Superlubricity of tetrahedral amorphous carbon (ta-C) coatings under boundary lubrication with organic friction modifiers is important for industrial applications, but the underlying mechanisms remain elusive. Here, combined experiments and simulations unveil a universal tribochemical mechanism leading to superlubricity of ta-C/ta-C tribopairs. Pin-on-disc sliding experiments show that ultra- and superlow friction with negligible wear can be achieved by lubrication with unsaturated fatty acids or glycerol, but not with saturated fatty acids and hydrocarbons. Atomistic simulations reveal that, due to the simultaneous presence of two reactive centers (carboxylic group and C=C double bond), unsaturated fatty acids can concurrently chemisorb on both ta-C surfaces and bridge the tribogap. Sliding-induced mechanical strain triggers a cascade of molecular fragmentation reactions releasing passivating hydroxyl, keto, epoxy, hydrogen and olefinic groups. Similarly, glycerol’s three hydroxyl groups react simultaneously with both ta-C surfaces, causing the molecule’s complete mechano-chemical fragmentation and formation of aromatic passivation layers with superlow friction.

The giant magnetoresistance ͑GMR͒ effect was studied on electrodeposited Co-Cu/Cu multilayers of 300 bilayer repeats which were produced in an electrochemical cell with homogeneous current distribution from a bath with two solutes (CoSO 4 ,CuSO 4 ). The preparation employed the conventional potentiostatic/potentiostatic and galvanostatic/galvanostatic, as well as an unprecedented galvanostatic/potentiostatic ͑G/P͒ control. We find that the specific deposition parameters rather than the deposition mode itself are decisive for the magnitude of the GMR which could be as high as 10% measured at 1 kOe on substrate-free multilayers in optimized G/P conditions. For this new deposition mode, detailed studies on the dependence of GMR on Co and Cu layer thicknesses as well as the bath pH were performed. No oscillatory behavior of the GMR as a function of the Cu layer thickness could be observed. The results suggest the importance of a Co-dissolution and/or a Co vs. Cu exchange reaction after completing the deposition of each magnetic layer. These reactions lead to the formation of a Cu or Cu-rich interface layer prior to the electrochemical deposition of the actual Cu layer during the subsequent pulse in either deposition mode. It turned out that the properties of this interfacial layer ͑thickness, degree of chemical intermixing͒ strongly influence the resulting GMR behavior of the multilayer.

Electrodeposited Co-Cu/Cu multilayers were prepared under a variety of deposition conditions on either a polycrystalline Ti foil or on a silicon wafer covered by a Ta buffer and a Cu seed layer. X-ray diffraction (XRD) revealed a strong (111) texture for all multilayers with clear satellite peaks for the multilayers on Si/Ta/Cu substrates, in some cases for up to three reflections. Cross-sectional transmission electron microscopy investigations indicated a much more uniform multilayer structure on the Si/Ta/Cu substrates. The bilayer periods from XRD satellite reflections were in reasonable agreement with nominal values. An analysis of the overall chemical composition of the multilayers gave estimates of the sublayer thickness changes due to the Co-dissolution process during the Cu deposition pulse. The XRD lattice spacing data indicated a behaviour close to a simple "multilayer" Vegard's law which was, however, further refined by taking into account elastic strains as well. In agreement with the structural studies, magnetoresistance data also indicated the formation of more perfect multilayers on the smooth Si/Ta/Cu substrates. An analysis of the magnetoresistance behaviour revealed the presence of superparamagnetic (SPM) regions in the magnetic layers. The contribution of these SPM regions to the total observed giant magnetoresistance was found to be dominating under certain deposition conditions, e.g., for magnetic layer thicknesses less than 1 nm (about 5 monolayers).

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We have shown recently that both the magnetization and the magnetoresistance of electrodeposited Co-Cu/ Cu multilayers can be decomposed by assuming the presence of both ferromagnetic ͑FM͒ and superparamagnetic ͑SPM͒ regions in the magnetic layers. In the present work, for two selected samples, one with a large SPM and another one with a large FM contribution to the giant magnetoresistance, low temperature magnetic and magnetoresistance measurements were performed in order to reveal the evolution of the FM and SPM terms with temperature. The average apparent magnetic moment of the SPM regions deduced from the two sets of data showed a good agreement. The role of electrochemical processes in the formation of the SPM regions is discussed. An attempt has also been made to elaborate on some models for the spatial distribution of the constituent elements ͑Co and Cu͒ leading to the occurrence of SPM regions. The results are discussed also in the framework of interacting SPM regions.

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