Study of Rare-Earth Magnetism Through Spectroscopic Studies of Lanthanide-Based Single Crystals

This dissertation presents a study of rare-earth magnetism using spectroscopic techniques. The features of strong spin-orbit coupling and the presence of hyperfine coupling which lead to coupled electro-nuclear crystal field states are typical of lanthanides, and contribute to distinct magnetic properties. In the work reported here electron paramagnetic resonance (EPR) and neutron scattering techniques have been employed for investigating the ground state magnetic properties of a few lanthanide-based single crystalline sample. The two kinds of samples investigated in the course of this work include single molecule magnets (SMMs) and geometrically frustrated systems. EPR studies on mononuclear lanthanide-based SMMs, which have attracted considerable attention due to their potential application in spintronic devices, form a major portion of the research reported in this dissertation. In these systems the magnetization is associated with a single rare-earth ion (holmium) which facilitates mitigation of spin decoherence due to nuclear hyperfine and electron dipolar interactions by dilution and isotope purification. High frequency EPR studies on HoPOM indicate considerable transverse spin orbit anisotropy, which considerably affects the magnetization relaxation properties by giving rise to a tunneling gap between excited electro-nuclear spin states. Electron spin echo measurements at 9 GHz demonstrate long relaxation times (~ 100 ns) for concentrated samples, with much longer values for diluted samples containing deuterated solvent. Besides a detailed study involving the evaluation of the spin Hamiltonian parameters and measurement of transverse relaxation times for HoPOM samples of various concentrations, we also attempt to investigate the mechanism leading to the observed long coherence times. We propose that there is mitigation of decoherence in this system due to the nature of the tunneling gap, which leads to an insensitivity of the spin dynamics to field fluctuations. The other area of focus of the work presented in this dissertation involves studying spin frustrated lattices which give rise to novel ground state properties. The low temperature behavior of the modified honeycomb lattice compound SrHo2O4 has been characterized by dc magnetic susceptibility, heat capacity and neutron scattering experiments indicating lack of long range ordering down to 1.8 K. Elastic neutron scattering measurements show diffuse scattering indicative of short range ordering between nearest neighbor Ho3+ spins. Inelastic neutron scattering experiments carried out at multiple temperatures show the presence of five crystal field levels up to 80 K in energy, which is in agreement with the specific heat measurement on the system. The distorted kagome lattice compounds, Pr3Ga5SiO14 and Nd3Ga5SiO14, were also studied as examples of spin frustrated systems. EPR experiments on single crystals of these isostructural samples show complex multi-peak spectra with strong systematic temperature dependence. The nature of the observed excitations in high frequency EPR measurements indicate that they correspond to collective excitations akin to spin-wave resonances, caused by the formation of spin clusters whose correlation length depends on field and frequency. This study potentially provides an experimental basis to the investigation of antiferromagnetic spin wave - like resonances in the kagome lattice. The two kinds of samples discussed demonstrate interesting ground state properties, which have been probed using spectroscopic techniques for an insight into rare earth magnetism.