Ba3NbFe3Si2O14 belongs to the family of non-centrosymmetric langasite materials providing with interesting geometrically frustrated networks of spins. In this layered compound, a triangular array of magnetic Fe3+ triangles are found in the (a,b) planes. We have shown by neutron diffraction that it orders magnetically below TN =28 K, the magnetic moments forming a 120 degrees spins structure within each triangle and showing a helical modulation along the perpendicular direction with a period of 7 lattice parameters [1]. This magnetic structure is single-domain, with a single chirality for the helical order and a single triangular chirality as shown by neutron polarimetry [1]. Additionally, it was proposed to give rise to magnetoelectric effect and multiferroism [2, 3]. Inelastic neutron scattering experiments revealed novel features in the magnon excitations. We observed two branches emerging from each magnetic satellite [4]. The lower one exhibits no chiral spin dynamics and the upper one is fully chiral over the whole spectrum, which appears not to have been reported so far. This remarkable result has been analyzed by linear spin wave calculation based on a Hamiltonian including antiferromagnetic exchange interactions and a Dzyalochinskii Moriya interaction. Finally, I will discuss the existence and nature of the magnetic fluctuations in the paramagnetic phase, which result in helical spin correlations.
[1] K. Marty, V. Simonet, E. ressouche, et al., Phys. Rev. Lett. 101, 247201 (2008). [2] K. Marty, P. Bordet, V. Simonet et al., Phys. Rev. B 81 054416 (2010). [3] H. D. Zhou et al., Chem. Mater. 21 156 (2009). [4] M. Loire, V. Simonet, S. Petit, et al., submitted to Phys. Rev. Lett., Arxiv :1010.2008
Institut Néel, Grenoble