Elastic anomalies of YbIrGe in magnetic fields

The Yb-based heavy-fermion compound YbIrGe, which has the orthorhombic structure, shows the crystal electric field effect at high temperatures and antiferromagnetic orderings at T_N = 2.4 K and T_m = 1.4 K. We previously found anomalous elastic softening in zero magnetic field originating from an indirect quadrupole interaction between the ground doublet and the excited doublets, and determined the crystal electric field level scheme: the ground doublet and an excited doublet at 138 K. To investigate the antiferromagnetic orderings at T_N and T_m, we performed ultrasonic measurements under magnetic fields along the a-axis on YbIrGe single crystals. Temperature dependence of elastic modulus C₁₁ exhibits elastic hardening below both T_N and T_m at 0 T. As increasing the magnetic field along the a-axis, both T_N and T_m decrease monotonically, suggesting that both transitions are antiferromagnetic ordering. T_N closes around 2 T, and T_m disappears above 1 T. We clarified the magnetic field- temperature phase diagram along the a-axis