Room-temperature ferromagnetism in Mn-implanted amorphous Ge

A 2 x 10(16) Mn+/cm(2) 100 keV ion implantation at liquid-nitrogen temperature onto Ge(100) surfaces produces a perfect Mn dilution into a completely amorphized Ge layer (155 nm thickness and 4% average Mn concentration) as directly demonstrated by Mn K-edge x-ray absorption spectroscopy. Superconducting quantum interference device (SQUID) investigations demonstrate that this dilutedmagnetic semiconductor system exhibits ferromagnetism up to room temperature. The magnetic response is explained within the model of percolation of bound polarons. Once the Mn fluence is doubled during the implantation, SQUID measurements clearly point to formation of Mn-Mn dimers and phase separation of Mn into Mn-rich amorphous clusters. First-principles calculations on Mn-doped amorphous Ge give a rationale to the experiments at the lowest fluence showing that disorder in the amorphous phase with the distortion of the Ge tetrahedra plays a crucial role, favoring the Mn substitutional inclusion and, correspondingly, enhancing the magnetic response of the system.