Correlation between the magnetic phase transition and the electric properties in substituted spinels Zn1-xCuxCr2Se4

The electric proper-ties of normal-type spinels Zn1-xCuxCr2Se4 have been studied by measuring the electric resistivity from 5 K to 300 K. It is found that the substituted compounds with x = 0.02 and 0.15 are semiconductors down to 5 K, while ones with x greater than or equal to 0.30 are metals with high conductivity. However, it is also found that the metallic properties of the latter are characterized by new electric modulations in a low-temperature region. Zn1-xCuxCr2Se4 with x = 0.3 and 0.5 show the electric phase transition from semiconductor to metal at about 60 K and 25 K, respectively. The metallic compounds with x = 0.75 and 1.0 show the kink of the resistivity at about 60 K with increasing temperature. The highly conductive Zn0.4Cu6Cr2Se4 slightly shows the weak semiconducting behavior in the region from 5 K to 280 K. It is considered for the substituted system Zn1-xCuxCr2Se4 with x < 0.75 that, only in the semiconducting phase (0.0 < x < 0.75) and the insulating phase (x = 0.0), there are partially localized Cr3+ spins showing the superexchange interaction between the Cr3+ and Cr3+ spins in the antiferromagnetic phase below about 22 K to 28 K. The antiferromagnetic to ferromagnetic phase transitions occur-ring at 24 K to 28 K in Zn1-xCuxCr2Se4 with 0.15 less than or equal to x < 0.75 are induced by the disappearance of the superexchange interaction between localized Cr3+ and Cr3+ spins, and by the predominance of the double-exchange interaction between Cr3+ spin and conductive Cr4+ spin polaron in the Se2- valence band.