  We consider a model of the dynamics of a magnetic grain, incorporating
interactions with nuclear and paramagnetic spins, conduction electrons, and
phonons. Decoherence comes both from the spins and the electrons, although
electron effects can be neglected if the substrate is insulating, and the grain
has spin $S \le 10^5$. A successful experiment will require both isotopic and
chemical purification, if coherence is to be seen; we suggest a "spin echo"
arrangement to do this.
