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Electron-doping effect on magnetic order and superconductivity in Nd2−xCexCuO4 single crystals
Physica C 378–381 (2002) 273–277 www.elsevier.com/locate/physc
Electron-doping effect on magnetic order and superconductivity in Nd2xCex CuO4 single crystals T. Uefuji
a,*
, K. Kurahashi b, M. Fujita a, M. Matsuda c, K. Yamada
a
a
Institute for Chemical Research, Kyoto University, Gokasho, Uji 611-0011, Kyoto, Japan Department of Physics, Tohoku University, Aramaki, Aoba-ku, Sendai 980-8578, Japan Advanced Science Research Center, Japan Atomic Energy Research Institute, Tokai 319-1195, Japan b
c
Accepted 22 January 2002
Abstract Doping dependences of magnetic properties such as the Neel temperature, staggered moment of Cu2þ spin and antiferromagnetic coherence length in the electron-doped Nd2x Cex CuO4 have been investigated using elastic neutron scattering measurement. Near the antiferromagnetic–superconducting phase boundary for the heat-treated samples antiferromagnetism remarkably degrades upon electron-doping. The effect of heat treatment on the Neel temperature is clearly seen when x exceeds 0.10 where fractional superconductivity appears by the heat treatment. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 74.25.Dw; 74.25.Ha; 74.72.)h; 74.72.Jt; 75.25.þz Keywords: High-Tc superconductivity; Electron-doped cuprate; Magnetic-superconducting phase diagram; Neutron scattering
1. Introduction The magnetic-superconducting (SC) phase diagram of the electron-doped Nd2x Cex CuO4 (NCCO) was firstly and only studied by muon spin relaxation/rotation (lSR) studies [1]. There, the ntype SC phase appears abruptly upon disappearing the antiferromagnetic (AF) order at x 0:14. On the other hand, coexistence or phase separation between the cluster spin glass and the SC phases has been proposed for the hole-doped La2x Srx -
*
Corresponding author. Tel.: +81-774-38-3112; fax: +81774-38-3118. E-mail address: [email protected] (T. Uefuji).
CuO4 in the vicinity of the phase boundary [2]. Therefore, in order to discuss the interplay between antiferromagnetism and superconductivity in the electron-doped cuprates in comparison with that in the hole-doped cuprates, more detailed phase diagram is highly required. However, in the previous phase diagram studied by lSR, effects of Nd moment are included particularly in the low temperature region as well as near the phase boundary between AF and SC phases. In order to study magnetism due to Cu moment in more detail, neutron scattering measurements serve as a powerful tool to separate the contribution from Cu and Nd moments. In the present work we systematically studied the magnetic Bragg peaks of NCCO as functions of electron-doping and temperature to evaluate the staggered moment of Cu2þ
0921-4534/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 2 ) 0 1 4 2 6 - 0
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T. Uefuji et al. / Physica C 378–381 (2002) 273–277
spin lCu , the Neel temperature TN and AF coherence length nM .
2. Experimental Series of single crystals of NCCO listed in Table 1 were grown by a travelling-solvent floating-zone method under air flow for x ¼ 0:13 and under O2 gas flow for all other samples. As-grown samples of about 5 g were heat-treated in Ar-gas flow at 920 °C for 12 h. The difference of oxygen concentration d per formula unit between as-grown and reduced samples was evaluated from the weight loss after the heat treatment and is listed in Table 1. The uncertainty of d is of the order of 104 by taking into account the resolution of microbalance 105 g and sample weight 5 g. Susceptibility measurements were performed by a SQUID magnetometer under zero-field-cooling (ZFC) and field-cooling (FC) processes. Elastic neutron scattering measurements were performed on the thermal neutron 3-axis spectrometer TOPAN (6G) for reduced samples with x ¼ 0:13 and 0.14 and on the cold neutron 3-axis spectrometer HER (C11) for all the reduced samples other than x ¼ 0:13 and for the as-grown samples. These spectrometers are installed at JRR3M reactor in JAERI, Tokai, Japan. The incident neutron energies were fixed at 14.7 meV and at 5.0 meV for TOPAN and for HER, respectively. The horizontal collimation sequency on TOPAN was 150 -PG-150 -S-PG-150 -Blank and that on HER was Guide-100 -Be-S-800 -800 for reduced samples with Table 1 The difference of oxygen concentration d per formula unit between as-grown and reduced samples described in Figs. 1 and 2; d for differently heat-treated x ¼ 0:13 sample is also listed Ce concentration, x
