Spin glass transition. A dynamical instability

The instability of a spin glass phase against an external magnetic field is discussed on the basis of a Landau‐type theory of phase transition with two order parameters. There are two different critical magnetic fields, both approaching zero for T → T SG ; the principal meaning of these critical fields is considered. The free energy F ( H, T ) develops different branches as a function of H and T with bifurcations typical for non‐equilibrium phase transitions (or dynamical instabilities).