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*similar to 45 K, T-C1 similar to 630-650 K, and T-C2 similar to 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 Mn4Si7 precipitates identified by electron diffraction. Possible origins of T-C1 and T-C2 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. (c) 2008 American Institute of Physics.