Collinear Antiferromagnet SmPt6Al3 with a Sm Honeycomb Structure Centered by Pt Triangles

We report a single-crystal study on the magnetism of the Sm honeycomb compound SmPt₆Al₃ with the trigonal NdPt₆Al₃-type structure by the measurements of the magnetic susceptibility χ, electrical resistivity, specific heat, and resonant X-ray diffraction. The trivalent state of the Sm ion is indicated by the fact that the lattice parameters fall on the lanthanoid contraction lines for the R³⁺Pt₆Al₃ series. For \(B\parallel c\), χ(T) shows a cusp at T_N = 6.5 K and strongly decreases on cooling below T_N, whereas χ(T) for \(B \perp c\) continues to increase without showing any anomaly at T_N. Application of \(B\parallel c\) = 12 T depresses T_N to 4.6 K, whereas T_N remains at 6.4 K in \(B \perp c\) = 14 T. The isothermal magnetization curves at 1.8 K for both \(B\parallel c\) and \(B \perp c\) have no spontaneous moment. These magnetic properties of SmPt₆Al₃ indicate a collinear antiferromagnetic structure of the ordered moments pointing toward the c axis. This magnetic structure is verified by the Sm resonant X-ray diffraction measurement which revealed the \([0,0,0]\) propagation vector. By comparing the magnetic properties of SmPt₆Al₃ with those for the canted antiferromagnet GdPt₆Al₃, we highlight the role of Dzyaloshinskii–Moriya interaction in the series of isostructural compounds with the centrosymmetric NdPt₆Al₃-type structure.