Quantum Coherence in Magnetic Grains

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.