Properties of magnetization tunneling in high-spin molecules at low temperature

Based upon several interesting experiments, this thesis aims to address some deeper properties of Quantum Tunneling of Magnetization in Mn₁₂ -acetate, a high-spin molecular magnet at low temperature. After a brief introduction (Chapter 1), I will show data obtained from Inelastic Neutron Scattering experiments in Mn₁₂ revealing the presence of higher order uniaxial anisotropy (Chapter 2), a term central to the discussion of the results presented later. Chapter 3 will deal with a study of the magnetic relaxation of single crystals of Mn₁₂ in the regime of thermally assisted tunneling; complex features were found within each resonance which are not fully understood at this time. In Chapter 4 I will describe a sensitive Hall bar technique for measuring the magnetization of micron-size samples at very low temperature. I will present experimental data of hysteresis obtained at temperatures down to 0.4 K in Mn₁₂, and then interpret the data in light of the higher-order anisotropy and the theory of the first-order transition in Mn₁₂. A separate study of the effect of a transverse magnetic field on the lineshape of the N = 0 resonance in Mn₁₂ will be presented in Chapter 5. It will help further identify the unsolved key issues related to the tunneling mechanism in Mn₁₂. I will conclude my dissertation with a summary and suggestions for future studies in Chapter 6.