Keywords

Single molecule magnets, berry phase interference, quantum tunneling of magnetization, spin dynamics

Abstract

The prominent features of single molecule magnets (SMMs), such as the quantum tunneling of the magnetization (QTM), are conventionally understood through the giant spin approximation (GSA) which considers the molecule as a single rigid spin. This model often requires the inclusion of high order anisotropy terms in the Hamiltonian, a manifestation of admixing of low lying excited states that can be more naturally understood by employing a multi-spin (MS) description i.e. considering the individual spins and the interactions between ions within the molecule. However, solving the MS Hamiltonian for high nuclearity molecules is not feasible due to the enormous dimensions of the associated Hilbert space that put it beyond the capability of existing computational resources. In contrast, low nuclearity systems permit the complete diagonalization of the MS Hamiltonian required to sample the effect of internal degrees of freedom, such as exchange interactions and single ion anisotropies, on the QTM. This dissertation focuses on the study of low nuclearity SMMs in view of understanding these subtle quantum effects. To accomplish this, we have developed a series of magnetic characterization techniques, such as integrated microchip sensors resulting from the combination of two dimensional electron gas (2DEG) Hall-Effect magnetometers and microstrip resonators, capable of performing measurements of magnetization and EPR spectroscopy simultaneously. The thesis bases on a comparative study of two low nuclearity SMMs with identical magnetic cores (Mn4 dicubane) but differing ligands. Notably, one of these SMMs lacked solvent molecules for crystallization; a characteristic that gives rise to extremely sharp resonances in the magnetization loops and whose basic QTM behavior can be well explained with the GSA. On the contrary, the second SMM exhibited mixed energy levels, making a MS description necessary to explain the observations. We have also examined the role of internal degrees of freedom on more subtle QTM phenomena, leading to the explanation of asymmetric Berry-phase interference patterns observed in a Mn4 SMM in terms of a competition between different intermolecular magnetic interactions, i.e. non-collinear zero-field splitting tensors and intramolecular dipolar iii interactions, resulting in astonishing manifestations of the structural molecular symmetry on the quantum dynamics of the molecular spin.

Notes

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Graduation Date

2012

Semester

Fall

Advisor

Gonzalez Garcia, Enrique

Degree

Doctor of Philosophy (Ph.D.)

College

College of Sciences

Department

Physics

Degree Program

Physics

Format

application/pdf

Identifier

CFE0004790

URL

http://purl.fcla.edu/fcla/etd/CFE0004790

Language

English

Release Date

June 2013

Length of Campus-only Access

None

Access Status

Doctoral Dissertation (Open Access)

Subjects

Dissertations, Academic -- Sciences, Sciences -- Dissertations, Academic

Restricted to the UCF community until June 2013; it will then be open access.

Included in

Physics Commons

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