Magnetic and Crystal Structures of CeC2, PrC2, NdC2, TbC2, and HoC2 at Low Temperatures

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Abstract
The light rare-earth dicarbides, CeC2, PrC2, and NdC2, having the tetragonal CaC2-type structure, have been shown by neutron diffraction to become a body-centered first-kind antiferromagnet with the Néel temperatures of 33°, 15°, and 29°K and with the ordered moments being 81%, 44%, and 90% of the free-ion values, respectively. The moment direction is parallel to the c axis in the above compounds. The isostructural heavy rare-earth dicarbides, TbC2 and HoC2, exhibit an antiferromagnetic, elliptic-helical spin alignment propagating along the a axis with the repetition quadrupole of the atomic spacing, where the Néel temperatures are 66° and 26°K, and the root-mean-square ordered moments are 5.1 and 6.9 Bohr magnetons, respectively. Here, the moments lie on the bc plane. A modulation of the above structure takes place below 40° and 16°K, with the maximum possible additional moments of 0.54 and 1.65 Bohr magnetons at 2°K in TbC2 and HoC2, respectively. In TbC2, another complex magnetic structure coexists below ∼33°K. Also presented are a discussion on the wavy diffuse backgrounds of TbC2 and HoC2 at temperatures down to 2°K, and the low-temperature crystallographic parameters.