad peaks as well as a small fraction of material with a
distribution of abrupt melting transitions.
