Insulating ferromagnetic (Ga,Mn)As with low Mn-doping

We probe the onset of ferromagnetism in (Ga,Mn)As near the vicinity of the metal-insulator transition by studying systematic series of low Mn-doped ($<$ 2.0 atomic {\%}) GaAs epilayers [growth described in R.C. Myers \textit{et al.} Phys. Rev. B \textbf{74}, 155203 (2006)]. We have studied the temperature dependent magnetization and electronic transport properties of these insulating samples. We fit the data using the variable-range-hopping conduction model and calculate the hopping energy from the logarithm of the resistivity versus T$^{-1/4}$ slope fit. Our results indicate that a cutoff in ferromagnetism is strongly correlated with critical values of the electric conductivity and the hopping energy. This work is supported by grants from NSF, ONR and DARPA.