Linewidth narrowing in the epr spectra of Gd3+ impurity ions due to the spin-lattice relaxation of lanthanide Kramers' host ions

Abstract Electron paramagnetic resonance (EPR) of Gd 3+ doped in some lanthanide Kramers' host ion single crytals of the sulfate octahydrates (Ln 2 (SO 4 ) 3 ·8H 2 O;Ln≡Nd,Sm,Dy and Er) and trichloride hexahydrates (LnCl 3 ·6H 2 O;Ln≡Nd, Sm, Dy, Er and Yb) has been studied at T ≅ 297 K using a 9.4 GHz EPR spectrometer. The effect of the Kramers' host Ln 3+ ions on the g -values and linewidths of Gd 3+ spectra has been determined by comparison with those for the isostructural diamagnetic La, Y lattices. At 297 K,in the EPR transitions of Gd 3+ ions have narrow linewidths in spite of the presence of paramagnetic host ions like Nd 3+ , Sm 3+ and Yb 3+ , whereas an unusual variation in the linewidth is observed in the Dy 3+ , Er 3+ hosts as well as a negative g -value shift. In these latter hosts, the linewidths of the Δ M = ± 1 transitions decrease progressively as the magnitude of M increases. The observation of resolved Gd 3+ spectra at 297 K in the above hosts has been interpreted in terms of a random modulation of the interactions between the Gd 3+ and the host Ln 3+ ions by the rapid spin-lattice relaxation of Ln 3+ ions following the generalized theory of magnetic resonance by Kubo and Tomita [15]. τ 1 for Ln 3+ has been estimated in the above mentioned Kramers' hosts from the observed EPR linewidths of Gd 3+ spectra. Values for τ 1 have also been computed for Ln 3+ ions in Ln(C 2 H 5 SO 4 ) 3 . 9H 2 O and LnF 3 from linewidth data in the literature. The results are consistent with an effective host spin-lattice time which is due to Orbach and/or Raman processes, depending upon the temperature and the ground state energy level scheme.