Magnetic anisotropy and spin dynamics in the kagome magnet Fe4Si2Sn7O16 : NMR and magnetic susceptibility study on oriented powder

${\mathrm{Fe}}_4{\mathrm{Si}}_2{\mathrm{Sn}}_7{\mathrm{O}}_{16}$ hosts an undistorted kagome lattice of ${\mathrm{Fe}}^{2+}$ ($3d^6, S=2$) ions. We present results of bulk magnetization and Sn nuclear magnetic resonance (NMR) measurements on an oriented ${\mathrm{Fe}}_4{\mathrm{Si}}_2{\mathrm{Sn}}_7{\mathrm{O}}_{16}$ powder sample oriented in geometries parallel ($\parallel$) and perpendicular ($\perp$) to the external applied magnetic field used for orienting the powder ($B_{\text{ori}}$). The bulk susceptibility $\chi$ shows a broad peak at $T_N\sim 3$ K associated with antiferromagnetic ordering. NMR spectra indicate the presence of planar anisotropy in the kagome planes. From an analysis of the static NMR shift ($K$) and dynamic spin-lattice relaxation rate ($1/T_1$) we conclude the presence of dominant magnetic fluctuations in the kagome planes. For the $\parallel$ orientation, $K$ scales linearly with the bulk susceptibility for temperatures down to $\sim 4$ K, while in the $\perp$ orientation $K$ starts to deviate strongly below $T\sim 30$ K. We associate this deviation with the onset of spin-tilting towards the kagome planes. These correlations are also reflected in the $1/T_1$ data for the $\parallel$ orientation, which starts to decrease below $T\sim 30$ K. In this correlated regime, $T_N<T< \sim 30$ K, we discuss the formation of positive chiral spin correlations in the kagome planes.