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Séminaire MCBT : Mardi 1 juin 2021 à 11h

En visio sur (webcast @) : https://univ-grenoble-alpes-fr.zoom.us/j/91904353692?pwd=Sm5uTnpFVk1waXBvZG5Ka3RjYnVrQT09

Mélanie Leger, UGA/Institut Néel


Field-temperature phase diagram of the enigmatic Nd2Zr2(1-x)Ti2xO7 pyrochlore magnets

In pyrochlore compounds, where the magnetic atoms are located on the vertices of a network of corner-sharing tetrahedra, the combination of strong Ising anisotropy along the local <111> axes with ferromagnetic interactions leads to a local organizing principle, the so-called ice-rule : two spins point in and two spins point out in each tetrahedron. The resulting spin ice state is called a Coulomb phase because the ice-rule can be interpreted as a divergence free condition of an emergent gauge field.
In the Nd2Zr207 pyrochlore, a positive Curie temperature indicates the existence of ferromagnetic interactions. Together with the Ising nature of the Nd3+ ion, a spin ice ground state should be stabilized. Polarized neutron scattering actually reveal the expected spin-ice pinch point pattern between 1K and 300mK. However, the spin-ice phase gives way to a partial AIAO antiferromagnetic ordering below 300mK.
To further elucidate the nature of these peculiar phases, we have studied the role of defects, substituting titanium instead of zirconium to affect the magnetic interactions. We show that defects have a low impact on the crystal electric field scheme. We establish the (H,T) phase diagram of these doped compounds by combining neutron scattering and magnetization measurements down to 80mK for the three high symmetry directions of applied field : [001], [1-10] and [111]. Our results show that the AIAO phase is reinforced by doping (larger Néel temperature and larger critical field) and that the critical fields are far lower than predicted by theory.