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Antiferromagnetism and superconductivity in YBa2Cu3Ox studied by μSR
Physica C 153 155 (1988) 759-760 Nor th-Holland, Amsterdam
ANTIFERROMAGNETISM AND SUPERCONDUCTIVITY IN YBa2Cu30x STUDIED BY #SR G. M. Luke 1, R. F. Kiefl1, J. H. Brewer 1, T. M. Riseman 1, D. L1. Williams 1, J. R. Kempton 1, S. R. Kreitzman 1, E. J. Ansaldo2, N. Kaplan 3, Y. J. Uemura 4, W. N. Hardy 1, J. F. Carolan 1, M. E. Haydenl,B. X. Yang 1 1Dept. of Physics, Univ. of British Columbia, Vancouver, B.C., Canada V6T 2A6 2Dept. of Physics, Univ. of Saskatchewan, Saskatoon, Saskatchewan, Canada 3Racah Inst. of Physics, Hebrew Univ. of Jerusalem, Jerusalem, Israel 4Brookhaven National Laboratories, Upton, NY, USA We have used the technique of muon spin relaxation to study the competition between antiferromagnetism (AFM) and superconductivity (SC) in oxygen deficient YBa2Cu30~ for 6.0 < x < 6.5. Average Ngel temperatures range from 450K for x = 6.0 down to 2.5K for x = 6.40. Samples with x near 6.4 were both SC and AFM. In the x = 6.1 sample, where there is a narrow distribution of TN'S, we have observed the critical slowing down of the Cu spins at the average TN.
Following the discovery of superconductivity at Tc = 90K in the perovskites RBa2Cu3Os.9s where tt is virtually any rare earth element (1), there has been great interest in possible new mechanisms for superconductivity. A number involve frustrated antiferromagnetic couplings (2) between the copper ions in the CuO2 planes which these materials all contain. Antiferromagetic ordering was first detected in the YBa2Cu30~ materials by #SR measurements (3) for x ~ 6.2 and confirmed later by neutron scattering (4) for x ~ 6.0 and 6.15. The #SR (Muon Spin Rotation) technique(5) is particularly suited for determining the phase diagram in these materials since it is sensitive' to and can distinguish between superconductivity (SC), antiferromagnetism (AFM) and paramagnetism. The spin-polarized muons come to rest at interstitial sites distributed over the bulk and sample the microscopic magnetic environment. The parity violating decay of the muon into a positron and two neutrinos (mean lifetime vu = 2.2#s) allows one to measure the time dependence of the muon polarization. In an ordered AFM powder the muon will experience a local field that is of well-defined magnitude, but random orientation. Thus in zero external field (ZF) a single spin precession frequency will be observed in the muon decay spectrum. In an applied external field, a more complicated field distribution will be observed. In a SC powder one sees a negative frequency shift of 5-10% as well as a T2 relaxation rate of 0.5 - 1.0#s -1 due to partial field exclusion and sample granularity(6). The samples in this study were prepared using a "slow anneal" method in which YBa2Cu30x powder from a parent batch was pressed into pellets and annealed in oxygen (1 atm.) at 450°C for 24 hours, producing fully oxygenated samples with x = 6.95[5]. Oxygen was then removed from the samples between 420°C and 460 °C using a helium cooled wand in a sealed system. Next, the isolated sample was held at 420 °C for 24 hours to remove any gradients in x. Finally,
0921-4534/88/$03.50 © ElsevierSciencePublishersB.V. (North-Holland PhysicsPublishingDivision)
the remaining oxygen gas was removed from the system and the sample was quickly cooled to room temperature. Samples were stored either under argon or in vacuum to prevent re-oxygenatlon. The difference in x between any two pellets is known to better than O.01, whereas there is an absolute error of 0.05 originating from the uncertainty in x for the fully oxygenated parent batch. The most oxygen-deficient samples, (x ~< 6.3) displayed strong ZF-#SR precession signals, indicating a local field at the muon site of approximately 300 G at low temperatures in agreement with ref. (3). This is considerably smaller thah for muons in other AFM oxides, indicating the moments are on the order of one/~B or less (7). The main purpose of the present study however was to determine the effect of oxygen deficiency on ordering temperature. An average TN for the various samples was extracted from low (~ 85 G) transverse field measurements. In this case, where the applied field is smaller than the local field in the ordered state, the amplitude of the precession signal corresponding to the applied field originates from the fraction of the sample which has not ordered, ie. the paramag: netic fraction. Muons in the ordered region see a completely different average field, with the applied field essentially a random perturbation causing a fast spin relaxation due to inhomogeneous broadening. The amplitude of the paramagnetie signal can therefore be used to measure the paramagnetic fraction of the sample. We have taken the temperature at which half the sample is ordered to be the average N~el temperature. < TN >is shown as a function of oxygen concentration x in Figure 1. It can be seen that is a extremely steep function of x around x = 6.25. The widths of the transitions are largest in this range which suggests that small inhomogeneities in the oxygen concentration are the main source of broadening the distribution in TN'S. In one sample (x = 6.1), which has a particularly nat-
G.M. Luke et al. / Antiferromagnetism and superconductivity in YBa2Cu30x
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Figure 1: Average N~el temperature 'IN for YBa2Cu30= for various values of x. Samples were made using the "slow anneal" method.
