  We systematically investigate the magnetic properties and local structure of
Ba2YIrO6 to demonstrate that Y and Ir lattice defects in the form of antiphase
boundary or clusters of antisite disorder affect the magnetism observed in this
$d^4$ compound. We compare the magnetic properties and atomic imaging of (1) a
slow cooled crystal, (2) a crystal quenched from 900\degree C after growth, and
(3) a crystal grown using a faster cooling rate than the slow cooled one.
Atomic imaging by scanning transmission electron microscopy (STEM) shows that
quenching from 900oC introduces antiphase boundary to the crystals, and a
faster cooling rate during crystal growth leads to clusters of Y and Ir
antisite disorder. STEM study suggests the antiphase boundary region is Ir-rich
with a composition of Ba2YIrO6. The magnetic measurements show that Ba2YIrO6
crystals with clusters of antisite defects have a larger effective moment and a
larger saturation moment than the slow-cooled crystals. Quenched crystals with
Ir-rich antiphase boundary shows a slightly suppressed saturation moment than
the slow cooled crystals, and this seems to suggest that antiphase boundary is
detrimental to the moment formation. Our DFT calculations suggest magnetic
condensation is unlikely as the energy to be gained from superexchange is small
compared to the spin-orbit gap. However, once Y is replaced by Ir in the
antisite disordered region, the picture of local non-magnetic singlets breaks
down and magnetism can be induced. This is because of (a) enhanced interactions
due to increased overlap of orbitals between sites, and, (b) increased number
of orbitals mediating the interactions. Our work highlights the importance of
lattice defects in understanding the experimentally observed magnetism in
Ba2YIrO6 and other J=0 systems.
