Origins of ferromagnetism in transition-metal doped Si

Abstract

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, TC, 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 (TC* ∼45 K, TC1 ∼630-650 K, and TC2 ∼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 TC* is attributed to the Mn4 Si7 precipitates identified by electron diffraction. Possible origins of TC1 and TC2 are also discussed. Our findings raise questions regarding the origin of the high- TC ferromagnetism reported in many material systems without a careful chemical analysis. © 2008 American Institute of Physics.