Crossover in magnetic properties of FeSi

The magnetization of high quality iron monosilicide single crystals has been studied in a wide temperature and magnetic-field range. These data are analyzed by taking into account strong Hubbard correlations between e_g and t_2g states of Fe and the formation of spin polarons in the cubic FeSi matrix at low temperatures (T<100 K). These heavy particles are transformed into ferromagnetic microregions at T_c~15 K, which result in the appearance of the low-temperature anomalies in transport and thermodynamic properties of this compound. The suppression of the many-body resonance at E_F occurs in FeSi at high temperatures, thus leading to the crossover in magnetic properties of FeSi. The transition to the temperature induced magnetic moments causes a dramatic increase of the magnetic susceptibility at temperatures T>100 K.