We examine the Mn concentration dependence of the electronic and magnetic properties of optimally annealed ${\mathrm{Ga}}_{1-x}{\mathrm{Mn}}_x\mathrm{As}$ epilayers for $1.35\%<~x<~8.3\%.$ The Curie temperature ${(T}_c),$ conductivity, and exchange energy increase with Mn concentration up to $x\sim 0.05,$ but are almost constant for larger x, with $T_c\sim 110\mathrm{K}.$ The ferromagnetic moment per Mn ion decreases monotonically with increasing x, implying that an increasing fraction of the Mn spins do not participate in the ferromagnetism. By contrast, the derived domain wall thickness, an important parameter for device design, remains surprisingly constant.

• Received 12 April 2002

DOI:https://doi.org/10.1103/PhysRevB.66.012408