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 chi(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 Gamma(7) CEF ground state. A peak in the easy-plane susceptibility at low temperatures demonstrates that the previously reported magnetic ordering at T-N approximate to 1.65K is of antiferromagnetic (AFM) nature. In addition, we observed sharp anomalies in all measured properties, evidencing a further phase transition at T-L approximate to 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, C-4f, shows a well-defined T-3 dependence, as expected for AFM spin-wave excitations. Surprisingly, the resistivity, rho(T), also shows a T-3 power law in the same temperature range, instead of the expected T-5. 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 T-2 to T = 0, the Sommerfeld coefficient gamma(0) = 0.13 J K-2 mol(-1) is extracted. Although magnetism in YbCo2Si2 is dominated by the RKKY exchange interaction, the enhanced gamma(0) and a weak Kondo-like increase in rho(T) indicate the presence of some interactions between the conduction electrons and the 4f moments.