Structure, magnetism and magnetoresistance effect of $cd _1-x Cu _x Cr ₂S ₄(x=0.01, 0.04, 0.1, 0.2)

A series of Cd _1-xCu _x Cr ₂ S ₄ ceramics have been prepared by a solid state reaction method. The X-ray diffraction patterns suggest that the samples have a cubic spinel phase with a space group $Fd3m$ and a tiny amount of Cr ₂O ₃ impurity phase. X-ray photoemission spectroscopy reveals a bivalence of Cu and trivalence of Cr in Cd _0.8 Cu _0.2 Cr ₂S ₄. The replacement of Cd by Cu leads to a slight shift of magnetic transition temperature $T _C. The saturation magnetic moment per formula unit at 2 K and effective moments in paramagnetic region decrease with Cu content, which is attributed to the coexistence of low spin state Cr ³⁺ with S=1/2 and high spin state Cr} ³⁺ with S=3/2 as deduced from the M-H and M- measurements. The transport measurements suggest that the Cu ²⁺ doping enhances the carrier density. In contrast, a significant decrease in magnetoresistance is observed upon Cu doping. The magnetoresistance effect is discussed based on the magnetic polaron theory.