Influence of oxygen on the interfacial stability of Cu on Co(0001) thin films

Vapor deposited Cu Co multilayers with high quality Co-Cu interfaces exhibit significant giant magnetoresistance. These multilayers are sometimes grown using process environments with significant background partial pressures of oxygen, which can impact the quality of film and its properties. Previously we found that oxygen preferentially stabilizes Co Cu(111). First principle density functional theoretical calculations are used herein to examine the effects of surface atomic oxygen on the stability of the Cu(111) Co(0001) interface. This interface can be grown with varying degrees of intermixing at the surface. We examine the significance of this by analyzing the first two Cu layers deposited on a Co(0001) substrate. The effects of oxygen are studied for the nonmixed Cu1 ML Co1 ML Co(0001) system, the mixed Co0.33 Cu0.67 Co0.67 Cu0.33 Co(0001) and Co0.66 Cu0.33 Co0.33 Cu0.67 Co(0001) surface alloys and the Co-capped Co1 ML Cu1 ML Co(0001) system. In the absence of oxygen, the nonmixed Cu1 ML Co1 ML Co(0001) system is found to be the most stable. The calculations show that the fraction of the surface comprised of Cu, in the absence of background oxygen, is maximized for Cu deposited onto a Co(0001) substrate. This reduces the tendency for intermixing. Submonolayer coverages of atomic oxygen preferentially bind at the three-fold fcc and hcp sites on all four Cu Co(0001) surfaces investigated as well as on the homogeneous Co(0001) and the Cu(111) surfaces. The difference in the oxygen-metal binding energy for the fcc and hcp sites appears to be negligible which is consistent with the minor changes that occur in the local structure (oxygen height above the surface and interlayer spacing). Total energy calculations indicate that the intermixing is unfavorable when the oxygen coverage is kept below 0.362 ML. This threshold coverage is hardly affected by the oxygen binding sites (hcp and fcc), stacking sequence of the metal layer or thicker Cu layers. This threshold oxygen coverage minimizes the influence of the surface oxygen on the magnetic properties of the system. These results, taken with other calculations for Co Cu(111), suggest that 0.362 ML of oxygen is optimal for stabilizing the Co Cu(111) interface without disrupting the stability of the Cu Co(0001) interface. The calculations show that the addition of oxygen to the Cu-segregated unmixed Cu1 ML Co1 ML Co(0001) surface increases the surface magnetization as a result of the unpaired electrons that arise from the surface oxygen atoms. The addition of atomic oxygen to the Co-capped Co1 ML Cu1 ML Co(0001) surface, on the other hand, results in an increase in the magnetization for oxygen coverages up to 0.33 ML. At higher oxygen coverages, however, there is a shifting of the minority states toward the majority states near the Fermi level which significantly reducing the magnetization of the surface Co layer.