Adiabatic Demagnetization in a Rotating Reference System

Abstract

Redfield has proposed that under some circumstances, a magnetic resonance should be described by saying the spin system has achieved a temperature in a reference system which rotates at the frequency of the applied alternating field. He based his proposal on experiments in which characteristic times of observation were long compared to the spin-lattice relaxation time. A theory of spin-lattice processes was necessary to analyze the results. We describe a set of experiments to verify his hypothesis, using times short compared to the spin-lattice relaxation, which test his hypothesis without need for a theory of spin-lattice relaxation. The experiments are shown to be similar to conventional adiabatic demagnetization, performed, however, in a rotating reference frame. Redfield's ideas are thereby presented in a particularly simple form. The difference between reversible and irreversible losses in magnetization are illustrated, and it is shown, for example, that one can invert the magnetization with respect to the static field by passing through the resonance using alternating fields much less than the linewidth. The studies were made using the Na resonance in NaCl.