  We present an investigation of Mn {\delta}-doped layers in Si(001) grown by
molecular beam epitaxy. We discovered that a Pb surfactant has significant
effect on the structural and magnetic properties of the submonolayer of Mn,
which depends on the Si capping layer growth temperature, T_Si, and the Mn
coverage, {\theta}_Mn. The results presented in this paper identify three
regions in the growth-phase-diagram characterized by distinct magnetic
behaviors and crystal structures. In one region, X-ray absorption fine
structure (XAFS) and transmission electron microscopy (TEM) experiments
indicate that MnSi nanocrystallites form with B2-like crystal type structures.
At the optimal growth conditions, T_Si = 200 C and {\theta}_Mn = 0.26
monolayer, a ferromagnetic phase develops with a Curie temperature T_C > 400 K
and a saturation moment m_sat = 1.56 {\mu}_B/Mn, whereas T_C drops to zero for
a control sample prepared without Pb. For T_Si > 200 C, MnSi-B20 type
precipitates form with a T_C ~ 170 K. Rutherford backscattering spectroscopy
shows that the increase in the remanent magnetization in these two phases is
possibly correlated with an increase in the Mn substitutional fraction, which
suggests that a Si_{1-x}Mn_x dilute magnetic semiconductor may be forming in
the matrix between the precipitates. Density functional calculations show that
Pb changes the pathway by which the Mn atoms access the Si substitutional
sites, Mn_Si. While the Pb increases the formation energy of Mn_Si at the Si
surface, it enables substitutional incorporation by lowering the formation
energy of Si vacancies by 0.92 eV.
