Strong magnetic coupling in the hexagonal R5Pb3 compounds (R=Gd–Tm)

We have synthesized the R5Pb3 (R=Gd–Tm) compounds in polycrystalline form and performed neutron scattering and magnetization measurements. For all R5Pb3 reported here the Weiss temperatures θW are several times smaller than the ordering temperatures TORD, while the latter are remarkably high (TORD up to 275 K for R=Gd) compared to other known R–M binaries (M=Si, Ge, Sn and Sb). The magnetic order changes from ferromagnetic (FM) in R=Gd, Tb to antiferromagnetic (AFM) in R=Dy–Tm. Below TORD, the magnetization measurements together with neutron powder diffraction show complex magnetic behaviors and reveal the existence of up to three additional phase transitions, believed to be a result of large anisotropic exchange and/or crystal electric field effects, induced high anisotropy. The R5Pb3 magnetic unit cells for R=Tb–Tm can be described with incommensurate magnetic wave vectors with spin modulation either along the c axis in R=Tb, Er and Tm, or within the ab plane in R=Dy and Ho.