Magnetic and Structural Characterization of Ultrathin Iron Films on Metallic, Semiconducting, and Insulating Surfaces

Correlation studies between thin film nanostructure and macroscopic magnetic properties in ultrathin Fe films grown on Cu(100) and CaF_2(111) substrates were performed in situ using an ultrahigh vacuum scanning transmission electron microscope and the surface magneto-optic Kerr effect. Thin Fe films were also deposited on Ag/Si(111) and Si(111) substrates and characterized with standard broad-beam surface science analytical tools such as Auger electron spectroscopy and reflection high -energy electron diffraction. Iron was grown on Cu(100) substrates held at room temperature. The onset of ferromagnetism occurred at 2 monolayers (ML) with a predominantly out-of-plane anisotropy. The out-of-plane anisotropy gradually changed to an in-plane anisotropy as Fe coverages were increased from 2 to 5 ML. Iron films with coverages between 5 and 10 ML were no longer magnetic. The Fe film remained fcc and matched to the Cu(100) specimen through at least 10 ML, as observed with reflection high-energy electron diffraction. A field-induced metastable magnetic state was observed for room-temperature grown 3.5 ML Fe films. Ultrathin films (1-6 ML) of Ag were not capable of inhibiting silicide formation after Fe was deposited. Low -temperature (100^circC) anneals greatly modified the surface atom concentration. Auger electron spectroscopy revealed a decreased Fe atom surface concentration following each anneal. The resulting morphology of ultrathin Fe films grown on CaF_2 was shown to be independent of the deposition temperature for rm20<T<300 C^circ. In addition, within the statistical uncertainties of the experiment, 7.5 ML Fe films grown on room-temperature 10, 25, and ~800 nm-thick CaF_2 yielded the same island structures. A similar result was obtained for bulk CaF_2, although insulator charging problems degraded the signal-to-noise ratio of the acquired secondary electron images. The onset of an observable magnetic signal occurred for 7 ML room -temperature grown films. With greater coverages, the paramagnetic signal changed to that of ferromagnetic behavior.