First-principles prediction of high Curie temperature for ferromagnetic bcc-Co and its relation to Co/MgO/Co magnetic tunnel junctions

We determine from first principles the Curie temperature of bulk Co in the ground state hcp phase and the metastable fcc and bcc phases. For fcc-Co we found a Curie temperature of $T_{\mathrm{C}}(\mbox{fcc-Co})=1280$~K, in reasonable agreement with experimental results. For bcc-Co, a Curie temperature of $T_{\mathrm{C}}(\mbox{bcc-Co})=1400$~K is predicted. This suggests that bcc-Co/MgO/bcc-Co tunnel junctions offer high tunneling magnetoresistance ratios even at elevated temperatures, giving them an advantage over Fe/MgO/Fe junctions. $T_{\mathrm{C}}(\mbox{bcc-Co})$ appears robust under tetragonalization upon epitaxial growth on MgO, in contrast to Fe for which $T_{\mathrm{C}}(\mbox{bcc-Fe})$ is found to drop by more than 20\% (from 970~K to 750~K) upon such a tetragonalization. We find that FeCo alloys have an even higher $T_{\mathrm{C}}$, as high as 1660~K for ordered FeCo. We discuss the origin of these effects in terms of the electronic structure and densities of states. The Curie temperatures are calculated by mapping {\it ab initio} results to a Heisenberg model, which is solved by a Monte Carlo method.