Fermer le menu



Mélanie Léger presents

Quantum fragmentation in frustrated magnetic systems

Thursday, 9th December 2021 at 9:30 am 

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

Zoom link : https://univ-grenoble-alpes-fr.zoom.us/j/92724765869?pwd=bTVmYmZ2RjlqdEFURG9hdCs0MHQxUT09

Meeting ID : 927 2476 5869
Code : 042514

The presentation will be in French and the slides will be in English.


Abstract :

In this thesis, we are interested in frustrated magnetism. Frustration is indeed able to induce a very rich physics, with original phenomena and in particular the stabilization of new ground states. We are more specifically interested in compounds with a pyrochlore structure (built on the basis of tetrahedra connected by their vertices). Magnetic ions are rare earth ions and are located at the vertices of these tetrahedra.

In this context, the compound Nd2Zr2O7 has aroused a particular attention. In spite of ferromagnetic interactions which should stabilize a spin ice state, a partial antiferromagnetic order is stabilized. This order is called “all in all out”, as all spins point into or out of a given tetrahedron. Moreover, measurements of the excitations show the coexistence of a flat mode at non-zero energy and characterized by spin ice correlations, with dispersive modes located at higher energy. The theoretical framework, likely to explain this distinction between modes (and more generally the physics of the compound Nd2Zr2O7) is that of the “dynamical fragmentation”. In order to better understand this phenomenon, it is interesting to investigate the effects induced by the presence of defects. In my thesis, I study the impact of two different substitutions: titanium instead of zirconium and lanthanum instead of neodymium. The goal of this thesis is to examine the evolution of the exchange parameters in these different systems as well as the evolution of the crystal field properties asscoiated with the modification of the environment of the Nd3+ ion. We also seek to understand the evolution of the dynamical fragmentation as well as of the ground state, through on the one hand, the construction of field and temperature phase diagrams and, on the other hand, the study of magnetic excitations.

This study was carried out by means of magnetization and neutron scattering measurements at very low temperature (<100 mK) as well as numerical analyses allowing to compare the experimental results with theoretical models.


Supervisors : 
Elsa Lhotel and Sylvain Petit

Jury’s Members :
Alain Pautrat
Fabienne Duc
Ludovic Jaubert
Nicolas Rougemaille
Grégory Chaboussant