J. Phys. France 49 (1988) 7-11 JANVIER 7 1988, Classification Physics Abstracts 76.80 - 74.70 A Mössbauer study YBa2Cu3O7201403B4 of a superconducting sample of 57Fe doped P. Imbert and G. Jéhanno Service de Physique du Solide et de Résonance 91191 Gif-sur-Yvette Cedex, France (Regu le QO aoict 1987, accepti sous Magnétique, forme d£finitive CEN - le 4 novembre Saclay, 1987) Résumé. 2014 A partir de l’étude des spectres Mössbauer d’impuretés de 57Fe substituées au cuivre dans le composé supraconducteur YBa2Cu3O7201403B4, nous concluons à l’absence d’ordre magnétique statique dans ce composé au-dessus de 4,2K. study of the Mössbauer absorption of 57Fe impurities substituted for Cu in superconducting YBa2Cu3O7_03B4 we conclude that there are no static ordered magnetic moments within the Cu sublattices in this compound down to 4.2K. Abstract. - From a "Magnetism and superconductivity are usually mutually exclusive, but they seem to be intimately related in the new high-temperature superconducting compounds", so writes A.L. Robinson [1]. He adds : "Superconductivity and antiferromagnetism are the Jekyll and Hyde of these systems" . A question of much current interest in these compounds is whether or not there is coexistence of superconductivity and antiferromagnetic ordering of the copper atoms. According to P.G. de Gennes, the high critical temperatures (T, ) could arise from an attractive interaction between carriers mediated by spin waves within the framework of a canted antiferromagnetic structure [2]. According to P.W. Anderson, the copper valence electrons could, in contrast, be associated as nearest neighbour singlet pairs [3]. In addition, it seems to be experimentally established that in the La2-,,Sr.,,CU04 system, the long-range antiferromagnetic order in non superconducting La2 Cu04 disappears in the superconducting compounds of the series [4]. Here we present an experimental contribution to the current discussion, this time concerning Y Ba2Cu307 _ h: a Mossbauer study of this compound doped with 57 Fe shows that ordered magnetic moments are absent at temperatures much lower than Tc. 1. Sample preparation and control. samples of YBa2(CU1-,,Fe,,)307-h were prepared under identical conditions with x 0, 0.8 and 5 % respectively. The Mossbauer study was carried out mainly on the sample with x 0.8 % which was prepared using 57 Fe enriched iron. A mixture of appropriate amounts of Y203, BaC03, CuO and Fe203 were cold pressed and sintered in air for 10h at 900C. The heating rate was near 300C/h and the cooling rate down to room temperature was near 150C/h. The samples were then finely crushed, cold pressed and again heat treated. We have verified that a third heat treatment at 900° C for the sample with x 0.8 % followed by an anneal at 500°C for 5h as suggested by P. Strobel et al. [5] so as to possibly increase the oxygen content, did not appreciably modify either the crystallographic properThree = = = ties or the Mossbauer data. X-ray study of the three samples showed the presence of a single phase and that the orAn Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:019880049010700 8 Tab. I) strongly decreases increases and becomes essentially equal to 1.0 for x = 5.0 %. This shows that the Fe is well incorporated into the YBa2Cu307_b lattice. 0 and 0.8 % are slightly Our b/a values for x smaller than the value (b/a 1.01821(4)) obtained by Cava et al. [6] for YBaZCu306,9. As the amount of orthorhombicity is a function of the oxygen content (following Bordet et al. [7] the lattice becomes tetragonal for the composition YBa2Cu306) we can estimate that the mean oxygen content in our samples is slightly lower than 6.9. thorhombicity ( b/a, as x = = Orthorhombicity (b/a ratios) measured samples YBa2(CU1-.Fe.).307-6 Table Lour as well as that at 77K and 4.2K clearly shows the presence of electric quadrupole interactions. Only the spectrum at 1.4K (Fig. 2) shows the presence of a magnetic hyperfine structure. From a more detailed analysis of the hyperfine interwe discuss below the number and the actions, nature of the sites occupied by the 57Fe probe as well as the electronic nature of these iron atoms. We conclude that there is no static magnetic ordering within the Cu sublattices at least down to 4.2K. in Fig.l.