Investigation of Molecule-Based Magnetic Materials via EPR Spectroscopy

This dissertation presents a series of electron paramagnetic resonance investigations on molecule-based magnetic materials. We begin with an introduction to the field of Single-Molecules magnets in the Ch. 1 introduction. In Ch. 2 we present angle-dependent high-frequency EPR studies on a single-crystal of a trigonal Mn_3^III cluster with an unusual structure in which the local magnetic easy-axes of the constituent MnIII ions are tilted significantly away from the molecular C3 axis towards the ‘magic-angle’ of 54.7 degrees, resulting in an almost complete cancelation of the 2nd-order axial magnetic anisotropy, DS ̂_z^2, associated with the ferromagnetically coupled total spin ST = 6 ground state. This contrasts the situation in many related Mn_3^III single-molecule magnets (SMMs) that have been studied intensively in the past, for which the local MnIII anisotropy tensors are reasonably parallel. As such, the tilted nature of this particular Mn_3^III compound, presented herein provides a unique opportunity to study how molecular geometry influences the interactions that are responsible for quantum tunneling of magnetization in high-symmetry SMMs. This is accomplished by angle-dependent EPR measurements that provide a full mapping of the molecular magneto-anisotropy. While chemists have routinely synthesized exchange coupled clusters consisting of transition metals in the context of SMMS for many years, only recently has the generation of magnetic exchange in lanthanide clusters been the subject of considerable interest. In Ch.3 we investigate a unique linear trinuclear lanthanide cluster, in which its neighbors are linked together by single carbon bridges via unique metallocene ligands. While Ch. 2 demonstrates a scenario in which a coupled geometry can be antagonistic to SMM behavior, here we show a trinuclear lanthanide cluster where the axial anisotropy is demonstrably larger compared to its analogous monomer sub-unit, suggesting the presence of a strong ferromagnetic exchange between neighboring lanthanides. As such, the study focuses on identifying the nature of anisotropy in the trimer from a series of powder EPR studies. Ch.4 continues on this line of investigation with regards to lanthanide SMMs, where we present magnetic resonance studies on an Yb(trensal) complex using a unique far infrared (FIR) spectroscopy set up. Previous AC susceptibility measurements of Yb(trensal) demonstrated a rather extreme scenario, where, despite possessing a relatively large zero-field energy barrier to magnetization reversal of ~900 cm-1¬, a Arrhenius fit to the temperature dependent relaxation yielded an unrealistic activation energy of just 38 cm-1. This suggests a rather sizeable contribution from Raman relaxation, which can be attributed to strong coupling between the crystal field and the vibration degrees of freedom available to the system. Here, we present frequency domain magnetic resonance (FDMR) measurements in the FIR range to spectroscopically characterize the spin-vibron coupling associated with the previously observed Raman relaxation in Yb(trensal). The last two chapters in this dissertation take quite a departure from the matter of SMMs, and as such are written to entirely be self-contained. In Ch.5 we present studies on bisdithiazolyl (and related) organic radicals. In the field of organic materials, these particular radicals are attractive to study since their magnetic properties can be carefully tuned by varying and substituting their chalcogen and halogen content. In this work, we focus specifically on heavy halide substitution in two families of molecule: ‘single-orbital’ IBSSEt (1) and ‘multi-orbital’ IBBO (2). Structural differences result in a ferromagnetic ground state in 1, while 2 orders as a spin canted antiferromagnet. Ferromagnetic resonance (FMR) measurements on 1 show that substitution of I for Cl has no effect on the exchange anisotropy (EA). However, antiferromagnetic resonance (AFMR) measurements show that substituting I for F has a dramatic effect on the EA. Our analysis demonstrates that the sensitivity to the halide’s larger spin-orbit coupling is dictated by the molecular orbitals that dominate the exchange. The last chapter presents instrumentation development in the context of Pulsed EPR. Recent advances in high-power microwave electronics allow for the integration of an Arbitrary Waveform Generators (AWG) into a high-frequency (94 GHz) spectrometer, providing a high level of control over the phase, frequency and amplitude of the excitation pulses over a bandwidth of up to 1 GHz. This makes fast/efficient EPR experiments feasible in the time domain, with dramatic improvements in sensitivity and resolution, providing access to microscopic information that is not accessible via previously existing methods. Here, we demonstrate preliminary results on a model radical and Gd(III) system to demonstrate the added benefits of pulse shaping afforded by the AWG.