Comment on Local Magnetism and Crystal Fields of Pr in PrBa2 Cu3 O7 Studied by 141 Pr NMR

VOLUME 77, NUMBER 22 PHYSICAL REVIEW LETTERS 25N OVEMBER 1996Comment on “Local Magnetism and Crystal Fields141of Pr in PrBa Cu O Studied by Pr NMR”2 3 7141Nehrke and Pieper presented a Pr NMR study [1]in which they report a tiny ordered magnetic moment ofPr in PrBa Cu O at low temperature. In addition, they2 3 7determined the crystal field (CF) splitting (potential) ofthe Pr ions. They propose that the magnetic transition at17 K is due to a ferromagnetic coupling between the CuO2planes of a bilayer induced by Pr. These conclusions areambiguous and in total disagreement with most of theother experimental techniques.The proposed reorientation of the Cu spins would1 1 1 FIG. 1. Neutron energy spectra of Pb Sr PrCu O , a double2 2 3 8indeed increase the magnetic , , s0d reflection but2 2 2layer cuprate similar as PrBa Cu O .1 1 3 1 1 5 2 3 7would strongly decrease the , , s1d and , , s2d2 2 2 2 2 2reflections, which are not observed for PrBa Cu O [2]2 3 7techniques. Neutron scattering is a fast probe, whichsT › 17 Kd, PrBa Cu O [3] sT › 10 Kd, nor forN 2 3 6 Nmeans it sees slowly fluctuating spins as static, whereasPb Sr PrCu O [4] sT › 7 Kd. There are only positive2 2 3 8 NNMR is much slower. Pr Mössbauer, which lies inBragg intensities observed in the neutron diffractionbetween these techniques, reports an ordered moment ofpatterns, indicating an additional magnetic moment in the0.32m [9], still much higher compared to those reportedBsystem.in the Letter ...