Role of Hyperfine Coupling in Magnetic and Quadrupolar Ordering of Pr₃Pd₂₀Si₆

We study the ternary clathrate Pr₃Pd₂₀Si₆ in specific heat and ac susceptibility measurements on a highquality single crystal, distinguishing antiferromagnetic and antiferroquadrupolar ordering, as well as a hitherto unknown magnetic low-temperature transition. The specific heat shows the direct involvement of nuclear spin degrees of freedom in the antiferromagnetic ordering, which is well supported by our calculation of the hyperfine level scheme without adjustable parameters. Pr₃Pd₂₀Si₆ is, therefore, one of the rare materials where the nuclear moments are involved in the formation of the magnetic ground state.