  The magnetism of the double perovskite compounds \SLFCOx\ ($x$ = 0, 1, 2) are
contrasted using magnetization, neutron diffraction and electron paramagnetic
resonance with the support from density functional theory calculations. \LFCO\
is identified as a long-range ordered antiferromagnet displaying a near-room
temperature transition at $T_N$ = 270~K, accompanied by a low temperature
structural phase transition at $T_S$ = 200~K. The structural phase
transformation at $T_S$ occurs from $R\overline{3}c$ at 300~K to $Pnma$ at
200~K. The density functional theory calculations support an insulating
non-compensated AFM structure. The long-range ordered magnetism of \LFCO\
transforms to short-range glassy magnetism as La is replaced with Sr in the
other two compounds. The magnetism of \LFCO\ is differentiated from the
non-equilibrium glassy features of \SFCO\ and \SLFCO\ using the {\em
cooling-and-heating-in-unequal-fields} (CHUF) magnetization protocols. This
contransting magnetism in the \SLFCOx\ series is evidenced in electron
paramegnetic resonance studies. The electronic density-of-states estimated
using the density functional theory calculations contrast the insulating
feature of \LFCO\ from the metallic nature of \SFCO. From the present suite of
experimental and computational results on \SLFCOx, it emerges that the
electronic degrees of freedom, along with antisite disorder, play an important
role in controlling the magnetism observed in double perovskites.