Difference of oxygen concentration, d
0.10 0.12 0.13
0.0285 0.0304 0.0165 (shown in Fig. 1) 0.018 (shown in Fig. 2) 0.0211 0.02 [8] 0.0200 0.0207
0.14 0.15 0.16 0.17
0:10 6 x 6 0:14 and all as-grown samples and Guide-400 -Be-S-800 -800 for reduced samples with 0:15 6 x 6 0:17. We define the reciprocal lattice space below with tetragonal notation of the I4/ mmm space group. nM was obtained from inverse of the full-width at half-maximum (FWHM) of the AF Bragg peak at ð0:5; 0:5; 3Þ considering the spectrometer resolution. Staggered moments of Cu2þ and Nd3þ spins, lCu and lNd , were evaluated from the AF Bragg peak intensities integrated at ð0:5; 0:5; 1Þ and ð0:5; 0:5; 3Þ and the nuclear Bragg peak intensity integrated at ð1; 1; 0Þ as follows. For these samples we assumed the same spin structure as that of the asgrown x ¼ 0:15 sample [3]. We further assumed a non-collinear spin structure [4], where the two equally populated domains exist in the whole sample. Then the magnetic structure factors are written as, 2 2 jF M ð0:5; 0:5; 1Þj ¼ 4ð0:93lCu 1:18lNd Þ ; 2 2 jF M ð0:5; 0:5; 3Þj ¼ 4ð0:84lCu þ 1:79lNd Þ :
ð1Þ
lCu and lNd were calculated from Eq. (1) using the nuclear and magnetic Bragg peak intensities. We note that lCu and lNd were calculated on an assumption of full volume fraction of the magnetically ordered phase in each sample. Therefore in the case of phase separation into the magnetic and nonmagnetic phases [5] as discussed in more detail in Section 4, lCu and lNd do not simply correspond to the intrinsic values. Nevertheless as discussed below these parameters give us important information on the interplay between the magnetic order and the superconductivity in this system and furthermore possible phase separation between the AF and SC regions near the phase boundary can only be discussed based on the doping dependences of these parameters.
3. Results Fig. 1 shows the temperature dependence of susceptibility for the reduced samples measured under ZFC process. We note that for the magnetization measurement small pieces of crystal of
T. Uefuji et al. / Physica C 378–381 (2002) 273–277
275
Fig. 1. Temperature dependence of susceptibility for reduced NCCO samples under ZFC process.
about 100 mg were cut from the samples used for neutron scattering experiment. In the doping region of 0:14 6 x 6 0:17 large diamagnetism of about 0.015 emu/g is observed and their Tc ’s are well-defined at 25 K, though the SC transition was rather broad in the x ¼ 0:17 sample. In the doping region of x 6 0:12 bulk SC diamagnetism was not observed. However, we note that for the samples with x ¼ 0:10 and 0.12 small diamagnetic susceptibility <1% of that for the x ¼ 0:14 sample was detected near T 20 K. Magnetic susceptibilities for the reduced samples under FC process are shown in Fig. 2. We note that the x ¼ 0:13 sample of about 100 mg in Fig. 2 was heat-treated under a slightly different condition from the other samples (at 900 °C for 10 h). Hence the oxygen loss by the heat treatment is slightly different between the two x ¼ 0:13 samples as shown in Table 1. Susceptibility at 5 K is independent on applied magnetic field at least up to 10 Oe in the x ¼ 0:15 but not in the x ¼ 0:13 and 0.17 samples. These facts indicate the bulk and uniform superconductivity appears for the former and the inhomogeneous or granular superconductivity for the latter. The phase diagram of NCCO studied by elastic neutron scattering studies is shown in Fig. 3. TN was defined as the temperature where lCu starts increasing upon cooling (see Fig. 4). TN for the asgrown x ¼ 0:00 sample was referred from Ref. [7].
Fig. 2. Temperature dependence of susceptibility under FC process for reduced NCCO samples with (a) x ¼ 0:13, (b) x ¼ 0:15, and (c) x ¼ 0:17. The heat treatment condition for x ¼ 0:13 was different from the one shown in Fig 1.
