Thermodynamics of Spin Systems in Solids

Abstract

Quasiequilibrium states of the spin system in a solid are described in terms of one "Zeeman" temperature for each spin species plus one "dipole-dipole" temperature, $T_D$. Energy and entropy are calculated and used to predict the steady state of processes such as cross relaxation. It is predicted and demonstrated by an experiment on the nuclear spins in LiF that the state of the "dipole-dipole" system has a strong influence on such steady states. Continuous wave (cw) and pulse spectroscopy are discussed for systems with low $T_D$. Techniques are proposed (and have been used) to measure $T_D$ and one Zeeman temperature simultaneously, using coherent pulse instrumentation, and for preparing a state of low $T_D$ in a large magnetic field by complete adiabatic demagnetization followed by sudden magnetization. A density matrix formalism is proposed for the description of quasiequilibrium situations in the case of "spins" with unequally spaced energy levels. Finally the influence of the "nonsecular" part of the spin-spin Hamiltonian on the quasi-equilibrium states is estimated by a perturbation calculation, and the resulting description includes the cases of low- or zero-magnetic field and partly or completely overlapping absorption lines.