Dangling-bond electron-paramagnetic-resonance spectra and relaxation rates have been measured in the (0.3 -4)-K temperature range on samples of amorphous silicon produced by sputtering, vacuum evaporation, and ion implantation of si1icon, argon, neon, oxygen, and nitrogen into crystalline silicon. Intensity measurements of the dangling-bond resonance associated with silicon made amorphous by Si implantation suggest that the Curie temperature is essentially zero (0& 8 &0.03 K) for temperatures down to 0.4 K. The relaxation rates follow unusual temperature dependencies that cannot be explained on the basis of conventional spin-phonon interactions. Instead, the relaxation rates obey a simple T" power law in temperature where n falls within two ranges: 2.09 -2.36 and 3.26 -3.47. A comparison of rates at microwave frequencies of 9.3 and 16.5 6Hz indicates no magnetic field dependence. A relaxation model involving spin coupling to a distribution of two-level states is consistent with the observed T" dependence.

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Published relaxation data from ten samples of paramagnetic proteins are compared to illustrate the uncertainty which existed in identifying the anomalous low temperature relaxation mechanism in frozen solutions of proteins. Relaxation involving localized two level tunneling states or a phonon-limited direct process can explain the T2 temperature dependence of the relaxation rate that is observed in some proteins at temperatures above 1 K. Relaxation data on myoglobin at a microwave frequency of 16.545 GHz and in the temperature range between 0.4 and 1.2 K are presented. These data exhibit a coth2(ℏω/2kBT) dependence upon temperature and identify the relaxation process as phonon limited.