Symmetry analysis of the antiferromagnetic phase transitions in hexagonal FeGe

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Abstract
In hexagonal FeGe the experimental data show existence of three ordered magnetic phases. A symmetry analysis based on the representations of the space group of the crystal structure is applied in order to predict the possible magnetic structures of hexagonal FeGe. The character of the transitions between the different magnetic structures is considered by expressing the phenomenological Ginzburg-Landau functionals in terms of the order parameters for the different phases. The magnetic modes for the wave-vector k allowed by the symmetry are also calculated. Along with a possible collinear magnetic ordering in the basal plane, clockwise and anti-clockwise triangular ordering of the basal-plane spin components are suggested. The presently available experimental data does not allow a choice between different types of basal-plane ordering. In addition, the spin wave modes for the wave-vector q along the c axis are calculated and the symmetry relations between the spin waves and the magnetic modes with wave-vector k are established. This allows the authors to suggest that the observed magnetic ordering of the basal-plane spin components at T2N may be caused by condensation of the corresponding spin wave in the antiferromagnetic phase. Such condensation is the result of a soft spin wave at wave-vector q=k which might be observed experimentally.

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