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A study of interface reactions between the magnetic layer and the protective carbon coating, and the influence of these interactions on magnetic properties of rigid disk media for magnetic data storage is presented. Quaternary CoPtCrB and CoPtCrTa alloy films and protective hydrogenated or nitrogenated carbon coatings were deposited by dc magnetron sputtering onto metal hard disks (AlMg substrate/electroplated NiP layer/Cr underlayer). Core level x-ray photoelectron spectroscopy of nitrogenated carbon films on B-containing alloys showed the formation of boron nitride and small amounts of chromium nitride at the interface, and also indicated the likely presence of chromium carbide. The amount of boron nitride at the interface varied depending on substrate bias voltage and temperature during deposition of the carbon coating. From a quantitative analysis of the x-ray photoelectron spectra it was inferred that boron nitride formation was controlled by the diffusion of boron from deeper regions of the magnetic layer to the interface. Surprisingly, compared to disks using the same magnetic alloy but with a hydrogenated or pure carbon coating, no effects of these interface reactions on the magnetic properties of the disks could be detected. In contrast, for a Ta-containing alloy the same comparison revealed a drop in the coercivity of up to 200 Oe in disks with nitrogenated carbon overcoats. Strong evidence for the formation of tantalum nitride as well as small amounts of chromium nitride was found in the photoelectron spectra. Thus, while the formation of boron nitride at the interface of CoPtCrB media and protective carbon coating does not affect the magnetic properties of the disks in the range of boron and nitrogen concentrations investigated here, small changes in the chemical environment of Ta and/or Cr can lead to significant changes in the magnetic properties of the CoPtCrTa media. © 2000 American Institute of Physics.