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Yann Alexanian presents

Couplings between different degrees of freedom in magnetic pyrochlores 

Friday 10th of December 2021 at 2:00 pm 

Seminar room, Building A, Institut Néel : number of person limited to 30

Lien visio : https://univ-grenoble-alpes-fr.zoom.us/j/98489722612?pwd=U09vc21lTWRCcnFpclVBSHFQOE45UT09

ID de réunion : 984 8972 2612
Code : 160588

The presentation and the slides will be in French.


Abstract :

In the magnetic pyrochlore oxides R2M2O7 where R3 + is a rare earth (Tb3 +, Ho3 +) and M4 + a transition metal (Ti4 +, Ir4 +, Nb5 + / Sc3 +), the pyrochlore lattice associated with R, the main magnetic element, is a corner sharing tetrahedra network which induces complex behaviors due to strong magnetic frustration effects. This thesis is focused on the couplings between different degrees of freedom (spin, orbit, charge, lattice) and their influence on the ground states, excitations and physical properties of these compounds. Several complementary techniques are used: specific heat, magnetometry, electrometry, synchrotron based THz spectroscopy, neutron scattering and numerical simulations.

The first study is devoted to dielectric and magnetoelectric effects in R2Ti2O7 (R3+ = Ho3+, Tb3+). Several contributions are observed and compared to the model proposed by Khomskii where a local electrical polarization is associated to magnetic monopoles in spin ices. Monte-Carlo simulations suggest the importance to go beyond this interpretation to understand all the observations.

The second study examines the magneto-elastic interactions in Tb2Ti2O7, a compound where the nature and origin of the ground state are still not understood. High-resolution THz spectroscopy measurements on synchrotron and under strong magnetic field have been performed and revealed the fundamental importance of these interactions in the low-temperature physics of this compound. A link between the amplitude of spin-lattice hybridizations and the nature of the ground state, spin liquid or quadrupolar, has been established by comparing samples of slightly different stoichiometry and numerically modeling of these couplings.

The following study is devoted to Tb2Ir2O7 which, unlike the other studied pyrochlores, has an Ir4 + magnetic ion at the site of the transition metal. Its magnetic properties are studied by elastic and inelastic neutron scattering but also by a numerical approach. It is shown that the complex interactions between and within the two magnetic sub-lattices of the system are responsible for the unconventional magnetic order observed at low temperature. The role of quadrupolar interactions is specifically discussed.

The last study is preliminary and is dedicated to the compound Tb2ScNbO7. Macroscopic magnetic measurement and neutron scattering are used to study the influence of the Sc3+ / Nb5+ charge disorder on the magnetic properties and the ground state.


Supervisors : 
S. De Brion (Institut Néel)
V. Simonet (Institut Néel)

Jury’s Members :
F. Bert (Université Paris-Saclay, rapporteur)
M. Cazayous (Université de Paris, rapporteur)
P. Dalmas de Réotier (CEA Grenoble, examinateur)
P. Foury-Leylekian (Université Paris-Saclay, examinateur)L. Jaubert (Laboratoire Ondes et Matière d’Aquitaine, examinateur)
R. Ballou (Institut Néel, invité)