Spin correlations and Kondo effect in a strong ferromagnet

The spin structure and electron-transport behavior of Pt-substituted MnBi thin films have been investigated. The electrical resistivity of these ferromagnetic films shows an unusual low-temperature resistance minimum or Kondo effect accompanied by positive magnetoresistance. First-principles calculations show that Mn atoms displaced to the bipyramidal interstitial sites are antiferromagnetically coupled to the Mn atoms on their regular lattice sites. We explain the observed Kondo effect and the positive magnetoresistance as the consequences of local spin correlations involving Mn atoms displaced to interstitial sites by Pt doping.