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NMR study of 19F resonance above Tc in fluorinated YBa2Cu3Ox
Physica C 304 Ž1998. 283–292
NMR study of 19 F resonance above Tc in fluorinated YBa 2 Cu 3 O x S.D. Goren
a,)
, C. Korn a , C. Perrin b, W. Hoffmann c , A. Privalov c , H.M. Vieth c , c K. Luders ¨ a
b
Department of Physics, Ben Gurion UniÕersity, Be’er SheÕa, Israel Laboratoire de Chimie du Solide et Inorganique Moleculaire, UniÕersite´ de Rennes-I, AÕenue du General Leclerc, 35402 Rennes Cedex, ´ France c Fachbereich Physik, Freie UniÕersitat ¨ Berlin, Arnimallee 14, D-14195 Berlin, Germany Received 10 November 1997; revised 10 February 1998; accepted 20 May 1998
Abstract The 19 F NMR spectra were obtained at 282.365 MHz, for a series of YBa 2 Cu 3 O x Fy samples at a number of different temperatures, where y s 0.1, 0.2 when x s 7; y s 0.10, 0.15, 0.29, 0.38 when x s 6.7; and y s 0.6, 0.98, 1.50 when x s 6. These values of x span the fully oxygenated superconductor, the partially oxygenated superconductor, and the antiferromagnetic non-superconductor when the material is undoped. The spectra were obtained using a phase cycled echo. By measuring the echoes using a series of different times between pulses, we were able to resolve the spectra into two separate lines, representing two different F sites where the occupancy of one is much larger than the other, their ratio depending on the concentrations. The major site is attributed to the chain sites along a or b or both and the minor site is tentatively assigned to the apical position between the chain copper and the plain copper. The frequency shift of the major component is independent of both the oxygen and the fluorine concentration. q 1998 Elsevier Science B.V. All rights reserved. Keywords: YBCO; NMR; Fluorination
1. Introduction The relative roles that crystallographic structural rearrangements and carrier concentration play in influencing the superconducting properties of high Tc superconductors are still an open question. The effect of fluorine doping has received considerable attention w1–13x. Besides shedding light on the problem of superconducting behavior, fluorine can be used as a probe of the magnetic vortex system below the
)
Corresponding author. Tel.: q972-7-646171; Fax: q972-76472903; E-mail: [email protected]
superconducting transition temperature. While muon spin rotation is useful in the low magnetic field regime, and the magnetic resonance of the nuclei of the parent material can be employed in the high magnetic field regime, 19 F, with its high gyromagnetic ratio, spin 1r2 and 100% natural abundance, is ideal for probing the intermediate magnetic field vortex system w14x. In this study, we obtained the 19 F resonance in fluorine doped YBCO above Tc , in order to obtain information on how fluorine behaves in the material. It is well known that the superconducting properties of YBa 2 Cu 3 O x ŽYBCO. are dependent on the oxygen concentration; thus at oxygen concentrations
0921-4534r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 Ž 9 8 . 0 0 2 5 8 - 5
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just above x s 6.0, the material is a non-superconductor with the plane Cu ions oriented antiferromagnetically. As the oxygen concentration is raised above x s 6.4, the material becomes a superconductor with increasing values of Tc , resulting in a maximum Tc of 92 K at x f 6.95. Hence doping with holes enhances the superconducting properties in this regime. There have been many studies conducted concerning the role of various dopants on the properties of YBCO. Since fluorine, like oxygen, is an anion, one may expect that doping with F can influence the superconducting properties in a manner similar to increasing the oxygen concentration. It has been found w1–5,12x that fluorine can have a restorative effect on the onset superconducting transition temperature of oxygen deficient material. Thus an oxygen deficient material YBa 2 Cu 3 O6.7 having an onset Tc of 63 K, increases its Tc progressively upon fluorination to its final value of 91 K at a concentration of y s 0.2 ŽYBa 2 Cu 3 O x Fy .. Increasing the fluorine content beyond this concentration, decreases the superconducting diamagnetic signal without further increasing Tc . Fluorination of a fully oxygenated material does not increase its Tc beyond 91 K, but has its diamagnetic signal decreased. This reduction is attributed to intergranular effects w2,15x. Similar results were reported for F in Hg compounds w16x. In order to elucidate the role of F more fully, we conducted nuclear magnetic resonance ŽNMR. measurements above Tc , on a series of YBa 2 Cu 3 O x Fy samples where x is ; 7, 6.7, and 6.0. This repre-
Fig. 1. The 19 F ambient temperature NMR spectra showing how the different oxygen and fluorine concentrations affect the line shape.
Table 1 Onset Tc in YBa 2 Cu 3 O x Fy xs7
x s6.7
x s6
y
Tc ŽK.
0.00 0.10 0.20 0.00 0.10 0.15 0.25 0.29 0.38 0.00 0.60 0.98 1.50
91.0 91.0 91.0 62.5 71.0 89.5 91.0 91.0 91.0 Non Non Non 50
Fig. 2. 19 F spectra at ambient temperature for YBa 2 Cu 3 O6.7 F0.1 obtained from the echo using different values of t . The solid lines are LSFs to two Lorentzians.
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sents respectively the fully oxygenated and partially oxygenated superconductors, and when the material is not fluorinated, the non-superconductor antiferromagnetic ŽAF. material. The superconducting properties of these samples are summarized in Table 1. The 89 Y NMR resonance was reported previously w17x. In this study we describe the results of a series of NMR measurements on the 19 F nucleus for the same series of samples. These nuclei have spin 1r2 so that the interpretation of the results is unencumbered with the complications of quadrupole effects.
