Anomalous Antiferromagnetic Phase Diagram In Horu2al10

To investigate an antiferromagnetic (AFM) phase transition of HoRu2Al10 with T-N = 5.2 K, we performed specific heat measurements on a single-crystalline sample under magnetic fields H. In H along the b- and c-axes, a sharp peak of the specific heat at TN, indicating the phase transition, shifts to lower temperature with increasing H. From these measurements, we clarified a magnetic phase diagram for H // b and c above 2 K. In H // b, TN decreases monotonically with increasing H, and then the phase boundary closes around 1.0 T, which is a usual behavior for an antiferromagnet. In H // c, although TN shows a monotonic decrease with increasing H at low H, the phase boundary exhibits an inflection point around 4.0 T, suggesting that additional interaction are enhanced or induced by H // c in HoRu2Al10. From the stand point of fluctuation of quadrupole moments in isomorphic compound HoFe2Al10, we propose that the inflection point is originated from a quadrupolar instability.