The spin wave excitations emerging from the chiral helically modulated 120$^{circ}$ magnetic order in a langasite Ba$_3$NbFe$_3$Si$_2$O$_{14}$ enantiopure crystal were investigated by unpolarized and polarized inelastic neutron scattering. A dynamica
l fingerprint of the chiral ground state is obtained, singularized by (i) spectral weight asymmetries answerable to the structural chirality and (ii) a full chirality of the spin correlations observed over the whole energy spectrum. The intrinsic chiral nature of the spin waves elementary excitations is shown in absence of macroscopic time reversal symmetry breaking.
The Fe-based langasites are the first reported compounds presenting a magnetic ordering in this rich family, besides being well known for piezo-electric properties and optical activity. The structural, magnetic and dielectric properties of the Fe lan
gasite compounds, with various substitution of non-magnetic cations, have been studied with x-ray and neutron diffraction, magnetostatic measurements, Mossbauer spectroscopy, and dielectric measurements. The title compounds (trigonal, space group P321) display a helical magnetic order with signatures of frustration below TN ~24- 35 K where an anomaly of the dielectric permittivity is observed. The influence of the cationic substitutions and the nature of the magnetoelectric coupling is hereafter addressed.
A novel doubly chiral magnetic order is found out in the structurally chiral langasite compound Ba$_3$NbFe$_3$Si$_2$O$_{14}$. The magnetic moments are distributed over planar frustrated triangular lattices of triangle units. On each of these they for
m the same triangular configuration. This ferro-chiral arrangement is helically modulated from plane to plane. Unpolarized neutron scattering on a single crystal associated with spherical neutron polarimetry proved that a single triangular chirality together with a single helicity is stabilized in an enantiopure crystal. A mean field analysis allows discerning the relevance on this selection of a twist in the plane to plane supersuperexchange paths.
Using magnetization, specific heat and neutron scattering measurements, as well as exact calculations on realistic models, the magnetic properties of the lacuvo compound are characterized on a wide temperature range. At high temperature, this oxide i
s well described by strongly correlated atomic $S$=1/2 spins while decreasing the temperature it switches to a set of weakly interacting and randomly distributed entangled pseudo spins $tilde S=1/2$ and $tilde S=0$. These pseudo-spins are built over frustrated clusters, similar to the kagome building block, at the vertices of a triangular superlattice, the geometrical frustration intervening then at different scales.
The static and dynamic magnetic properties of the Nd$_3$Ga$_5$SiO$_{14}$ compound, which appears as the first materialization of a rare-earth kagome-type lattice, were re-examined, owing to contradictory results in the previous studies. Neutron scatt
ering, magnetization and specific heat measurements were performed and analyzed, in particular by fully taking account of the crystal electric field effects on the Nd$^{3+}$ ions. One of the novel findings is that the peculiar temperature independent spin dynamics observed below 10 K expresses single-ion quantum processes. This would short-circuit the frustration induced cooperative dynamics, which would emerge only at very low temperature.
