Low temperature spin excitation spectrum of FeSi by polarized neutron spectroscopy

We investigated the spin excitation spectrum of FeSi using the thermal triple axis neutron spectrometer IN20 at the Institute Laue Langevin, Grenoble, with polarization analysis. FeSi displays a behavior which has, so far, only been observed in f-electron containing Kondo insulators. Therefore, it has often been claimed to be the first 3d system belonging to this class of materials. A temperature dependent energy gap was observed in optical spectroscopy, but measurements of inelastic magnetic scattering via neutrons have been restricted to T≥150 K so far and did not report gap signatures. We measured magnetic scattering between room temperature and T=15 K over a large energy range up to energies of 80 meV. Particular attention was devoted to possible spin-exciton formation at energy transfers below the gap value at low temperatures as observed in the Kondo-insulator YbB12.