Elastic Softening in the Orthorhombic Compound YbPdGe

The Yb-based compound YbPdGe with the orthorhombic structure shows a ferromagnetic phase transition at 11.4 K. YbPdGe has localized 4f-electrons of Yb3+ ions and the crystal electric field (CEF) effect at high temperatures. On the other hand, a heavy-fermion state is reported at low temperatures, indicating that YbPdGe is a heavy-fermion ferromagnet. To investigate the phase transition at 11.4 K and the CEF effect, we carried out ultrasonic measurements on a single-crystalline sample. Characteristic elastic softening below 170 K is observed at high temperatures in the transverse elastic modulus C66 in contrast to the longitudinal modulus C22 without the softening. In the orthorhombic compound YbIrGe, the transverse moduli exhibit a similar elastic softening originating from the CEF effect, suggesting that the softening of C66 in YbPdGe is also caused by the CEF effect. At the phase transition, C22 and C66 show abrupt elastic hardening and softening, respectively, due to the magnetostriction.