Phase formation kinetics of metastable fcc cobalt

• Sputter deposition in N 2 at ≥ 300 °C yields phase-pure metastable fcc Co without N. • Deposition in Ar gives substrate-dependent hcp or fcc Co phases. • N 2 enables coherent epitaxial hcp / fcc stacking on Al 2 O 3 (0001). • Nitridation stabilizes metastable fcc Co and coherent hcp / fcc for spintronic application. The kinetics for phase formation of metastable face-centered cubic (fcc) cobalt is explored using a combination of epitaxial thin film deposition and first-principles calculations. Co layers are deposited by magnetron sputtering while systematically varying the substrate, substrate temperature T s , and processing gas (Ar or N 2 ). Deposition in N 2 results in nitrogen-free fcc Co on amorphous SiO 2 /Si for T s ≥ 300 °C but causes residual nitrogen incorporation for T s ≤ 200 °C. Templating with hexagonal Al 2 O 3 (0001) leads to nucleation and growth of epitaxial hexagonal close-packed (hcp) Co(0001) in Ar while N 2 causes stacking faults and the formation of a coherent mixed hcp/fcc epitaxial microstructure. MgO(001) substrates facilitate nucleation of metastable fcc Co and growth of epitaxial Co(001)/MgO(001) in both Ar and N 2 for T s ≥ 100 and T s ≥ 300 °C, respectively. Density functional calculations support experimental trends, suggesting that nitrogen stabilizes the cubic phase with a predicted hcp-to-fcc transition at 10 at.%N for T = 0 K and 2 at.%N at 300 °C. They also indicate a negligible hcp(0001)/fcc(111) phase-boundary energy, facilitating the experimentally observed mixed hcp/fcc microstructure. The overall results demonstrate that N 2 -assisted Co deposition provides an effective approach to synthesize metastable fcc Co layers.