Paramagnetic Acoustic Resonance and Relaxation of Rare Earths in CaF2

Abstract

Using the group-theoretical approach of Blume and Orbach, a Hamiltonian is derived for interaction between coherent acoustic waves and rare-earth ions in cubic coordination. This Hamiltonian is found in partial agreement with the phenomenological spin-phonon Hamiltonian for effective spin 5=\ ions. Observations were made of the transmission of longitudinal and transverse 10-kMc/sec acoustic waves through rare-earth-doped CaF 2 as a function of an applied magnetic field. Magnetic-field-dependent attenuation of acoustic waves was observed for only two ions, Eu 2+ and Dy 3+ . In other cases only the upper limits of the acoustic absorption could be determined, and from these, lower limits of the direct-process relaxation time T\ were estimated. These lower limits were found approximately three orders of magnitude longer than the T\ values measured in CaF 2 at liquid-helium temperatures by the conventional pulse-saturation technique; an explanation of the discrepancy for some cases is suggested.146 268

Two-Phonon Absorption in Ultrasonic Paramagnetic Resonance of Uranium-Doped Calcium Fluoride

“…Ultrasonic paramagnetic resonance (UPR), which involves measurement of the additional attenuation of a microwave-frequency elastic wave in a paramagnetic crystal when the crystal is placed in an external magnetic field of magnitude corresponding to electron paramag- 6 netic resonance at the frequency of the elastic wave, has been employed by several investigators to study the one-phonon process of spinlattice interaction (see the review article by Tucker 4 ). Most of the effort has been devoted to iron-group ions; however, UPR investigations on rare-earth ions have also been reported.…”

“…5 ? 6 The results reported here were obtained during a general investigation of spin-lattice interactions in calcium fluoride containing various rare-earth ions with particular emphasis on the comparison of electronically similar non-Kramers lanthanide and actinide ions.…”