Abstract
In dhcp Pr, the Pr ions (J=4) experience a crystal field which produces singlet ground states at both the locally hexagonal and locally cubic sites: the coupling between the ions on the hexagonal sites is just below the critical value for the development of a magnetically ordered phase at low temperatures. The magnetic excitations propagating on the hexagonal sites are composed of linear combinations of singlet-doublet transitions. The lowest energy mode is along TM and has a pronounced minimum at a wavevector of 0.25 Å−1. The large orbital angular momentum of Pr leads to a strong magnetoelastic coupling between the spin and lattice systems. From the field dependence of the excitation energies, Jensen1 deduced the strength of this coupling and suggested that magnetic ordering might be induced by an appropriate uniaxial stress. In a series of neutron scattering studies at the Institut Laue-Langevin, we have observed a dramatic stress dependence of the magnetic excitations2. With increasing stress along an a-direction, the lowest energy mode (longitudinal optic) exhibits a clear soft-mode behaviour. Above a critical stress, elastic satellite reflections corresponding to long-range magnetic ordering in a longitudinally modulated structure were observed. This critical stress is 800 bar (80 MPa) at 7.5 K.
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