Ultrasonic Investigation of Magnetic Ordering with Higher-Order Interactions in the Cage-Structured Compound U3Pd20Si6

Abstract

The elastic properties of localized 5f-electron system U3Pd20Si6 were studied by means of ultrasonic measurements under magnetic fields. We have measured elastic constants C11, (C11-C12)/2, C44 and CL110 = CB + (C11-C12)/6 + C44 in order to check a possible contribution of electric quadrupoles on the magnetically ordered phases of this compound for Γ1 ⊕ Γ3, Γ3, Γ5 and Γ1 ⊕ Γ3 ⊕ Γ5 symmetries elastic response, respectively. We discovered that an elastic anomaly, which is accompanied by ultrasonic attenuation maximum of Γ5- symmetry mode, at around T* ~ 16 K in zero magnetic field and shifts to lower temperatures with increasing magnetic fields. Since the T* seems to be continuously connected to the lower boundary of a 'spin-flop' region in the magnetic field-temperature phase diagram of U3Pd20Si6, we conclude that a fluctuation of the spin-system of the 8c-site ion exists even just below TN and couples to elastic wave via magneto-elastic coupling with Γ5-symmetry quadrupoles of U's 5f electron or local charge distributions of the guest U ion.