NQR and NMR studies in superconducting La2CuO4+δ

Journal of Magnetism and Magnetic Materials 104-107 (1992) 523-524 North-Holland

NQR and NMR studies in superconducting La2CuO4+ d T. Kohara, K. Ueda, Y. Kohori and Y. Oda Department of Material Science, Himeji Institute of Technology, Akoh-gun, Hyogo 678-12, Japan In the superconducting La2CuO4+6, Cu NQR signals were observed at around 33.1 and 36.0 MHz together with the antiferromagnetic Cu NMR signals similar to those in La2CuO4. The NQR intensity on oxygen-loaded samples increases with increasing the loaded contents, d, of oxygen. The nuclear spin relaxation time, Tl, of Cu NQR indicates that the paramagnetic phase is in the superconducting state. La NMR was also measured to study microscopicallya phase separation and magnetic behaviors in oxygen-loaded La2CuO4+6. La2CuO 4 is a Mott insulator in which Cu moments undergo three dimensional antiferromagnetic ordering at T N ~ 325 K. Hole doped La2CuO4+ ~ by adding excess oxygen have both the reduced TN and the induced superconductivity below 40 K. This material has been studied extensively because it embodies much of the physics and the chemistry generic to the hightemperature conductors and the magnetic superconductors. Jorgensen et al. recently have reported that a reversible phase-separation into two crystallographic distinct structures occurs near 320 K in powdered La2CuO4+,~ [1]. Diffraction studies on single crystal La2CuO4+ ~ have also suggested that the phase-separation temperature, Tp, is about 280 K [2]. Below Tp, the oxygen loaded material is reported to be composed of both a non-superconducting, oxygen-poor phase (Bmab; presumably with high resistance) and a superconducting, oxygen-rich phase (Fmmm; presumably with less resistance) with a closely related orthorhombic structure. Here, we report Cu NMR and NQR and La NMR measurements on oxygen-loaded La2CuO4+,~ employed as a probe of the local crystalline environment which is very sensitive to hole doping. The stoichiometric amounts of materials (5N purity) were well mixed and pressed into pellets. They were fired at 950 ° C for 24 h, successively at ~ 1100 ° C for 24 h in air after regrinding. The pulverized samples were annealed further at 600 ° C for 96 h under pure oxygen gas of about 800-1200 bar. By nondisperive IR analyzer, the d-values are determined to be typically about 0.012 and 0.022 on 800 bar and 1200 bar of annealing pressure, respectively. All the samples examined by X-ray diffraction were found to be a single phase of K2NiF4 structure. The susceptibility measurements on all the oxygen-loaded pellets showed T N, of about 255 K reduced by oxygen-loading and almost the complete diamagnetism well below Tc of 36 K. The frequency- or the magnetic field-swept spectrum was obtained by integrating the spin echo signals. T 1 was measured by the saturation pulse method. Shown in fig. 1 is Cu spin echo spectrum of La2CuO4+ 8 annealed under oxygen gas of 1200 bar.

Since the frequency ratio and the integrated intensity ratio of the two lines correspond to the ratios of electric quadrupole moment and natural abundances of 63Cu and 65Cu, these peaks are assigned to be Cu NQR signals with no internal field. Comparing with that of (La-Sr)2CuO 4 [3], the spectrum has rather wider width and a tail observed in higher frequency, which may be caused by a random distribution of EFG due to excess oxygen. As the NQR signal was approximately proportional to the oxygen pressure in annealing, this paramagnetic signal comes from the oxygenrich (Fmmm) phase. On the contrary, the antiferromagnetic Cu NMR signal in La2CuO4+ ~ was also observed under zero external field in the same frequency-range (75-115 MHz) as that in undoped La2CuO 4 [4], that means the antiferromagnetic NMR signal comes from the oxygen-poor (Bmab) phase. These results show that there coexist paramagnetic (Fmmm) and antiferromagnetic (Bmab) phases in the oxygen-loaded sample. Fig. 2 shows the T-dependence of T 1 of 63Cu in La2fuO4+ 6 annealed under 800 bar of oxygen. In the figure neither the enhancement just below Tc nor the exponential decrease in 1 / T 1 is observed in the superconducting state, both of which are characteristic of the BCS superconductors. It is reported that the T-dependence of 1 / T 1 below Tc is explained by a d-wave model with 2A (0) = 10kBTc Icos 01 [5]. This T-depenI

