Spin excitations of the correlated semiconductor FeSi probed by THz   radiation
  By direct measurements of the complex optical conductivity $\sigma(\nu)$ of FeSi we have discovered a broad absorption peak centered at frequency $\nu_{0}(4.2 K) \approx 32 cm^{-1}$ that develops at temperatures below 20 K. This feature is caused by spin-polaronic states formed in the middle of the gap in the electronic density of states. We observe the spin excitations between the electronic levels split by the exchange field of $H_{e}=34\pm 6 T$. Spin fluctuations are identified as the main factor determining the formation of the spin polarons and the rich magnetic phase diagram of FeSi. 
V. V. Glushkov, B. P. Gorshunov, E. S. Zhukova, S. V. Demishev, A. A.   Pronin, N. E. Sluchanko, S. Kaiser, M. Dressel
cond-mat.str-el