Physical Review B vol:56 issue:20 pages:13103-13112
Using a low-temperature molecular-beam epitaxy growth procedure, Ga1-xMnxAs-a III-V diluted magnetic semiconductor-is obtained with Mn concentrations up to x similar to 9%. At a critical temperature T-c (T-c approximate to 50 K for x=0.03-0.05), a paramagnetic to ferromagnetic phase transition occurs as the result of the interaction between Mn-h complexes. Hole transport in these compounds is strongly affected by the antiferromagnetic exchange interaction between holes and Mn 3d spins. A model for the transport behavior both above and below Tc( )is given. Above T-c, all materials exhibit transport behavior which is characteristic for systems near the metal-insulator transition. Below T-c, due to the rising spontaneous magnetization, spin-disorder scattering decreases and the relative position of the Fermi level towards the mobility edge changes. When the magnetization has reached its saturation value (below similar to 10 K) variable-range hopping is the main conduction mechanism. The negative magnetoresistance is the result of the expansion of the hole wave functions in an applied magnetic field.