Hyperfine coupling in singlet ground state magnets

The influence of hyperfine coupling to nuclear spins and of their quadrupolar splitting on the induced moment order in singlet ground state magnets is investigated. The latter are found among non-Kramers f electron compounds. Without coupling to the nuclear spins, these magnets have a quantum critical point (QCP) separating the paramagnetic and induced moment regimes. The hyperfine interaction suppresses the QCP and leads to a gradual crossover between induced electronic and nuclear hyperfine coupling dominated magnetic order. It is shown how the critical temperature depends on the electronic and nuclear control parameters, including the nuclear spin size and its possible nuclear quadrupole splitting. In particular, the dependence of the specific heat on the control parameters and applied field is investigated for ferro- and antiferromagnetic order. It is shown that the three-peak structure in the electronic-induced moment regime gradually changes to a two-peak structure in the hyperfine coupling dominated nuclear moment order regime or for increasing field strength. Most importantly, the possibility of a reentrance behavior of magnetic order or likewise nonmonotonic critical fields due to hyperfine coupling influence is demonstrated. Finally, the systematic evolution of the phase diagram under the influence of nuclear quadrupole coupling is clarified.