Mn-implanted GeC: An Amorphous Ferromagnetic Material*

High energy (20 keV) Mn ions were implanted in two samples: 1) bulk Ge (100) and 2) a 250 nm thick epitaxial GeC film, grown on a Si (100) wafer. The GeC thin film was grown by UHV chemical vapor deposition using a mixture of germane (GeH$_4$) and methylgermane (CH$_3$GeH$_3$) gases and contains about 1\% carbon. The Mn implant dose was $1.1\times10^{16}$/cm$^2$ at a temperature of 300$^{\circ}$C for both samples. A SQUID magnetometer study shows ferromagnetism in both samples. The Curie temperature of the first samples is about 150 K, while that of the second sample is about 170 K. The in-plane saturated magnetic moment for the first sample is about $2.2\times10^{-5} $emu/cm$^2$ and that for the second sample is about $3.0\times10^{-5} $emu/cm$^2$. These results show clear enhancement of magnetic properties of the Mn-implanted GeC thin film over the identically implanted Ge layer due to the presence of a small amount of carbon. Further, high-resolution transmission electron microscopy showed that Mn implanted region is amorphous, without any precipitates. It is believed that it is this amorphous phase that is responsible for ferromagnetism.