  A combination of competing exchange interactions and substitutional disorder
gives rise to magnetic inhomogeneities in the
[Y$_{2-x}$Mn$_x$]MnMnMn$_4$O$_{12}$ $x = 0.23$ and $x = 0.16$ quadruple
perovskite manganites. Our neutron powder scattering measurements show that
both the $x = 0.23$ and $x = 0.16$ samples separate into two distinct magnetic
phases; below T$_{1}$ = 120 $\pm$ 10 K the system undergoes a transition from a
paramagnetic phase to a phase characterised by short range antiferromagnetic
clusters contained in a paramagnetic matrix, and below T$_{2}$ $\sim$ 65 K, the
system is composed of well correlated long range collinear ferrimagnetic order,
punctuated by short range antiferromagnetic clusters. A sharp increase in the
antiferromagnetic phase fraction is observed below $\sim$ 33 K, concomitant
with a decrease in the ferrimagnetic phase fraction. Our results demonstrate
that the theoretically proposed AFM phase is stabilised in the
[Y$_{2-x}$Mn$_x$]MnMnMn$_4$O$_{12}$ manganites in the presence of dominant B-B
exchange interactions, as predicted.
