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

U. Staub

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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 ...

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• Publié par: Shiem
• Nombre de lectures: 12
• Langue: English

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V OLUME 77, N UMBER 22

P H Y S I C A L R E V I E W

Comment on “Local Magnetism and Crystal Fields of Pr in PrBa 2 Cu 3 O 7 Studied by 141 Pr NMR” Nehrke and Pieper presented a 141 Pr NMR study [1] in which they report a tiny ordered magnetic moment of Pr in PrBa 2 Cu 3 O 7  In addition, theyat low temperature. determined the crystal eld (CF) splitting (potential) of the Pr ions. They propose that the magnetic transition at 17 K is due to a ferromagnetic coupling between the CuO 2 planes of a bilayer induced by Pr. These conclusions are ambiguous and in total disagreement with most of the other experimental techniques. The proposed reorientation of the Cu spins would indeed increase the magnetic 1 2 , 2 1 , 2 1 s0dre ection but would strongly decrease the 1 2 , 2 1 , 3 2 s1dand 1 2 , 2 1 , 5 2 s2d re ections, which are not observed for PrBa 2 Cu 3 O 7 [2] sT N 17Kd, PrBa 2 Cu 3 O 6 [3]sT N 10Kd, nor for Pb 2 Sr 2 PrCu 3 O 8 [4]sT N 7Kd. There are only positive Bragg intensities observed in the neutron diffraction patterns, indicating an additional magnetic moment in the system. Figure 1 shows inelastic neutron scattering (INS) data from Pb 2 Sr 2 PrCu 3 O 8 [5], which are very similar compared to PrBa 2 Cu 3 O 7 [6] because of the same local structure around the Pr ions. The magnetic scattering is obviously peaked around 3 meV, which is directly related to the splitting of the quasitriplet. The proposed energy level scheme of Nehrke and Pieper (11 meV) is in strong contrast to this result. We note that INS is the most direct probe for a determination of the CF in these optically opaque systems. In addition, the tabulated CF parameters are totally different from those presented for the other RBa 2 Cu 3 O 7 systems (e.g., R ratio of the TheHo [7]). fourth and sixth order CF parameters do not even have the same sign compared with the results retrieved with INS or structural modeling [7]. However, what is even more strange is that we were not able to reproduce the tabulated splittings with the presented parameters. The proposed susceptibility (x) is in strong disagree- ment with the observations for temperature below 20 K [6].xis not constant below 20 K (due to the large en- ergy separation of the proposed quasitriplet), but increases further despite the small hump due to the ordering of the Pr sublattice, which is consistent with the INS results. The experimental results of specic heat studies [6] are in good agreement with the CF proposed from the INS and the magnetic ordering of the Pr sublattice, but are in contradiction to the presented interpretation of this Letter. The cited Mössbauer study [8] found a magnetic eld of 0.5 – 1 T at the rare earth site which can never be explained by the proposed Cu spin ordering (it cancels due to symmetry) but is well understood by the magnetic ordering of the Pr sublattice below 17 K. A possible explanation for these results may be re- lated to the different time scale probed by the different

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0031-9007y96y77(22)y(8864000.ƒ11)

L E T T E R S

25 N OVEMBER 1996

FIG. 1. Neutron energy spectra of Pb 2 Sr 2 PrCu 3 O 8 , a double layer cuprate similar as PrBa 2 Cu 3 O 7 . techniques. Neutron scattering is a fast probe, which meansitseesslowlyuctuatingspinsasstatic,whereas NMR is much slower. Pr Mössbauer, which lies in between these techniques, reports an ordered moment of 0.32m B [9], still much higher compared to those reported in the Letter [1] but half of the value obtained by neutron diffraction. In summary, the presented conclusions in this Letter are very ambiguous because they are in disagreement with all the results obtained by other experimental techniques. U. Staub Swiss Light Source Project Paul Scherrer Institute CH-5232 Villigen PSI, Switzerland

Received 23 April 1996 [S0031-9007(96)01729-2] PACS numbers: 75.25.+ z, 75.10.Dg, 76.60.Lz

[1] K. Nehrke and M. W. Pieper, Phys. Rev. Lett.76, 1936 (1996). [2] W.-H. Li, J. W. Lynn, S. Skanthakumar, and T. W. Clinton, Phys. Rev. B40, 5300 (1989). [3] M. Guillaume, P. Fischer, B. Roessli, P. Podlesnyak, J. Schefer, and A. Furrer, J. Appl. Phys.75, 6331 (1994). [4] W. T. Hsieh, W.-H. Li, K. C. Lee, J. W. Lynn, J. H. Shieh, and H. C. Ku, J. Appl. Phys.76, 7124 (1994). [5] U. Staub, S. Skanthakumar, L. Soderholm, and R. Osborn (to be published). [6] G. Hilscher, E. Holland-Moritz, T. Holubar, H.-D. Jostarndt, V. Nekvasil, G. Schaudy, U. Walter, and G. Fillion, Phys. Rev. B49, 535 (1994), and references therein. [7] U. Staub, J. Mesot, M. Guillaume, P. Allenspach, A. Furrer, H. Mutka, Z. Bowden, and A. D. Taylor, Phys. Rev. B50, 4068 (1994), and references therein. [8] J. A. Hodges, G. l. Bras, P. Bonville, P. Impert, and G. Jéhanno, Physica (Amsterdam)218C, 283 (1993). [9] A. A. Moolenaar, P. C. M. Gubbens, J. J. Loef, M. J. V. Menken, and A. A. Menovsky, Hyperne Interact.93, 1717 (1994).

Ž 1996 The American Physical Society