RUI: Quantum Micromagnetism

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9978882ChudnovskyThis is an RUI award to support theoretical research on micromagnetism in quantum and disordered systems. In particular, two projects will be undertaken:(1) Macroscopic quantum tunneling of magnetization in solids. This research will follow up on the PI's suggestion that there are two kinds of crossover on temperature from quantum to thermal decay of a metastable magnetic state. This work has induced significant recent activity on computing crossover diagrams for spin Hamiltonians of different symmetry. Experimental evidence of the first-order crossover has been found in Mn-12. The vicinity of the crossover from tunneling to thermal activation will be worked out, with account of the tunneling prefactor and dissipation, by mapping the spin problem onto a particle problem and including the interaction with the environment via the Caldeira-Leggett method. The PI will also attempt to develop an equivalent of the Caldeira-Leggett approach for the Landau-Lifshitz equation for the magnetic moment. The study of two kinds of crossover between quantum and thermal behavior will be extended to problems of quantum depinning of domain walls and quantum nucleation of magnetic domains.(2) Research will also continue on spin tunneling in high-spin molecular clusters, Mn-12 and Fe-8, and on the possibility of resonant spin tunneling and its experimental detection in single-domain magnetic particles. For the molecular clusters, the emphasis will be on the theory of thermally assisted resonant and non-resonant tunneling, quantum avalanches, the possibility of quantum coherence and Berry-phase oscillations. Such oscillations have been recently reported by Wernsdorfer and Sessoli in Fe-8. The effect of dissipation on these oscillations will be studied. The continuous and discrete-level models will be developed to explain quantitatively the quantum-classical crossover diagrams in Mn-12 and Fe-8. The PI will also study the possibility of the laser effect on spin levels, triggered by the external magnetic field. For single domain particles the question is whether one can detect quantization of spin levels in a many-particle system with a certain size distribution. The field dependence of the relaxation and of the blocking temperature will be computed, with account of level quantization, for ferro-and antiferromagnetic particles.%%% This is an RUI award to support theoretical research on micromagnetism in quantum and disordered systems. In particular, two projects will be undertaken: (1) research will be done on macroscopic quantum tunneling of magnetization in solids, and (2) research will be done on spin tunneling in high-spin molecular clusters and on the possibility of resonant spin tunneling in single-domain magnetic particles. Generally, this work investigates the physics of systems in which quantum effects manifest themselves at the macroscopic length scales.***