Multiferroic materials where magnetic and ferroelectric orders are coupled have become a topic of much interest recently owing to the possible technological applications [1]. This coupling is often introduced through the presence of a Dzyaloshinskii-Moriya interaction term which breaks inversion symmetry. I will present a neutron scattering study of the langasite compound Ba3NbFe3Si2O14 where the structural and magnetic properties are directly coupled. The low temperature magnetic structure is defined by a single domain spiral phase characterized by magnetic Bragg peaks at q=(0,0, tau=1/7) onset at TN=27 K [2]. The nature of the fluctuations and temperature dependence of the order parameter is consistent with a classical second order phase transition for a two dimensional triangular antiferromagnet. We will show that the physical properties and energy scales including the ordering wave vector, Curie-Weiss temperature, and the spin-waves can be explained through the use of only symmetric exchange constants without the need for the Dzyaloshinskii-Moriya interaction. This is accomplished through a set of “helical” exchange pathways along the c direction imposed by the chiral crystal structure and naturally explains the magnetic diffuse scattering which displays a strong vector chirality up to high temperatures well above the ordering temperature. This illustrates a strong coupling between magnetic and crystalline chirality in this compound [3].
References:
[1] S.-W. Cheong et al. Nature Materials, 6, 13 (2007).
[2] K. Marty et al. Phys. Rev. Lett. 101, 247201 (2008).
[3] C. Stock et al. cond-mat/arXiv:1007.4216.