Magnetic behaviour of the intermetallic compound YbCo₂Si₂

We report on a comprehensive study of the magnetic properties of YbCo₂Si₂ investigated using a single crystal. All data confirm a stable trivalent Yb state carrying a well-localized 4f moment, as well as a non-magnetic Co state. We observed a sizeable anisotropy in the magnetic susceptibility χ(T), with the moment in the basal plane twice as large as that along the c-direction. Combining this result with published neutron-scattering data puts strong constraints on the values of the crystalline electrical field (CEF) parameters. Our calculation yields one possible solution with a Γ₇ CEF ground state. A peak in the easy-plane susceptibility at low temperatures demonstrates that the previously reported magnetic ordering at T_N≈1.65 K is of antiferromagnetic (AFM) nature. In addition, we observed sharp anomalies in all measured properties, evidencing a further phase transition at T_L≈0.9 K, found to be first order, which corresponds to a change in the AFM structure. Both transitions are suppressed by a magnetic field B=2 T. Below T_L the specific heat, C^4f, shows a well-defined T³ dependence, as expected for AFM spin-wave excitations. Surprisingly, the resistivity, ρ(T), also shows a T³ power law in the same temperature range, instead of the expected T⁵. This discrepancy might be related to more complex magnetic interactions, as also evidenced by the multiple transitions. By extending the linear fit of C^4f/T versus T² to T=0, the Sommerfeld coefficient γ₀=0.13 J K^{− 2} mol⁻¹ is extracted. Although magnetism in YbCo₂Si₂ is dominated by the RKKY exchange interaction, the enhanced γ₀ and a weak Kondo-like increase in ρ(T) indicate the presence of some interactions between the conduction electrons and the 4f moments.