  We report an experimental investigation of the two-dimensional ${J}_{\rm
eff}$ = 1/2 antiferromagnetic Mott insulator by varying the interlayer exchange
coupling in [(SrIrO$_3$)$_1$, (SrTiO$_3$)$_m$] ($m$ = 1, 2 and 3)
superlattices. Although all samples exhibited an insulating ground state with
long-range magnetic order, temperature-dependent resistivity measurements
showed a stronger insulating behavior in the $m$ = 2 and $m$ = 3 samples than
the $m$ = 1 sample which displayed a clear kink at the magnetic transition.
This difference indicates that the blocking effect of the excessive SrTiO$_3$
layer enhances the effective electron-electron correlation and strengthens the
Mott phase. The significant reduction of the Neel temperature from 150 K for
$m$ = 1 to 40 K for $m$ = 2 demonstrates that the long-range order stability in
the former is boosted by a substantial interlayer exchange coupling. Resonant
x-ray magnetic scattering revealed that the interlayer exchange coupling has a
switchable sign, depending on the SrTiO$_3$ layer number $m$, for maintaining
canting-induced weak ferromagnetism. The nearly unaltered transition
temperature between the $m$ = 2 and the $m$ = 3 demonstrated that we have
realized a two-dimensional antiferromagnet at finite temperatures with
diminishing interlayer exchange coupling.
