Aging and Barkhausen noise in the relaxor ferroelectrics
"Relaxor ferroelectrics are disordered crystalline materials whose polar order is limited to mesoscopic-scaled nanodomains. There is no ferroelectric phase transition, but a faster than Arrhenius cooperative freezing into a glassy relaxor regime instead. It is not understood how the random fields and random interactions present in these dielectrics inhibit the formation of long-range ferroelectric order. This thesis presents two types of experi- ments aimed at shedding light on this issue: aging and Barkhausen noise. Aging shows low-temperature regimes with spin-glass-like behavior in re- laxors with cubic perovskite crystalline structure that are not present in those with uniaxial tungsten-bronze crystalline structure. In particular, the cubic relaxor PMN/PT (90/10) [(PbMn1=3Nb2=3O3)1¡x(PbTiO3)x, x = 0:1] shows aging that directly parallels that in re-entrant spin-glasses, with ""hole- like"" aging at low-temperature, where the uniaxial relaxor SBN:La (60/40) [Srx¡yLayBa1¡xNb2O6, x = 0:6; y = 0:01] shows cumulative, not ""hole-like"" aging. The Barkhausen experiments measure the noise from the abrupt reori- entation of polar clusters driven by an ac field, giving a measure of a typical dynamic dipole step size in PMN/PT (90/10) of about 100 nanodomains in the paraelectric regime, which then abruptly freezes out on cooling into the relaxor regime. This suggests an abrupt growth of barriers associated with the dipoles. The presence of complicated spin-glass-like aging, requiring cooperativity between many aging units, combined with the relative insensitivity of aging effects to small field perturbations suggest that units much smaller than nanodomains are responsible for the aging. This points to a picture of the cubic relaxors where polar regions are coupled to canted moments orthogonal to the mean mid-scale polarization (Egami, 1999; Dkhil et al., 2001) which provide the glassy freezing, much like a re-entrant xy spin-glass. These canted moments are the unit cell analogs to tweed domain-patterning seen in PMN/PT compositions with large ferroelectric doping (Viehland et al., 1995; Xunhu et al., 1994). We note the relevance of a theory mapping the pre-martensitic tweed Hamiltonian onto a spin-glass Hamiltonian (Kartha et al., 1991; Sethna et al., 1992). We also present evidence of possible discrete polarization/depolarization steps in the pyroelectric current of SBN:La (60/40)."
Physics, Condensed Matter, Relaxor Ferroelectrics, Barkhausen noise, Aging (physics)
University of Illinois - Urbana-Champaign
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