Zero-field relaxation pathways

The quantum tunnelling relaxation pathway is expected to be dominant at very low temperatures (when there are very few phonons). At higher temperatures the thermally activated regime, up-and-over the barrier, is dominant. At intermediate temperatures both mechanisms are important. This is easily understood from Figure 5.24 at zero field Ms = +9 and -9, 4-8 and -8, and so on are in resonance in addition to Ms = 10. Hence, the spin can be thermally promoted to, for example, -9... 

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