Resonance Paramagnetic Relaxation and Alignment of Small Grains

Abstract

We show that the energy-level splitting arising from grain rotation ensures that paramagnetic dissipation acts at its maximum rate, i.e., the conditions for paramagnetic resonance are automatically fulfilled. We refer to this process as ``resonance relaxation''. The differences between the predictions of classical Davis-Greenstein relaxation and resonance relaxation are most pronounced for grains rotating faster than 1 GHz, i.e., in the domain where classical paramagnetic relaxation is suppressed. This mechanism can partially align even very small grains, resulting in linearly polarized microwave emission which could interfere with efforts to measure the polarization of the cosmic microwave background.