9992
J. Phys. Chem. 1992,96, 9992-9994
Observation of Conduction Electron Spln Resonance In the Nd1.85Ce0.15C~OCy Superconductor Marceline Bonvalot and Larry Kevan* Department of Chemistry and the Texas Center for Superconductivity, University of Houston, Houston, Texas 77204-5641 (Received: February 19, 1992; In Final Form: July 28, 1992)
Normal and superconducting Nd,,Ce,CuO, ( x = 0.00,0.15) compounds have been studied by electron spin resonance (ESR). In contrast to previous reports that the high-temperature superconducting oxocuprate compounds are ESR silent, a conduction ESR (CESR) signal has been observed in the superconductingNdl.&~,lsCuOcycompound. The temperature dependence of the CESR signal intensity, g value, and line width are consistent with a two-dimensional spin structure in Ndl,8,C~,l,Cu0,. A possible reason for the lack of a CESR signal in other high-temperature superconducting systems is that they have more efficient spin-lattice relaxation which is associated with CESR lines too broad to be detected.
Introduction Although electron spin resonance (ESR) should, in principle, provide information on the local environment of the Cu2+ions present in high-temperature superconductingoxocuprate materials, it is widely accepted that the ESR signal observed in these compounds originates from an impurity ESR-active phase present in small amount in the superconducting phase.' The absence of a copper ESR signal intrinsic to a superconducting phase has been the subject of several papers.'-" However, no attention has been given to the lack of an ESR signal originating from the conduction electrons rather than from localized copper species. In this work, we report the observation of a conduction ESR (CESR) signal in the 24 K superconducting N d l . 6 5 ~ . 1 5 C u 0compound 4-y as well as its temperature dependence, g value, and line width. This appears to be the first report of a CESR signal from a superconducting copper oxide material. Experimental Section Nd,CuO, and Ndl.85C%.15C~04-y were prepared by a solidstate reaction of NdzO3 (99.9% purity from Alfa Chemicals), CeOz (99.9% from Aldrich), and CuO (99% from Fluka Chemicals). The starting chemicals were initially preheated at 870 OC for nominal purification. Stoichiometric amounts of the oxides were ground in an agate mortar for a minimum of 20 min and sintered at 950 OC for 16 h in a muffle furnace. The sintered powder was slowly furnace-cooled, reground, and pressed at a pressure of 7.8 X lo3 kg into disks with a diameter of 7 mm and a thickness of 1 mm. These disks were resintered at 1100 OC for 20 h, furnace-cooled to room temperature, and cut into rectangular pieces with a diamond wheel saw. Finally, these pieces were annealed under an argon atmosphere at 925 OC for 24 h in a quartz tube furnace and argon-quenched to room temperature by rapidly removing the flow system out of the split-hinge furnace. The phase formation and purity of annealed Nd2CuO4, and Nd1.65;C~.15C~O+, compounds were checked by X-ray diffraction on a Siemens D5000 powder diffractometer equipped with Cu Ka radiation and nickel filters and operating at 40 kV and 30 mA. No impurity phases were observed. A standard four-probe technique at 15 lrHz was used for resistance measurements and indicated that annealed NdzCuO+ is semiconducting, while annealed Nd,,5Ce,,15Cu0, is superconducting with an onset superconducting transition temperature of 24 K. ESR spectra were recorded with a Bruker ESP 300 ESR spectrometer operating at X-band with 100-kHz magnetic field modulation and equipped with an Oxford Instruments ESR-900 helium flow cryostat. In addition to the internal thermocouple mounted in the cryostat, a calibrated Au-doped Fe versus chrome1 thermocouple was placed in the immediate vicinity of the sample to monitor the temperature at the sample position. Approximately 5 mg of finely powdered sample was vacuum-sealed in a Suprasil quartz tube, which was placed in the middle of the ESR cavity. The reproducibility of the results was checked by scanning the 0022-365419212096-9992$03.00/0
compound at least three times under the same experimental conditions. Each time, identical ESR characteristics were obtained.
