  The effect of electron doping on the magnetic properties of the
Brownmillerite-type bilayered compounds has been investigated by neutron powder
diffraction in La substituted Ca2.5-xLaxSr0.5GaMn2O8 compounds (x = 0.05 and
0.1), in comparison with the undoped compound (x = 0). In all compounds, a
long-range three dimensional collinear antiferromagnetic (AFM) structure is
found below the N\'eel temperature TN of respective compound, whereas, well
above TN, three dimensional short-range magnetic ordering is observed. In the
intermediate temperature range just above TN, a strong effect of electron
doping (La substitution) on the magnetic correlations has been observed. Here,
a short-range AFM correlation with possible dimensionality of three has been
found for substituted compounds (x = 0.05 and 0.1) as compared to the reported
two dimensional long-range AFM ordering in the parent compound. With increasing
the electron doping, a decrease in TN is also observed. The short-range
magnetic correlations set in over a large temperature range above TN. A
magnetic phase diagram in the x-T plane is proposed from these results.
