Reentrant phenomena in relaxors

This chapter provides a comprehensive review on the reentrant relaxors, that is the materials in which both types of reentrant and relaxor behaviors are observed. The reentrant phenomena were studied elaborately in dilute magnets, where the sequence of phases from paramagnetic to ferro-, or antiferro-, magnetic and further to reentrant spin glass was observed within a particular concentration range of magnetic atoms. Relaxor ferroelectrics are often considered as an electric analog of magnetic spin glasses because they exhibit similar features, such as a quenched disorder in the structure, frustrated interactions, ultra-slow relaxation dynamics, phase transition to a glassy phase under conditions far from equilibrium, aging, rejuvenation and memory effects. The analogy between the reentrant spin glasses and relaxor ferroelectrics is not very surprising, as both are characterized by partially frustrated interactions and both exhibit a mesoscopic or even macroscopic collinear order that exists alongside a glassy order.