Irreversibility in Ising and Heisenberg spin glasses (invited)

Abstract

Spin glasses are rather unique systems which show simultaneously apparent phase transitions as well as metastable or glassy behavior. The existence of irreversible behavior appears to be intimately connected with the phase transition. Here, we will review experiments which show irreversible phenomena and will deduce from the data a simple heuristic picture of the free energy surface F[mi], where mi is the thermally averaged spin at site i. The picture that follows from this analysis is then made more rigorous within a calculational scheme, in which, for large-size systems, we numerically compute the evolution of minima of mean field models for F[mi] as the magnetic field H and temperature T are changed. For Ising spins, magnetic hysteresis, field-cooled, zero-field-cooled, and remanent magnetizations are computed and found to be in good qualitative agreement with experiment. For Heisenberg spins, we find no irreversibility unless anisotropy is present. We discuss the re-entrant ferromagnet–spin glass transition as well as the effects of various kinds of anisotropy, on vector spin glasses. The overall good qualitative agreement between theory and experiment lends support to our hypothesis that, on intermediate time scales, the behavior of spin glasses reflects the properties of the free energy surface: that irreversibility occurs when minima of F are destroyed with changing H or T.