STRUCTURE AND MAGNETISM OF Mn-DOPED Si THIN FILMS GROWN BY MOLECULAR BEAM EPITAXY
The structure and magnetism of ultrathin Mn layers grown by molecular beam epitaxy (MBE) on Si(001) are investigated. X-ray absorption fine structure (XAFS) shows that a metastable MnSi phase with a B2-like (CsCl) crystal structure forms and superconducting quantum interference device (SQUID) magnetometry measurements reveal that the B2 structure is ferromagnetic with a Mn moment of 0.33 µB and a Curie temperature TC > 400 K. A change in the Si capping layer growth temperature, TSi, produces a MnSi phase with a B20-like structure, which exhibits a small moment and a TC below 20 K. Surfactant mediated growth of Mn ?-doped Si films and co-deposited Si1-xMnx alloys shows that a Pb surfactant strongly alters the structure and magnetism of these systems. For the ?-doped films, analysis of the crystal structure and magnetic properties over a range of growth parameters identified three distinct Mn-Si phases. With Pb, a sample with a coverage ?Mn = 0.26 monolayer and TSi = 200 ºC develops a ferromagnetic phase with a Mn moment of 1.56 µB and a TC > 400 K, whereas TC drops to zero for a sample grown without Pb. For TSi > 200 ºC, nano-disks with MnSi-B20 type structure form with a TC ? 170 K. A possible correlation exists between the remanent moment and the fraction of Mn occupying Si substitutional sites, which suggests that a dilute Si1-xMnx may be forming in the matrix. Density functional theory (DFT) shows that Pb lowers the formation energy of Si vacancies by 0.92 eV, which enhances the substitutional incorporation of Mn. In the absence of Pb, the co-deposited Si1-xMnx films undergo a 2D spinodal decomposition and form nanocolumns. The nanocolumns are amorphous and paramagnetic. In contrast, in the samples grown with Pb, MnSi1.7 nanorods form in the plane of the films and exhibit two ferromagnetic transitions at TC1 ? 40 K and TC2 > 400 K when x = 0.5%. While TC1 is consistent with TC of bulk MnSi1.7 crystals, TC2 is believed to originate from Mn diluted in the Si matrix.