Personal web page : http://iramis.cea.fr/Phocea/Membres/Annuaire/index.php?uid=spetit
Laboratory link : http://www-llb.cea.fr/
Magnetic frustration is one of the modern routes in condensed matter physics leading to the discovery of new states of matter. The “spin ice” and more generally, the “Coulomb phases” are celebrated examples of this physics. In contrast with classical magnetically ordered phases, these states remain disordered down to the lowest temperatures, yet form a correlated paramagnet with specific spin-spin correlations. In this context, a new concept has been recently proposed, called “magnetic fragmentation” [PRX 4, 011007 (2014)]. This is an original state where the magnetic moment fragments into two sub-fragments: one of them forms an antiferromagnetic phase with a reduced ordered moment, while the other keeps fluctuating and forms a Coulomb phase.
In combining magnetization measurements, elastic and inelastic neutron scattering experiments, we have shown that the pyrochlore compound Nd2Zr2O7 could be a realization of this theory [1,2], even if experimental evidences suggest that still not understood quantum phenomena are at play.
This thesis work aims at understanding the origin of fragmentation in this system. We especially plan to determine its stability range by studying doped samples. Actually, replacing part of the Zirconium (Zr) by Titanium (Ti), or Neodymium (Nd) by Lanthanum (La), magnetic interactions can be modified. Varying the substitution, we will explore the phase diagram and probe the possible existence of a quantum critical point predicted by theory. The complementarity between macroscopic and neutron scattering measurements is one of the key points to determine the quantum Hamiltonian and beyond, understand the microscopic mechanisms of magnetic fragmentation, along with the nature of the spin dynamics that emerge from this peculiar ground state.
The thesis work will take place in France both at the Institut Néel (Grenoble) and at LLB (Saclay). It consists in measuring both the magnetization and specific heat down to base temperature (100 mK) (Institut Néel) and to finely determine the magnetic structures as well as the spin excitations spectrum by the different neutron techniques. The latter will be carried out at LLB (Saclay) and at ILL (Grenoble). A large part of the data analysis will be based on numerical simulation tools. Most of them exist today but may be further developed.