Spin-Lattice Relaxation

Abstract

The measurement of electron spin-lattice relaxation times for paramagnetic crystals at low temperatures is complicated by the fact that the spin specific heat can be much larger than the lattice specific heat. At low temperatures direct processes dominate in the spin-lattice relaxation mechanism and evidence exists that indicates that only a narrow portion of the phonon spectrum takes part in the relaxation processes. This situation is not encompassed by usual treatments of the spin-lattice problem and a microscopic treatment is presented which allows for this selective excitation of the phonon spectrum. It is pointed out that phonon relaxation times can be the dominant quantity measured in the usual saturation spin-lattice relaxation measurements. The analysis indicates how pulse measurements may be used to evaluate the actual spin-lattice relaxation time independent of the phonon relaxation time. A discussion of some conditions under which the concept of temperature may be applied to quantum-mechanical systems interacting with electromagnetic fields, such as in solid-state amplifiers or absorbers, is given.