%X The magnetic structure and electronic ground state of the layered perovskite 
Ba
2
IrO
4
 have been investigated using x-ray resonant magnetic scattering. Our results are compared with those for 
Sr
2
IrO
4
, for which we provide supplementary data on its magnetic structure. We find that the dominant, long-range antiferromagnetic order is remarkably similar in the two compounds and that the electronic ground state in 
Ba
2
IrO
4
, deduced from an investigation of the x-ray resonant magnetic scattering 
L
3
/
L
2
 intensity ratio, is consistent with a 
J
eff
=
1
/
2
 description. The robustness of these two key electronic properties to the considerable structural differences between the Ba and Sr analogues is discussed in terms of the enhanced role of the spin-orbit interaction in 
5
d
 transition metal oxides.
%O This version is the author accepted manuscript. For information on re-use, please refer to the publisher’s terms and conditions.
%I AMER PHYSICAL SOC
%L discovery10134049
%J Physical Review Letters
%D 2013
%V 110
%A S Boseggia
%A R Springell
%A HC Walker
%A HM Ronnow
%A C Rueegg
%A H Okabe
%A M Isobe
%A RS Perry
%A SP Collins
%A DF McMorrow
%T Robustness of Basal-Plane Antiferromagnetic Order and the J(eff)=1/2 State in Single-Layer Iridate Spin-Orbit Mott Insulators
%N 11