Magnetotransport properties of the diluted ferromagnetic semiconductor (Ga,Mn)As

We have studied experimentally the magnetotransport properties of a p-type diluted ferromagnetic semiconductor Ga_1–xMn_xAs by measuring the resistivity and Hall-effect as a function of temperature in various magnetic fields. The Ga_1–xMn_xAs thin films with thickness 1µm were grown on semi-insulating GaAs substrates by using a low temperature molecular beam epitaxy (MBE) technique and varying the Mn concentration between x=0.02 and x=0.05. The samples with x ≤ 0.03 showed insulating behaviour at low temperatures (resistivity ~1/T), whereas the samples with x ≥ 0.04 showed metallic behaviour (a finite resistivity at low temperatures). In the metallic samples the exchange interaction between the hole spins in the valence band and the spins of the Mn 3d electrons was manifested in the anomalous Hall-effect (AHE), from which the ferromagnetic transition temperature T_C = 30K was estimated. Further, a resistivity peak appeared at T_C in the metallic samples. The AHE can be explained by the wellknown theory for the Hall-effect in ferromagnetic conductors, where the AHE is proportional to the average spin-polarization of the magnetic lattice. The resistivity vs. temperature can be described accurately by a model, where a transport mechanism for disordered heavily doped semiconductors is combined with the spin disorder scattering processes due to the exchange interaction. This model explains both the resistivity peak at T_C in the metallic samples and the temperature dependence of the resistivity in the semiconducting samples.