Pressure Effect on the Chiral Helimagnetic Order in YbNi3Al9

The monoaxial chiral magnet YbNi3Al9 undergoes a chiral helimagnetic (CHM) order at around T-M = 3.4 K. By applying magnetic field perpendicular to the chiral c-axis, the magnetic phase transition to the forced ferromagnetic state manifests itself at around B-c = 0.1 T. In this work, we have studied pressure effect on the CHM order by measuring magnetization and electrical resistivity for single crystals of YbNi3Al9 under hydrostatic pressures P up to 2.5 GPa and magnetic fields B. We have found that both T-M for 0 T and B-c for 0.3K monotonically decrease by applying pressures in the ratios of -0.22K/GPa and -0.014 T=GPa, respectively. The suppression of the Kondo scattering against pressures suggests the effective increase of Ruderman-Kittel-Kasuya-Yosida interactions. We discuss the possible origin of the reductions in T-M and B-c in relation to the mean-field theory on monoaxial chiral magnets. Two times larger decreasing ratio in B-c(P)/B-c(0) than that in T-M(P)/T-M(0) is also discussed.