Nuclear Magnetic Resonance of 169Tm (Enhanced) and 31P in TmPO_4

The enhanced nuclear magnetic resonance (n.m.r.) spectrum of ¹⁶⁹Tm (I = 1/2 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 276 MHzT^-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 ³¹P (I = 1/2) is shown to exhibit an anisotropic paramagnetic shift, mainly dipolar in origin, exactly proportional to the electronic susceptibility of the Tm³⁺ ions, and this is used to extrapolate the measurement of γ for ¹⁶⁹Tm 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.