Origins of ferromagnetism in transition-metal doped Si

We present results of the magnetic, structural, and chemical characterizations of Mn + -implanted Si displaying n -type semiconducting behavior and ferromagnetic ordering with Curie temperature, T C , well above room temperature. The temperature-dependent magnetization measured by superconducting quantum interference device from 5 to 800 K was characterized by three different critical temperatures ( T C * ∼ 45 K , T C 1 ∼ 630 - 650 K , and T C 2 ∼ 805 - 825 K ). Their origins were investigated using dynamic secondary ion mass spectroscopy and transmission electron microscopy (TEM) techniques, including electron energy loss spectroscopy, Z -contrast scanning TEM imaging, and electron diffraction. We provided direct evidences of the presence of a small amount of Fe and Cr impurities which were unintentionally doped into the samples together with the Mn + ions as well as the formation of Mn-rich precipitates embedded in a Mn-poor matrix. The observed T C * is attributed to the Mn 4 Si 7 precipitates identified by electron diffraction. Possible origins of T C 1 and T C 2 are also discussed. Our findings raise questions regarding the origin of the high- T C ferromagnetism reported in many material systems without a careful chemical analysis.