- M6ssbauer spectrum of YBa2 (CUO.992 at 295K (see Tab. IIa for the fit- ,57Feo.008 )3 07-6 ted parameters). Resistivity measurements show that for the sample with x = 0, Tc is slightly lower than 90K and that for x = 0.8 %, Tc is reduced by about 4K with the transition now spread out over several degrees. A test of the Meissner effect for 5 % shows that it is not the sample with x at 77K. The Meissner effect is superconducting on each of the other two samples clearly visible at this temperature. The observed rapid vari= ation of Tc with Fe content shows that the Fe probably substitute for the Cu atoms whose sublattices are thought to be responsible for the superconducting properties. This substitution would be expected for two 3d elements having comparable atomic volumes. atoms very Finally, a part of the x = 0.8 Mossbauer spectrum of YBa2 (Cuo.992 "FeO.008)3 07-6 at 1.4K (Note the change in the velocity scale compared with Fig. 1). The solid line was fitted using a Zeeman sextuplet with He ff 230kOe, on a residual quadrupole paramagnetic superimposed doublet. Fig.2.- = % sample annealed at 650 C for 10h in an argon atmosphere. From the observed weight loss, the oxygen content 7 - 8 of the resulting tetragonal non superconducting phase was estimated to be about 6.2. was 2. 57Fe M6ssbauer results. hyperfine structure observed on the x % superconducting sample at 295K (Fig. 1) The 0.8 = At 295K and 77K, that is either side of the Mossbauer spectra are practically identiT, cal. Satisfactory line fits are obtained in terms of three quadrupole doublets (two strong and one weak). However the four prominent experimental lines associated with the two dominant quadrupole doublets can be paired equally well in two different ways. This gives for the first choice an outer and an inner doublet hav- 9 the same isomer shift value (a "symmetric lines" fit, leading to the doublets D1 and D2 in Tab. IIa), or for the second choice two doublet$ with quite different isomer shift values (a "crossed lines" fit, leading to the doublets Di and D2 in Tab. IIb). In both types of fit the components of each doublet were allowed to have different linewidths. Although both procedures give fits of equally good quality, the "symmetric lines" fit is favoured for the following For x = 5 %, the "symmetric lines" reasons. fit leads to the pairing of lines with the same widths, whereas very different linewidths are required for two lines linked by the "crossed lines" fit. Besides, preliminary results obtained on the x = 0.8 % sample treated at 650C in an argon atmosphere, where the relative areas of the external lines are much enhanced, are consistent with a "symmetric lines" fit. Finally, when comparing the two fitting procedures, we have to keep in view that the two dominant quadrupole doublets are very likely related to the two possible sites where the iron impurities can substitute for the Cu : the 5-oxygen coordinated pyramidal site and the 4-oxygen coordinated planar site [7]. In this respect, it is difficult to understand the hyperfine parameters (almost identical quadrupole splittings and widely different isomer shifts) provided by the "crossed lines" fit. In contrast, it is easier to understand the hyperfine parameters (very different quadrupole splittings and almost equal isomer shifts) provided by the "symmetric lines" fit. The very different quadrupole splittings are in accord with nearest neighbours point charge calculations, which show that the electric field gradient is much larger (in absolute value) for the planar configuration than for the pyramidal configuration [8]. In addition, NQR measurements on 63Cu in YBa2Cu307 [4,8] show that ing nearly the planar ous as the sites Cu 1, which are half as numerpyramidal sites Cu 2, give the largest quadrupole frequency (v(’) 32MHz, V(2) For all these reasons we conclude in f avour of the ’asymmetric lines" fit (Tab. IIa) and we assign the largest quadrupole splitting, ?S’(Di) - 1.96mm/s, to the 57Fe impurities substituted for Cu in the planar sites and the small- 22MHz). est quadrupole splitting QS(D2) = 1.15mm/s to the 57Fe impurities substituted an the pyramidal sites. This assignment is further corroborated by the fact that the doublet D2 is approximately twice as intense as the doublet Dl for the x = 0.8 % sample (however the D21D, area 5 %). ratio is close to 1 for x = The isomer shift values of the doublets Di and D2 are both close to zero, relative to Fe metal. Such a low isomer shift value is not linked to the metallic character of the matrix, because a comparable value is observed in the nonmetallic sample obtained after argon annealing. In a purely ionic model, this value would suggest either a Fe4+ charge state or a low spin Fe3+ state [9] (a low spin Fe2+ state is excluded because it is diamagnetic, which is not compatible with the magnetic spectrum observed at 1.4K). But as a large hybridization between the Cu or Fe d-orbitals with the oxygen p-orbitals is likely to occur in this material, we suggest rather the presence of a high spin Fe3+ state strongly modified by covalency effects. The large and temperature independent quadrupole splittings of the doublets DI and D2 would thus reflect directly the highly anisotropic charge distribution around the two copper sites substituted by 57Fe. The low intensity doublet D3, which has a smaller quadrupole splitting (QS(D3) isomer shift (about and a larger 0.6mm/s) Table IIa.