Magnetic properties of HoCoC2, HoNiC2 and their solid solutions

Magnetic properties of single crystalline HoCoC2 and the evolution of magnetic and structural features in a series of polycrystalline solid solutions HoCo1−xNixC2 (0⩽x⩽1) are investigated by means of X-ray diffraction, magnetization, magnetic susceptibility and specific heat measurements. The crystal structures of all investigated samples refers to the CeNiC2-type structure (space group Amm2 and Pearson symbol oS8). Non-isoelectronic substitution of Co by Ni causes a non-linear increase of the unit cell volume and especially a non-monotonous variation of the a and c lattice parameters as well as a pronounced reduction of the C–C bond length of carbon dimers. Temperature dependent magnetization and specific heat measurements reveal a crossover from a ferromagnetic for HoCoC2 with TC=10.6(1)K to an antiferromagnetic ground state for HoNiC2 with TN=2.78(6)K and a non-monotonous variation of the magnetic ordering temperature with a minimum at intermediate compositions. Crystalline electric field effects of HoCoC2 and HoNiC2 are analysed using combined thermodynamic and magnetic susceptibility data. The electrical resistivity of HoNiC2 displays a distinct anomaly near room temperature which indicates the formation of a charge density wave (CDW) state as earlier reported for several other rare earth nickel dicarbides.