Abstract
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 TN≈1.65 K is of antiferromagnetic (AFM) nature. In addition, we observed sharp anomalies in all measured properties, evidencing a further phase transition at TL≈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 TL the specific heat, C4f, 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 C4f/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.
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