  The temperature-dependent behavior of magnetic domains plays an essential
role in the magnetic properties of materials, leading to widespread
applications. However, experimental methods to access the three-dimensional
(3D) magnetic domain structures are very limited, especially for
antiferromagnets. Over the past decades, the spin-orbit Mott insulator iridate
$Sr_2IrO_4$ has attracted particular attention because of its interesting
magnetic structure and analogy to superconducting cuprates. Here, we apply
resonant x-ray magnetic Bragg coherent diffraction imaging to track the
real-space 3D evolution of antiferromagnetic ordering inside a $Sr_2IrO_4$
single crystal as a function of temperature, finding that the antiferromagnetic
domain shows anisotropic changes. The anisotropy of the domain shape reveals
the underlying anisotropy of the antiferromagnetic coupling strength within
$Sr_2IrO_4$. These results demonstrate the high potential significance of 3D
domain imaging in magnetism research.
