Magnetic and structural properties of epitaxial c-FeSi films grown on MgO(100)

Conversion electron Mossbauer spectroscopy (CEMS) and superconducting quantum interference device (SQUID) magnetometry have been applied to study the metastable iron monosilicide phase (c-FeSi) with the B2 (or CsCl) lattice structure synthesized by molecular beam epitaxy (MBE). Thin films of nominal composition of c-FeSi0.85 were grown by codeposition of Fe-57 and Si onto MgO(100) carrying a thin Fe or Cr buffer layer. X-ray diffraction was performed to determine the structure and epitaxial relationship of the c-FeSi0.85(100) films. The B2 structure was observed after different thermal annealing steps. The lattice parameter perpendicular to the film plane was found to be 2.77(5) A in each case. The CEM spectra at room temperature could be decomposed into two components: (i) a weakly quadrupole-split doublet assigned to nonmagnetic stoichiometric c-FeSi, and (ii) a weakly ferromagnetic component characterized by a distribution of hyperfine magnetic fields, P(B-hf), assigned to a fraction of nonstoichiometric c-FeSix with excess Fe. CEMS and SQUID magnetometry demonstrate the occurrence of magnetic ordering effects with decreasing temperature down to 4.2 K. Our results reveal that, contrary to expectation, the stoichiometric c-FeSi phase is paramagnetic at room temperature and ferromagnetically ordered below similar to 30 K, while c-FeSix is ferromagnetic at and below 300 K. At 5 K we find small ground-state Fe atomic magnetic moments mu(Fe) of (0.10 +/- 0.02) mu(B) for c-FeSi and (0.13 +/- 0.03) mu(B) for c-FeSix. These small moments are reflected in the observed small hyperfine magnetic fields of 2 similar to 4 T in the ground state.