Re and Ru induced CMR effect in CaMnO3: the prime role of valency

The substitution of rhenium and ruthenium for manganese in the perovskite CaMnO3 significantly lowers the resistivity in the low temperature region and simultaneously induces ferromagnetism, and, consequently, a colossal magnetoresistance effect is observed. This behavior is explained in terms of valency effect: Re and Ru are mainly pentavalent, thus their substitution for Mn generates Mn3+ species, allowing double-exchange interaction between Mn3+ and Mn4+. Thermoelectric power measurements confirm this model, showing a shift from −350 μV·K−1 for the undoped phase, to −100 μV·K−1 for the doped phase (6% Re or Ru) at 300 K.