Temperature-induced local moments in MnSi and FeSi

A unified theory is given of the magnetic and thermal properties of the itinerant-electron magnet MnSi and the non-magnetic semiconductor FeSi. The calculations are based on the spin-fluctuation theory of Hasegawa (1981) and employ a model density of states derived from an existing band calculation. The exchange parameter is determined to give the observed spontaneous moment in MnSi at T=0. The finite-temperature properties are then calculated without adjustable parameters and semi-quantitative agreement with experiment is obtained for the temperature dependence of the susceptibility of MnSi and for the unusual susceptibility and specific heat of FeSi. In both materials the root-mean-square atomic moment increases rapidly with increasing temperature to values of 1.0-1.5 mu B . The associated fluctuating exchange fields destroy the band gap in FeSi which explains the similarity in the resistivity of FeSi and MnSi for T>400K.