= Spin relaxation
Spin relaxation is the approach of nonequilibrium <spin> populations and coherences toward their equilibrium values through coupling to other degrees of freedom or environmental fluctuations. A spin polarized away from equilibrium can lose its net polarization even when the total <angular momentum> of the larger closed system is conserved.
For a simple ensemble in a fixed <magnetic field>, longitudinal relaxation with time $T_1$ restores the population difference along the field, while transverse relaxation with time $T_2$ damps the rotating transverse <spin polarization>. Static field inhomogeneity can produce an additional reversible spread of precession phases, so an observed free-induction decay time need not equal the intrinsic $T_2$. <Spin-flip scattering> changes populations; phase-randomizing processes can also suppress transverse coherence without individual flips. A <Hanle spin-precession measurement> probes the competition of precession, transport and relaxation.
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