Nonlocalized Magnetic Moments in the Weak Van Vleck Paramagnet PrF 3

We have observed for the first time a small, linear Zeeman effect of low-lying crystal field excitations in insulating PrF 3 by means of Raman spectroscopy. As the crystal field of low symmetry in PrF 3 completely quenches the magnetic moments of the single Pr 3+ -ions, this observation strongly supports the previously given interpretation of the Raman and farinfrared spectra of PrF 3 using the Frenkel-exciton model for the 4 f -excitations of the lowest multiplet component interacting with the system of optical phonons. By a simple technique, which uses the orthogonal polarization of the light scattered by two Zeeman-components and a computerized spectrometer, we were able to resolve Zeeman splittings smaller than 0.1 cm -1 . The results are in agreement with theoretical predictions based on a purely phonon-mediated Pr-Pr interaction.