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 ͑ϳ10-100 K͒ 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. Amorphous metal-semiconductor alloys ͑a-M-Si͒ offer unique insight into the metal insulator transition ͑MIT͒ as a comparison to doped crystalline materials. It has been widely documented that amorphous alloys undergo a MIT with identical low temperature behavior but at much greater dopant concentration compared to their crystalline counterparts due to significant additional disorder. The doping of local magnetic moments into semiconductors near the MIT causes dramatic effects in the magnetic and transport properties, including enormous negative magnetoresistance, fielddependent anomalous ͑nonspectral weight conserving͒ optical conductivity, and a magnetic susceptibility with a near-Curie law temperature dependence but a nonmonotonic dependence on composition, including a large peak at the MIT. 1-3 The enormous magnetic field dependence has allowed measurements of scaling behavior continuously through the 3D MIT on a single sample, including tunneling determination of the electron density of states. While there has been some success in describing the low temperature properties of amorphous doped semiconductors on both the metallic and insulating sides of the MIT, including the magnetically doped semiconductors, 6-8 a strikingly unresolved question is the nature of the higher temperature behavior where the effects upon the charge carriers due to the magnetic dopants "turns on." This temperature, which we call T * , is clearly seen in a sharp decrease of dc conductivity dc ͑T͒ for the magnetically doped a-Gd-Si alloys below that of a comparable nonmagnetic a-Y-Si and also reflects an upper limit of significant magnetoresistance. A magnetic temperature in our system analogous to the Curie temperature, T C , found in DMS materials would be the spin glass freezing temperature, T f . However T * represents a fundamentally different magnetic thermal energy scale, i.e., the onset of interactions between the moments and the charge carrying electrons ͑whereas T C or T f sets the temperature where the moments interact͒, and in fact T * will be shown here to follow a different trend with dopant concentration, indicating underlying physics unique from that of T f . T * has been noted in previous work; 1,9 however, the dependence on carrier and moment concentration, from which an understanding of the essential interactions can be developed, was not determined. In this work, we study the effects of separate tuning of charge carrier and magnetic dopant concentration, as well as the semiconducting matrix. We find a strong dependence of T * on metallicity ͑independent of magnetic moment concentration͒ and on the semiconductor band gap. These results are strongly suggestive that the crucial factor in determining