Low frequency dependent elastic modulus in UCo_1-xOs_xAl

UCoAl with the hexagonal ZrNiAl-type structure shows an itinerant metamagnetic phase transition at 0.7 T in the magnetic field along the c-axis. Whereas, this compound becomes to undergo a ferromagnetic phase transition in zero field by substituting Os for Co. UCo_0.995Os_0.005Al exhibits the ferromagnetic phase transition at T_C = 8 K. At high temperatures, the temperature dependence of the transverse elastic modulus C₄₄ in UCo_0.995Os_0.005 Al shows a slight softening below 50 K. The softening turns to an abrupt hardening below 30 K. With further decreasing temperature, a bending is observed at T_C. Although T_C changes sensitively by applying the magnetic fields, the temperature of the elastic hardening is robust in the magnetic fields. This elastic hardening is not caused by a magnetic origin. We measured ultrasonic frequency dependences of C₄₄ and found that the temperature of the elastic hardening increases with increasing ultrasonic frequency. We propose that this ultrasonic frequency dependence is due to a large amplitude atomic motion of constituent atoms.