Silicon (Si)-based spintronics material is attractive for creating a new class of device. Manganese (Mn)-doped Si is one of the major candidates for such spintronics materials. Our experiments showed that Mn-silicide nanoparticles, whose structure is presumably MnSi1.7, are formed in Mn-implanted Si after thermal annealing and that they exhibit ferromagnetism [1]. When the annealing temperature is lower, smaller Mn-nanosilicide particles are formed and the saturation magnetization is increased. These results suggest that the magnetic properties depend on the size of the Mn-nanosilicide particles formed in Si. However, no clear explanation of how ferromagnetism appears in MnSi1.7 nanosilicide has been provided. We thus investigated the magnetic properties of Mn-silicide by using the first-principles calculation.
The physical properties of bulk MnSi1.7 have not been clarified yet. As a first step, we examined bulk Mn4Si7, which is known as the prototype of MnSi1.7 [2]. The results indicate that bulk Mn4Si7 shows semiconducting properties and is nonmagnetic, which support one previous experimental report [3]. Then, we examined the strain and charging effects on Mn-silicide, because it is plausible that Mn-nanosilicide is affected by such effects when present in Si. However, we found that both effects yield nonmagnetism. These findings suggest that nanosilicide magnetism does not originate from bulk but rather from the interface structure or defects within Mn-nanosilicide.
Therefore, we next studied the interface effects on magnetism. Among various interface arrangements between Si(001) and Mn4Si7(001), we found that magnetism appears on particular metallic interfaces, while semiconducting interfaces and other metallic interfaces show nonmagnetism. These findings suggest that Mn-nanosilicide magnetism originates from the interface between the host Si and Mn-nanosilicide. The observed trend, that smaller Mn-nanosilicide particles have larger saturation magnetization, suggests that interface irregularities yield metallic interfaces that trigger magnetism.
[1] S. Yabuuchi et al., unpublished.
[2] S. Okada et al., J. Alloys Comp. 317-318 (2001) 315.
[3] I. Kawasumi et al., J. Mater. Sci. 16 (1981) 355. |