Effect of transition metal doping on magnetism and superconductivity in Nd1.85Ce0.15Cu0.99M0.01O4 (M=Co, Ni and Zn

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PhysicaC 193 (1992) 449--454 North-Holland

Effect of transition metal doping on magnetism and superconductivity in Nd t.gsCeo.~ 5CUo.99Mo.0!O4 (M = Co, Ni and Zn) J. S u g i y a m a a, J.H. B r e w e r b, G . D . Morris b, C. N i e d e r m a y e r b,c, T . M . R i s e m a n b, I. S h i n k o d a b, H. Zhou b E.J. Ansaldo d, H. Glueker ¢, H. Yamauehi ~ and S. Tanaka a a Superconductivity Research Laboratory. International Superconductivity Technology Center. 10-1~Shinonome l-chome. Kotoku, Tokyo 135. Japan b Canadian Institute for Advanced Research and Department of Physics, Universityof British Columbia, 6224 Agricultural Road. Vancouver. B.C., Canada ¢ Department of Physics, University ofKonstanz, Konstanz. Germany d Department of Physics. University ofSaskatchewan, Saskatoon. Sask.. Canada

Received 27 January 1992

Positive muon spin rotation/relaxation (Ix+ SR) was used to study the effectsof substitution of small amounts of transition metals (Co, Ni and Zn) for Cu in superconducting Ndt.ssCeo.tsCuO4.AS expected, Zn doping has only a small effect upon T¢, presumably due to randomness and/or a change in the charge carrier density; however, the same quantity (!%) of Co or Ni suppresses T¢dramatically. In the case of Co, this may be due to the observedonset of disordered but near-static magnetism below 50 K; in the case of Ni there is no evidence of static paramagnetic moments down to 4 K, so rapidly fluctuating moments (r, < I0 ns, too fast to affect the muon spin) must be present, if pair-breaking is the predominant mechanism for suppression of T,.

1. Introduction Numerous studies [ 1-4 ] o f the substitution of a 3d transition metal (such as Co, Ni or Zn) for Cu in the electron-doped superconducting cuprate Ndt.ssCeo.LsCuO4 have shown that Tc was more sensitive to the paramagnetic impurity content than to nonmagnetic impurities. It was found that only 0.7% Co or 0.8% Ni substitution for Cu destroyed superconductivity [ 1-3 ]. In contrast, for the Zn-substituted samples, only a weak decrease o f Tc (proportional to the Zn content) was observed, with T¢ ~ 9 K even for a 2% Zn-substituted sample [ 2-4 ]. This result suggested that a pair-breaking effect [ 5 ] played the dominant role in the suppression of superconductivity in electron-doped superconductors containing paramagnetic impurities. However, there is another possible explanation for the suppression of T¢ by substitution for Cu: impurity doping is often used to alter the carrier concentration in such materials, with marked effects on

To; it may be that Co a n d / o r Ni are merely more efficient carrier dopants than Zn. Although the results o f measurements o f the Hall coefficient (RH) in the Co-, Ni- and Zn-substituted samples [2] indicated no clear relationship between the value of RH and the suppression of To, it is difficult to estimate the slight change in the carrier concentration due to the dilute substitution effects. When the carrier concentration is reduced sufficiently that superconductivity disappears entirely, one may expect to observe antiferromagnetic correlations between Cu moments, as reported for Nd2_xCexCuO4 [9], YBa2Cu3Ov_x [ 10] and Y~ _=Pr:Ba2Cu307 [ 11 ]. On the other hand, such antiferromagnetic correlations should not be observed in the non-superconducting samples if pair-breaking is the predominant mechanism for suppression of T¢. We report here the results of a muon spin rotation/relaxation (~t+SR) study of the interplay between superconductivity and static magnetic order-

0921-4534/92/$05.00 © 1992 Elsexier Sci nce Publishers B.V. All rights reserved.

450

ing in polycrystaUine (M = Co, Ni and Zn).

