Microscopic View of Exchange Narrowing

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Abstract
In this paper two methods are presented for calculating the spin-pair correlation functions for an exchange-narrowed system of spins. These methods are applied to an isotropic Heisenberg paramagnet with a relatively small anisotropic interaction. The first method consists of a microscopic derivation of the spin correlation function itself. By neglecting successively higher correlation functions, a usable equation is obtained. In the second method, sum-rule moment expansions are used to determine the frequency-dependent linewidth rather than the spin correlation function itself. Electron-paramagnetic-resonance line shapes are evaluated using these techniques and are compared with experimental data and with other theoretical work.