Orientational dependence of the tails of dipole-broadened NMR spectra in crystals

This paper describes experimental and theoretical studies of the tails of t he dipole-broadened nuclear magnetic resonance (NMR) absorption spectra of F-19 in isomorphic single crystals of BaF2 and CaF2 with the magnetic field directed along three crystallographic axes. The results obtained by direct ly measuring the derivative of the tail of the NMR absorption spectrum and the falloffs of the Engelsberg-Lowe free precession after Fourier transform ation qualitatively agree. It is shown that the shape of the tail is well d escribed by an exponential function in which the orientational dependence o f the exponent does not reduce to variation of the second moment. The obser ved shape of the tail and the orientational dependence of its parameters ar e explained on the basis of a self-consistent fluctuating-local-field theor y. Nonlinear integral equations are derived for the correlation functions, taking into account the changes of the actual number of nearest neighbors c aused by the anisotropy of the dipole-dipole interaction and the contributi on of lattice sums with loops. The equations are solved numerically. Good a greement is obtained for the computed dropoffs of the free precession, the NMR spectra, and the cross-polarization rates with the experimental results . (C) 1999 American Institute of Physics. [S1063-7761(99)02401-4].