High-temperature ferromagnetism in Si$_{1-x}$ Mn$_x$ $(x\approx 0.5)$ nonstoichiometric alloys

It has been found that the Curie temperature ($T_{\rm C}\sim 300\,$K) in nonstoichiometric Si$_{1-x}$Mn$_x$ alloys slightly enriched in Mn ($x\approx0.52$–$0.55$) in comparison to the stoichiometric manganese monosilicide MnSi becomes about an order of magnitude higher than that in MnSi ($T_{\rm C}\approx30\,$K). Deviations from stoichiometry lead to a drastic decrease in the density of charge carries (holes), whereas their mobility at about $100$ K becomes an order of magnitude higher than the value characteristic of MnSi. The high-temperature ferromagnetism is ascribed to the formation of defects with the localized magnetic moments and by their indirect exchange interaction mediated by the paramagnetic fluctuations of the hole spin density. The existence of defects with the localized magnetic moments in Si$_{1-x}$Mn$_x$ alloys with $x\approx0.52$–$0.55$ is supported by the results of numerical calculations performed within the framework of the local-density-functional approximation. The increase in the hole mobility in the nonstoichiometric material is attributed to the decay of the Kondo (or spin-polaron) resonances presumably existing in MnSi.