Ferroic Order for Anisotropic Magnetic Dipole Term in Collinear Antiferromagnets of (t_{2g})^{4} System.

We present the possibility of x-ray magnetic circular dichroism on RuO_{2} with collinear antiferromagnetism (AFM). Given that the crystal symmetry breaks the time reversal symmetry when the antiparallel spin aligns along the [100] direction, the expectation vector of the anisotropic magnetic dipole operator ⟨t⟩ remains uncanceled along the [010] direction. Our Letter reveals that the magnetic dipole (T_{z}) term in the x-ray magnetic circular dichroism is induced by the residual ⟨t⟩. Because the features of the magnetic moment can be detected via absorption measurements even in the AFM, this technique will be useful for determining the magnetic phase, the Van Vleck-type paramagnet or the excitonic AFM in (t_{2g})^{4} system.