2. Experimental technique The measurements were performed at 282.365 MHz, the reference resonance frequency of 19 F in C 6 F6 at this field. The phase cycled Hahn echo was obtained at room temperature as a function of time t between the 908 and 1808 pulses, where in general t s 10, 20, 35, 55, 80, 120 ms. This was useful in separating multiple resonance lines. The pulse sepa-
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ration at other temperatures was limited to 10 and 55 ms. The number of scans for each sample ranged from about 2000 to 160,000, with the number of scans increasing with increasing t , in order to compensate for the lower signal to noise ratio. Sample preparation and characterization are described in Refs. w1,3x.
3. Experimental results Before going into the details, let us first get an overview of the influence of fluorine and oxygen concentrations on the 19 F NMR spectra. Fig. 1 shows the room temperature spectra for four different concentrations. The zero of the frequency was taken at the frequency of the measurements and coincides with the resonance frequency of our standard C 6 F6 . The maxima have been normalized to the same value for easy comparison. The YBa 2 Cu 3 O 7 F0.1 sample is superconducting below 91 K, and gives the narrowest line. The YBa 2 Cu 3 O6.7 F0.15 sample also becomes
Fig. 3. The ambient temperature 19 F resonance shift of the two components, as a function of fluorine concentration, and different oxygen concentrations. ŽThe reference frequency is 282,365 kHz, hence wppmx s Ž10 6r282,365. wkHzx.. The full and empty symbols represent respectively the major and minor occupation site.
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Fig. 4. The ambient temperature 19 F linewidths of the two components, as a function of fluorine concentration, and different oxygen concentrations. Note change of scale for the YBa 2 Cu 3 O6 samples. The full and empty symbols represent respectively the major and minor occupation site.
superconducting at similar temperatures, and the spectrum is almost the same as that of YBa 2 Cu 3 O 7 F0.1 , except that an additional feature appears at the low frequency region. The YBa 2 Cu 3 O6 F0.6 is non superconducting, and gives a wider NMR line. When more fluorine is added at this oxygen concentration Ž y s 1.5., the material becomes a superconductor below 50 K with small superconducting amplitude. The NMR line however is not narrowed, but on the contrary, the line is even wider. A detailed analysis w18x shows that the 19 F spectra we obtain, cannot have their origin in possible decomposition products caused by the fluorination w19x. 3.1. YBa2 Cu 3 O6 .7 Fy (y s 0.10, 0.15, 0.25, 0.29, 0.38) We first present the results of the partially oxygenated sample YBa 2 Cu 3 O6.7 , since here the fact that the signal consists of two resonances is most obvious. This is illustrated in Fig. 2 which shows the room temperature 19 F resonance in YBa 2 Cu 3O6.7 F0.10 , obtained from the Hahn echo for a series of different values of t . The solid lines are the least
Fig. 5. The ratio of signal intensity of the two components representing the two sites in YBa 2 Cu 3 O6.7 Fy , as a function of fluorine concentration, at ambient temperature. This ratio is a measure of the relative F occupancy at the two sites.
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square fits ŽLSF. to two Lorentzians. None of the spectra for any of the samples were compatible with a Gaussian fit. The two lines represent fluorine at two different sites. It is seen that the spin–spin relaxation times T2 , of the two lines are different; the higher frequency line disappearing faster with increasing t than the lower frequency one. In what follows we will designate the lower frequency line and the atomic site it represents by the subscript 1, and the other one by subscript 2. A least squares fit analysis was performed for all the spectra in order to obtain the positions, linewidths, and intensity ratios A 2rA1 of the two lines as a function of fluorine concentration y. These are shown in Figs. 3–5. The analysis and separation of the contributions of the two sites is facilitated by the fact that at low values of t , the contribution of site 2 dominates while at the higher t , site 1 dominates. The ratio A 2rA1 takes into account the linewidths and is thus proportional to the ratio of the signal intensities of the two lines. This value depends on the spin–spin lattice relaxation time so that A 2rA1 is a function of t . When t Fig. 7. Comparison of the ambient temperature lineshapes of YBa 2 Cu 3 O6 Fy upon changing t . The spectra have been normalized to give the same maximum amplitude.
Fig. 6. An example of the change in relative intensity of the components representing the two sites, as a function of t , the interval between the two pulses. The ratio changes due to the different values of T2 for the two sites. The extrapolated value of A 2 r A1 at t s 0 is indicative of the ratio of the occupancy at the two sites, and is the value of A 2 r A1 plotted in Fig. 5.
is equal to zero Ži.e., before relaxation takes place., A 2rA1 is proportional to the ratio of number of nuclei located at the respective sites. Hence A 2rA1 was plotted as a function of t and the curve was extrapolated to t s 0 Žsee Fig. 6 for an example.. These extrapolated values were used in the plot of A 2rŽ A1 q A 2 . shown in Fig. 5. The results show that site 2 is much more populated than site 1. In all the figures, the line representing the major contribution is indicated by a filled symbol, and the other by an open one. Fig. 3 shows that the frequency shifts for the two sites are of the order of y60 ppm and 85 ppm and independent of the F concentration. The respective linewidths W1 and W2 are of the order of 45 kHz. While the intensity ratios A 2rŽ A1 q A 2 . are only indicative of the fluorine concentrations at the respective sites, it is obvious from Fig. 5 that site 2 is overwhelmingly favored over site 1, and this preference is increased as additional fluorine is introduced.
