PHONON INTERACTIONS IN CRYSTALS

Spin lattice relaxation to a phonon bottlenecked lattice has been observed in Fe(2+) and Ni(2+) doped MgO. The Ni(2+) spin system was studied extensively. Avalanche relaxation was observed for both the delta M = 2 and delta M = 1 transitions at 2 K with characteristics avalanche times of approx. 4 to the minus 7th power sec and 0.000002 sec respectively in material having a doping concentration of 8 x 10 to the 18th power/cc. The expected line shape 'inversion' has also been observed. For Fe(2+) with a concentration of 2 x 10 to the 18th power/cc the calculated avalanche time constant is 4 x 10 to the minus 8th power sec. This time is short enough to allow avalanche formation in a small volume of crystal and phonons generated by an avalanche at one end of a 2.5 cm long rod were detected at the other end. It was found that only a few phonons modes were excited, most of the modes remaining cold. The decay of the hot phonons then occurs primarily through spin scattering to the cold modes at the spin resonance frequency. Results of heat pulse measurements between 4 - 54 K are reported for sapphire and show that the propagation is ballistic below 18 K and diffusive above 40 K. In the intermediate range, the propagation is a superposition of the two. No second sound is observed throughout this temperature range.