Schematic representation of the spin-density-wave and commensurate phases. In the former case, the figure shows three sets of spin chains. In the latter case, the moments are aligned ferromagnetically perpendicular to the page.
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A powder diffraction study reveals an unexpected structural transformation of the compound Ca3Co2O6 below 12K.
Temperature-induced phase transitions between two magnetically ordered states usually correspond to lock-in transitions, transitions from collinear to non-collinear order, or magnetic ordering of symmetry-independent ionic sites. Very rarely, transitions between two magnetically ordered phases involve a change of translational symmetry, especially at low temperatures. A recent time-resolved powder diffraction study of the frustrated Ca3Co2O6 compound has shown that below 12 K, the known magnetic structure (a nearly 600 Å longitudinal spin-density wave) transforms slowly into a commensurate magnetic phase (in which the magnetic structure follows the atomic structure).
The broadening of diffraction features suggests that this new phase forms as an inter-growth within the matrix of the initial magnetic phase, i.e., the sample does not undergo macroscopic phase
separation. This phenomenon appears below the freezing point of 12 K, where bulk dynamical behaviour is also known to change dramatically and steps appear in the magnetization. Both effects, however, still defy explanation at the atomic level.
S Agrestini (Max-Planck Institut CPfS), LC Chapon (ISIS), C Mazzoli (ESRF), A Bombardi (Diamond Light Source), CL Fleck, MR Lees, OA Petrenko (Warwick University)
Research date: August 2011
Contact: Dr S Agrestini, firstname.lastname@example.org
Further reading: S. Agrestini, et al., Phys. Rev. Lett. 106 (2011) 197204
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