Étude des propriétés magnétiques des aimants frustrés Ba(Dy,Ho)2O4 et SrHo2O4 par diffusion de neutrons

Frustrated magnets are systems where localized magnetic moments interact through competing exchange interactions, creating a degeneracy of the ground state. While normally introduced by the geometry of the systems, the frustration can originate from the delicate balance of the strength of the interactions J1–J2 between the nearest and the next-nearest neighbours. The systems studied belong to the AkLn2O4 family (where Ak = alkaline earth metal and Ln = lanthanide) for which the lattices of magnetic rare earth ions are built from hexagonal tiles in the ab-plane, and zigzag chains along the c-axis. In this structure, each rare earth ion is located inside a differently distorted oxygen octahedron, resulting in two inequivalent rare earth sites with a distinct crystal field environment. It has been argued that the unusual magnetism in these materials originates from the chains, which can be described as an effective one-dimensional system.   Using novel techniques in neutron diffraction and spectroscopy, it is possible to probe the magnetic interactions between the atoms within a system and to determine the excitations of the ground states. Measurements were performed with the new polycrystalline samples Ba(Dy,Ho)2O4, prepared by solid state reaction. Combined with the detailed calculation of the crystal electric field, it was possible to demonstrate the independence of the magnetic orders located on each rare earth site. Thus, the refinement of the magnetic structures of both system shows the coexistence of two types of long range orders. In the two cases, a vast diffuse scattering is observed, which was modelled using a one- dimensional interaction model between the nearest and next-nearest neighbours. These correlations persist at temperatures well below the magnetic transitions temperatures, a sign that there are sites where magnetic order and fluctuations are in cohabitation.   Another characteristic of this family of compounds is the assumption of field induced magnetic phase. In SrHo2O4, measurements of the magnetic field dependence of the magnetization show the presence of a large plateau located at one third of the total magnetization. This induced phase was characterized using neutron diffraction on single crystals grown with an optical floating zone furnace. Indeed, the application of a magnetic field uncovers a transition of one of the magnetic sites from a double Néel to a short range intermediary phase. The refinement of the structure, complex due to the short range nature of the interactions, suggests on one hand the presence of a non-uniform magnetization of a site, and on the other, the presence of a transversal magnetic moment.   The first chapter of this thesis reviews the concepts of theoretical magnetism to de- fine the frustration. The next chapter is a short literature review of the systems studied, outlining the principal characteristics of the crystalline and magnetic structures, as well as the development of plateaux in magnetization. The third chapter presents the characterization of the magnetic structure of BaDy2O4 and BaHo2O4 through neutron diffraction and neutron spectroscopy. At last, the magnetic field-induced phase in SrHo2O4 is described.

Keyphrases: Condensed matter physics, Magnetism, Magnetic frustration, Neutron scattering, Neutron diffraction, Spectroscopy, Crystalline electric field, Low-dimensional magnetism, Phase transition, Crystal growth