Effect of Structure and Electron Configuration on the Magnetic Properties of La0.7Ca0.3–xSrxMn0.95Al0.05O3 Manganites

La
_0.7
Ca
$_{0.3-x}$
Sr
$_{x}$
Mn
_0.95
Al
_0.05
O
₃
manganites doped in
A
and
B
positions of a perovskite
AB
O
₃
cell are studied in this letter. Replacing Mn
³⁺
by Al
³⁺
decreases the density of itinerant
$e_{\mathrm g}$
electrons. It weakens the double exchange and decreases the Curie temperature (
$T_{\mathrm C}$
). Continuous substitution of Ca
²⁺
ion with larger Sr
²⁺
ion leads to increase of average and local crystal structure parameters of the perovskites and causes an increase in
$T_{\mathrm C}$
. Change of electron configuration, due to the substitution of Mn
³⁺
by Al
³⁺
, affects the
$T_{\mathrm C}$
more than does the change in local structure, caused by the substitution of Ca
²⁺
by Sr
²⁺
. It leads to a decrease in
$T_{\mathrm C}$
in general despite some increase in
$T_{\mathrm C}$
caused by the smaller Al
³⁺
ionic radius than that of Mn
³⁺
.