Characteristic temperature in magnetically doped amorphous semiconductors

The introduction of magnetic moments such as Gd into amorphous Si produces dramatic effects in electrical transport below a characteristic temperature T*. Below T*, the conductivity of the magnetically doped systems is strongly suppressed compared to equivalent nonmagnetic Y doped samples, and displays enormous negative magnetoresistance. T* occurs at relatively high temperatures (similar to 10-100 K) and decreases sharply with increasing Gd concentration, passing smoothly through the metal-insulator transition. In ternary samples with both Gd and nonmagnetic Y, T* decreases strongly with increasing metallization, whether due to the addition of Gd alone or a mixture of Gd and Y. These results cannot be explained by simple magnetic interaction models, suggest the crucial role of electron screening and are reminiscent of mass enhancement behavior.