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Measurements were also performed at T s 150, 200, 250 and 350 K for samples having x s 0.10 and 0.38 and at T s 150, 225 and 350 K for samples having x s 0.15 and 0.29. These were performed for only two values of t , namely t s 10 and 55 ms. An analysis of the results showed a slight shift towards lower frequencies as the temperature was raised. 3.2. YBa2 Cu 3 O6 Fy (y s 0.60, 0.98, 1.50) The 19 F spectra of samples having an oxygen concentration of x s 6, give wider lines ŽFig. 1., the widest being that for y s 1.5. The samples having fluorine concentrations of y s 0.6 and 0.98 are somewhat narrower and almost the same. Samples YBa 2 Cu 3 O6 Fy also show evidence of the existence of more than one site although the differences in frequency shift are much smaller than those for YBa 2 Cu 3 O6.7 Fy . They are distinguishable mainly by the difference in their linewidth. This is
seen from Fig. 7 which shows the spectra for various values of t . They have been normalized to give the same maximum for easy comparison of the linewidth. If we limit ourselves on physical grounds to two well defined sites, then some consistency is found to a two line fit of the spectra for different values of t . This is supported by a monotonic change in A1rA 2 as a function of t , as would be expected from two lines having different T2 ’s. Their shifts and linewidths are plotted in Figs. 3 and 4. Using these values of the line positions and widths, a least squares analysis for the respective intensities of the two lines was performed for the different values of t ŽFig. 8.. It is seen that at low values of t , the wide line gives very little contribution. As t is increased, the relative value of the wide line increases, until at t ( 75 ms, only the wide line contribution remains. Hence, while the major occupancy is the narrow line site, its T2 is much shorter than that due to the wide line site. This indicates that the broadness of the wide line is due to
Fig. 8. The relative contributions of narrow and wide lines as a function of t for YBa 2 Cu 3 O6 F0.60 and YBa 2 Cu 3 O6 F0.98 . It can be seen that the wide line relaxes much slower than the narrow line.
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concentration is of the order of the main component ŽFig. 4.. The line shifts were essentially independent of temperature, within the experimental error. Although the temperature dependence of A 2rA1 would be an important parameter, indicating shifting of occupation sites with temperature, the results showed too much scatter to yield useful information. The width of the lines however were narrowed with increasing temperature as indicated in Fig. 9. 3.3. YBa2 Cu 3 O 7 Fy (y s 0.10, 0.20) Although the line shape as a function of t showed that there is an additional component to the resonance line also for the fully oxygenated samples YBa 2 Cu 3 O 7 Fy , the signal to noise was such that we were unable to obtain consistent parameters in the least square fitting process. A fair fit was obtained assuming a single Lorentzian for the shortest t Ž10 Fig. 9. The linewidths of the main Žfilled symbols. and minor Žopen symbols. NMR lines of samples YBa 2 Cu 3 O6 F0.6 Žcircles. and YBa 2 Cu 3 O6 F0.98 Žtriangles. as a function of temperature. The relatively large reduction of width of the minor line is attributed to a reduction of the AF field intensity with temperature.
inhomogenous broadening, and it has an intrinsic T2 much larger than that of the narrow line. By extrapolating the values of A 2rA1 to t s 0, we find that for YBa 2 Cu 3 O6 F0.6 the occupancy of the narrow line site is of the order of 20 times more than the wide line site, and this ratio increases to about 100:1 for YBa 2 Cu 3 O6 F0.98 . When y s 1.5, the weak line is more difficult to separate from the strong line since the linewidth for both components are similar, and the weak line could not be discerned at t s 10 ms. The line shifts and widths for all these samples are given in Figs. 3 and 4. In contrast to the minor wide line, the resonance frequency and the linewidth of the major component changes only slightly with changes in the F concentration. The minor line narrows considerably with increasing F concentration. The narrowing phenomenon can be seen quite clearly by comparing the widths of the lines for y s 0.60 and 0.98 at t s 55 ms in Fig. 8. We were able to detect a trace of a second line in the y s 1.50 sample only for large values of t . Its linewidth at this
Fig. 10. The room temperature 19 F resonance lines for the fully oxygenated samples having fluorine concentrations y s 0.1 and 0.2. The solid line is a Lorentzian fit to the data.
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Fig. 11. The temperature dependence of the 19 F resonance shift for the fully oxygenated samples having y s 0.1 Žsquares. and 0.2 Žtriangles.. The full curves are aids to the eye and have no physical significance.
of 50 kHz, it is reduced to about 30 kHz at 1.3 T w14x. Let us first consider the samples having an oxygen concentration of x s 6.7. Neutron diffraction of YBa 2 Cu 3 O6.7 Fy w5x showed that in the regime where Tc increases from 63 K in the undoped material to 90 K when y s 0.28, both the OŽ4. and OŽ5. occupancies increase, with the former seeming to saturate at a value of 0.79. In contrast, by increasing the O concentration to x s 7 the OŽ4. occupancy approaches unity. Neutron diffraction cannot distinguish between O and F occupancy. Thus, while it is certain that F enters either one of, or both of the OŽ4. or OŽ5. sites, it is not clear how they are distributed. Extended Huckel tight binding calculations per¨ formed on YBa 2 Cu 3 O6.75 F0.25 w10x show that energetically these are very favorable sites and there is very little difference between them. One would have hoped that NMR would help decide the situation. Had we obtained two strong distinct resonances, one could have attributed each one to F at the OŽ4. and OŽ5. site respectively. But our results show that
ms.. The room temperature results are shown in Fig. 10, and the resonance shifts and linewidths are plotted in Figs. 3 and 4. The position of the resonance lines is the same as the other samples. There is a steady decrease in the resonance frequency with increasing temperature for the y s 0.1 sample, while the y s 0.2 sample shows a relative insensitivity to a change in temperature ŽFig. 11.. Fig. 12 indicates some narrowing for both fluorine concentrations, with increasing temperature.