Figure 2: Paramagnetic Asymmetry and Frequency for YBa2Cu306.40. < TN >corresponds to half of the sample being in the ordered state.
row AFM transition (ATN = 75K), measurements of the muon T1 relaxation rate were made in a longitudinal field of 1 kG. A peak in the relaxation rate of 0.4#s -1 was observed at < TN > = 341 K, which is attributed to critical slowing down of the moment fluctuations. The absence of a sharp cusp is presumably due to small inhomogeneities in TN. From the peak relaxation rate and using the observed spontaneous field on the muon in ZF at low temperature, (300 G) we estimate the average correlation time vc of the moment fluctuations to be about O.5ns at T = < T N > . In two of the slow annealed samples (x = 6.35 and x = 6.40), both SC and AFM were observed. These samples had sharp SC transitions at Tc = 25K and 33K respectively and ordered magnetically below 10K and 5K. The effects of these two phenomena are easily distinguished; superconductivity causes large negative frequency shifts and moderate relaxation due to partial flux exclusion whereas AFM ordering causes a decrease in the signal amplitude. For < T N > < T < TC a single component relaxation is observed in the x = 6.40 sample, indicating that the entire sample is in the SC phase in this temperature range. The observation of a significant AFM fraction at lower temperature can only be explained if the sample switches from SC to AFM or if the SC and AFM coexist. The muon asymmetry and frequency are shown in Figure 2 for a sample of YBa2Cu306.40. This work was supported by NSERC under Strategic Grant #80099. One of us (DLW) also acknowledges support from the THEANON Foundation. We would like to thank Keith Hoyle and John Worden for their technical assistance during the experiment.
REFERENCES (i) C.W. Chu, P.H. }[or, R.L. Meng, L. Gao, Z.J. Huang and Y.Q. Wang, Phys. Rev. Lett. 58,405 (1987); S. Hikami, T. Hirai and S. Kagoshima, Jpn. J. Appl. Phys. 26, L314 (1987). (2) P.W. Anderson, Science 235, 1196 (1987); P.W. Anderson, G. Baskaran, Z. Zou and T. Hsu, Phys. Rev. Lett. 58, 2790 (1987); V.J. Emery, ibid. 58, 2794 (1987); P.A. Lee and M. Read, ibid. 58, 2691 (1987); J.E. Hirsch, ibid. 59,228 (1987); Y. Hasegawa and H. Fukayama, Jpn. J. Appl. Phys. 26, L322 (1987); S.A. Kivelson, D.S. Rokhsar and J.P. Sethna, Phys. Rev. B35, 8865 (1987). (3) N. Nishida et al., Jpn. J. Appl. Phys. 26, L1856 (1987); N. Nishida et al., J. Phys. Soc. Japan 57,599 (1988). (4) J. M. Tranquada et. al., Phys. Rev. Letts. 60, 156 (1988). (5) S.F.J. Cox, J. Phys. C: Solid State Phys. 20, 3187 (1987); A. Schenck, Muon Spin Rotation Spectroscopy: Principles and Applications in Solid State Physics (Adam Hilger Ltd., Bristol and Boston, 1985); Y.J. Uemura and T. Yamazaki, Physica 109-110B, 1915 (1982). (6) G. Aeppli et. al., Phys. Rev. B35, 7129 (1987). (7) Y. J. Uemura et. al., Phys. Rev. Letts. 59, 1045 (1987).
