Impact of hyperfine contributions on the ground state of spin-ice compounds

We examined the magnetic ground state of the pyrochlore spin-ice compounds Pr₂Hf₂O₇ and Ho₂Ti₂O₇ by means of specific-heat, magnetization, and ac-susceptibility measurements in the mK regime. At these low temperatures, we observe an unexpected large specific heat and corresponding entropy, which diminish in applied magnetic fields. This evidences the presence of additional states beyond the electronic spin and orbital degrees of freedom. We can qualitatively explain the large specific heat by the coupling of the nuclear spins of ¹⁴¹Pr and ¹⁶⁵Ho with their electronic counterparts, which leads to a complex hyperfine-coupled term scheme. With increasing fields, the nuclear and electronic spins decouple leaving only the electronic excitations in the measured temperature window. At intermediate fields, a rather evolved term scheme emerges that may explain the unusual hysteretic magnetization and a remarkable state with a negative magnetization found for Ho₂Ti₂O₇. Our findings bring deep insights to the complex ground state of pyrochlore spin-ice compounds and their low-energy excitations.