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In this paper we will present a study of several Tm and Pr compounds, where the rare earth atoms have as their two lowest energy levels a quasi-doublet ground state. These compounds are also compared with Er and/or Dy compounds, which have a Kramers doublet as ground state. Four compounds (TmNi5, TmCu2, TmCuAl and PrRu2Si2) are discussed in sequence, where the energy separation of the two levels of the quasi-doublet ground state doublet is increasing. These compounds were studied with rare earth Mossbauer spectroscopy and "SR as the main measuring techniques. 1. Introduction In intermetallic compounds of Tm and Pr, the trivalent rare earth atoms are non-Kramers atoms with an integer angular momentum quantum number (J =6 and 4 for Tm and Pr respectively). For hexagonal local symmetry (or lower), the electronic ground state of these atoms often consists of a pair of non-degenerate, non-magnetic singlets. Nevertheless, a very large cross term between the two singlets can lead to magnetism with an easy direction of magnetisation parallel to the c axis. Therefore, the two singlets can be regarded as a quasi-doublet. This behaviour is in contrast with Kramers doublets of Dy and Er (J = 15 2 ), which are already magnetic and whose degeneracy is lifted only by the magnetic interaction. In this paper we consider results accumulated over the last twelve years for several Tm and Pr intermetallic compounds and compare them with results for isostructural compounds of Er. The results will be presented in the sequence of increasing energy separation of the quasi-doublet ground-state levels. First, results for ErNi5 (TC =9 ¢2 K) and TmNi5 (TC =4 ¢5 K) show that the dynamic ∞uctuations of the magnetic spins above TC are comparably slow compared with the nuclear Larmor precession frequency (Gubbens et al. 1985, 1989). In the case of TmNi5 the splitting between the two levels of the quasi-doublet ground state is only 0¢4 K as determined recently by Kayzel (1997). For TmNi5, the temperature dependence of the magnetic relaxation above TC shows an exponential behaviour. ⁄ Refereed paper based on a contribution to the International Workshop on Nuclear Methods

Magnetic Behaviour of Systems with a Quasi-doublet Ground State