ANTIFERROMAGNETISM AND SUPERCONDUCTIVITY IN YBa2Cu30x STUDIED BY #SR G. M. Luke 1, R. F. Kiefl1, J. H. Brewer 1, T. M. Riseman 1, D. L1. Williams 1, J. R. Kempton 1, S. R. Kreitzman 1, E. J. Ansaldo2, N. Kaplan 3, Y. J. Uemura 4, W. N. Hardy 1, J. F. Carolan 1, M. E. Haydenl,B. X. Yang 1 1Dept. of Physics, Univ. of British Columbia, Vancouver, B.C., Canada V6T 2A6 2Dept. of Physics, Univ. of Saskatchewan, Saskatoon, Saskatchewan, Canada 3Racah Inst. of Physics, Hebrew Univ. of Jerusalem, Jerusalem, Israel 4Brookhaven National Laboratories, Upton, NY, USA We have used the technique of muon spin relaxation to study the competition between antiferromagnetism (AFM) and superconductivity (SC) in oxygen deficient YBa2Cu30~ for 6.0 < x < 6.5. Average Ngel temperatures range from 450K for x = 6.0 down to 2.5K for x = 6.40. Samples with x near 6.4 were both SC and AFM. In the x = 6.1 sample, where there is a narrow distribution of TN'S, we have observed the critical slowing down of the Cu spins at the average TN.
Following the discovery of superconductivity at Tc = 90K in the perovskites RBa2Cu3Os.9s where tt is virtually any rare earth element (1), there has been great interest in possible new mechanisms for superconductivity. A number involve frustrated antiferromagnetic couplings (2) between the copper ions in the CuO2 planes which these materials all contain. Antiferromagetic ordering was first detected in the YBa2Cu30~ materials by #SR measurements (3) for x ~ 6.2 and confirmed later by neutron scattering (4) for x ~ 6.0 and 6.15. The #SR (Muon Spin Rotation) technique(5) is particularly suited for determining the phase diagram in these materials since it is sensitive' to and can distinguish between superconductivity (SC), antiferromagnetism (AFM) and paramagnetism. The spin-polarized muons come to rest at interstitial sites distributed over the bulk and sample the microscopic magnetic environment. The parity violating decay of the muon into a positron and two neutrinos (mean lifetime vu = 2.2#s) allows one to measure the time dependence of the muon polarization. In an ordered AFM powder the muon will experience a local field that is of well-defined magnitude, but random orientation. Thus in zero external field (ZF) a single spin precession frequency will be observed in the muon decay spectrum. In an applied external field, a more complicated field distribution will be observed. In a SC powder one sees a negative frequency shift of 5-10% as well as a T2 relaxation rate of 0.5 - 1.0#s -1 due to partial field exclusion and sample granularity(6). The samples in this study were prepared using a "slow anneal" method in which YBa2Cu30x powder from a parent batch was pressed into pellets and annealed in oxygen (1 atm.) at 450°C for 24 hours, producing fully oxygenated samples with x = 6.95[5]. Oxygen was then removed from the samples between 420°C and 460 °C using a helium cooled wand in a sealed system. Next, the isolated sample was held at 420 °C for 24 hours to remove any gradients in x. Finally,
0921-4534/88/$03.50 © ElsevierSciencePublishersB.V. (North-Holland PhysicsPublishingDivision)
the remaining oxygen gas was removed from the system and the sample was quickly cooled to room temperature. Samples were stored either under argon or in vacuum to prevent re-oxygenatlon. The difference in x between any two pellets is known to better than O.01, whereas there is an absolute error of 0.05 originating from the uncertainty in x for the fully oxygenated parent batch. The most oxygen-deficient samples, (x ~< 6.3) displayed strong ZF-#SR precession signals, indicating a local field at the muon site of approximately 300 G at low temperatures in agreement with ref. (3). This is considerably smaller thah for muons in other AFM oxides, indicating the moments are on the order of one/~B or less (7). The main purpose of the present study however was to determine the effect of oxygen deficiency on ordering temperature. An average TN for the various samples was extracted from low (~ 85 G) transverse field measurements. In this case, where the applied field is smaller than the local field in the ordered state, the amplitude of the precession signal corresponding to the applied field originates from the fraction of the sample which has not ordered, ie. the paramag: netic fraction. Muons in the ordered region see a completely different average field, with the applied field essentially a random perturbation causing a fast spin relaxation due to inhomogeneous broadening. The amplitude of the paramagnetie signal can therefore be used to measure the paramagnetic fraction of the sample. We have taken the temperature at which half the sample is ordered to be the average N~el temperature
G.M. Luke et al. / Antiferromagnetism and superconductivity in YBa2Cu30x
760
500
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Oxygen Formule Content "x"
10
I
I
I
I
20
30
40
50
Temperature
60
(K)
Figure 1: Average N~el temperature 'IN for YBa2Cu30= for various values of x. Samples were made using the "slow anneal" method.