R€3dtS At temperatures between 4 and 300 K, no ESR signal is detected in the undoped, argon-annealed, nonsuperconducting Nd,CuO+ parent compound. In contrast, broad, somewhat asymmetric ESR signals with no hyperfine structure are obtained for the 24 K superconductingNdl.85C%.15C~04-y compound as shown in Figure 1. The base line is also somewhat sloping even though a standard run before and after did not show such a base line. Thus, the asymmetry may be characteristic of the line shape. The 40 K spectrum seems to show greater asymmetry for unknown reasons; perhaps there is some dispersion contribution to the spectrum. Curve fitting calculations indicate that a Dysonian function5fits the line shapes of these spectra reasonably well as shown in Figure 2. Because of the asymmetry, neither Gaussian nor Lorentzian functions for an absorption ESR signal fit the spectra. The doubly integrated intensity is temperature independent from 4 to 300 K. The temperature dependence of the g value and line width of the resonant signals is displayed in Figures 3 and 4. The g value has been evaluated from the relationship hv = &H, where h is Planck's constant, 0 is the Bohr magneton, and v is the frequency of the incident microwave radiation. The magnetic field for resonance H has been evaluated as H = '/Z(Hmin + H-), where Hminand H,,, respectively are determined graphically as the magnetic field at which the derivative signal line shape function reaches its minimum and maximum. The experimental error in the calculated g value has been evaluated from the spectra to be fO.lO,which is fairly large because of the broad line width of the spectra. Mscupsion Dysonian ESR line shapes typically arise from highly mobile and delocalized electron^.^ Since the experimental line shapes fit a Dysonian function reasonably well as shown in Figure 2, the ESR signals in Figure 1 are assigned as CESR signals. This assignment is supported more strongly by the temperature independence of the intensity. Three temperature regions can be distinguished in Figures 3 and 4. The first one ranges from 300 to 50 K,the second one from 50 to 25 K, and the third one from 25 to 4 K. In the following, we discuss these three temperature regions separately. 50 I T (K)I 300. Under the standard mode of ESR operation, the first derivative of the imaginary part of the paramagnetic susceptibility dx"/dH is recorded as a function of the external magnetic field, and then the doubly integrated ESR signal intensity is directly proportional to the paramagnetic susceptibility. Thus, the approximately temperatureindependent CESR signal intensity observed at temperatures 50 5 T (K)5 300 is indicative of a temperature-independentparamagnetic susceptibility. This bcQ 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No.24, 1992 9993
ESR in the Ndl.85Ceo,15Cu04-y Superconductor
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1600
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1200 I4O0l
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8000
50
100
150
200
250 300 T(K)
Tc
Figure 4. Temperature dependence of the line width of the CESR signal of Nd I .s&~o.I 0 0 4 - y
2
.
?
p
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H(G) 30 5970 Figure 1. ESR response of Ndl,85Ce,,15CuOeyat several temperatures.
the g value from that of the free electron value of g, = 2.0023 (Ag 1 0.2) suggests spin-orbit coupling of the conduction electrons.' This could imply a spin-lattice relaxation mechanism involving phonon coupling? but the line width variation with temperature goes the wrong way to support a phonon coupling mechanism. An order of magnitude of the spin-lattice relaxation time TI can be obtained from the line width AH of the CESR signal from the equation9
AH a 1.5/yeT1 (1) where yeis the electron gyromagnetic ratio (1.76 X lo7 s-l G-l). One obtains from eq 1 TI = 6 X lo-" s-l at room temperature and T1 = 4.5 X lo-" s-I at 50 K. A linear regression calculation gives eq 2 for the empirical AH (G) = -1.4T (K) + 1819 (2)
Figure 2. Dysonian line shape fits (+) to the CESR signals from Figure 1.