Near the AF–SC phase boundary for reduced samples, TN remarkably decreases upon electrondoping, though the AF correlation still remains in the reduced x ¼ 0:16 sample. The effect of heat treatment on TN is clearly seen in the doping region of x > 0:10. The temperature dependence of lCu is shown in Fig. 4. In the reduced x ¼ 0:17 sample we detected magnetic intensity only at ð0:5; 0:5; 3Þ but not at
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Fig. 3. Magnetic-SC phase diagram of NCCO. TN for as-grown x ¼ 0:00 sample was referred from Ref. [7]. It is noted that TN is plotted against the Ce concentration x, not against the effective doping concentration. TN1 (closed triangle) is defined by lSR measurements [6]. Tc in the figure denotes the onset SC transition temperature. The inset shows the onset and the middlepoint of Tc for 0:14 6 x 6 0:17.
Fig. 5. Temperature dependences of nM along the (a) in-plane and (b) out-of-plane directions for all reduced NCCO samples.
spectively. In the region of x 6 0:13 the AF Bragg peak width is almost resolution-limited (nM > 200 ). nM also remarkably decreases upon electronA doping near the AF–SC phase boundary.
4. Discussions
Fig. 4. Temperature dependences of lCu for all reduced NCCO samples.
ð0:5; 0:5; 1Þ within the experimental sensitivity and evaluated the staggered moments from Eq. (1) lCu lNd ð0:006 0:004ÞlB at 7 K. The temperature dependences of nM along the in-plane and the out-of-plane directions for reduced samples are shown in Fig. 5(a) and (b), re-
The results obtained in the present study clearly demonstrate that the AF correlations remarkably degrade upon electron-doping near the AF–SC phase boundary. In reduced samples with x smaller than 0.12, bulk superconductivity does not appear, though the SC property is very sensitive to the heat treatment condition [8]. These results suggest a competitive relationship between antiferromagnetism and superconductivity in the electron-doped system. However, the magnetic order with a finite value of nM is observable even in the optimally doped (x ¼ 0:15) SC sample. The value for x P 0:15 suggests the exof nM ¼ 50 100 A
T. Uefuji et al. / Physica C 378–381 (2002) 273–277
istence of isolated AF regions in the SC background. We note again that in inhomogeneous system the calculated value of lCu does not simply correspond to the intrinsic value of staggered moment but to the averaged value over the sample volume. Then the decrease of lCu near phase boundary upon doping originates in the decrease of staggered moment and/or AF volume fraction. According to our recent lSR measurements AF volume fraction at 5 K changes abruptly near the phase boundary from (80 5%) in the samples showing only fractional superconductivity to (15 10%) in the bulk SC samples [5]. Therefore, the effect of volume fraction on the evaluated value of lCu possibly dominates near the phase boundary. However, whether the AF and SC phases coexist or phase separate cannot be concluded at present. As shown in Fig. 2(c), over-doping degrades both AF order and superconductivity. Our recent lSR measurements on NCCO [6] defined two characteristic temperatures TN1 and TN2 ðTN1 > TN2 Þ related with the magnetic order of this system: upon cooling below TN1 a fast exponential type relaxation firstly appears in the time spectrum in addition to the slow relaxation due to the fluctuation of Nd spins and below TN2 an oscillation in the time spectrum or an another faster exponential type relaxation additionally appears. As shown in Fig. 3, TN determined by the neutron scattering measurements nearly corresponds to TN1 by lSR measurements. From lSR measurements under a longitudinal magnetic field [6], we concluded that the AF order between TN1 and TN2 is quasi-static and the one below TN2 is static, while the magnetic order below TN TN1 is static when seen by neutron scattering. The effect of the heat treatment on TN seems to be different between samples with x < 0:10 and with x > 0:10. Appreciable effect is seen in the latter region. According to the recent resistivity measurement on NCCO thin films, superconductivity is observed in x down to 0.10 [9]. Although the present result does not confirm bulk superconductivity in x 6 0:12, the finite heat treatment
277
effect on the magnetic properties for x > 0:10 may connect with the onset of superconductivity.
Acknowledgements We greatly acknowledge K. Nemoto and S. Watanabe for technical support for the experiments in JAERI. This research was in part supported by the Japanese Ministry of Education, Culture, Sports, Science and Technology, Grantin-Aid for Scientific Research on Priority Areas (Novel Quantum Phenomena in Transition Metal Oxides), 12046239, 2000, for Scientific Research (A), 10304026, 2000, for Encouragement of Young Scientists, 13740216, 2001 and for Creative Scientific Research (13NP0201) ‘‘Collaboratory on Electron Correlations––Toward a New Research Network between Physics and Chemistry––’’ and by the Japan Science and Technology Corporation, the Core Research for Evolutional Science and Technology Project (CREST).
