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

Vapor deposited $\mathrm{Cu}∕\mathrm{Co}$ multilayers with high quality $\mathrm{Co}\text{\ensuremath{-}}\mathrm{Cu}$ 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 $\mathrm{Co}∕\mathrm{Cu}(111)$. First principle density functional theoretical calculations are used herein to examine the effects of surface atomic oxygen on the stability of the $\mathrm{Cu}(111)∕\mathrm{Co}(0001)$ 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 $\mathrm{Co}(0001)$ substrate. The effects of oxygen are studied for the nonmixed ${\mathrm{Cu}}_{1\phantom{\rule{0.3em}{0ex}}\mathrm{ML}}∕{\mathrm{Co}}_{1\phantom{\rule{0.3em}{0ex}}\mathrm{ML}}∕\mathrm{Co}(0001)$ system, the mixed ${\mathrm{Co}}_{0.33}{\mathrm{Cu}}_{0.67}∕{\mathrm{Co}}_{0.67}{\mathrm{Cu}}_{0.33}∕\mathrm{Co}(0001)$ and ${\mathrm{Co}}_{0.66}{\mathrm{Cu}}_{0.33}∕{\mathrm{Co}}_{0.33}{\mathrm{Cu}}_{0.67}∕\mathrm{Co}(0001)$ surface alloys and the Co-capped ${\mathrm{Co}}_{1\phantom{\rule{0.3em}{0ex}}\mathrm{ML}}∕{\mathrm{Cu}}_{1\phantom{\rule{0.3em}{0ex}}\mathrm{ML}}∕\mathrm{Co}(0001)$ system. In the absence of oxygen, the nonmixed ${\mathrm{Cu}}_{1\phantom{\rule{0.3em}{0ex}}\mathrm{ML}}∕{\mathrm{Co}}_{1\phantom{\rule{0.3em}{0ex}}\mathrm{ML}}∕\mathrm{Co}(0001)$ 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 $\mathrm{Co}(0001)$ 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 $\mathrm{Cu}∕\mathrm{Co}(0001)$ surfaces investigated as well as on the homogeneous $\mathrm{Co}(0001)$ and the $\mathrm{Cu}(111)$ 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 $0.362\phantom{\rule{0.3em}{0ex}}\mathrm{ML}$. 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 $\mathrm{Co}∕\mathrm{Cu}(111)$, suggest that $0.362\phantom{\rule{0.3em}{0ex}}\mathrm{ML}$ of oxygen is optimal for stabilizing the $\mathrm{Co}∕\mathrm{Cu}(111)$ interface without disrupting the stability of the $\mathrm{Cu}∕\mathrm{Co}(0001)$ interface. The calculations show that the addition of oxygen to the Cu-segregated unmixed ${\mathrm{Cu}}_{1\phantom{\rule{0.3em}{0ex}}\mathrm{ML}}∕{\mathrm{Co}}_{1\phantom{\rule{0.3em}{0ex}}\mathrm{ML}}∕\mathrm{Co}(0001)$ 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 ${\mathrm{Co}}_{1\phantom{\rule{0.3em}{0ex}}\mathrm{ML}}∕{\mathrm{Cu}}_{1\phantom{\rule{0.3em}{0ex}}\mathrm{ML}}∕\mathrm{Co}(0001)$ surface, on the other hand, results in an increase in the magnetization for oxygen coverages up to $0.33\phantom{\rule{0.3em}{0ex}}\mathrm{ML}$. 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.