J. Sugiyama et al. / Effect of transition metal doping NdLssCeo.=sCUo.99Mo.mO4

2. Experimental Sintered polycrystaliine discs (2 cm d i a m x 2 m m thick) of NdLssCeo.tsCuO4 and NdLssCe,asCuo.99Mo.otO4 (M=Co, Ni and Zn) were prepared using conventional solid-state reaction techniques [ 2 ]. To improve the homogeneity, stoichiometric mixtures of starting oxides were calcined three times at 900°C for 10 h in air and three more times at 1000 °C for 10 h in air. After regrindin~ they were pressed into pellets and sintered at 1100°C for 100 h in air. Then, the samples were annealed at 900°C for 15 h in flowing nitrogen gas and rapidly cooled to room temperature. X-ray diffraction studies and ekctron-probe microanalysis observations indicated that the samples were nearly single phase comprising the Nd2CuOa-type tetragonal structure. Both resistivity and DC magnetic susceptibility measurements indicated superconducting transition temperatures Tc of 22 K, < ? K, < 2 K and 19 K, respectively, for the pure, Co-, Ni- and Zn-doped samples. The preparatior, of samples and the results of sample characterization are reported in detail elsewhere [2 ]. The samples were then brought to T R I U M F to be tested by positive m u o n spin rotation/relaxation (II+SR) [6], a sensitive probe of superconducting [7 ] and magnetic [8 ] properties. Measurements were performed at the M 13 secondary channel using weak (100 G) transverse magnetic fields (wTF~t+SR) and zero field (ZF-~t+SR). t~ne former method is sensitive to local superconductivity via the negative shift of the ~t+ precession frequency (due to a Meissner effect on individual crystaUites) and the enhanced !~+ spin relaxation (due to field inhomogeneities caused by formation of flux vortices in a type 11superconductor), while ZF-IJ+SR is sensitive to weak local magnetic [dis]order in samples exhibiting quasi-static paramagnetic moments.

fig. 1. The paramagnetism of Nd ions causes a significant field inhomogeneity below about 100 K, but the resultant relaxation rate due to this mechanism (about 0.2 l~S- ~ at low T) is the same for all samples at temperatures above the onset of either superconductivity or other magnetic behavior. Although the enhanced relaxation rate in the superconducting state cannot be translated into a magnetic penetration depth as easily in weak applied field

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Fig. 1. Time evolution of the muon polarization in a sintered pob'cDsmlline sample of Ndt ssCeo ~CuO4 in a transverse external field of 100 G.

J. Sugiyama et al. / Effecl ,~j ,ansioon metal doping

(wTF) as in high field (hTF), the cause of relaxation (formation of a vortex stz~cture) is qualitatively the same and previous experience with other sintered copper oxide superconductors has shown only a small ( < 25%) difference between wTF and h T F results. As can be seen from the temperature dependence of the muon spin precession frequency (J,.) and the relaxation rate (z£ ' ) in the pure and Zn-doped samples (fig. 2), the effect of Zn doping is to suppress the onset of superconductivity (as detected by Ix+SR) from about 20 K to about 15 K (25%) and to reduce the vortex-induced relaxation rate A at T ~ 0 from about 1.06 to about 0.73X106 s -~ (31%), corresponding to an increase of the London penetration depth AL by about 20°/0. This is approximately the expected behavior predicted by the empirical "Uemura law" [ 12 ]

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TEMPERATURE(K) Fig. 2. Muon spin precession frequency v. and relaxation rate r£ ~ in undoped NdLssCeo.tsCuO4 (To~20 K) (circles) and in Ndt.~Ceoa~Cuo99Zno.mO4 (T¢~ 15 K) (triangles).

