Theoretical T₁ Calculation for Isotropic High Spin Molecules

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We calculate the molecular-spin (S), temperature (T), and field (H) dependence of 1/T₁ for a local magnetic probe coupled to an isotropic high spin molecule, based on spin-phonon interaction. We compare the calculation to recent NMR and μSR experiments in CrCu₆ (S = 9/2), CrNi₆ (S = 15/2) and CrMn₆ (S = 27/2). Although we can account for the high and intermediate temperature regimes, the calculation is fundamentally different from the data at T 0. Since 1/T₁ must be due to coupling of the molecular spin to an external heat bath, and since phonon contribution is ruled out at low T, we conclude that at these temperatures hyperfine interactions must play an important role in the molecular spin dynamic.