Like water molecules in ice, the elements making up spin ice are organized into networks of tetrahedrons connected by their apexes. Each node is occupied by a magnetic rare earth ion (terbium, dysprosium, praseodymium, etc.). This particular structure is responsible for an astonishing property: these materials potentially possess an infinity of stable states (of minimal energy)! 

Physicists have found a formal analogy between the equation that describes the magnetic state of the system – the orientation of the spins as a function of their interaction – and the fundamental equation of quantum electrodynamics. They deduced from this the existence of a magnetic excitation spectrum of the spin ice, which had not been suspected until now. To demonstrate this, they selected a material containing a magnetic ion whose spin orientation is extremely sensitive to minute constraints induced by crystalline defects, such as the substitution of one ion for another. They chose the praseodymium ion Pr³⁺ within the compound Pr₂Zr₂O₇.

The study of this compound by polarized neutron scattering shows a level of crystalline defects induced by the chemical disorder around 0.001. The constraints associated with these defects can be evaluated, as well as their impact on the fluctuations in the spin orientation of the Pr³⁺ ions. In parallel, the spectrum of magnetic fluctuations associated with this disorder is observed starting from the inelastic neutron scattering spectra. The measurements are in perfect agreement with the predictions of the model.

This remarkable correspondence between theory and experimentation validates the established link between chemical disorder and spin dynamics, opening the way to the custom-made crystallogenesis of doped spin ice with targeted quantum properties.

This work was conducted in collaboration with the Léon-Brillouin Laboratory (Iramis) in Saclay, the Institut Néel (CNRS, Université Grenoble Alpes), the Institut Laue-Langevin (Grenoble), the Université Paris-Sud and the University of Warwick (Great Britain).