Effect of pressure on the field-induced ordered phase in the heavy-fermion compound YbCo ₂ Zn ₂₀

The effect of pressure on the field-induced ordered phase for H {norm of matrix} 〈111〉 in the heavy-fermion compound YbCo ₂ Zn ₂₀ , which is presumably a field-induced antiferro-quadrupolar (FI-AFQ) phase, was investigated in the pressure range up to 4.5 GPa and under magnetic fields up to 80 kOe. When pressure is applied, the metamagnetic-like transition at H _m =6 kOe shifts to lower fields and disappears around the quantum critical pressure P _c ∼1.8 GPa. A pressure-induced antiferromagnetic (PI-AFM) phase appears at pressures above about 2 GPa, and the critical field H _c of the PI-AFM phase increases with increasing pressure. On the other hand, the transition field H _Q of the FI-AFQ phase decreases gradually without showing any anomalous behavior around P _c and becomes obscure around 4 GPa, where H _c of the PI-AFM phase and H _Q of the FI-AFQ phase become comparable. The magnetic field versus pressure, H-P, phase diagram for H {norm of matrix} 〈111〉 at 0.1 K was constructed in the pressure range up to 4.5 GPa.