Enhancement of the colossal magnetoresistance in Eu_(1-x) Ca _x B_6

摘要

The transport and magnetic properties of single crystal samples of substitutional solid solutions Eu_(1-x) Ca _x B_6 (0 ≤ x ≤ 0.26) have been studied at temperatures 1.8-300 K in magnetic fields up to 80 kOe. It has been shown that an increase in the calcium concentration results in the suppression of the charge transport accompanied by an increase in the amplitude of the colossal magnetoresistance (CMR) up to the value (ρ(0)-ρ(H))/ρ(H) ≈ 7 × 10~5 detected for x = 0.26 at liquid-helium temperature in a field of 80 kOe. The transition from the hole-like conductivity to the electron-like conductivity has been observed in the Eu_(0.74)Ca_(0.26)B_6 solid solution in the CMR regime at T < 40 K. The Hall mobility values μH = 200-350 cm ~2/(V s) estimated for charge carriers in the strongly disordered matrix of the Eu_(0.74)Ca_(0.26)B_6 solid solution are comparable with the charge carrier mobility μH = 400-600 cm~2/(V s) for the undoped EuB_6 compound. The anomalous behavior of the transport and magnetic parameters of the Eu_(1-x) Ca _x B _6 solid solutions is discussed in terms of a metal-insulator transition predicted within the double exchange model for this system with low carrier density.