with a chiral crystal structure: AC-calorimetric measurements up to 12 GPa*

YbNi₃X₉ (X=Al, Ga) crystallize in the trigonal ErNi₃Al₉-type structure with a chiral space group R32. YbNi₃Al₉ undergoes a magnetic transition at T_M=3.4 K. By substituting Cu for Ni, a chiral soliton lattice (CSL) is realized under magnetic fields B⊥c. While YbNi₃Ga₉ resides in an intermediate-valence regime under ambient pressure, application of pressure P is expected to drive this compound into a magnetic ordered state. Indeed, a magnetic order above P_c=9 GPa was inferred from the electrical resistivity and AC magnetic susceptibility measurements.
In this work, we have investigated the magnetic order in YbNi₃Ga₉ using AC-calorimetric measurements under P up to 12 GPa. With applying P, the Sommerfeld coefficient dramatically increases and reaches to 1 J/K²mol at 8.6 GPa≈P_c. A broad maximum in C/T at 1.6 K for P=9.3 GPa>P_c shifts to higher temperatures and becomes a sharp λ-type peak at 5 K for P≥11 GPa. The sets of data of C(T, P) under B//c and B⊥c at P≧11 GPa revealed that another field-induced ordered phase (FIOP) appears only for B⊥c. We discuss the origin of the FIOP in relation to the CSL for Yb(Ni_1−xCu_x)₃Al₉ and A-phase for MnSi and EuPtSi.