Grazing-angle x-ray reflectivity (XRR) is described as an efficient, nondestructive, parameter-free means to measure the mass density of various types of amorphous carbon films down to the nanometer thickness range. It is shown how XRR can also detect layering if it is present in the films, in which case the reflectivity profile must be modeled to derive the density. The mass density can also be derived from the valence electron density via the plasmon energy, which is measured by electron energy-loss spectroscopy (EELS). We formally define an interband effective electron mass $m^{*},$ which accounts for the finite band gap. Comparison of XRR and EELS densities allows us to fit an average $m^{*}=0.87m$ for carbon systems, m being the free-electron mass. We show that, within the Drude-Lorentz model of the optical spectrum, $m^{*}=\left[1-{n\left(0\right)\mathrm{}}^{-2}\right]m,$ where $n\left(0\right)\mathrm{}$ is the refractive index at zero optical frequency. The fraction of ${\mathrm{sp}}^2$ bonding is derived from the carbon K-edge EELS spectrum, and it is shown how a choice of “magic” incidence and collection angles in the scanning transmission electron microscope can give ${\mathrm{sp}}^2$ fraction values that are independent of sample orientation or anisotropy. We thus give a general relationship between mass density and ${\mathrm{sp}}^3$ content for carbon films.

The nanoscale magnetic structure of FeRh epilayers has been studied by polarized neutron reflectometry. Epitaxial films with a nominal thickness of $500\text{Å}$ were grown on MgO (001) substrates via molecular-beam epitaxy and capped with $20\text{Å}$ of MgO. The FeRh films show a clear transition from the antiferromagnetic (AF) state to the ferromagnetic (FM) state with increasing temperature. Surprisingly the films possess a FM moment even at a temperature 80 K below the AF-FM transition temperature of the film. We have quantified the magnitude and spatial extent of this FM moment, which is confined to within $\sim 60–80\text{Å}$ of the FeRh near the top and bottom interfaces. These interfacial FM layers account for the unusual effects previously observed in films with thickness $<100\text{Å}$. Given the delicate energy balance between the AF and FM ground states we suggest a metastable FM state resides near to the interface within an AF matrix. The length scale over which the FM region resides is consistent with the strained regions of the film.