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Effect of Exchange Interaction on Magnetic Thermal Fluctuation and Spin Susceptibility

The expression of the thermal fluctuation parameter in the stochastic Landau-Lifshitz-Gilbert equation has been derived from a fundamental quantum theory of spins and phonons, in which the exchange interaction between nearest atoms has been included. Our studies show that the thermal fluctuation decreases exponentially with increasing exchange interaction. The non-uniform fluctuation of local spins make the spin susceptibility much different from the result derived by the macro-spin model or single spin model. The related spin susceptibility depends not only on the strength of exchange interaction, but also on the lattice structure. The non-uniform fluctuation can lead to an extra broadening of the resonance line width along with the broadening arisen from the Gilbert damping.