4. Discussion All the spectra were fit to a Lorentian line shape. None were compatible to a Gaussian. The shape becomes Gaussian only at temperatures below Tc w14x. This may mean that the origin of the linewidth is due to a large extent to interaction with the Cu ions rather than 19 F dipole–dipole interactions. This is bolstered by the fact that while the linewidth measured at a field of 7 T Žthis study. is of the order
Fig. 12. The temperature dependence of the 19 F linewidth for the fully oxygenated sample having y s 0.1 Žsquares. and 0.2 Žtriangles.. The full curves are aids to the eye and have no physical significance.
S.D. Goren et al.r Physica C 304 (1998) 283–292
except for a very small contribution of a second line ŽFig. 5., there is only one major resonance line, and the frequency of this line is independent of both oxygen concentration and fluorine concentration ŽFig. 3.. Since the environments of the OŽ4. and OŽ5. sites are so similar, one is at a loss to say whether the strong line is due to F at only one site, or whether both sites contribute. The fractional occupation of the minor site was found to be very small, and it decreases with increasing F concentration ŽFig. 5.. The ratios A 2rA1 used in the figure were obtained by assuming two different T2 ’s for each site. Then the ratio A 2rA1 should vary exponentially with t . Hence the fit was to an exponential decay that was extrapolated to t s 0. This gave A 2rA1 values significantly larger than that for the smallest t , Žt s 10 ms.. The resulting large values of this ratio are thus dependent on this assumption. But even if a linear extrapolation from the lowest values of t were taken, the A 2rA1 values would be too high to assume that the minor site has a significant influence on the recovery of Tc observed upon fluorination. The origin of this minor line will now be discussed. While the O 7 and O6.7 samples were superconductors for all F concentrations, YBa 2 Cu 3 O6 is an AF nonsuperconductor and remains a nonsuperconductor for y s 0.60 and 0.98. At y s 1.50 it is a Tc s 50 K superconductor. Figs. 3 and 4 show the resonance positions and linewidths of the major and minor sites. The values of A 2rA1 obtained from an exponential extrapolation were about 25 for y s 0.6 and in the thousands for y s 0.98 and even greater for the y s 1.50 sample. The fact that the minor line was detectable seems to indicate that a strictly exponential extrapolation may be an exaggeration. Nevertheless the occupancy of the second site seems to be negligible as far as having an effect on the superconducting properties. Its linewidth is 170 kHz for y s 0.6 and decreases to that of the main component as y s 1.50 is reached and the material is a superconductor. Such a wide line can be attributable to the AF interaction which exists between the CuŽ2. in the nonsuperconductor. The width decreases as the fluorine concentration is increased, due to the weakening of the AF field, until at y s 1.5, there is no AF interaction. The AF interaction also probably contributes to the shift in resonance frequency of the
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weak line. The fact that the large linewidth is due to the AF interaction is also supported by the temperature dependence of the linewidth ŽFig. 9.. As the temperature is increased from 175 K to 450 K the line narrows significantly due to a weakening of the AF interaction. It is unlikely that this narrowing is due to kinetics since one would expect a narrower line to narrow more than a wide line for the same thermal frequency dependence. The main component is however hardly effected by the increase in temperature. The fact that the minor lines have a width of the order of 170 and 100 kHz for the AF samples YBa 2 Cu 3 O6 F0.6 and YBa 2 Cu 3 O6 F0.98 , while in the same samples the main line linewidth remains at about 50 kHz, shows that the site of the minor line must be much closer to the CuŽ2. plains than the major line and also in a non-symmetric site as far as the AF field is concerned. A possible site may be one where some F have replaced the bridging OŽ1. atoms along the c axis. Energy calculations have shown this to be a possibility w10x, especially if the bridging is to a CuŽ1. that has an adjacent vacancy. This site would place the F close to the magnetic CuŽ2. atoms. The fact that it yields different resonance frequencies at the different oxygen concentrations, can be explained by the change in bonding due to the large shifts of the O–Cu distances upon changing the oxygen concentration and also by a change in the AF interaction. Since the frequency shift of the major line is practically independent of the oxygen and fluorine concentration within the experimental error, it is probable that the F site Žor sites. is the same for all the oxygen concentrations. These are the OŽ4. andror OŽ5. positions when x s 6.7, as shown by neutron diffraction, so they are also the F sites for all the O concentrations. The YBa 2 Cu 3 O6 samples that have been restored to superconductors have a small Meissner volume. One should not interpret this as the introduction of a small amount of superconductor into an unchanged matrix of the original AF material. This can be seen from the linewidth behavior of the minor line in YBa 2 Cu 3 O6 ŽFig. 4., which shows that the AF interaction is diminished as y increases, and the linewidth approaches that of the major line at y s 1.5. If the region were multiphased with a large population of
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the original AF material, then the minor line should have remained wide. This does not preclude the existence of a changed AF phase coexisting with a small amount of superconductor. For example, the data would be compatible with an AF having its T N lowered below the temperature of our measurements Ž; 200 K. or of a disordered magnetic phase Žwith a low spin glass temperature. coexisting with a superconductor phase with a highly distributed Tc .
5. Conclusions 1. When F is added to YBa 2 Cu 3 O x , only one major NMR line is obtained at a frequency that is independent of oxygen or fluorine concentration. Hence if more than one site is occupied, their local environments are similar Žsuch as OŽ4. and OŽ5... 2. The AF behavior can be followed as a function of F concentration and temperature from the NMR line contribution from F at the sparsely occupied site in YBa 2 Cu 3 O6 Fy . 3. Although the Meissner volume in samples that have its superconducting property restored by F is small, the material is not simply mixed phase of the original AF and a superconductor, since it is seen that the AF interaction is reduced in the entire sample even before the superconducting phase is reached. The resulting material may or may not be a single phase.
