Ferroic superglasses: Polar nanoregions in relaxor ferroelectric PMN versus CoFe superspins in a discontinuous multilayer

Abstract

Superdipolar glass properties of polar nanoregions (PNRs) in relaxor ferroelectric $\mathrm{PbM}{\mathrm{g}}_{1/3}\mathrm{N}{\mathrm{b}}_{2/3}{\mathrm{O}}_{3}$ (PMN) are compared to those of ferromagnetic nanoparticles (FNPs) in the superspin glass ${[\mathrm{C}{\mathrm{o}}_{80}\mathrm{F}{\mathrm{e}}_{20}(0.9\phantom{\rule{0.16em}{0ex}}\mathrm{nm})/\mathrm{A}{\mathrm{l}}_{2}{\mathrm{O}}_{3}(3\phantom{\rule{0.16em}{0ex}}\mathrm{nm})]}_{10}$. Both the dynamic critical properties at $Tg{T}_{g}$ and nonergodicity phenomena at $Tl{T}_{g}$ are comparable, but this does not apply to their polydispersivity at $T\ensuremath{\le}{T}_{g}$. Due to the quenched random size and position distribution of the FNP, the superspin glass exhibits standard Cole-Cole broadening of the spectrum of relaxation frequencies at all temperatures. Contrastingly, the relaxation spectrum of the PNRs in PMN is subject to a crossover from Lacroix-B\'en\'e to Cole-Davidson statistics on cooling toward ${T}_{g}$. Surprisingly, it becomes replaced by relaxation and creeplike domain-wall dynamics below ${T}_{g}$ as a consequence of volume percolation of the PNR at ${T}_{g}$ and formation of a ferroelectric microdomain state under the simultaneous control by quenched random electric fields and the softening ferroelectric ${F}_{1u}$ lattice mode.