A Group-IV Ferromagnetic Semiconductor: Mn _x Ge _{1− x}

We report on the epitaxial growth of a group-IV ferromagnetic semiconductor, Mn _x Ge _{1− x} , in which the Curie temperature is found to increase linearly with manganese (Mn) concentration from 25 to 116 kelvin. The p -type semiconducting character and hole-mediated exchange permit control of ferromagnetic order through application of a ±0.5-volt gate voltage, a value compatible with present microelectronic technology. Total-energy calculations within density-functional theory show that the magnetically ordered phase arises from a long-range ferromagnetic interaction that dominates a short-range antiferromagnetic interaction. Calculated spin interactions and percolation theory predict transition temperatures larger than measured, consistent with the observed suppression of magnetically active Mn atoms and hole concentration.