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The hexagonal close packed phase of cobalt (hcp-Co) is associated with numerous stacking faults while the face centered cubic phase of cobalt (fcc-Co) has considerably less stacking faults, as shown e.g. in [1]. Two domains of hcp-Co stacked one above the other with a stacking fault in between are usually delimited by a small interface region of a few fcc-Co layers as shown e.g. in [2,3]. The crystal structures of hcp-Co and fcc-Co should be considered together because the crystallites of these two phases are often clustered together in the same grains of cobalt. The gradual ferromagnetic spin reorientation in hcp-Co between 230 $^\circ$C and 330 $^\circ$C reported in [3] suggests that this phase could not have a hexagonal symmetry. This hypothesis is verified positively by synchrotron radiation and neutron powder diffraction [4]. The crystal structure of the hcp phase of cobalt is described by the monoclinic symmetry with the magnetic space group C2'/m'. In this monoclinic crystal structure the former hexagonal [001] axis is no longer perpendicular to the hexagonal layers. The monoclinic structure is an approximate description of the multitude of stacking faulted hcp-Co domains coexisting with fcc-Co domains.
[1] O. S. Edwards and H. Lipson, Proc. R. Soc. Lond. Ser. A-Math. Phys. Sci. 180, 268 (1942).
[2] O. Blaschko, G. Krexner, J. Pleschiutschnig, G. Ernst, C. Hitzenberger, H. P. Karnthaler, and A. Korner, Phys. Rev. Lett. 60, 2800 (1988).
[3] E. Bertaut, A. Delapalme, and R. Pauthenet, Solid State Commun. 1, 81 (1963).
[4] P. Kozlowski, P. Fabrykiewicz, I. Sosnowska, F. Fauth, A. Senyshyn, E. Suard, D. Oleszak and R. Przenioslo - in preperation.
