Elastic Softening and Phase Transition Characteristics in YbPtGe

YbPtGe with the orthorhombic ɛ-TiNiSi-type structure is reported as a heavy-fermion ferromagnet with TC = 5.4 K. Recent research suggested that its magnetic anisotropy can be ascribed to the crystal electric field (CEF) effect which acts on the eight-fold multiplet of Yb3+ ions. In order to clarify the CEF effect and to investigate the magnetic phase transition in this compound, ultrasonic measurements of YbPtGe and related theoretical calculation have been performed. The transverse elastic modulus C66 exhibits a softening characteristic with a peak at 180 K while the longitudinal modulus C33 shows a continuous hardening in the whole temperature range. According to the theoretical strain-susceptibility fitting of the transverse elastic modulus C66 by using the CEF parameters reported by Katoh et al., we argue the softening at higher temperatures (>50K) to be a result of quadrupole interaction between the CEF ground and excited Kramers doublets. With further decreasing the temperature below TC, C66 mode exhibits a step-like softening which is quite different from that of the C33 mode and the possible mechanism is discussed.