Longitudinal Nuclear Spin-Spin Relaxation

A theoretical and experimental study is made of the nuclear magnetization which appears after a sudden unidirectional step magnetic field is applied nonadiabatically to an ordered spin system which has been prepared by the process of adiabatic demagnetization. The magnetization exhibits damped oscillations about a nonvanishing equilibrium value. The oscillation frequencies correspond to the fundamental and harmonic components of Larmor frequency determined by the combined effect of local dipole and external step fields. The oscillations arise as the internally ordered dipole-dipole interaction energy exchanges with the suddenly imposed Zeeman energy reservoir. A density matrix calculation carried to second order in time-dependent perturbation theory, combined with a Gaussian decay model, accounts for the observed oscillations within times comparable to the decay time. Fluorine nuclei in single Ca${\mathrm{F}}_2$ crystals are studied for various crystal orientations with respect to the applied step field. For long times after step field application the attainment of thermal equilibrium between dipole-dipole and Zeeman reservoirs permits prediction, using the spin temperature concept, of magnetization developed along the step field.