In this article, we present the transport properties of W/C multilayers which are generally studied for their optical properties (X-ray mirrors). For the thinnest tungsten layers the magnetoresistance is clearly governed by weak localization. We point out the importance of electron-electron interactions.

We studied the magnetoresistance of Ni/Ti, NiC/Ti and Co/Cu sputtered multilayers. In fields below 2 T, the Co/Cu films exhibit previously described giant magnetoresistance. We observe an unusual negative contribution to the magnetoresistance in high fields whose origin is not well understood at this time.

Period expansion after annealing in tungsten/carbon (W/C) multilayer films has been observed by several authors. Although most results have emphasized that the carbon layers play the important role in such thermal evolution, it is important to clarify the structure of the carbon atoms in those multilayers both before and after annealing in order to explain such period expansion more clearly. In this paper, Raman scattering is carried out on the W/C multilayers fabricated by various methods, all of which showed different period expansion values ranging from 0% to 20%. We show that the initial carbon in those multilayers becomes graphitized differently depending on the fabrication methods. The carbon fabricated by rf sputtering under a reactive gas has the highest amount of graphite component as compared with those prepared under a pure argon gas atmosphere. After annealing at 1000 °C, graphitization can be observed, but there are no obvious structural differences for the carbon in all the annealed W/C multilayer films. Such graphitization of the carbon layer causes a decrease in its density, and thus an increase in its thickness. Due to the different initial states of the carbon in the multilayers produced by the different methods and to the identical final state after annealing, the different carbon layer thickness expansions are expected and do agree with the small angle x-ray diffraction measurements.

W/C multilayers were grown by the sputtering technique. For tungsten layers 10 nm thick, the resistivity and the magnetoresistance exhibit a 3D classical behavior while the transport properties, for thickness of about 2 nm, can be described in the framework of 2D weak localization theory.

A series of W/C multilayers has been fabricated using a magnetron sputtering device. In order to establish comparison with other multilayers produced by several laboratories, their atomic structure and thermal evolution has been studied by small- and wide-angle x-ray scattering, respectively, and Raman spectroscopy. By plotting the nominal thickness determined by the sputtering speed versus the thickness obtained by simulation of small-angle x-ray scattering spectra the existence has been confirmed of a WC interface compound of about 0.7 nm formed during the multilayer deposition. The multilayers fabricated by the magnetron sputtering device showed higher thermal stability and less period expansion than those produced by different apparatus [Dupuis et al., J. Appl. Phys. 68, 5146 (1990), Jiang et al., J. Appl. Phys. 65, 196 (1989)]. Raman spectra indicated that the carbon in the W/C multilayers is more graphitized than in the previous W/C multilayer. The carbon is further graphitized after annealing at 1000 °C. The graphitization of carbon induced its density to decrease, that is, thickness to increase. Consequently, in the multilayers a smaller expansion of carbon is expected after annealing at 1000 °C since there is more graphitized carbon in the initial as-deposited state than in the previous W/C multilayer. This conclusion agrees very well with the thickness variation measured by x-ray diffraction.

New magnetic materials and x-ray mirrors have lead to the preparation of multilayered structures (MLS) with ultrathin periodic layers. However, physical limitations appear for periods below some tens of A. In particular, fabrication tolerances relative to these ultrathin layer thicknesses are critical for the realization of effective MLS. In sputtering, the deposition rate is easily reproducible and can be maintained at a constant and low value for several hours; for this reason we are able to perform a large number (more than 100) of periodic ultrathin layers by this system (3 to 1.5 nm period). High Resolution Transmission Electron Microscopy (HRTEM) and x-ray reflectivity measurements are used to compare interfaces and stacking regularities for different samples with 140 layers. The W/Si MLS present a better quality of interfaces and structures compared to the W/C MLS.

Multilayers consisting of alternating thin bilayers of W and Si (period: 1.5

Ultrathin and thin bilayers of W coupled with C and Si have been deposited by rf magnetron sputtering. Behavior of the multilayer structures (MLS) has been analyzed through two combined techniques: x-ray reflectivity and electron microscopy study and conventional (TEM) and high-resolution transmission (HRTEM). The experimental results we present provide average informations on the reflective characteristics of the mirrors in agreement with theory, comparative values of parameters (thickness and interfacial roughness of the bilayers), as well as structural characteristics of the stackings. We focused on ultrathin W/Si MLS with a bilayer thickness of 1.5 nm that present excellent regularity with effective roughness less than 3 Å. They offer reflective qualities to be used as reflectors with an angle 2θ>6° in the first order for the medium x-ray range.