The hyperfine coupling may either stabilize a magnetic order, when they align with the ordered moments, or destabilize it, if the coupling to the components transverse to the ordering moment dominates. Only the first form has been observed experimentally. For example, in certain rare earth magnets, this is signaled by an upturn of the critical transverse field at low temperatures.
We understand when and where to expect the opposite where hyperfine coupling destabilizes an existing order. This requires the spin interactions to favor an order away from the easy axis. This then enables the hyperfine interactions, which are strongest along the easy axis, to compete with the magnetic order. We have established that this arises with dipolar coupled RE materials on certain crystal lattices. In a representative H-T phase diagram of a tetragonal system we show the phase boundary modifications due to hyperfine coupling. Here, by weakening the hyperfine coupling (e.g. simply by heating above the HF energy scale), one can induce magnetic order close enough to a quantum phase transition.
Laboratory for Scientific Computing and Modelling