Elastic softening due to the quadrupole interaction and anomalous magnetic phase diagram under the magnetic field in HoRu2Al10

To investigate an influence of the quadrupole interaction to an antiferromagnetic phase transition at TN=5 K in HoRu2Al10 under magnetic fields H, we measured the specific heat, magnetization, and elastic moduli. From these measurements, we clarified the magnetic phase diagram for the three principal axes. Under H along the b axis, TN decreases monotonically with increasing H. Although TN in H along the a and c axes is also reduced with increasing H, the phase boundary shows an anomalous inflection point around 5 T. With increasing H, we found an enhancement of elastic softening toward TN due to the quadrupole interaction in the longitudinal and transverse moduli under H||c. We proposed the 4f-level scheme in HoRu2Al10, of which the ground state singlet and the first excited singlet at 0.5 K form a quasidoublet, from orthorhombic crystal electric field analyses. An interlevel quadrupole transition exists between the ground and exited singlets under fields. These suggest a possibility that the quadrupole interaction plays a central role for the inflection point in the phase diagram.