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NMR study of 19 F resonance above Tc in fluorinated YBa 2 Cu 3 O x S.D. Goren
a,)
, C. Korn a , C. Perrin b, W. Hoffmann c , A. Privalov c , H.M. Vieth c , c K. Luders ¨ a
b
Department of Physics, Ben Gurion UniÕersity, Be’er SheÕa, Israel Laboratoire de Chimie du Solide et Inorganique Moleculaire, UniÕersite´ de Rennes-I, AÕenue du General Leclerc, 35402 Rennes Cedex, ´ France c Fachbereich Physik, Freie UniÕersitat ¨ Berlin, Arnimallee 14, D-14195 Berlin, Germany Received 10 November 1997; revised 10 February 1998; accepted 20 May 1998
Abstract The 19 F NMR spectra were obtained at 282.365 MHz, for a series of YBa 2 Cu 3 O x Fy samples at a number of different temperatures, where y s 0.1, 0.2 when x s 7; y s 0.10, 0.15, 0.29, 0.38 when x s 6.7; and y s 0.6, 0.98, 1.50 when x s 6. These values of x span the fully oxygenated superconductor, the partially oxygenated superconductor, and the antiferromagnetic non-superconductor when the material is undoped. The spectra were obtained using a phase cycled echo. By measuring the echoes using a series of different times between pulses, we were able to resolve the spectra into two separate lines, representing two different F sites where the occupancy of one is much larger than the other, their ratio depending on the concentrations. The major site is attributed to the chain sites along a or b or both and the minor site is tentatively assigned to the apical position between the chain copper and the plain copper. The frequency shift of the major component is independent of both the oxygen and the fluorine concentration. q 1998 Elsevier Science B.V. All rights reserved. Keywords: YBCO; NMR; Fluorination
1. Introduction The relative roles that crystallographic structural rearrangements and carrier concentration play in influencing the superconducting properties of high Tc superconductors are still an open question. The effect of fluorine doping has received considerable attention w1–13x. Besides shedding light on the problem of superconducting behavior, fluorine can be used as a probe of the magnetic vortex system below the
)
Corresponding author. Tel.: q972-7-646171; Fax: q972-76472903; E-mail: [email protected]
superconducting transition temperature. While muon spin rotation is useful in the low magnetic field regime, and the magnetic resonance of the nuclei of the parent material can be employed in the high magnetic field regime, 19 F, with its high gyromagnetic ratio, spin 1r2 and 100% natural abundance, is ideal for probing the intermediate magnetic field vortex system w14x. In this study, we obtained the 19 F resonance in fluorine doped YBCO above Tc , in order to obtain information on how fluorine behaves in the material. It is well known that the superconducting properties of YBa 2 Cu 3 O x ŽYBCO. are dependent on the oxygen concentration; thus at oxygen concentrations
0921-4534r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 Ž 9 8 . 0 0 2 5 8 - 5
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just above x s 6.0, the material is a non-superconductor with the plane Cu ions oriented antiferromagnetically. As the oxygen concentration is raised above x s 6.4, the material becomes a superconductor with increasing values of Tc , resulting in a maximum Tc of 92 K at x f 6.95. Hence doping with holes enhances the superconducting properties in this regime. There have been many studies conducted concerning the role of various dopants on the properties of YBCO. Since fluorine, like oxygen, is an anion, one may expect that doping with F can influence the superconducting properties in a manner similar to increasing the oxygen concentration. It has been found w1–5,12x that fluorine can have a restorative effect on the onset superconducting transition temperature of oxygen deficient material. Thus an oxygen deficient material YBa 2 Cu 3 O6.7 having an onset Tc of 63 K, increases its Tc progressively upon fluorination to its final value of 91 K at a concentration of y s 0.2 ŽYBa 2 Cu 3 O x Fy .. Increasing the fluorine content beyond this concentration, decreases the superconducting diamagnetic signal without further increasing Tc . Fluorination of a fully oxygenated material does not increase its Tc beyond 91 K, but has its diamagnetic signal decreased. This reduction is attributed to intergranular effects w2,15x. Similar results were reported for F in Hg compounds w16x. In order to elucidate the role of F more fully, we conducted nuclear magnetic resonance ŽNMR. measurements above Tc , on a series of YBa 2 Cu 3 O x Fy samples where x is ; 7, 6.7, and 6.0. This repre-
Fig. 1. The 19 F ambient temperature NMR spectra showing how the different oxygen and fluorine concentrations affect the line shape.
Table 1 Onset Tc in YBa 2 Cu 3 O x Fy xs7
x s6.7
x s6
y
Tc ŽK.
0.00 0.10 0.20 0.00 0.10 0.15 0.25 0.29 0.38 0.00 0.60 0.98 1.50
91.0 91.0 91.0 62.5 71.0 89.5 91.0 91.0 91.0 Non Non Non 50
Fig. 2. 19 F spectra at ambient temperature for YBa 2 Cu 3 O6.7 F0.1 obtained from the echo using different values of t . The solid lines are LSFs to two Lorentzians.
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sents respectively the fully oxygenated and partially oxygenated superconductors, and when the material is not fluorinated, the non-superconductor antiferromagnetic ŽAF. material. The superconducting properties of these samples are summarized in Table 1. The 89 Y NMR resonance was reported previously w17x. In this study we describe the results of a series of NMR measurements on the 19 F nucleus for the same series of samples. These nuclei have spin 1r2 so that the interpretation of the results is unencumbered with the complications of quadrupole effects.