- Values obtained from the 295K Mössbauer spectra using the "symmetric lines" fit (see text). IS : isomer shift relative to SfiFe - metal ; G : full linewidth ; QS : quadrupole splitting ; P : relative area of the three quadrupole doublets. 10 Table IIb.- Values obtained from the 295K Mossbauer spectra, Table IIa for definition of symbols. relative to Fe-metal) is attributed to high spin ionic Fe3+ state located in a site less deformed than the normal Cu sites (ex. Fe3+ substituting for y3+) ; but it is not clear whether this site belongs in fact to the matrix or to some separate phase. If such an extra phase exists, it must correspond to an impurity with a relatively high iron concentration, as it is not detected by X-ray diffraction. 0.31mm/s a At 4.2K the doublets Dl and D2 are still 0.8 % suvisible in the spectrum of the x perconducting sample, but they have increased linewidths. At 1.4K (Fig. 2), they give rise to a magnetic hyperfine structure, which can be fitted to a first approximation using a mean effective field Hff -- 230kOe on the 57Fe nucleus. Such a = magnetic ture structure appears at (between 4.2K a higher tempera- and 10K) in the sample with (x 5 %). This concentra- a larger iron content tion dependence suggests that it is the coupling between the iron impurities which is responsible for their magnetic ordering. However the magnetization of the 57Fe probe could also reflect the presence of a long range or a short range magnetic order within the Cu sublattices at these low temperatures. The dynamic nature of the magnetic hyperfine interaction can also be envisaged : the low temperature magnetic splitting can be due either to blocked magnetic Fe moments or to slow paramagnetic relaxation. However, whatever the exact origin and nature of the magnetic hyperfine structure observed at 1.4K, the existence of this structure clearly shows that the electronic configuration of the Fe atoms which substitute for the Cu is not diamagnetic and that these Fe impurities are not in a spin-compensated using the "crossed lines" fit (see text). See Kondo state, as quently the 57Fe for example in Cr [10]. Conseprobe should be sensitive to any magnetic ordering within the Cu sublattices (we mention that the existence of magnetic ordering in antiferromagnetic La2Cu04 is clearly visible on a 0.5 % 57Fe probe [11]). The fact that the main paramagnetic quadrupole doublets D1 and D2 remain visible down to 4.2K thus excludes the existence of static magnetic ordering sublattices at 4.2K and above. of the Cu This conclusion agrees with that obtained from recent NMR and NQR results on Cu in superconducting YBa2Cu307 [4]. It is also to be compared to our 170Yb and 166Er Mossbauer analysis of superconducting YbBa2Cu307- b and ErBa2CU307-,b which shows low rare-earth magnetic-ordering temperatures of 0.35 and 0.7K respectively ( 12) . = Further experiments using Mossbauer emis- sion spectroscopy, which can be carried out at much lower 57Co doping concentrations than are required for Mossbauer absorption spectroscopy with 57Fe, could perhaps elucidate the origin of the low temperature magnetic hyperfine interaction and determine whether the Cu sublattices are magnetically ordered at 1.4K. Let us mention too that specific heat measurements between 30 and 200mK strongly suggest the existence of a hyperfine field at the copper nuclei in supercon- ducting Lal.g5Sro.1,5CU04-h 1131. Acknowledgments. The authors resistivity Hodges indebted to J.M. Delrieu for and to A. G6rard and for useful discussions. are measurements J.A. 11 References [1] ROBINSON, A.L., Science 267 (1987) 780. [2] DE GENNES, P.G., C.R. Hebd. Scéan. Acad. Sci., Paris 305, Série II (1987) 345348. [3] ANDERSON, P.W., Science 235 (1987) 1196. [4] LÜTGEMEIER, L., PIEPER, M.W., Solid State Commun. 64 (1987) 267. [5] STROBEL, P., CAPPONI, J.J., CHAILLOUT, C., MAREZIO, M. and THOLENCE, J.L., ture 327 (1987) Na- 306. [6] CAVA, R.J., BATLOGG, B., (1987) (1987) 1676. [7] BORDET, P., CHAILLOUT, C., CAPPONI, 309. [9] GREENWOOD, N.N. and GIBB, T.C., Mössbauer Spectroscopy (Chapman and Hall Ltd, London) 1971, p. 91. [10] HERBERT, I.R., CLARK, P.E. and WILSON, G.V.H., J. Phys. Chem. Solids 33 (1972) VAN DOVER, R.B., MURPHY, D.W., SUNSHINE, S., SIEGRIST, T., REMEIKA, J.P., RIETMAN, E.A., ZAHURAK, S. and ESPINOSA, G.P., Phys. Rev. Lett. 58 J.J., CHENAVAS, J. and MAREZIO, M. Preprint. [8] RIESEMEIER, H., CRABOW, Ch., SCHEIDT, E.W., MÜLLER, V., LÜDERS, K. and RIEGEL, D., Solid State Commun. 64 979. To be published. [11] HODGES, J.A., IMBERT, [12] P. and JÉHANNO, G., Commun., to be published. GUTSMIEDL, P., WOLFF, G., and ANDRES, K., Phys. Rev. B36 (1987) 4043. Solid State [13]
© Copyright 2024