Magnetization Process of Narrow-Gap Semiconductor FeSb2

We have measured the magnetization on a high-quality single crystal of a narrow-gap semiconductor, FeSb 2 , which displays an anomalous temperature dependence of magnetic susceptibility. In the traditional Stoner–Wohlfath theory, the fourth expansion coefficient (γ) of the free energy in magnetization is mainly determined from the density of states curve at around Fermi energy, and a negative value of γ is predicted for narrow-gap semiconductors. However, we found that, in FeSb 2 , γ is positive and shows a rapid change with temperature. These behaviors resemble those seen in FeSi and are qualitatively in accordance with Takahashi's spin fluctuation theory. This implies that the magnetization process of an itinerant electron system is governed by the correlated mode of spin fluctuations.