2. Experimental technique The measurements were performed at 282.365 MHz, the reference resonance frequency of 19 F in C 6 F6 at this field. The phase cycled Hahn echo was obtained at room temperature as a function of time t between the 908 and 1808 pulses, where in general t s 10, 20, 35, 55, 80, 120 ms. This was useful in separating multiple resonance lines. The pulse sepa-
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ration at other temperatures was limited to 10 and 55 ms. The number of scans for each sample ranged from about 2000 to 160,000, with the number of scans increasing with increasing t , in order to compensate for the lower signal to noise ratio. Sample preparation and characterization are described in Refs. w1,3x.
3. Experimental results Before going into the details, let us first get an overview of the influence of fluorine and oxygen concentrations on the 19 F NMR spectra. Fig. 1 shows the room temperature spectra for four different concentrations. The zero of the frequency was taken at the frequency of the measurements and coincides with the resonance frequency of our standard C 6 F6 . The maxima have been normalized to the same value for easy comparison. The YBa 2 Cu 3 O 7 F0.1 sample is superconducting below 91 K, and gives the narrowest line. The YBa 2 Cu 3 O6.7 F0.15 sample also becomes
Fig. 3. The ambient temperature 19 F resonance shift of the two components, as a function of fluorine concentration, and different oxygen concentrations. ŽThe reference frequency is 282,365 kHz, hence wppmx s Ž10 6r282,365. wkHzx.. The full and empty symbols represent respectively the major and minor occupation site.
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Fig. 4. The ambient temperature 19 F linewidths of the two components, as a function of fluorine concentration, and different oxygen concentrations. Note change of scale for the YBa 2 Cu 3 O6 samples. The full and empty symbols represent respectively the major and minor occupation site.
superconducting at similar temperatures, and the spectrum is almost the same as that of YBa 2 Cu 3 O 7 F0.1 , except that an additional feature appears at the low frequency region. The YBa 2 Cu 3 O6 F0.6 is non superconducting, and gives a wider NMR line. When more fluorine is added at this oxygen concentration Ž y s 1.5., the material becomes a superconductor below 50 K with small superconducting amplitude. The NMR line however is not narrowed, but on the contrary, the line is even wider. A detailed analysis w18x shows that the 19 F spectra we obtain, cannot have their origin in possible decomposition products caused by the fluorination w19x. 3.1. YBa2 Cu 3 O6 .7 Fy (y s 0.10, 0.15, 0.25, 0.29, 0.38) We first present the results of the partially oxygenated sample YBa 2 Cu 3 O6.7 , since here the fact that the signal consists of two resonances is most obvious. This is illustrated in Fig. 2 which shows the room temperature 19 F resonance in YBa 2 Cu 3O6.7 F0.10 , obtained from the Hahn echo for a series of different values of t . The solid lines are the least
Fig. 5. The ratio of signal intensity of the two components representing the two sites in YBa 2 Cu 3 O6.7 Fy , as a function of fluorine concentration, at ambient temperature. This ratio is a measure of the relative F occupancy at the two sites.
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square fits ŽLSF. to two Lorentzians. None of the spectra for any of the samples were compatible with a Gaussian fit. The two lines represent fluorine at two different sites. It is seen that the spin–spin relaxation times T2 , of the two lines are different; the higher frequency line disappearing faster with increasing t than the lower frequency one. In what follows we will designate the lower frequency line and the atomic site it represents by the subscript 1, and the other one by subscript 2. A least squares fit analysis was performed for all the spectra in order to obtain the positions, linewidths, and intensity ratios A 2rA1 of the two lines as a function of fluorine concentration y. These are shown in Figs. 3–5. The analysis and separation of the contributions of the two sites is facilitated by the fact that at low values of t , the contribution of site 2 dominates while at the higher t , site 1 dominates. The ratio A 2rA1 takes into account the linewidths and is thus proportional to the ratio of the signal intensities of the two lines. This value depends on the spin–spin lattice relaxation time so that A 2rA1 is a function of t . When t Fig. 7. Comparison of the ambient temperature lineshapes of YBa 2 Cu 3 O6 Fy upon changing t . The spectra have been normalized to give the same maximum amplitude.
Fig. 6. An example of the change in relative intensity of the components representing the two sites, as a function of t , the interval between the two pulses. The ratio changes due to the different values of T2 for the two sites. The extrapolated value of A 2 r A1 at t s 0 is indicative of the ratio of the occupancy at the two sites, and is the value of A 2 r A1 plotted in Fig. 5.
is equal to zero Ži.e., before relaxation takes place., A 2rA1 is proportional to the ratio of number of nuclei located at the respective sites. Hence A 2rA1 was plotted as a function of t and the curve was extrapolated to t s 0 Žsee Fig. 6 for an example.. These extrapolated values were used in the plot of A 2rŽ A1 q A 2 . shown in Fig. 5. The results show that site 2 is much more populated than site 1. In all the figures, the line representing the major contribution is indicated by a filled symbol, and the other by an open one. Fig. 3 shows that the frequency shifts for the two sites are of the order of y60 ppm and 85 ppm and independent of the F concentration. The respective linewidths W1 and W2 are of the order of 45 kHz. While the intensity ratios A 2rŽ A1 q A 2 . are only indicative of the fluorine concentrations at the respective sites, it is obvious from Fig. 5 that site 2 is overwhelmingly favored over site 1, and this preference is increased as additional fluorine is introduced.
