A giant enhancement of CMR in Eu0.6Ca0.4B6

The transport and magnetic properties of Eu0.6Ca0.4B6 single crystals have been studied at temperatures 1.8-300 K in magnetic fields up to 80 kOe. It was found that lowering of temperature results in a drastic increase of magnetoresistance up to the values of ρ(0)/ρ(H)∼7·105 detected below 6K. The Hall and Seebeck effect measurements showed that colossal magnetoresistance (CMR) observed in Eu0.6Ca0.4B6 is accompanied by a crossover from hole-like to electron-like regime of charge transport induced by applied magnetic field. Hall mobility values μH∼200-350 cm2/(Vs) estimated for the high conductive state of Eu0.6Ca0.4B6 in the presence of strong substitutional disorder were proved to be comparable with these ones measured for undoped EuB6. The anomalous behaviour of transport and magnetic parameters is discussed in terms of metal-insulator transition earlier predicted for this low carrier density system within double exchange model by V M Pereira et al., Phys. Rev. Lett. 93 147202 (2004).