YbNi₃Al₉ crystalizes in the trigonal ErNi₃Al₉-type structure with a space group of R32. This compound undergoes the chiral helimagnetic (CHM) order at T_M = 3.4 K. According to the chiral sine-Gordon model, the CHM order is realized by the competition between the Dzyaloshinskii-Moriya interactions and the ferromagnetic (FM) interactions, both of which work along the helical axis. By applying magnetic field perpendicular to the helical c-axis, the magnetic phase transition to the forced FM state manifests itself at B_c = 0.1 T in association with the divergence of the helical pitch.
In this work, we have studied pressure effect on the CHM order in YbNi₃Al₉ by measuring magnetization, electrical resistivity, and magnetoresistance under hydrostatic pressures. Both of T_M for 0 T and B_c for 0.3 K monotonically decrease by applying pressures up to 2.5 GPa in the ratios of -0.22 K / GPa and -0.013 T / GPa, respectively. By the application of pressures, the ordered moment increases and the Kondo scattering is suppressed, suggesting that the Ruderman-Kittel-Kasuya-Yosida interaction dominates the Kondo effect. Based on these results, we attribute the reduction in T_M and B_c to the destabilization of helical structures owing to the development of the FM correlations.