Nuclear magnetic resonance of
169
Tm (enhanced) and
31
P in TmPO
4
The enhanced nuclear magnetic resonance (n. m. r.) spectrum of 169 Tm ( I = ½) in the tetragonal compound TmPO 4 has been measured from 1.6 K to 30 K. It is highly anisotropic, with γ /2π = 11.34 and 276MHzT -1 , parallel and perpendicular to the c -axis respectively, at the lowest temperatures. The corresponding values of the Van Vleck electronic susceptibility per ion are 0.0231 and 0.805 μ B T -1 . Above 4 K the values of γ /2π become temperature dependent as excited levels become populated, and the results are discussed in the light of a crystal field calculation of the energy levels and wave functions. The n. m. r. spectrum of 31 P ( I = ½) is shown to exhibit an anisotropic paramagnetic shift, mainly dipolar in origin, exactly proportional to the electronic susceptibility of the Tm 3+ ions, and this is used to extrapolate the measurement of γ for 169 Tm up to 64 K in the perpendicular direction. A simple calculation of the direct process for spin-lattice relaxation shows that this is orders of magnitude too slow to account for the observed relaxation rates, which must be ascribed to the presence of paramagnetic impurities.