  We study experimentally and theoretically the effects of disorder, nonlinear
screening, and magnetism in semiconductor heterostructures containing a
$\delta$-layer of Mn, where the charge carriers are confined within a quantum
well and hence both ferromagnetism and transport are two-dimensional (2D) and
differ qualitatively from their bulk counterparts. Anomalies in the electrical
resistance observed in both metallic and insulating structures can be
interpreted as a signature of significant ferromagnetic correlations. The
insulating samples turn out to be the most interesting as they can give us
valuable insights into the mechanisms of ferromagnetism in these
heterostructures. At low charge carrier densities, we show how the interplay of
disorder and nonlinear screening can result in the organization of the carriers
in the 2D transport channel into charge droplets separated by insulating
barriers. Based on such a droplet picture and including the effect of magnetic
correlations, we analyze the transport properties of this set of droplets,
compare it with experimental data, and find a good agreement between the model
calculations and experiment. Our analysis shows that the peak or shoulder-like
features observed in temperature dependence of resistance of 2D
heterostructures $\delta$-doped by Mn lie significantly below the Curie
temperature $T_{C}$ unlike the three-dimensional case, where it lies above and
close to $T_{C}$. We also discuss the consequences of our description for
understanding the mechanisms of ferromagnetism in the heterostructures under
study.
