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Slow Magnetic Relaxation in f-Element Complexes: A Tale of Dipolar Interactions, Symmetry, and Magnetic Hysteresis


This dissertation describes the results of research into some of the subtle and not-so-subtle factors that govern slow magnetic relaxation in lanthanide and actinide complexes. Single-molecule magnetism is only now entering its third decade, while the lanthanides and actinides in particular have only been a part of the field for about the last decade and half a decade, respectively. Within this time, the study of slow magnetic relaxation in f-element systems has proven to be a rich and complex area of research. Chapter 1 sets out to provide some historical context for the state of the f-elements in the field today as well as to highlight some of the most impressive systems in the literature. Some of the themes introduced in Chapter 1 will be relevant to the research discussed in subsequent chapters.

Chapter 2 describes the detailed magnetic characterization of U(H2BPz2)3, only the second actinide molecule found to exhibit slow magnetic relaxation, in magnetically dilute form. The results of variable-temperature and variable-field ac susceptibility measurements reveal that dipolar interactions are implicit in speeding up molecular slow relaxation and also facilitate very slow intermolecular relaxation that leads to magnetic hysteresis at low temperatures. When placed in the context of some relevant literature, the results of this study suggest that the intermolecular relaxation and observed hysteresis may be unrecognized, though common, phenomena among low-nuclearity U(III) molecules exhibiting slow magnetic relaxation.

Chapter 3 describes a detailed study of the role of donor atom influence and dipolar interactions on slow magnetic relaxation in two series of uranium and lanthanide compounds with nitrogen and carbon donor atoms, respectively. Through a combination of magnetic susceptibility characterization and lanthanide M5,4-edge XANES, EPR, and 1H NMR spectroscopies, it is found that the carbon donor facilitates slower magnetic relaxation for all metal ions in the investigated temperature and frequency range. Thus, in addition to symmetry, the identity of the donor atom is revealed to be another tunable parameter in the design of f-element complexes exhibiting slow magnetic relaxation.

Chapter 4 describes full magnetic characterization of concentrated and magnetically dilute samples of the bis(cyclooctatetraenide) complex [Er(COT)2]−. The high symmetry ligand field afforded by the two (COT)2− groups leads to exceptionally slow magnetic relaxation for Er(III), with magnetic blocking at 9.25 K and magnetic hysteresis as high as 10 K. Magnetic dilution also leads to an unprecedented opening of the hysteresis loop for this molecule at low temperatures, demonstrating that dipolar or intermolecular interactions can affect slow magnetic relaxation in some anomalous ways, by analogy with Chapter 2.

Finally, Chapter 5 changes gears slightly and investigates slow magnetic relaxation in the series of (N2)3− radical-bridged complexes {[(Me3Si)2N)2Ln(THF)]2(μ-N2)K} (for Ln = Gd(III), Tb(III), Dy(III)). Inner-sphere coordination of the K+ counter-ion in these complexes leads to bending of the Ln-radical-Ln unit, which is planar in the previously reported parent complexes {[(Me3Si)2N)2Ln(THF)]2(μ-N2)}− exhibiting an outer-sphere potassium ion. While the parent complexes hold records for both blocking temperature and magnetic coupling strength, bending of the core in {[(Me3Si)2N)2Ln(THF)]2(μ-N2)K} introduces non-negligible intramolecular lanthanide-lanthanide coupling that competes with the lanthanide-radical interaction. This competition results in depressed magnetic moments and faster magnetic relaxation for the anisotropic lanthanides, revealing that even a seemingly simple counter-ion can be used to tune slow magnetic relaxation.

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