Abstract
For quantum phenomena involving a macrovariable such as the total moment of a ferromagnetic particle, or the Néel vector of an antiferromagnetic one, environmental couplings play a crucial role. The effect of such couplings, or dissipation, is studied, with emphasis on the nuclear spin environment. It is shown that magnetic macroscopic quantum tunneling (MQT) and coherence (MQC) both become harder to see. Quantitative calculations are presented for the tunneling rate in MQT, and the tunnel splitting and ac susceptibility for MQC. A recent claim to have seen MQC in a resonance experiment on ferritin is critically discussed and the absorption is found to be at least 1500 times larger than predicted by MQC.
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