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Anomalous behavior is observed in the time dependence of paramagnetic relaxation for dilute cerous magnesium nitrate at Zeeman frequencies of 11 GHz, temperatures 1.2-4.2°K, and nominal cerium concentrations of 0.2%. Immediately after excitation of the paramagnetic transition to a negative spin temperature, direct relaxation is accompanied by heating of the resonant lattice modes, and a "phonon avalanche" occurs. The decay profile of the center of the paramagnetic resonance line is distorted in a characteristic manner, and certain peculiarities in the shape of the line appear. Later in the decay, after the spin temperature has become positive, the decay approaches the behavior customarily identified with a phonon bottleneck, and the line shape becomes normal. The avalanche builds up on a time scale of microseconds, while the decay at positive spin temperature requires tens of milliseconds. Concentration, size, and surface effects are found. A simple theoretical model is developed and shown to agree qualitatively, but not quantitatively, with experiment. 157 3 J. A.

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