Pressure effect on superconductivity and antiferroquadrupolar order in PrOs ₄Sb ₁₂

We report the results of the low-temperature dc magnetization measurements of PrOs ₄Sb ₁2 under hydrostatic pressures up to 1.45 GPa. The pressure effect on the temperature dependence of magnetization and the magnetic phase diagram reveals that the crystal field energy splitting of a Γ ₁ singlet ground state and a Γ ₄ ⁽²⁾ triplet first exited state decreases with an increase in pressure, whereas the two superconducting transition temperatures along with the upper critical field decrease significantly. Our results suggest that the low-field superconducting state competes with the high-field antiferroquadrupolar ordered state.