spinnrelaxation

Spin relaxation, sometimes misspelled as spinnrelaxation in informal contexts, is the process by which non-equilibrium spin populations and coherences return to thermal equilibrium with their surroundings after perturbation. In magnetic resonance techniques such as NMR and ESR, relaxation is described by two characteristic time constants: T1, the spin-lattice or longitudinal relaxation time, and T2, the spin-spin or transverse relaxation time. T1 describes the recovery of the component of magnetization along the external field as spins exchange energy with the lattice. T2 describes the decay of coherence perpendicular to the field due to spin-spin interactions and local field inhomogeneities; it sets an upper limit to signal decay independent of field inhomogeneity, though T2* includes inhomogeneities.

Relaxation mechanisms vary by system. Nuclear spin relaxation in solids and liquids is often governed by dipole-dipole interactions and, at higher temperatures, by coupling to lattice vibrations (phonons). The direct process (one-phonon) and the Raman and Orbach two-phonon processes are common temperature-dependent pathways; in metals, electron-nucleus coupling can proceed via the Korringa mechanism. Electron spin relaxation involves additional processes such as the Elliott–Yafet and Dyakonov–Perel mechanisms, as well as Orbach and Raman processes in magnetic or paramagnetic materials.

Experimentally, T1 is commonly measured by inversion-recovery or saturation-recovery sequences, while T2 is obtained from spin-echo decay (Hahn echo or CPMG sequences). The ratio T1/T2 provides information about the dynamics of the local environment, molecular motion, and interactions with paramagnetic centers.

Spin relaxation plays a central role in determining spectral resolution and contrast in NMR and MRI, as well as in characterizing materials, liquids, and biological systems.