Growth and electronic and magnetic structure of iron oxide films on Pt(111)

Ultrathin (111)-oriented polar iron oxide films were grown on a Pt(111) single crystal either by the reactive deposition of iron or oxidation of metallic iron monolayers. These films were characterized using low-energy electron diffraction, scanning tunneling microscopy, and conversion electron Mössbauer spectroscopy. The reactive deposition of Fe led to the island growth of Fe${}_3$O${}_4$, in which the electronic and magnetic properties of the bulk material were modulated by superparamagnetic size effects for thicknesses below 2 nm, revealing specific surface and interface features. In contrast, the oxide films with FeO stoichiometry, which could be stabilized as thick as 4 nm under special preparation conditions, had electronic and magnetic properties that were very different from their bulk counterpart wüstite. Unusual long-range magnetic order appeared at room temperature for thicknesses between 3 and 10 monolayers, the appearance of which requires severe structural modification from the rock-salt structure.

Figure 1

The stacking of the (111)-atomic layers in spinel (magnetite, top) and rock-salt (wüstite, bottom) structures. Denoted by Fe${}_A$ are iron atoms in tetrahedral positions, by Fe${}_B$ iron atoms in tetrahedral positions, by O oxygen atoms. The element symbols are preceded by fractions indicating layer occupation.

Figure 2

LEED patterns and STM images (100 $\times$ 100 nm${}^2$) for iron oxide films grown on Pt(111) by reactive deposition of iron in an atmosphere of molecular oxygen. Nominal doses of iron are 0.17 nm for (a) and (d), 0.5 nm for (b) and (e), and 0.8 nm for (c) and (f), corresponding to the nominal Fe${}_3$O${}_4$ thickness of 0.3, 1.0, and 1.7 nm, respectively.

Figure 3

Evolution of room temperature CEMS spectra with increasing thickness for iron oxide films grown on Pt(111) by reactive deposition of iron. The inset at the 1-nm spectrum shows the effect of low temperature (108 K). The lines show the result of the best fit and the deconvolution into spectral components. Green (dotted) and red (solid) lines correspond to components unambiguously identified as coming from magnetite, denoted $A$ (tetrahedral Fe) and $B$ (octahedral Fe), respectively, and the blue (shaded) components identified as coming from interfacial oxide-Pt(111) phases.

Figure 4

LEED patterns and STM images (400 $\times$ 400 nm${}^2$, middle; and 100 $\times$ 100 nm${}^2$, bottom) for iron oxide films grown on Pt(111) by oxidation of iron monolayers. The columns correspond to the nominal oxide thickness of 3, 4, 5, and 7 ML, from left to right, respectively.

Figure 5

Room temperature CEMS spectra for iron oxide films grown on Pt(111) by oxidation of iron monolayers as a function of increasing thickness. The lines show the result of the best fit and the deconvolution into spectral components: blue (shaded) and red (solid) are the low isomer shift (LIS) components in the nonmagnetic and magnetic state, respectively, and green (dotted) is the high isomer shift (HIS) component.

Figure 6

Results of CEMS analysis for iron oxide films grown on Pt(111) by oxidation of iron monolayers: (top) isomer shift (relative to α-Fe), (middle) fraction of spectral component, and (bottom) hyperfine magnetic field as a function of the increasing thickness. Squares and circles are used for the low isomer shift (LIS) component in magnetic and nonmagnetic state, respectively; diamonds are used for the high isomer shift (HIS) component.

Figure 7

Hysteresis loop measured using magnetooptic Kerr effect for a 5-ML FeO film on Pt(111).