Magnetic properties of Tb doped in YBa2Cu3Ox

Physica B 234-236 (1997) 841-842

ELSEVIER

Magnetic properties of Tb doped in YBa2Cu3Ox U. Staub a'*, F. Fauth b'c, M. Gutmann b, W. Kagunya d " Swiss Light Source Project, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland b Laboratory for Neutron Scattering, E T H & PSI, CH-5232 Villigen PSI, Switzerland c Institute Lane-Langevin, 156 X, F-38042 Grenoble Cedex, France d ISIS, Rutherford Appleton Laboratory Didcot, O X l l OQX, UK

Abstract

We present inelastic neutron scattering results on the temperature dependence of the magnetic response of Tb in YBa2Cu30~. Two magnetic components were observed in the spectra; a quasi-elastic (Lorentzian line shape) and an elastic (resolution-limited) contribution, but no crystal field transition was observed as expected. The temperature dependence of the full-width at half-maximum (F) of the quasi-elastic contribution can be well described by a power law with an exponent of 2.6 ___0.1. No anomalies were found at either Tc nor above. K e y words: Crystal fields; High-T~ superconductors; Quasi-elastic scattering

Inelastic neutron scattering (INS) is able to determine directly the imaginary part of the spin susceptibility Im z(q, co), which is an important quantity for the understanding of the fundamental mechanism of high-To superconductivity. However, there are still some discrepancies between these experiments and N M R results. Another approach is to study the temperature (T) dependence of the full-width at half-maximum (F) of crystalline electric field (CEF) transitions, which were performed in BCS-type superconductors I-1] as well as in RBa2CuaOv (R = Tm and H o [2, 3]). For a quasi-elastic transition (co --, 0) F is proportional to T Im{Yq z ( q , e)CEF)} and therefore leads to similar results as obtained by NMR. By far the best candidate for such an investigation is R = Tb with a "quasi-doublet" ground state (two singlets separated by only 5 peV [4]) separated from the higher lying CEF states by at least 18meV. We note here that Tb is one of the exceptions which does not form the YBazCu3Ox *Corresponding author.

structure, but just a partial substitution of 20% is possible. Recently, we have shown that Tb doped in YBa2Cu3Ox is in a trivalent oxidation state and is not significantly hybridized with the CuO2 bands 1,4]. Yo.9Tbo.lBa2Cu307 was prepared by standard procedures and found to be of single phase. The inelastic neutron scattering experiments were performed using the IRIS spectrometer at ISIS of the Rutherford Appleton Laboratory in U K and the IN5 spectrometer at the Institut Laue-Langevin in France. On IRIS the P G (0 0 2) and Mica (0 0 2) analyzer reflections were used giving rise to energy resolutions of 15 and 4 laeV, respectively. On IN5, incident wavelengths of 5 and 7 A were used corresponding to energy resolutions of 110 and 42 ~teV, respectively. Fig. 1 shows a typical spectra of Yo.9Tbo.1 Ba2Cu307 taken from IRIS at 80 K. We did not observe any indication of the CEF transition expected at 5 ~teV, even after using the improved resolution (4 tieV). At low T, only magnetic elastic scattering is observed. With increasing T the elastic

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The T dependence of F of the quasi-elastic contribution, which is well described with a Lorentzian line shape, is shown in Fig. 2 in double logarithmic scale. The linearity of F(T) implies a power-law behavior. This indicates that other processes for the broadening are dominant over the interaction with the carriers, which would result in a Korringa-lawtype behavior above Tc (92 K). In addition, no anomaly in F(T) is observed at either Tc or above. Such anomalies have been interpreted as an indication of an opening of a spin gap. Our results indicate that the s-f interaction is too small to be observed in this case. This imposes questions about the reliability of the studies determining F(T) for the inelastic transitions in the RBa2CuaO7system, where such anomalies have been observed [2, 3]. A recent ESR work on Gd:EuBa2CuaO7 did not find any anomaly above T~ either, however, a Korringa type of behavior of the Gd ~<--~ 7 5 transition was observed [7]. In part, financial support by the Swiss National Science Foundation is gratefully acknowledged.

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Fig. 2. Temperature dependence of the width of the quasi-elastic component in Yo.9Tbo.lBa2Cu3OT.

magnetic contribution decreases and a quasi-elastic contribution appears, increasing with T. We can describe the system in the mean-field approximation for an effective two-level system. The elastic component corresponds to the I(01J±10)l 2 and the quasi-elastic to I(0[Jl[1)[ 2, 0 and 1 label the wave function of the two states. A non-zero value of I(01J±10)l 2 would imply a magnetic field at the Tb site. Similar results were found for Pb2Sr2Tbl-xCaxCu3Os I-5, 6].

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