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Measurements were also performed at T s 150, 200, 250 and 350 K for samples having x s 0.10 and 0.38 and at T s 150, 225 and 350 K for samples having x s 0.15 and 0.29. These were performed for only two values of t , namely t s 10 and 55 ms. An analysis of the results showed a slight shift towards lower frequencies as the temperature was raised. 3.2. YBa2 Cu 3 O6 Fy (y s 0.60, 0.98, 1.50) The 19 F spectra of samples having an oxygen concentration of x s 6, give wider lines ŽFig. 1., the widest being that for y s 1.5. The samples having fluorine concentrations of y s 0.6 and 0.98 are somewhat narrower and almost the same. Samples YBa 2 Cu 3 O6 Fy also show evidence of the existence of more than one site although the differences in frequency shift are much smaller than those for YBa 2 Cu 3 O6.7 Fy . They are distinguishable mainly by the difference in their linewidth. This is
seen from Fig. 7 which shows the spectra for various values of t . They have been normalized to give the same maximum for easy comparison of the linewidth. If we limit ourselves on physical grounds to two well defined sites, then some consistency is found to a two line fit of the spectra for different values of t . This is supported by a monotonic change in A1rA 2 as a function of t , as would be expected from two lines having different T2 ’s. Their shifts and linewidths are plotted in Figs. 3 and 4. Using these values of the line positions and widths, a least squares analysis for the respective intensities of the two lines was performed for the different values of t ŽFig. 8.. It is seen that at low values of t , the wide line gives very little contribution. As t is increased, the relative value of the wide line increases, until at t ( 75 ms, only the wide line contribution remains. Hence, while the major occupancy is the narrow line site, its T2 is much shorter than that due to the wide line site. This indicates that the broadness of the wide line is due to
Fig. 8. The relative contributions of narrow and wide lines as a function of t for YBa 2 Cu 3 O6 F0.60 and YBa 2 Cu 3 O6 F0.98 . It can be seen that the wide line relaxes much slower than the narrow line.
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concentration is of the order of the main component ŽFig. 4.. The line shifts were essentially independent of temperature, within the experimental error. Although the temperature dependence of A 2rA1 would be an important parameter, indicating shifting of occupation sites with temperature, the results showed too much scatter to yield useful information. The width of the lines however were narrowed with increasing temperature as indicated in Fig. 9. 3.3. YBa2 Cu 3 O 7 Fy (y s 0.10, 0.20) Although the line shape as a function of t showed that there is an additional component to the resonance line also for the fully oxygenated samples YBa 2 Cu 3 O 7 Fy , the signal to noise was such that we were unable to obtain consistent parameters in the least square fitting process. A fair fit was obtained assuming a single Lorentzian for the shortest t Ž10 Fig. 9. The linewidths of the main Žfilled symbols. and minor Žopen symbols. NMR lines of samples YBa 2 Cu 3 O6 F0.6 Žcircles. and YBa 2 Cu 3 O6 F0.98 Žtriangles. as a function of temperature. The relatively large reduction of width of the minor line is attributed to a reduction of the AF field intensity with temperature.
inhomogenous broadening, and it has an intrinsic T2 much larger than that of the narrow line. By extrapolating the values of A 2rA1 to t s 0, we find that for YBa 2 Cu 3 O6 F0.6 the occupancy of the narrow line site is of the order of 20 times more than the wide line site, and this ratio increases to about 100:1 for YBa 2 Cu 3 O6 F0.98 . When y s 1.5, the weak line is more difficult to separate from the strong line since the linewidth for both components are similar, and the weak line could not be discerned at t s 10 ms. The line shifts and widths for all these samples are given in Figs. 3 and 4. In contrast to the minor wide line, the resonance frequency and the linewidth of the major component changes only slightly with changes in the F concentration. The minor line narrows considerably with increasing F concentration. The narrowing phenomenon can be seen quite clearly by comparing the widths of the lines for y s 0.60 and 0.98 at t s 55 ms in Fig. 8. We were able to detect a trace of a second line in the y s 1.50 sample only for large values of t . Its linewidth at this
Fig. 10. The room temperature 19 F resonance lines for the fully oxygenated samples having fluorine concentrations y s 0.1 and 0.2. The solid line is a Lorentzian fit to the data.
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Fig. 11. The temperature dependence of the 19 F resonance shift for the fully oxygenated samples having y s 0.1 Žsquares. and 0.2 Žtriangles.. The full curves are aids to the eye and have no physical significance.
of 50 kHz, it is reduced to about 30 kHz at 1.3 T w14x. Let us first consider the samples having an oxygen concentration of x s 6.7. Neutron diffraction of YBa 2 Cu 3 O6.7 Fy w5x showed that in the regime where Tc increases from 63 K in the undoped material to 90 K when y s 0.28, both the OŽ4. and OŽ5. occupancies increase, with the former seeming to saturate at a value of 0.79. In contrast, by increasing the O concentration to x s 7 the OŽ4. occupancy approaches unity. Neutron diffraction cannot distinguish between O and F occupancy. Thus, while it is certain that F enters either one of, or both of the OŽ4. or OŽ5. sites, it is not clear how they are distributed. Extended Huckel tight binding calculations per¨ formed on YBa 2 Cu 3 O6.75 F0.25 w10x show that energetically these are very favorable sites and there is very little difference between them. One would have hoped that NMR would help decide the situation. Had we obtained two strong distinct resonances, one could have attributed each one to F at the OŽ4. and OŽ5. site respectively. But our results show that
ms.. The room temperature results are shown in Fig. 10, and the resonance shifts and linewidths are plotted in Figs. 3 and 4. The position of the resonance lines is the same as the other samples. There is a steady decrease in the resonance frequency with increasing temperature for the y s 0.1 sample, while the y s 0.2 sample shows a relative insensitivity to a change in temperature ŽFig. 11.. Fig. 12 indicates some narrowing for both fluorine concentrations, with increasing temperature.
