Optical and magnetic studies of TmAsO 4

Abstract

In the tetragonal paramagnet thulium arsenate, TmAsO 4 , the lowest manifold of the Tm 3+ ion, 4f 12 , 3 H 6 is split by the crystal field, which leaves a ground doublet with a nearby singlet at 13.8 cm -1 . Below a critical temperature T D = 6.13(3) K, a cooperative Jahn-Teller distortion splits the ground doublet, which produces domains with principal axes [110], [11̄0]. The behaviour of the three lowest levels has been studied by using high resolution optical absorption spectroscopy with magnetic fields up to 9 T along the [001] axis and in the (001) plane. Enhanced nuclear magnetic resonance of the single stable isotope 169 Tm has also been observed in these directions. Parallel to the c -axis the results confirm that from 1.6 to 4.2 K the magnitude of the Jahn-Teller splitting follows mean field theory to within 0.2 %. New values of the parameters give good agreement with mean field theory, but in the (001) plane the n. m. r. and optical measurements disagree with each other and with calculations based on the crystal field parameters of Bingham et al . (1984), which indicates that other interactions may be present.