Paramagnetic hyperfine splitting in the Eu-151 Mössbauer spectra of CaF2:Eu2+

Eu-151 Mossbauer spectra in zero magnetic field of highly dilute (0.1 mol%) Eu2+ ions in CaF2 showed an almost temperature-independent asymmetrically split pattern, arising from the paramagnetic hyperfine interaction AS.I in a cubic crystal field with slow electron spin relaxation; in a small external magnetic field B of 0.2 T such that g mu B-B>A an almost symmetrical pattern was observed. Both the spectra with and without external field are well described using the spin Hamiltonian and previous electron paramagnetic resonance data. A more concentrated (2 mol% Eu2+) sample exhibited a strongly broadened symmetrical resonance line due to an increased Eu-Eu spin relaxation rate; in an external magnetic field of 0.2 T the Mossbauer spectra exhibited further broadening and additional magnetic structures due to the reduced relaxation rate. When a large field of 6 T was applied such that g mu B-B is much larger than the crystal field splitting, a fully resolved hyperfine pattern was observed at 2.5 K, with an effective field at the Eu nuclei of -33.7 T; at higher temperatures superimposed patterns originating from excited electronic states were observed in the spectra. The present results on the highly dilute CaF2:0.1%Eu2+ sample deliver a straightforward explanation for previous observations of a seemingly large dependence of the Eu2+ isomer shift on europium concentration.