- Home
- Institut Néel
- Research teams
- Technical Groups & Services
- Work at the Institut
- Partnerships
The defence will be in English.
Abstract: Frustrated magnetism focus in magnetic system in which the interactions between magnetic
moments (spins) cannot be simultaneously satisfied, due to the geometry of the lattice or competition
of interactions. This frustration prevents from the stabilization of conventional magnetic order at low
temperature and favors the emergence of exotic states such as spin ices or quantum spin liquids. Among
geometrically frustrated lattices, the pyrochlore lattice, composed of corner-sharing tetrahedra, hosts a
large number of original ground states. Experimentally, this lattice is realized in rare-earth oxides with
the general formula R2M2O7, where R3+ is a rare-earth ion and M 4+ a metallic ion. In this thesis, we
focus on two cases: the spin ice state with R = Ho, Dy and the dipolar-octupolar states with R = Nd.
In the spin ice state, effective ferromagnetic interactions impose a local constraint, called the ice rule,
which generates a macroscopically degenerate correlated state. When thermal energy is large enough,
the ice rule can be broken locally, generating fractionalized excitations: the magnetic monopoles. These
magnetic charges can propagate almost freely through the lattice, interacting with each other via Coulomb
interactions. Their complex dynamics have been extensively studied in Dy2Ti2O7 and, to a lesser extent,
in Ho2Ti2O7, and remain poorly understood. At very low temperature, it has been observed that the
dynamics slow down drastically, resulting in a freezing in magnetization measurements, suggesting an out-
of-equilibrium state. The first part of this thesis focuses on the strongly correlated dynamics of monopoles
down to very low temperature (150 mK) and on the nature of this out-of-equilibrium state by probing
the fluctuation-dissipation theorem. It relies on the design and optimization of a cryogenic experiment
allowing the simultaneous measurement of the ac susceptibility and the magnetic noise associated to the
monopoles. Three regimes of temperature and frequency have been identified: an equilibrium regime, a
regime where local equilibrium is preserved, and an out-of-equilibrium regime where deviations from the
fluctuation-dissipation relation are observed. This experiment also made it possible to demonstrate and
quantify the role of demagnetizing field effects on magnetic noise measurements.
In the second part, this thesis studies the dipolar-octupolar pyrochlore compounds Nd2Ru2O7 and
Nd2Ir2O7. In these systems, crystal electric field effects confer unusual symmetry properties to the rare-
earth ion, generating both dipolar and octupolar magnetic components which can couple to each other.
These couplings act as a quantum perturbation allowing the realization of new magnetic phases. In the
family of Nd-based pyrochlores, a mixed dipolar-octupolar all-in/all-out order is observed. The objective of
this study is to understand the effect of a magnetic metallic ion (M = Ir, Ru) on this state. For this purpose,
neutron scattering and low-temperature magnetization measurements were carried out and analyzed by
determining the symmetry-allowed Nd-M interaction terms. The parameters of the Hamiltonian were then
estimated by numerically optimizing the model parameters in comparison with the experimental data