c 0
I
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I
2.31
I
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8
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150 I
200 I
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Fgue 3, Temperature dependence of the g value of the CESR signal of Ndi.s5C~.15CuOcp
havior is characteristic of Pauli-type paramagnetism rather than Curie-type paramagnetism which has an inverse temperature dependence of the paramagnetic susceptibility: This supports the assignment of the resonance signals displayed in Figure 1 to CESR signals. In the 50-300 K temperature range, the g values of the CESR spectra do not show any significant temperature variation (Figure 3), while the line width shows a definite small decrease with increasing temperature (Figure 4). The significant departure of
temperature dependence of the CESR line width in the 50-300 K temperature range with a correlation coefficient of 0.945. When combined with eq 1, the temperature dependence of the spin-lattice relaxation time Tl can be expressed by eq 3. Since over the 50-300 K temperature range 11.47l < 1819, the spin-lattice relaxation time is nearly temperature independent. 1.5 TI (3) ye(-l.4T + 1819) 25 I T (I() I 50. Figures 3 and 4 indicate that the g value and line width of the CESR signals of Ndl,~~C%,l~CuOey exhibit an abrupt variation around 50 K. The g value variation is quite large and corresponds to about 580 G. A small g shift of about 10 Gl0is expected near the superconducting transition temperature due to the partial reduction of the internal magnetic field in this type I1 superconductor. This effect is much too small to account for the large g shift actually observed. It is noteworthy that the onset superconducting transition temperature in this compound is only 24 K. It then seems puzzling that these abrupt changes in the CESR spectra take place at a temperature as high as 50 K. This apparent discrepancy can be interpreted by considering the theory of Nagata and Tazuke" concerning the effect of short range order spin fluctuations on the ESR spectrum of a paramagnetic substance in a twedhensional spin system. This theory predicts that the spin dynamics near a magnetic phase transition are strongly dependent on the dimensionality of the spin arrangement."-" Studies of such critical phenomena by ESR have revealed that the position and width of the paramagnetic resonance line are influenced by the development of short range order fluctuations within the spin system as the critical temperature is approached. An understanding of the effects of short range order of ESR transitions requires a knowledge of the correlation function of the spin system in a finite magnetic field." A quantum mechanical treatment of this problem has shown that the short range order spin fluctuations at wave vector q = 0 undergo a slower decay in a reduced dimensional spin system, as compared with a threedimensional spin ~ystem.lI-~~ Thus, the temperature range above T, at which spin fluctuations and short range order coexist
9994 The Journal of Physical Chemistry, Vol. 96, No. 24, 1992
Bonvalot and Kevan
Other high-temperature superconducting copper oxide systems increases with decreasing dimensionality of the spin system. As do not give rise to a detectable CESR signal, presumably because a consequence, the magnetic phase transition detected by ESR such signals are too broad to observe.I4 This may indicate that takes place at a temperature T above T, such that T - T, increases electrons occupying the conduction band of the Nd,Ce,CuO,, with decreasing dimensionality of the spin system. For instance, superconducting system have a less efficient spin-lattice relaxation Cr203undergoes a three-dimensional antiferromagnetic phase than electrons or holes in other high-temperature superconducting transition at TN = 300 K,but the ESR line broadening and g shift materials. More efficient relaxation processes lead to broader associated with this transition are detected about 10 K above TN.I3 resonant lines which become undetectable. By assuming that Similarly, in CsMnCI3-2H2Owhich undergoes a onedimensional CESR line widths beyond 5000 G are no longer detectable at 9 antiferromagnetictransition at