Structure, Magnetism and Magnetoresistance Effect of ${\rm Cd}_{1-{\rm x}}{\rm Cu}_{\rm x}{\rm Cr}_{2}{\rm S}_{4}$ ($x=0.01$, 0.04, 0.1, 0.2)

A series of Cd
_1-x
Cu
_x
Cr
₂
S
₄
(
x
=0.01, 0.04, 0.1, and 0.2) 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
Fd
3
m
and a tiny amount of Cr
₂
O
₃
impurity phase. X-ray photoemission spectroscopy reveals a bivalence of Cu and trivalence of Cr in Cd
_0.8
Cu
_0.2
Cr
₂
S
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
-
T
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.