4. Discussion All the spectra were fit to a Lorentian line shape. None were compatible to a Gaussian. The shape becomes Gaussian only at temperatures below Tc w14x. This may mean that the origin of the linewidth is due to a large extent to interaction with the Cu ions rather than 19 F dipole–dipole interactions. This is bolstered by the fact that while the linewidth measured at a field of 7 T Žthis study. is of the order
Fig. 12. The temperature dependence of the 19 F linewidth for the fully oxygenated sample having y s 0.1 Žsquares. and 0.2 Žtriangles.. The full curves are aids to the eye and have no physical significance.
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except for a very small contribution of a second line ŽFig. 5., there is only one major resonance line, and the frequency of this line is independent of both oxygen concentration and fluorine concentration ŽFig. 3.. Since the environments of the OŽ4. and OŽ5. sites are so similar, one is at a loss to say whether the strong line is due to F at only one site, or whether both sites contribute. The fractional occupation of the minor site was found to be very small, and it decreases with increasing F concentration ŽFig. 5.. The ratios A 2rA1 used in the figure were obtained by assuming two different T2 ’s for each site. Then the ratio A 2rA1 should vary exponentially with t . Hence the fit was to an exponential decay that was extrapolated to t s 0. This gave A 2rA1 values significantly larger than that for the smallest t , Žt s 10 ms.. The resulting large values of this ratio are thus dependent on this assumption. But even if a linear extrapolation from the lowest values of t were taken, the A 2rA1 values would be too high to assume that the minor site has a significant influence on the recovery of Tc observed upon fluorination. The origin of this minor line will now be discussed. While the O 7 and O6.7 samples were superconductors for all F concentrations, YBa 2 Cu 3 O6 is an AF nonsuperconductor and remains a nonsuperconductor for y s 0.60 and 0.98. At y s 1.50 it is a Tc s 50 K superconductor. Figs. 3 and 4 show the resonance positions and linewidths of the major and minor sites. The values of A 2rA1 obtained from an exponential extrapolation were about 25 for y s 0.6 and in the thousands for y s 0.98 and even greater for the y s 1.50 sample. The fact that the minor line was detectable seems to indicate that a strictly exponential extrapolation may be an exaggeration. Nevertheless the occupancy of the second site seems to be negligible as far as having an effect on the superconducting properties. Its linewidth is 170 kHz for y s 0.6 and decreases to that of the main component as y s 1.50 is reached and the material is a superconductor. Such a wide line can be attributable to the AF interaction which exists between the CuŽ2. in the nonsuperconductor. The width decreases as the fluorine concentration is increased, due to the weakening of the AF field, until at y s 1.5, there is no AF interaction. The AF interaction also probably contributes to the shift in resonance frequency of the
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weak line. The fact that the large linewidth is due to the AF interaction is also supported by the temperature dependence of the linewidth ŽFig. 9.. As the temperature is increased from 175 K to 450 K the line narrows significantly due to a weakening of the AF interaction. It is unlikely that this narrowing is due to kinetics since one would expect a narrower line to narrow more than a wide line for the same thermal frequency dependence. The main component is however hardly effected by the increase in temperature. The fact that the minor lines have a width of the order of 170 and 100 kHz for the AF samples YBa 2 Cu 3 O6 F0.6 and YBa 2 Cu 3 O6 F0.98 , while in the same samples the main line linewidth remains at about 50 kHz, shows that the site of the minor line must be much closer to the CuŽ2. plains than the major line and also in a non-symmetric site as far as the AF field is concerned. A possible site may be one where some F have replaced the bridging OŽ1. atoms along the c axis. Energy calculations have shown this to be a possibility w10x, especially if the bridging is to a CuŽ1. that has an adjacent vacancy. This site would place the F close to the magnetic CuŽ2. atoms. The fact that it yields different resonance frequencies at the different oxygen concentrations, can be explained by the change in bonding due to the large shifts of the O–Cu distances upon changing the oxygen concentration and also by a change in the AF interaction. Since the frequency shift of the major line is practically independent of the oxygen and fluorine concentration within the experimental error, it is probable that the F site Žor sites. is the same for all the oxygen concentrations. These are the OŽ4. andror OŽ5. positions when x s 6.7, as shown by neutron diffraction, so they are also the F sites for all the O concentrations. The YBa 2 Cu 3 O6 samples that have been restored to superconductors have a small Meissner volume. One should not interpret this as the introduction of a small amount of superconductor into an unchanged matrix of the original AF material. This can be seen from the linewidth behavior of the minor line in YBa 2 Cu 3 O6 ŽFig. 4., which shows that the AF interaction is diminished as y increases, and the linewidth approaches that of the major line at y s 1.5. If the region were multiphased with a large population of
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the original AF material, then the minor line should have remained wide. This does not preclude the existence of a changed AF phase coexisting with a small amount of superconductor. For example, the data would be compatible with an AF having its T N lowered below the temperature of our measurements Ž; 200 K. or of a disordered magnetic phase Žwith a low spin glass temperature. coexisting with a superconductor phase with a highly distributed Tc .
5. Conclusions 1. When F is added to YBa 2 Cu 3 O x , only one major NMR line is obtained at a frequency that is independent of oxygen or fluorine concentration. Hence if more than one site is occupied, their local environments are similar Žsuch as OŽ4. and OŽ5... 2. The AF behavior can be followed as a function of F concentration and temperature from the NMR line contribution from F at the sparsely occupied site in YBa 2 Cu 3 O6 Fy . 3. Although the Meissner volume in samples that have its superconducting property restored by F is small, the material is not simply mixed phase of the original AF and a superconductor, since it is seen that the AF interaction is reduced in the entire sample even before the superconducting phase is reached. The resulting material may or may not be a single phase.
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