Incommensurate-commensurate magnetic phase transition in SmRu2Al10

Magnetic properties of single crystalline SmRu2Al10 have been investigated by electrical resistivity, magnetic susceptibility, and specific heat. We have confirmed the successive magnetic phase transitions at T-N = 12.3 K and T-M = 5.6 K. Resonant x-ray diffraction has also been performed to study the magnetic structures. Below TN, the Sm3+ moments order in an incommensurate structure with q(1) = (0,0.759,0). The magnetic moments are oriented along the orthorhombic b axis, which coincides with the magnetization easy axis in the paramagnetic phase. A very weak third harmonic peak is also observed at q(3) = (0,0.278,0). The transition at T-M is a lock-in transition to the commensurate structure described by q(1) = (0,0.75,0). A well-developed third harmonic peak is observed at q(3) = (0,0.25,0). From the discussion of the magnetic structure, we propose that the long-range RKKY interaction plays an important role, in addition to the strong nearest-neighbor antiferromagnetic interaction.