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 07 CEF ground state. A peak in the easy-plane susceptibility at low temperatures demonstrates that the previously reported magnetic ordering at TN ≈ 1.65K is of antiferromagnetic (AFM) nature. In addition, we observed sharp anomalies in all measured properties, evidencing a further phase transition at TL ≈ 0.9K, 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 T 3 dependence, as expected for AFM spin-wave excitations. Surprisingly, the resistivity, ρ(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 C4f/T versus T 2 to T = 0, the Sommerfeld coefficient γ0 = 0.13 JK−2mol −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. 1 Present address: Max-Planck Institute of Biochemistry, Am Klopferspitz 18, 82152 Martinsried, Germany. 2 Author to whom any correspondence should be addressed. New Journal of Physics 13 (2011) 083024 1367-2630/11/083024+13$33.00 © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft