Resistivity size effect in epitaxial face-centered cubic Co(001) layers

Metastable face-centered cubic (fcc) Co layers are deposited by reactive magnetron sputtering in 5 mTorr N2 at 400 °C followed by vacuum annealing at 500 °C. The resulting phase-pure Co(001)/MgO(001) layers contain negligible nitrogen and exhibit a surface roughness <0.8 nm and a cube-on-cube epitaxial relationship with the substrate with Co[100]ǁMgO[100]. The measured resistivity vs thickness d = 10–1000 nm indicates a bulk resistivity ρo = 6.4 ± 0.3 μΩ cm for fcc Co at room temperature and ρo = 1.3 ± 0.1 μΩ cm at 77 K, and an effective electron phonon mean free path λ = 27 ± 2 nm and 79 ± 6 nm at 295 and 77 K, respectively. The resulting ρo × λ benchmark quantity is 3–5 times larger than that predicted from first principles, suggesting a breakdown of the Fuchs–Sondheimer model at small dimensions. The overall results indicate that fcc Co exhibits no intrinsic conductance benefit over stable hcp Co nor conventional Cu for narrow interconnects. The developed method for growth of epitaxial fcc Co(001) layers provides opportunities to study this metastable material for potential spintronic applications.