Successive Phase Transitions And Anisotropic Magnetic Field-Temperature Phase Diagram In Ndru2al10

To investigate the successive phase transitions in NdRu2Al10, we carried out ultrasonic measurements on single-crystalline samples. We found that the elastic moduli show an abrupt hardening at 2.4K without hysteresis and a two-step softening at 1.3 and 1.0K with clear hysteresis in the temperature sweep. The hardening at 2.4K is very similar to that observed in CeRu2Al10 and CeOs2Al10, suggesting that the second-order transition at 2.4K originates from a similar mechanism of the transitions in CeRu2Al10 and CeOs2Al10. Both transitions at 1.3 and 1.0K would originate from an antiferromagnetic first-order spin ordering. We also clarified the anisotropic magnetic field H dependences of the transition temperatures in H along the a-, b-, and c-axes, and found a new phase above 2.5 T in H parallel to a.