  We have measured the dissipation and frequency of nanocrystalline-diamond
nanomechanical resonators with resonant frequencies between 13.7 MHz and 157.3
MHz, over a temperature range of 1.4-274 K. Using both magnetomotive network
analysis and a novel time-domain ring-down technique, we have found the
dissipation in this material to have a temperature dependence roughly following
T^0.2, with Q^-1 = 10^-4 at low temperatures. The frequency dependence of a
large dissipation feature at ~35-55 K is consistent with thermal activation
over a 0.02 eV barrier with an attempt frequency of 10 GHz.