451

where ns and m* are, respectively, the number density and effective mass of the superconducting electron. There is a marked difference between the samples doped with Co and Ni: while the effect of each is to suppress Tc to below ~ 5 K, the It+SR results show very little (if any) evidence for static paramagnetic moments on the Ni ions above 4 K, whereas the dramatic increase of It+ relaxation below about 50 K in NdLssCeo.tsCuo.99Coo.olO4 indicates strong (though disordered) magnetism of the Co moments. Examples of wTF-It+SR and ZF-It+SR time spectra for the Co-doped sample are shown in fig. 3. A summary of the temperature dependence o f v , and zE ~ in all four samples is shown in fig. 4, where the relaxation rate is shown on logarithmic scale to allow meaningful display of both high and low values of z£ ~. While we cannot conclude, based only on the It+SR results, that Co doping has less effect on carder density than Ni doping (it is impossible to disentangle the effects of strong magnetic moments from the effects of charge carder doping), we can be sure that the suppression of Tc by Ni doping is due either to very efficient charge carder doping by Ni or to rapidly fluctuating paramagnetic moments (% too short to affect the muon spin) - for moments of roughly the same magnitude as in the Co-doped sample, this would require re< 10 ns. Since the results [2 ] of RH measurements have revealed approximately equal values of Ra for the Ni- and Zn-doped samples in the temperature range 30-300 K, it is difficult to see how the stronger suppression of T¢ in the Ni-doped sample could be due to more efficient charge carder doping. Thus, the rapidly fluctuating moments of Ni ions seem to play a key role. In the BCS approximation, such fluctuating moments could affect the superconducting electrons through the pair-breaking mechanism, because the required zc ( < 10 ns) is rather long compared with the period estimated from the Debye frequency (tog ~~ l0 -4 ns). There is no way to be certain (from II+SR measurements alone) whether the magnetism observed in the Co-doped sample involved Cu moments or only Co moments. However, the dissimilarity of the magnetic behavior of the Co-doped and Ni-doped samples argues against a common role of Cu moments. Moreover, we did not detect any sign of coherent ~+ precession at the frequencies observed in

J. Sugiyama et al. /Effect of transition metal doping

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quantity of Co or Ni suppresses Tc dramatically ( < 5 K). In none of the samples there is any evidence of antiferromagnetic correlations between Cu moments due to the reduced concentration of charge carriers. In particular, only for the Co-doped sample, disordered but near-static magnetism is observed below 50 K; and this magnetism seems to be closely related to the suppression of To. In the case of the Ni-doped sample, there is no indication of static paramagnetic moments down to 4 K. If the pair-breaking mechanism is the predominant cause of the suppression of To, rapidly fluctuating moments (%< 10 ns, too short to affect the muon spin) must be present in the Nidoped sample.

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TEMPERATURE (K) Fig. 4• Muon spin precession frequency i,, and relaxation rate T£ t in transition metal doped Ndt.ssCeo.tsCut_xM~O,; circles: undoped Ndl.ssCeo.tsCuO4 (To~20 K); triangles: Nd t.ssCe0.tsCuo.99Zno.oiO, (To~ 15 K); squares: Nd t.ssCeo,i sCuo.99Coo.o104 (Tc<5 K); diamonds: Ndt.ssCeo i sCuo 99NiomO4 ( Tc< 5K).

antiferromagnetic Nd2CuO4, so we suspect that only Co moments participate in the magnetic properties at these concentrations. In addition, the magnetism observed for the Co-doped sample in the temperature range below ~ 50 K should cause the characteristic behavior of RH observed [2] only for the Codoped samples below ~ 90 K.

4. Summary Positive muon spin rotation/relaxation (~t+SR) was used to study the effects of substitution of Co, Ni and Zn for Cu in superconducting Ndl.ssCeo. i sCuO4 ( Tc ~ 20 K). Although 1% Zn doping has only a small effect (To~ 15 K), the same

This work was supported by the Canadian Institute for Advanced Research, the Natural Sciences and Engineering Research Council and (through TRIUMF) the National Research Council of Canada. We would like to thank Keith Hoyle and Curtis Ballard for technical support and Shinya Tokuono for his helpful advice.

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J. Sugiyama et al. /Effect of transition metal doping

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