Geometric magnetic frustration in rare earth spinels and stuffed rare earth pyrochlores

Geometric magnetic frustration occurs when the physical arrangement of electron spins prevents the simultaneous satisfaction of all nearest neighbor interactions. Degeneracy of energetically equivalent magnetic configurations can lead to complex ordering schemes and structural distortions. This thesis focuses on the synthesis and properties of new geometrically frustrated magnets containing lattices of corner sharing tetrahedra.;The second strategy involves doping extra Ho onto the non-magnetic Ti site to form the continuous solid solution Ho2(Ti2-xHo x)O7-x/2 (0 ≤ x ≤ 0.67). The 'stuffing' of additional Ho atoms alters the original lattice of corner-sharing tetrahedra by introducing new Ho interactions through edge-sharing tetrahedra. Despite the increase of spin connectivity, the measured spin entropy remains unchanged from the parent Ho2Ti2O7 compound. It is shown that short ranged pyrochlore ordering exists in stuffed spin ice, and that the details of the local ordering are complicated. Solid solutions of the smaller rare earth titanate pyrochlores exist for Tb through Lu, and short range ordering is likely present in each of these series. Single crystal growth of stuffed spin ice and other geometrically frustrated magnets are discussed.;Pyrochlore materials with general formula A2 B2O7 possess two distinct interpenetrating sublattices of corner-sharing tetrahedra with one comprised solely of A cations and the other with B. In Ho2Ti 2O7, magnetic frustration of the Ho3+ spins on the A lattice are known to mimic the positional frustration of hydrogen atoms in water ice, and is thus termed 'spin ice.' This thesis explores two ways to chemically study the magnetic properties of spin ice. The first involves the synthesis of rare earth containing materials with the closely related spinel and olivine structures. The cubic spinel structure also contains a magnetic sublattice of corner-sharing tetrahedra. However, the local coordination around the rare earth atoms is different between spinel and pyrochlore structures. It is shown that CdLn2S4 and ZnLn2S4 (Ln = Ho-Yb) possess geometric magnetic frustration with no long range order down to 2 K, and that the potential exists for more interesting magnetic behavior at lower temperatures.