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

Superdipolar glass properties of polar nanoregions (PNRs) in relaxor ferroelectric PbMg1/3Nb2/3O3 (PMN) are compared to those of ferromagnetic nanoparticles (FNPs) in the superspin glass [Co80Fe20(0.9 nm)/Al2O3(3 nm)](10). Both the dynamic critical properties at T > T-g and nonergodicity phenomena at T < T-g are comparable, but this does not apply to their polydispersivity at T <= 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-Bene 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.