Local structure and magnetic properties of B2- and B20-like ultrathin Mn films grown on Si(001)

The structural and magnetic properties of ultrathin Mn layers deposited onto Si(001) by molecular beam epitaxy at low temperature are reported. X-ray absorption fine structure studies reveal that the structure of the silicide layer that forms depends on the growth temperature of the capping layer. A capping layer grown at 200 \ifmmode^\circ\else\textdegree\fi{}C on 0.35-monolayer (ML) Mn results in a metastable MnSi phase with a B2-like (CsCl) structure, whereas a cap grown at room temperature on 0.5 ML followed by annealing at 200 \ifmmode^\circ\else\textdegree\fi{}C produces a lower coordinated MnSi phase with a B20-like structure. Increasing the Mn thickness from 0.5 to 4 monolayers does not trigger a structural transformation but drives the structure closer to MnSi-B20. The sample with B2-like structure has the largest Mn magnetic moment of 0.33 ${\ensuremath{\mu}}_{B}$/Mn at $T$ $=$ 2 K, and a Curie temperature ${T}_{C}$ above 250 K. MnSi-B20 layers showed lower moments and much lower ${T}_{C}$'s, in line with those reported for MnSi-B20 thin films.