Low-temperature magnetic phase diagram of the heavy-fermion compound YbCo2Si2

We report on the first high-resolution DC magnetization M(H,T) measurements on a single crystal of YbCo2Si2. M was measured down to 50 mK and fields up to 12 T with the field H perpendicular to c axis. The magnetic phase diagram of YbCo2Si2 was deduced from the isothermal curves along with susceptibility and heat-capacity measurements. Two antiferromagnetic phase transitions have been detected at TN=1.65 K and TL=0.9 K. The signatures at TN suggest that the phase transition is 2nd order and it is suppressed by a field μ0HN=1.9 T. At TL latent heat has been observed in the heat capacity in zero field and hysteresis effects in the M vs. H confirm the 1st order nature of the transition. The entropy below both transitions and the M value at HN point to a local character of the Yb 4f quasi-hole, resulting in a very small Kondo temperature compared to the one in YbRh2Si2. However, the data at 50 mK reveals the presence of a kink in M vs. H at μ0HN=10.6 T, very similar to the one observed YbRh2Si2 at μ0HN=9.9 T, where a suppression of the Kondo fluctuations or a Lifshitz transition have been proposed. The T-H phase diagram as well as the nature of the high-field transition are discussed.