Theoretical and experimental study of the magnetic structure ofTlCo2Se2

The magnetic properties of ${\mathrm{TlCo}}_{2}{\mathrm{Se}}_{2}$ are investigated by means of single-crystal neutron diffraction experiments and first principles calculations. The previous suggestion of a noncollinear incommensurate magnetic structure based on powder neutron diffraction data is confirmed by single-crystal diffraction. Our theoretical calculations find that ${\mathrm{TlCo}}_{2}{\mathrm{Se}}_{2}$ adopts an antiferromagnetic ordering. The magnetism is dominated by the contribution from the $\mathrm{Co}$ atoms, where the magnetic moments are arranged ferromagnetically within the ab plane. The magnetic moments from experiments are reproduced very well by our theoretical calculations. The origin of the magnetic ordering is analyzed by a detailed inspection of the energy band structure and the Fermi surface. The small calculated energy difference $(\ensuremath{\approx}0.1\phantom{\rule{0.3em}{0ex}}\mathrm{mRy})$ between the two magnetic arrangements, the experimental noncollinear and the antiferromagnetic, is viewed as the most likely origin of this discrepancy. The influence of nonstoichiometric samples on the magnetic state is also argued to be a possible source for the observed small discrepancy between experiment and theory.