We have investigated spin fluctuations in the langasite compound Ba${}_3$NbFe${}_3$Si${}_2$O${}_{14}$ both in the ordered state and as a function of temperature. The low-temperature magnetic structure is defined by a spiral phase characterized by magnetic Bragg peaks at $\mathrm{q⃗}=(0,0,\tau ~1/7$) onset at $T_N=27$ K as previously reported by Marty et al. [Phys. Rev. Lett. 101, 247201 (2008)]. 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 show that the physical properties and energy scales including the ordering wave vector, Curie-Weiss temperature, and spin waves can be explained through the use of only symmetric exchange constants without the need for a dominant 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.

• Received 21 July 2010

DOI:https://doi.org/10.1103/PhysRevB.83.104426