The magnetic structure of the intermetallic antiferromagnet Sm2IrIn8 was determined using x-ray resonant magnetic scattering (XRMS). Below TN = 14.2, Sm2IrIn8 has a commensurate antiferromagnetic structure with a propagation vector (1/2,0,0). The Sm magnetic moments lie in the ab plane and are rotated roughly 18 degrees away from the a axis. The magnetic structure of this compound was obtained by measuring the strong dipolar resonant peak whose enhancement was of over two orders of magnitude at the L2 edge. At the L3 edge both quadrupolar and dipolar features were observed in the energy line shape. The magnetic structure and properties of Sm2IrIn8 are found to be consistent with the general trend already seen for the Nd-, Tb- and the Ce-based compounds from the RmMnIn3m+2n family (R = rare earth; M=Rh or Ir, m = 1, 2; n = 0, 1), where the crystalline electrical field (CEF) effects determine the direction of magnetic moments and the TN evolution in the series. The measured Neel temperature for Sm2IrIn8 is slightly suppressed when compared to the TN of the parent cubic compound SmIn3.
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