Local structure and magnetic properties of ultrathin Mn films grown on Si(001)

We report on the structural and magnetic properties of ultrathin Mn layers deposited onto Si(001) by molecular beam epitaxy (MBE) at low temperature. X-ray absorption fine structure (XAFS) 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 $^{\circ}$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 $^{\circ}$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. Using SQUID magnetometry, we show that the sample with B2-like structure has the largest Mn magnetic moment of 0.33$\mu _{B}$/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.