H - T phase diagram of YbCo2Si2 with H // [100]

We report on the first high-resolution dc-magnetisation ($M$) measurements on a single crystal of \ycs. $M$ was measured down to 0.05 K and in fields up to 12 T, with the magnetic field $H$ parallel to the crystallographic direction [100]. Two antiferromagnetic (AFM) phase transitions have been detected in a field $\mu_{0}H = 0.1$ T at $T_{N} = 1.75$ K and $T_{L} = 0.9$ K, in form of a sharp cusp and a sudden drop in $\chi = M/H$, respectively. These signatures suggest that the phase transitions are $2^{nd}$ order at $T_{N}$ and $1^{st}$ order at $T_{L}$. The upper transition is suppressed by a critical field $\mu_{0}H_{N} = 1.9$ T. The field-dependent magnetisation shows two hysteretic metamagnetic-like steps at the lowest temperature, followed by a sharp kink, which separates the AFM region from the paramagnetic one. The magnetic $H - T$ phase diagram of \ycs has been deduced from the isothermal and isofield curves. Four AFM regions were identified which are separated by $1^{st}$ and $2^{nd}$ order phase-transition lines.