References [1] G.M. Luke, L.P. Le, B.J. Sternlieb, Y.J. Uemura, J.H. Brewer, R.F. Kiefl, S.R. Kreitzman, T.M. Riseman, C.E. Stronach, M.R. Davis, et al., Phys. Rev. B 42 (1990) 7981. [2] Ch. Niedermayer, C. Bernhard, T. Blasius, A. Golnik, A. Moodenbaugh, J.I. Budnick, Phys. Rev. Lett. 80 (1998) 3843. [3] M. Matsuda, Y. Endoh, K. Yamada, H. Kojima, I. Tanaka, R.J. Birgeneau, M.A. Kastner, G. Shirane, Phys. Rev. B 45 (1992) 12548. [4] S. Skanthakumar, J.W. Lynn, J.L. Peng, Z.Y. Li, Phys. Rev. B 47 (1993) 6173. [5] T. Uefuji, T. Kubo, K. Yamada, M. Fujita, K. Kurahashi, I. Watanabe, K. Nagamine, Physica C 357–360 (2001) 208. [6] T. Kubo, T. Uefuji, M. Fujita, K. Yamada, I. Watanabe, K. Nagamine, in: Proceedings of ISS2001, 2001. [7] M. Matsuda, K. Yamada, K. Kakurai, H. Kadowaki, T.R. Thurston, Y. Endoh, Y. Hidaka, R.J. Birgeneau, M.A. Kastner, P.M. Gehring, et al., Phys. Rev. B 42 (1990) 10098. [8] K. Kurahashi, Doctor Thesis, Tohoku University, 1999. [9] B. Prijamboedi, S. Kashiwaya, A. Sawa, Y. Tanaka, 19aTE8, in: Proc. Annual Meeting of Phys. Soc. Japan, Tokushima, 2001.
Electron-doping effect on magnetic order and superconductivity in Nd2xCex CuO4 single crystals T. Uefuji
a,*
, K. Kurahashi b, M. Fujita a, M. Matsuda c, K. Yamada
a
a
Institute for Chemical Research, Kyoto University, Gokasho, Uji 611-0011, Kyoto, Japan Department of Physics, Tohoku University, Aramaki, Aoba-ku, Sendai 980-8578, Japan Advanced Science Research Center, Japan Atomic Energy Research Institute, Tokai 319-1195, Japan b
c
Accepted 22 January 2002
Abstract Doping dependences of magnetic properties such as the Neel temperature, staggered moment of Cu2þ spin and antiferromagnetic coherence length in the electron-doped Nd2x Cex CuO4 have been investigated using elastic neutron scattering measurement. Near the antiferromagnetic–superconducting phase boundary for the heat-treated samples antiferromagnetism remarkably degrades upon electron-doping. The effect of heat treatment on the Neel temperature is clearly seen when x exceeds 0.10 where fractional superconductivity appears by the heat treatment. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 74.25.Dw; 74.25.Ha; 74.72.)h; 74.72.Jt; 75.25.þz Keywords: High-Tc superconductivity; Electron-doped cuprate; Magnetic-superconducting phase diagram; Neutron scattering
1. Introduction The magnetic-superconducting (SC) phase diagram of the electron-doped Nd2x Cex CuO4 (NCCO) was firstly and only studied by muon spin relaxation/rotation (lSR) studies [1]. There, the ntype SC phase appears abruptly upon disappearing the antiferromagnetic (AF) order at x 0:14. On the other hand, coexistence or phase separation between the cluster spin glass and the SC phases has been proposed for the hole-doped La2x Srx -
*
Corresponding author. Tel.: +81-774-38-3112; fax: +81774-38-3118. E-mail address: [email protected] (T. Uefuji).
CuO4 in the vicinity of the phase boundary [2]. Therefore, in order to discuss the interplay between antiferromagnetism and superconductivity in the electron-doped cuprates in comparison with that in the hole-doped cuprates, more detailed phase diagram is highly required. However, in the previous phase diagram studied by lSR, effects of Nd moment are included particularly in the low temperature region as well as near the phase boundary between AF and SC phases. In order to study magnetism due to Cu moment in more detail, neutron scattering measurements serve as a powerful tool to separate the contribution from Cu and Nd moments. In the present work we systematically studied the magnetic Bragg peaks of NCCO as functions of electron-doping and temperature to evaluate the staggered moment of Cu2þ
0921-4534/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 2 ) 0 1 4 2 6 - 0
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T. Uefuji et al. / Physica C 378–381 (2002) 273–277
spin lCu , the Neel temperature TN and AF coherence length nM .
