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Aspects of quantum dynamics in magnets

University of British Columbia

We investigate the quantum dynamics of a pair of magnetic grains and of a ferromagnetic domain wall in presence of both a Caldeira-Leggett type of environment and of nuclear spins, from which we consider the "degeneracy blocking" effect. For the grains, we find that dissipation is extremely disruptive to any kind of coherent tunneling, as most of the dynamics is composed of incoherent relaxation. We distinguish two regimes: the locked phase and the correlated phase. This last phase is quite interesting as we establish the existence of a region of "mutual coherence", where coherence is restored due to the interaction between the grains. In the simplified case where any bias on the grains is ignored, we give analytical results for the dynamics of the system in most of the regions of interest. In particular, we can obtain the values of the frequency of the oscillations in the occupancy probability functions as well as the corrections brought by the environment. We also examine the effect of phonons on the tunneling of a magnetic domain wall. We show that 1-phonon processes cause superohmic dissipation, as expected, but that the dominant term comes from the coupling of the phonons to the velocity of the wall. 2-phonon processes are found to give ohmic dissipation, with a temperature dependent friction coefficient η[sub 2ph](Τ) ~ T[sup d+3] where d is the dimensionality of the phonons. These two results are then used to establish the temperature dependence of the tunneling rate of the wall. At finite temperature, there is a decrease of the effect of 1-phonon processes proportional to T⁴. However, 2-phonon processes give an increasingly large effect as the temperature is raised, causing a competition between the two mechanisms in the overall temperature dependence of the tunneling exponent. Finally, we briefly examine the recently proposed phenomenon of band diffusion of a wall and of tunneling of its chirality. A very limited form of diffusion for small walls is possible, but the random potential created by the nuclear spins blocks any kind of coherent propagation for large walls. The motion then takes place by incoherent tunneling. As for the chirality, we find that its quantum behaviour is severely restricted, due to an ohmic coupling to the phonons.

Thesis/Dissertation

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2009-03-27

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http://hdl.handle.net/2429/6610

Physics

University of British Columbia

UBCV

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