Further studies of the enhanced nuclear magnet HoVO 4 - II. Experiments with acoustic waves

Acoustic measurements on single crystals of HoVO 4 at a frequency of 800 MHz (Bleaney et al., Proc. R. Soc. Lond . A 388, 479 (1983 a )) revealed the presence of both non-resonant acoustic absorption at low appliedfield strengths and resonant absorption at the enhanced nuclear magnetic resonance frequency. In a recent paper (Bleaney & Gregg Proc. R. Soc. Lond . A 413, 313 (1987)) the theory of such acoustic resonance in enhanced nuclear paramagnets has been extended to include other acoustic modes for ions at sites of tetragonal symmetry. In this paper the advantages of using acoustic waves rather than conventional nuclear magnetic resonance for the region 1-2 GHz is described, together with the preparation of transducers of ZnO and the experimental apparatus. Measurements of the acoustic velocities at 1 GHz in HoVO 4 are compared with those of (Goto et al., J. Phys. Soc. Japan 55, 1613 (1986)) at 10 MHz. Further measurements of the non-resonant and resonant attenuation will be presented in papers III and IV (Bleaney et al., Proc. R. Soc. Lond . A 416, 83; 93 (1988)), together with the appropriate theory.

Further studies of the enhanced nuclear magnet HoVO 4 - I. The crystal field and the Zeeman spectrum

A novel approach is adopted to fit the experimental results for the Van Vleck paramagnet HoVO 4 . Within the ground manifold 5 I 8 , J = 8, the five parameters for a crystal field of tetragonal symmetry are adjusted to give values in agreement with the optical spectrum for the lowest energy levels: the ground singlet, the first excited doublet at 21 cm -1 , and the (accidental) triplet at 47 cm -1 . Within experimental error (of order 1 cm -1 ), this agreement is not impaired by a small modification in which all the crystal field parameters are multiplied by a factor 1.0225. This factor is introduced to give the correct value of the enhanced nuclear magnetic resonance frequency for the stable isotope 165 Ho ( I = 7/2), known to 0.3% (Bleaney et al. Proc. R. Soc. Lond . A 362, 179 (1978)). The optical Zeeman effect, calculated therefrom, is in good agreement with that observed experimentally for the lowest levels in magnetic fields up to 15 T, directed along the [100], [110] and [001] axes (Battison et al. Phys. Lett . A 55, 173 (1975); J. Phys . C 10, 323 (1977)).