Further Studies of the Enhanced Nuclear Magnet TmPO_4

In thulium phosphate, TmPO_4, the lowest levels of the manifold ^3H_6, split by the crystal field, are a singlet, and a doublet, followed by another singlet. Measurements by Bleaney et al. (Proc. R. Soc. Lond. A 387, 75 (1983)) of enhanced nuclear magnetic resonance were fitted to these levels by using a set of crystal-field parameters and wave functions based on optical absorption measurements of Brecher et al. (J. chem. Phys. 49, 3303 (1968)) on Eu³⁺ in YPO_4. Two other sets of parameters (Hodges: J. Phys. 44, 833 (1983); Becker et al.: J. chem. Phys. 81, 2872 (1984)) give similar levels, except that the two lowest singlets are interchanged. A fresh set of parameters (adjusted from those of Hodges) is shown to give the correct Zeeman splitting for the doublet, and to fit the enhanced nuclear resonance measurements. Measurements of phonon velocities and of acoustic magnetic resonance at 0.7 GHz are also reported at field strengths up to 3 T. Values of the direct spin-lattice relaxation rates derived from these measurements are compared with results from other methods. The Orbach-Aminov relaxation rate is discussed, together with that in some other enhanced nuclear paramagnets.