Macroscopic Quantum Phenomena in Magnetic Systems

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The quantum mechanical tunneling of macroscopic degrees of freedom will be studied in magnetic systems, such as the total moment in ferromagnetic and the Neel vector in antiferromagnetic particles of size approx. 100 Angstroms. If such exists, it would significantly enlarge the known range of applicability of the superposition principle. Special emphasis will be placed upon studying dissipation due to environmental degrees of freedom with a view to guiding and understanding experiments. Examples of dissipative agents are nuclear spins, eddy currents and spin waves. Certain quantum phenomena resulting from the topological phase in the spin path integral will also be studied. Magnetic domain wall motion will also be studied from a tunneling perspective. A combination of path integral and perturbative methods will be employed involving both numerical and analytical work. %%% This proposal involves the theoretical study of quantum mechanics applied to systems with a large number of degrees of freedom. The question is whether systems whose state is determined by a superposition of macroscopically distinct states (eg. a magnetic moment and its orientation) can behave in a similar way to those with few degrees of freedom - that is exhibit tunneling across classically forbidden barriers or exhibit resonance. Good candidates for these phenomena are small magnetic particles, but, to date, experiments aiming at proof of existence have been controversial. This work aims to guide and critique experiment and to study those mechanisms (energy loss) which could mask the effect. The work has value because of its possible relevance to magnetic recording media where such effects would be undesirable.