High-field magnetotransport studies of ferromagnetic GaAs/Mn digital alloys

Magnetotransport properties of ferromagnetic GaAs/Mn digital alloys have been investigated in fields up to 33 T. A series of four GaAs/Mn digital alloys with different Mn coverages (0.15 ML - 0.5 ML) at fixed GaAs spacer thickness (9 ML), with Curie temperatures, T_C, between 20 and 40 K shows hopping conduction behavior in the zero-field sheet resistance below T_C. Analysis of the high field magnetotransport measurements on these samples reveals hole densities between 0.45 × 10¹³ and 1.8 × 10¹³ cm^-2/Mn layer at 5 K. In contrast, a GaAs/Mn digital alloy with slightly different parameters (0.5 ML Mn and 14 ML GaAs spacer layers) and growth conditions shows essentially metallic behavior and much higher T_C (60 K). The zero-field sheet resistance, although decreasing weakly with T at low temperatures, cannot be fit by a hopping expression. From analysis of Shubnikov-de Haas oscillations observed in this sample, an effective mass of ≈ 0.31m₀ was determined, close to the heavy hole mass of GaAs. The hole density extracted from fits to R _{H all} at high fields is comparable to that of the insulating GaAs/Mn digital alloys at the same Mn coverage. This suggests that the increased metallicity is the most important factor in significantly enhancing T _C.