Study of domain structure and evolution in PMN-30%PT single crystals by means of piezoresponse force microscopy

Abstract

Relaxor ferroelectric material, PMN-xPT, is a solid solution of Pb(Mg1/3Nb2/3)O3 and PbTiOs, in which the substitution of Ti4+ cations in B sites tends to reduce the random field induced by Mg2+ and Nb5+ cations and form long-range ordered ferroelectric domains. In order to understand the origin of extremely large piezoelectric coefficient and electromechanical coupling factor, the evolutions of ferroelectric domains under different temperatures and electric fields etc. have attracted a great deal of attention, and recently there has been an increasing interest in studying the ferroelectric domains by means of piezoresponse force microscopy (PFM). In this project, ferroelectric domain evolution in (Pb(Mg1/3Nb2/3)O3)0.7(PbTiO3)0.3 (PMN-30%PT) single crystal, grown by using the modified Bridgman technique with (001)- and (111)-cut crystal orientations, has been studied by means of PFM. As an important method to study ferroelectric domains, PFM is able to observe the domains with high spatial resolution. For three-dimensional polarization analyses in ferroelectric materials, in-plane polarization (IPP) and out-of-plane polarization (OPP) PFM imaging techniques can be implemented simultaneously. In addition, Nanometer-sized domain imaging and local poling by polarization switching can be achieved via tip scanning with an electric field applied between the bottom electrode of sample and the tip. Ferroelectric domain structure, especially the domain boundary polarization, in as-grown and poled (111)-cut PMN-30%PT single crystals has been studied by means of IPP- and OPP-PFM. It reveals that the as-grown crystal exhibits speckle-shaped microdomains, and the domain number distribution decreases exponentially as the domain size increases; by contrast, the poled crystal shows lamellar ferroelectric domains. Capacitive-force-free PFM reveals in-plane polarization at domain boundary. A time-dependent development of the lamellar ferroelectric domains from a single domain structure in the just-poled (111)-crystal has also been investigated by PFM imaging. It shows that the formation of lamellar macrodomains is via accumulation of well-aligned speckle-shaped nanodomains grown from polar nanosized regions (PNRs). The domain evolutions from macrodomain to microdomain and from ferroelectric to paraelectric phase at different temperatures have been revealed in temperature-dependent PFM, and the results are consistent with temperature-dependent relative permittivity measurement. Random field induced PNRs and their effect on domain evolution during thermal cycles with heating temperature higher than the Curie temperature have been studied, and it turns out that in the as-grown (001)-cut PMN-30%PT single crystal, the domain pattern is repeatable after thermal cycle; but it is not for the (111)-crystal. The different domain configuration and evolution between (001)- and (111)-crystals may be attributed to the existence of preferred orientation of random field. In summary, the ferroelectric domains in PMN-30%PT single crystal have been studied by means of PFM, and it reveals its ferroelectric domain configuration and evolution during aging and temperature change.