Nuclear Relaxation as a Probe of Electron Spin Correlation

Adiabatic demagnetization of a nuclear spin system produces a state in which there is considerable correlation between neighboring spins. This correlation affects the nuclear relaxation rate and, in metals, where nuclei relax via interaction with conduction electrons, can be used as a probe of spin correlation in the electronic system. For a given lattice temperature the ratio δ of low- to high-field relaxation rates can be written in the form δ=2+η, where η is essentially the electron spin correlation function at the nearest neighbor distance. η can also be related to the nonlocal electron spin susceptibility.For noninteracting electrons, calculated values of η are more than an order of magnitude too small to explain the observations (η~0.2 in the alkalis). The electronic spin correlation is considerably enhanced by exchange. Calculations which include this effect yield the value 0.1 for η.