Resistivity extrema in double exchange ferromagnetic nondegenerate   semiconductors
  A version of the magnetoimpurity theory of the colossal magnetoresistance materials suitable for the double exchange ferromagnetic nondegenerate semiconductors is presented. It provides an explanation of the nonmonotonic temperature dependence for the charge carrier density in them when it displays first a maximum and then a minimum, on increase in temperature. Respectively, the resistivity displays first a minimum and then a maximum. The theory is based on the relation between the charge carrier activation energy and the change in the magnon free energy caused by the ionization of an impurity. This is tantamount to the relation between the charge carrier density and the so called giant red shift of the optical absorption edge. 
E.L. Nagaev, A.I. Podel'shchikov, and V.E. Zil'bervarg