TN = 4.9 K,the ESR broadening GHz, the limiting spin-lattice relaxation time below which suand g shift have been observed as high as 40 K above TN.lIa perconducting oxocuprate materials will be CESR silent, is esThe two-dimensional nature of the magnetic properties of = 1.7 X lo-" s. We have timatedg as T I l.5/(~eAHpp) Nd,CuO, have been well established.I4 This property originates evaluated the spin-lattice relaxation time of Ndl.85C~,15Cu0, from its layered structure along the (ab) plane, in which each layer to be of the order of 5 X lo-" s in the 50-300 K temperature is magnetically isolated from the others, because of weak dipolar range. Thus, the Nd&exCu0+ superconductingsystem appears interactions along the c dire~ti0n.I~ Because of the two-dimensional nature of the magnetic properties of N d l . & ~ , 1 ~ C ~ 0 4 - y ' 4 to be near the limit of detection of a CESR signal at 9 GHz. and a d i n g to the spin dimensionality effect of short range order Acknowledgment. This material is based upon work supported spin fluctuations on the resonant line proposed by Nagata and by the Texas Center for Superconductivityat the University of Tazuke,ll variations in the CESR signal parameters of Houston under the prime grant MDA 972-88-G-0002 from the N d 1 , 8 5 C ~ , 1 5 C ~are 0 4 -expected y within a temperature interval Defense Advanced Research Projects Agency and the State of between 10 and 35 K above its superconducting transition temTexas. perature of 24 K. Thus,the abrupt variations in the CESR signal parameters observed near 50 K (Figures 3 and 4) seem consistent Referand Notes with the behavior of a two-dimensional magnetic spin structure (1) Owens, F. J.; Iqbal, Z.; Zakhidov, A. A.; Khairullin, I. I. Physica C of Ndl.85Ce&uO~y near a superconducting phase transition. 1991, 174, 309. (2) (a) McKinnon, W. R.; Morton, J. R.; Pleizier, G. Solid State Com4 I T (K)I25 K. All three parameters-intensity, g value, mun. 1988.66, 1093. (b) Mehran, F.; Anderson, P. W. Solid State Commun. and line width-of the Ndl.85C%.15C~Oky CESR signal remain 1989, 71, 29. (c) Mehran, F.; Barnes, S. E.; Giers, E. A.; McGuire, T. R. constant below 25 K. It seems that the conduction ESR line Solid Stare Commun. 1988, 67, 55. remains unaffected by the establishment of bulk superconductivity (3) Chakravarty, S.; Orbach, R. Phys. Reu. Lett. 1990,61, 224. (4) Janes, R.; Singh, K.K.;Burnside, S. D.; Edwards, P. P. Solid State in Nd1.85CQ,15Cu04-y. Since the superconducting electrons are Commun. 1991, 79, 241. spin-paired, they should not contribute to the CESR signal ob(5) Poole, C. P., Jr. Electron Spin Resonance. A Comprehemiue Treatise served below T,. The CESR signal seen thus seems due to on Experimental Techniques; Wiley: New York, 1983; pp 494-503. nonsuperconducting regions of this inhomogeneous type I1 su(6) Ashcroft. N. W.; Mennin, N. D. Solid Srare Physics; Holt, Rinehart and Winston: Philadelphia, 1986; pp 655-664. perconductor.
Conclusions ESR differentiates Nd2-xCexCuOeyinsulating (x = 0.00) and superconducting (x = 0.15) compounds. The fvst is ESR silent, whereas the second gives rise to a CESR signal. The origin of this difference lies in the nature of their respective band structures. Since the extra electrons introduced by cerium doping in this system are highly mobile and are delocalized into a conduction band, they give rise to a CESR signal. In contrast, the undoped insulating parent compound has antiferromagnetically ordered electron spins3,4J4contributing to it ESR silence.
(7) Jones, R.; Janes, R.; Armstrong, R.; Pypcr. N. C.; Edwards, P. P.; Keeble, D. J.; Harrison, M. R. J. Chem. Soc., Faraday Trans. 1990,86,675. (8) Elliott, R. J. Phys. Reo. 1954, 96, 266. (9) Feher, G.; Kip, A. F.Phys. Reu. 1955, 98, 337. (10) Masiakowski, J. T.; Puri, M.; Kevan, L. J. Phys. Chem. 1991, 95, 8968. (11) (a) Nagata, K.; Tazuke, Y. J . Phys. Soc. Jpn. 1972, 32, 337. (b) Tazuke, Y.; Nagata, K. J. Phys. Soc. Jpn. 1975, 38, 1003. (12) Owens, F.J. In Magnetic Resonance of Phase Transitions; Owens, F.J., Poole, C. P., Farach, H. A., Eds.; Academic: New York, 1985; Chapter 6. (13) Owens, F. J. Solid State Commun. 1989, 70, 173. (14) Seaz-Puche, R.; Norton, M.; Glaunsinger, W. S. Mater. Res. Bull. 1982, 17, 1523.
