  Recent experimental investigations of individual magnetic nanoparticles
reveal a diverse range of magnetic relaxation times which cannot be explained
by considering their size, shape, and surface anisotropy, suggesting other
factors associated with the internal microstructure of the particles are at
play. In this letter, we apply Langer's theory of thermal activation to single
magnetic domain fcc Co nanoparticles, whose experimental microstructures are
characterized by planar defects, and derive an analytic expression for the
relaxation time. The obtained Arrhenius exponential and its prefactor, which is
often assumed to be a constant, are here found to both depend exponentially on
system size and the number of defects. Together they provide a quantitative
prediction of the experimental findings, and more generally highlight the
importance of structural defects when considering magnetic stability.
