Ferromagnetism in Mn-implanted epitaxially grown Ge on Si(100)

We have studied ferromagnetism of Mn-implanted epitaxial Ge films on silicon. The Ge films were grown by ultrahigh vacuum chemical vapor deposition using a mixture of germane (GeH4) and methylgermane (CH3GeH3) gases with a carbon concentration of less than 1 at. %, and observed surface rms roughness of similar to 0.5 nm, as measured by atomic force microscopy. Manganese ions were implanted in epitaxial Ge films grown on Si (100) wafers to an effective concentration of similar to 16, 12, 6, and 2 at. %. Superconducting quantum interference device measurements showed that only the three highest Mn concentration samples are ferromagnetic, while the fourth sample, with [Mn] = 2 at. %, is paramagnetic. X-ray absorption spectroscopy and x-ray magnetic circular dichroism measurements indicate that localized Mn moments are ferromagnetically coupled below the Curie temperature. Isothermal annealing of Mn-implanted Ge films with [Mn] = 16 at. % at 300 degrees C for up to 1200 s decreases the magnetization but does not change the Curie temperature, suggesting that the amount of the magnetic phase slowly decreases with time at this anneal temperature. Furthermore, transmission electron microscopy and synchrotron grazing incidence x-ray diffraction experiments show that the Mn-implanted region is amorphous, and we believe that it is this phase that is responsible for the ferromagnetism. This is supported by our observation that high-temperature annealing leads to recrystallization and transformation of the material into a paramagnetic phase.