Sprecher
Beschreibung
The gradual ferromagnetic spin reorientation in the hcp phase of cobalt between 230°C and 330°C reported for a Co single crystal [1] suggests that this phase cannot have a hexagonal symmetry [2,3]. This hypothesis is verified positively by synchrotron radiation diffraction and neutron diffraction on powder of cobalt [4]. 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 [5,6]. The analysis of diffraction data has been done by using a specific set of Bragg peaks, which are not affected by the stacking faults. The crystal structure of the hcp-type ordered areas of cobalt is described by a monoclinic symmetry with the magnetic space group C2'/m', where the former hexagonal [001] axis is no longer perpendicular to the hexagonal layers. The hexagonal [001] and [010] axes make an angle equal α≈90.10(1)°, while the angle between in-plane [100] and [010] axes equals γ≈120.11(1)°. In addition, we report [4] a one order of magnitude smaller lattice mismatch in the hexagonal planes between hcp-Co and fcc-Co than the mismatch of about 0.5% between the hexagonal layers in hcp-Co and fcc-Co layers [5, 6]. Williamson-Hall analysis shows that microstrains are larger inside hexagonal planes than along the stacking faults direction [4]. Moreover, a higher content of the fcc-Co phase reduces microstrains in hcp-Co [4].
[1] E. Bertaut, A. Delapalme and R. Pauthenet, Solid State Commun. 1 (1963) 81
[2] R. Przeniosło, P. Fabrykiewicz and I. Sosnowska, Acta Cryst. A74 (2018) 705
[3] P. Fabrykiewicz, R. Przeniosło and I. Sosnowska, Acta Cryst. A77 (2021) 327
[4] P. Kozłowski, P. Fabrykiewicz, I. Sosnowska, F. Fauth, A. Senyshyn, E. Suard, D. Oleszak and R. Przeniosło, Phys Rev. B107 (2023) 104104
[5] O. S. Edwards and H. Lipson, Proc. R. Soc. Lond. Ser. A-Math. Phys. Sci. 180 (1942) 268
[6] O. Blaschko, G. Krexner, J. Pleschiutschnig, G. Ernst, C. Hitzenberger, H. P. Karnthaler and A. Korner, Phys. Rev. Lett. 60 (1988) 2800
