Suppression of macroscopic quantum coherence in magnetic particles by nuclear spins

The Néel vector of a small antiferromagnetic particle can in principle resonate between opposite directions. Nuclear spins strongly suppress this and related macroscopic quantum coherence (MQC), even when magnetic nuclei have only 2% abundance. The resonance signal breaks up into well separated groups of lines, and the highest frequency signal height increases with temperature as exp(-T02/T2). For a recent claim to have seen MQC in ferritin, less than 3.5% of the particles are found capable of contributing to the signal.