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dence of 1/T~ below Tc is similar to those of other (LaSr)- [3], Y- [6], Bi- [7], Pb-systems [8]. These results seem to support a d-wave pairing. The T-dependences of the penetration depth and the Knight shift of Cu in Y and T1 compounds, however, seem to support a s-wave pairing [9]. This discrepancy still remains for future problems. As seen in this figure, 1 / T ] above Tc has weaker T-dependence than T-linear. The T-dependence of T 1 in the normal state is as important as that in the superconducting state to study the T-dependence of Xo, which is expected to follow a C u r i e - W e i s s law at high temperature becoming constant below a characteristic temperature with further decreasing temperature. So we prepare the La2CuO4+ a with more loaded oxygen to measure the Cu N Q R signal in CuO 2 plane with high accuracy in the wider temperature range of the normal state. In fig. 3 we show La N M R spectrum taken at 195 K in the magnetic field of approximately 7.8 T applied to the c-axis. The spectrum is composed of a nearly unshifted line ( K = 0) and a broad line having a substantial shift, which may originate from oxygen-rich region and from oxygen-poor region, respectively. The broad line may be caused by the incomplete alignment of c-axis oriented parallel to the magnetic field. The ratio of the integrated intensity in a nearly unshifted line to that in a broad line (i.e. the ratio of the volume in both regions) was found to be about 1 : 9 at 195 K. Unshifted N M R signal, gradually increasing the intensity above 280 K, was still observed up to room temperature. On the contrary, the intensity of the broad line

drastically decreased accompanied by decreasing the shift with increasing temperature, and the signal vanished upon warming above 270 K. The abrupt disappearence of the broad line may partly be due to the warming through T N of 255 K. The precise intensity measurements on both La N M R signals are in progress to clarify microscopically the magnetic behaviors in poor-oxygen region and a phase-separation around 300 K. References

[1] J.D. Jorgensen, B. Dabrowski, S. Pei, D.G. Hinks, L. Solderholm, H. Morosin, J.E. Schirber, E.L. Venturini and D.S. Ginley, Phys. Rev. B 38 (1988) 11337. [2] C. Chaillout, J. Chenavas, S.W. Cheong, Z. Fisk, M.S. Lehmann, M. Marezio, B. Morosin and J.E. Schirber, Physica C 162-164 (1989) 57. [3] K. Ishida, T. Kondo, Y. Kitaoka and K. Asayama, J. Phys. Soc. Jpn. 58 (1989) 2638. [4] T. Tsuda, T. Shimizu, H. Yasuoka, K. Kishio and K. Kitazawa, J. Phys. Soc. Jpn. 57 (1988) 2908. [5] K. Fujiwara, Y. Kitaoka, K. Asayama, Y. Shimakawa, T. Manako and Y. Kubo, J. Phys. Soc. Jpn. 59 (1990) 3459. [6] K. Fujiwara, Y. Kitaoka, K. Asayama, H. KatayamaYoshida, Y. Okabe and T. Takahashi, J. Phys. Soc. Jpn. 57 (1988) 2893. [7] K. Fujiwara, Y. Kitaoka, K. Asayama, H. Sasakura, S. Minamigawa, K. Nakahigashi, M. Kogachi, N. Fukuoka and A. Yanase, J. Phys. Soc. Jpn. 58 (1989) 380. [8] T. Kohara, K. Ueda, H. Takenaka, Y. Kohori and Y. Oda, Physica B 165 & 166 (1990) 1307. [9] See for example, S.E. Barret, D.J. Durand, C.H. Pennington, C.P. Slichter, T.A. Friedmann, J.P. Rice and D.M. Ginsberg, Phys. Rev. B 41 (1990) 6283.