Thesis presented September 25, 2009

Abstract:
This PhD thesis is about geometrically frustrated magnetic materials which crystallise in the pyrochlore structure, made of corner sharing regular tetrahedra. Firstly, we show that the growth of the spin liquid Tb₂Ti₂O₇ presents many difficulties: some physical properties are sample dependent. Anyhow, two characteristic temperatures (2 and 50 K) are deduced by different experimental techniques, irrespective of the sample. Then, about the ordered spin ice Tb₂Sn₂O₇, the study of spin dynamics suggests the existence of a complex relaxation mechanism, characterized by several fluctuation times. One of them is t = 10^-10 s. Microscopic and macroscopic measurements carried out on these two compounds show that the lowest two crystal field levels are singlets, and they are separated by an energy of about 2 K. About the XY antiferromagnet Er₂Ti₂O₇, some results suggest the existence of excitations, responsible for low temperature spin dynamics. Finally, there are inconsistencies between the interpretation of some experimental results about the existence of a low temperature spin dynamics in the Heisenberg antiferromagnet Gd₂Sn₂O₇.

Keywords:
Geometric frustration, phase transition and dynamic order

On-line thesis. ​