Ultrasonic Study on the Hexagonal Antiferromagnet Dy₃Ru₄Al₁₂

In the distorted kagome lattice antiferromagnet Dy₃Ru₄Al₁₂ with T_N = 7 K, a crystal electric field (CEF) effect is expected at high temperatures. To investigate the CEF effect and the phase transition at T_N, we performed ultrasonic measurements on a single-crystalline sample. At high temperatures, both the longitudinal elastic modulus C₁₁ and the transverse modulus C₄₄ increase monotonically with decreasing temperature. Below 60 K a characteristic elastic softening is observed in C₄₄ in contrast to C₁₁ with monotonic hardening down to T_N. We analyzed C₄₄ using the Curie-Weiss-type equation and obtained a negative parameter: Θ which is proportional to a quadrupole-quadrupole coupling constant under the hexagonal CEF. With further decreasing temperature, both moduli exhibit abrupt elastic hardening at T_N due to a magnetostriction.