Figure 2: Paramagnetic Asymmetry and Frequency for YBa2Cu306.40. < TN >corresponds to half of the sample being in the ordered state.
row AFM transition (ATN = 75K), measurements of the muon T1 relaxation rate were made in a longitudinal field of 1 kG. A peak in the relaxation rate of 0.4#s -1 was observed at < TN > = 341 K, which is attributed to critical slowing down of the moment fluctuations. The absence of a sharp cusp is presumably due to small inhomogeneities in TN. From the peak relaxation rate and using the observed spontaneous field on the muon in ZF at low temperature, (300 G) we estimate the average correlation time vc of the moment fluctuations to be about O.5ns at T = < T N > . In two of the slow annealed samples (x = 6.35 and x = 6.40), both SC and AFM were observed. These samples had sharp SC transitions at Tc = 25K and 33K respectively and ordered magnetically below 10K and 5K. The effects of these two phenomena are easily distinguished; superconductivity causes large negative frequency shifts and moderate relaxation due to partial flux exclusion whereas AFM ordering causes a decrease in the signal amplitude. For < T N > < T < TC a single component relaxation is observed in the x = 6.40 sample, indicating that the entire sample is in the SC phase in this temperature range. The observation of a significant AFM fraction at lower temperature can only be explained if the sample switches from SC to AFM or if the SC and AFM coexist. The muon asymmetry and frequency are shown in Figure 2 for a sample of YBa2Cu306.40. This work was supported by NSERC under Strategic Grant #80099. One of us (DLW) also acknowledges support from the THEANON Foundation. We would like to thank Keith Hoyle and John Worden for their technical assistance during the experiment.
REFERENCES (i) C.W. Chu, P.H. }[or, R.L. Meng, L. Gao, Z.J. Huang and Y.Q. Wang, Phys. Rev. Lett. 58,405 (1987); S. Hikami, T. Hirai and S. Kagoshima, Jpn. J. Appl. Phys. 26, L314 (1987). (2) P.W. Anderson, Science 235, 1196 (1987); P.W. Anderson, G. Baskaran, Z. Zou and T. Hsu, Phys. Rev. Lett. 58, 2790 (1987); V.J. Emery, ibid. 58, 2794 (1987); P.A. Lee and M. Read, ibid. 58, 2691 (1987); J.E. Hirsch, ibid. 59,228 (1987); Y. Hasegawa and H. Fukayama, Jpn. J. Appl. Phys. 26, L322 (1987); S.A. Kivelson, D.S. Rokhsar and J.P. Sethna, Phys. Rev. B35, 8865 (1987). (3) N. Nishida et al., Jpn. J. Appl. Phys. 26, L1856 (1987); N. Nishida et al., J. Phys. Soc. Japan 57,599 (1988). (4) J. M. Tranquada et. al., Phys. Rev. Letts. 60, 156 (1988). (5) S.F.J. Cox, J. Phys. C: Solid State Phys. 20, 3187 (1987); A. Schenck, Muon Spin Rotation Spectroscopy: Principles and Applications in Solid State Physics (Adam Hilger Ltd., Bristol and Boston, 1985); Y.J. Uemura and T. Yamazaki, Physica 109-110B, 1915 (1982). (6) G. Aeppli et. al., Phys. Rev. B35, 7129 (1987). (7) Y. J. Uemura et. al., Phys. Rev. Letts. 59, 1045 (1987).