Griffiths phase and temporal effects in phase separated manganites

• Non equilibrium phenomena near the magnetic transition temperature are discussed.
• Temporal effects can be considered as a key feature of the Griffiths phase existence.
• Time relaxation effects are able to unveil a dynamic nature of phase separated state.

Phenomenological description of relaxation phenomena in magnetic and transport properties of perovskite manganites has been presented. The approach is based on generalization of some hypotheses appropriate to the Preisach picture of magnetization process for half-metallic ferromagnets and on an assumption that in doped manganites the phase separated state exists near the magnetic ordering temperature. For systems with the percolation type of a ferromagnet–paramagnet transition, distinctive features in relaxation of magnetization and resistivity have been found. The relaxation is shown to be most pronounced near the transition temperature, and to be an approximately logarithmic function of time. The theoretical results replicate a broad spectrum of behavior observed experimentally on time dependence of magnetization and resistivity of CMR systems and allow a direct comparison with available experimental data. We propose an additional experimental test to distinguish between the percolation scenario of magnetic and transport transitions in doped manganites, and the ferromagnetic polaron picture. In particular, an anomalously slow relaxation to zero of the order parameter can be considered as a key feature of the Griffiths-like phase transition in doped manganites. It is also shown that a system with the Griffiths-like state will exhibit nonequilibrium aging and rejuvenation phenomena, which in many aspects resemble that of a spin glass. We hope that experimental observation of a set of time decay properties will provide a settlement of apparently conflicting results obtained for different characteristics of phase-separated manganites.

Temperature dependence of the FC and ZFC magnetization response upon warming in magnetic field at different sweeping rates: (a) a system with the conventional (second order) ferromagnet–paramagnet phase transition and (b) a system with the Griffiths-like phase above the magnetic ordering temperature.Figure optionsDownload as PowerPoint slide