The structural and magnetic properties of Mn5Ge3Cx films prepared at elevated substrate temperatures TS are investigated. In particular, films with x ≥ 0.5 and TS = 680 K exhibit a strongly enhanced Curie temperature TC = 440 K compared to Mn5Ge3 with TC = 304 K. Structural analysis of these films suggests that the carbon is interstitially incorporated into the voids of Mn octahedra of the hexagonal Mn5Si3-type structure giving rise to a lattice compression. The enhanced ferromagnetic stability in connection with the lattice compression is interpreted in terms of an Mn-Mn interaction mediated by C based on a change in the electronic structure.

Si films prepared by magnetron sputtering at elevated substrate temperatures exhibit ferromagnetic order for compositions around Mn 5 Si 3 C x (xϾ0) in contrast to the antiferromagnetic Mn 5 Si 3 compound. In these sputtered samples the carbon concentration can be strongly enhanced compared to sintered powder samples. The local structural order around the Mn site in Mn 5 Si 3 C x films was investigated for xϭ0 and 0.75 by x-ray-absorption spectroscopy at different temperatures T. At low T, both films retain their hexagonal structure and do not exhibit the orthorhombic distortion observed for bulk Mn 5 Si 3 powder. For the carbon-doped film a local distortion of the octahedra formed by six Mn atoms of the Mn 5 Si 3 structure is found when surrounding a C atom. This indicates an anisotropic change of the atomic Mn environment by C, in contrast to a simple lattice expansion as inferred from previous structural analysis.

We report on scanning tunneling microscopy ͑STM͒ measurements on ultrathin Gd films in the thickness range 0.5рd Gd р10 ML ͓1 monolayer ͑ML͒ϭ2.89 Å͔ grown on a Y ͑0001͒ buffer layer at substrate temperatures T s ϭ473 K and T s ϭ573 K. The magnetic properties were investigated on these films capped with a 100-Å Y layer with the transverse magneto-optical Kerr effect. A ferromagnetic signal was found down to the sub-ML regime d Gd Ϸ0.5 ML for T s ϭ473 K. In this regime, the STM images show a lateral growth of ML thick Gd islands of hexagonal shape. The depression of the Curie temperature T C follows a power law ⌬T C ϰd Gd Ϫ with an exponent Ϸ1 up to d Gd Ϸ3-5 ML, where changes to ϭ1.6. This transition is also observed in the same thickness range for the exponent ␤ of the magnetization M .

The effect of carbon doping on the structural and magnetic properties of Mn5Si3Cx films is investigated for different substrate temperatures TS and concentrations x. Samples with x≈0.75 prepared at TS=650–750 K exhibit ferromagnetic order with enhanced ordering temperatures TC well above room temperature in contrast to the undoped antiferromagnetic Mn5Si3 compound. Structural analysis shows that C is incorporated interstitially in the hexagonal Mn5Si3-type structure with a lattice expansion with respect to the undoped compound. The enhanced ferromagnetic order is presumably not simply due to a change of the interatomic Mn–Mn distances but due to a change of the electronic structure and/or exchange interactions.

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