Hall effect of spin-chirality origin in a triangular-lattice helimagnet $\mbox{Fe}_{1.3} \mbox{Sb}$

We report on a topological Hall effect possibly induced by scalar spin chirality in a quasi-two-dimensional helimagnet$\mbox{Fe}_{1+x} \mbox{Sb}$. In the low-temperature region where the spins on interstitial-Fe (concentration $x\approx 0.3)$ intervening the $120^\circ $ spin-ordered triangular planes tend to freeze, a non-trivial component of Hall resistivity with opposite sign of the conventional anomalous Hall term is observed under magnetic field applied perpendicular to the triangular-lattice plane. The observed unconventional Hall effect is ascribed to the scalar spin chirality arising from the heptamer spin-clusters around the interstitial-Fe sites, which can be induced by the spin modulation by the Dzyaloshinsky-Moriya interaction.