| Summary: | Gd3Ga5O12 (GGG) is an insulating Heisenberg antiferromagnet. It is highly frustrated owing to its garnet structure, which is composed of elementary triangles with dominant antiferromagnetic interactions. We studied the effect of disorder in this geometrically frustrated system by changing the level of disorder in GGG:Ndx with x ranging from 0 to 1%. We measured the AC magnetic susceptibility in both the linear and nonlinear regimes and characterized the low temperature phases.
As a result, we observed signatures of short range order in the 0% and 0.1% crystals, but not in the 1% sample, indicating that the order is suppressed in the 1% sample because of increasing frustration. The Nd doping compensates for the Gd-Ga off stoichiometry and pushes the system to a more perfectly frustrated state. We used the non-linear response to characterize the evolution of isolated spin clusters as a function of x. The Nd doping effectively relieves the net disorder. Hence the sample with the highest doping presents the smallest net correlated moments and the smallest onset field to activate the clusters from the background spin bath.
The second project we report addresses the nonequilibrium dynamics of two related spin glasses. The model systems are the Ising magnets LiHo xY1--xF4 with x = 0.167 and 0.198. Although both systems undergo spin glass transitions, the mechanism that drives the phase transition is known to be different. The diluted sample is more affected by entanglement and in the concentrated system, the random field plays a more important role.
In this thesis, we performed preliminary thermal and quantum aging experiments to compare the two systems. In the classical aging measurement, with temperature as the pertinent variable, the decay is exponential and measures primarily the response of the lattice. However quantum aging measurements using transverse field probe the spin relaxation character. The x = 0.167 and 0.198 crystals decay in opposite directions, reflecting the different natures of their ground states.
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