Mn-silicide nanoparticles: the origin of ferromagnetism in Mn-implanted Si?

Recently Si and Ge semiconductors doped with Mn are reported to be ferromagnetic [1, 2]. The ferromagnetism is attributed to the coupling between Mn ions and carriers. However some MnxSiy and MnxGey compounds are ferromagnetic. A careful structural analysis is crucial to clarify the ferromagnetism in Mn doped Si or Ge. In this work, Mn ions were introduced to p-Si by ion implantation. Structural and ferromagnetic properties of Mn-implanted Si were investigated by Rutherford backscattering, x-ray diffraction (XRD), transmission electron microscopy (TEM) and superconducting quantum interference device magnetometer (SQUID). Synchrotron irradiation XRD and TEM revealed the formation of Mn4Si7 nanoparticles already in the as implanted samples (Fig. 1). Depending on the Mn-fluence they grew up from 5 nm to 20 nm upon rapid thermal annealing. No significant evidence is found for Mn substituting Si sites either in the as-implanted or annealed samples. By SQUID, hysteretic loops were observed, while the coecivity and remanence are drastically decreased with increasing temperature. The magnetization upon zero-field cooled and field cooled procedure shows a characteristic of a magnetic nanoparticle system. Together with structural analysis, we attributed the observed ferromagnetism to Mn-silicides nanoparticles.

Reference
(1) Y. D. Park, et al., Science 295, 651 (2002).
(2) M. Bolduc, et al., Phys. Rev. B 71, 033302 (2005).