Complex Magnetism in Gd₅Pb₃ single crystals

The intermetallic R₅M₃ compounds (R = rare earth, M = Si, Sn, Sb, Ge, Pb) crystallize in a hexagonal P6₃/mcm structure, where the rare earth ions are located at two inequivalent crystallographic sites. These compounds are, therefore, expected to display complex magnetic ordering due to the coupling between the two different magnetic sublattices, leading to anisotropic exchange and possible geometric frustration arising from the hexagonal symmetry. We have synthesized the first single crystals of Gd₅Pb₃, a member of this family of Mn₅Si₃-type structures. Using a combination of magneto-transport, specific heat, and high-field magnetization measurements, we find anisotropic magnetization within the ferromagnetic order (T_C = 285 K), with up to four additional magnetic transitions down to 2 K. The Hall data shows complex behavior, not expected for a simple ferromagnet, especially one based on Gd³⁺ (with a half-full f shell, 4f⁷, L = 0). A comparison of the ordering temperatures across the R₅M₃ series shows that T_C values for M = Pb are consistently larger than for the other transition metals series. This presents an opportunity to explore the role of spin-orbit coupling, anisotropic exchange, geometric frustration, and magnetic interactions and the interplay of these competing interactions in the R₅M₃ hexagonal compounds.