  Anti-site disorder is one of the most important issues that arises in
synthesis of double perovskite for spintronic applications. Although it is
known that anti-site disorder leads to a proliferation of structural defects,
known as the anti-phase boundaries that separate ordered anti-phase domains in
the sample, little is known about the magnetic correlation across these
anti-phase boundaries on a microscopic level. Motivated by this, we report
resonant elastic X-ray scattering study of room temperature magnetic and
structural correlation in a thin-film sample of Sr$_2$CrReO$_6$, which has one
of the highest $\mathrm{T_C}$ among double perovskites. Structurally, we
discovered existence of anti-phase nanodomains of $\sim$15~nm in the sample.
Magnetically, the ordered moments are shown to lie perpendicular to the $c$
direction. Most remarkably, we found that the magnetic correlation length far
exceeds the size of individual anti-phase nanodomains. Our results therefore
provide conclusive proof for existence of robust magnetic correlation across
the anti-phase boundaries in Sr$_2$CrReO$_6$.
