Unusual behavior of the low-moment magnetic ground state ofYbRh2Si2under high pressure

High-pressure investigations of the ground state properties of the non-Fermi-liquid system ${\mathrm{YbRh}}_2{\mathrm{Si}}_2$ ${(}^{170}\mathrm{Yb}$-Mössbauer effect, electrical resistance and x-ray diffraction) reveal unusual features that are different from those found in Ce-related systems. We show that the ordered state with a low magnetic moment persists in a very large pressure range up to $\approx 10\mathrm{GPa}$ and is associated with spin fluctuations. At 10 GPa the system undergoes a first-order magnetic phase transition to a high-moment state with ${\mu }_{\mathrm{Yb}}=1.9\mathrm{}\left(1\right)\mathrm{}{\mu }_B$ and magnetic ordering temperature $T_m=7.5\mathrm{}\left(4\right)\mathrm{}\mathrm{K}$ at 16.5 GPa. A magnetic phase diagram for ${\mathrm{YbRh}}_2{\mathrm{Si}}_2$ in the $\mathrm{}(p,T)\mathrm{}$ space is suggested.