The existence of an Almeida-Thouless (AT) instability surface below the upper critical six dimensions is demonstrated in the generic replica symmetric field theory. Renormalization flows from around the zero-field fixed point are investigated. By introducing the temperature and magnetic-field dependence of the bare parameters, the fate of the AT line can be followed from mean field $\left(d=\infty \right)$ down to $d=6-ϵ$.

• Received 22 September 2008

DOI:https://doi.org/10.1103/PhysRevB.78.220401