Quenched magnetic moment in Mn-doped amorphous Si (textit{a}-Mn_{x}Si_{1-x}) across the metal-insulator transition

The magnetic and electrical transport properties of Mn-doped amorphous silicon (\textit{a-}Mn$_{x}$Si$_{1-x}$) thin films have been measured. The magnetic susceptibility obeys the Curie-Weiss law for a wide range of $x$ (0.005-0.175) and the saturation moment is small. While all Mn atoms contribute to the electrical transport, only a small fraction (interstitial Mn$^{2+}$ states with $J$=$S$=5/2) contribute to the magnetization. The majority of the Mn atoms do not possess any magnetic moment, contrary to what is predicted by the Ludwig-Woodbury model for Mn in crystalline silicon. Unlike \textit{a-}Gd$_{x}$Si$_{1-x}$ films which have an enormous \textit{negative} magnetoresistance, \textit{a-}Mn$_{x}$Si$_{1-x}$ films have only a small \textit{positive} magnetoresistance, which can be understood by this quenching of the Mn moment.