2. Experimental Series of single crystals of NCCO listed in Table 1 were grown by a travelling-solvent floating-zone method under air flow for x ¼ 0:13 and under O2 gas flow for all other samples. As-grown samples of about 5 g were heat-treated in Ar-gas flow at 920 °C for 12 h. The difference of oxygen concentration d per formula unit between as-grown and reduced samples was evaluated from the weight loss after the heat treatment and is listed in Table 1. The uncertainty of d is of the order of 104 by taking into account the resolution of microbalance 105 g and sample weight 5 g. Susceptibility measurements were performed by a SQUID magnetometer under zero-field-cooling (ZFC) and field-cooling (FC) processes. Elastic neutron scattering measurements were performed on the thermal neutron 3-axis spectrometer TOPAN (6G) for reduced samples with x ¼ 0:13 and 0.14 and on the cold neutron 3-axis spectrometer HER (C11) for all the reduced samples other than x ¼ 0:13 and for the as-grown samples. These spectrometers are installed at JRR3M reactor in JAERI, Tokai, Japan. The incident neutron energies were fixed at 14.7 meV and at 5.0 meV for TOPAN and for HER, respectively. The horizontal collimation sequency on TOPAN was 150 -PG-150 -S-PG-150 -Blank and that on HER was Guide-100 -Be-S-800 -800 for reduced samples with Table 1 The difference of oxygen concentration d per formula unit between as-grown and reduced samples described in Figs. 1 and 2; d for differently heat-treated x ¼ 0:13 sample is also listed Ce concentration, x
Difference of oxygen concentration, d
0.10 0.12 0.13
0.0285 0.0304 0.0165 (shown in Fig. 1) 0.018 (shown in Fig. 2) 0.0211 0.02 [8] 0.0200 0.0207
0.14 0.15 0.16 0.17
0:10 6 x 6 0:14 and all as-grown samples and Guide-400 -Be-S-800 -800 for reduced samples with 0:15 6 x 6 0:17. We define the reciprocal lattice space below with tetragonal notation of the I4/ mmm space group. nM was obtained from inverse of the full-width at half-maximum (FWHM) of the AF Bragg peak at ð0:5; 0:5; 3Þ considering the spectrometer resolution. Staggered moments of Cu2þ and Nd3þ spins, lCu and lNd , were evaluated from the AF Bragg peak intensities integrated at ð0:5; 0:5; 1Þ and ð0:5; 0:5; 3Þ and the nuclear Bragg peak intensity integrated at ð1; 1; 0Þ as follows. For these samples we assumed the same spin structure as that of the asgrown x ¼ 0:15 sample [3]. We further assumed a non-collinear spin structure [4], where the two equally populated domains exist in the whole sample. Then the magnetic structure factors are written as, 2 2 jF M ð0:5; 0:5; 1Þj ¼ 4ð0:93lCu 1:18lNd Þ ; 2 2 jF M ð0:5; 0:5; 3Þj ¼ 4ð0:84lCu þ 1:79lNd Þ :
ð1Þ
lCu and lNd were calculated from Eq. (1) using the nuclear and magnetic Bragg peak intensities. We note that lCu and lNd were calculated on an assumption of full volume fraction of the magnetically ordered phase in each sample. Therefore in the case of phase separation into the magnetic and nonmagnetic phases [5] as discussed in more detail in Section 4, lCu and lNd do not simply correspond to the intrinsic values. Nevertheless as discussed below these parameters give us important information on the interplay between the magnetic order and the superconductivity in this system and furthermore possible phase separation between the AF and SC regions near the phase boundary can only be discussed based on the doping dependences of these parameters.
3. Results Fig. 1 shows the temperature dependence of susceptibility for the reduced samples measured under ZFC process. We note that for the magnetization measurement small pieces of crystal of
T. Uefuji et al. / Physica C 378–381 (2002) 273–277
275
Fig. 1. Temperature dependence of susceptibility for reduced NCCO samples under ZFC process.
about 100 mg were cut from the samples used for neutron scattering experiment. In the doping region of 0:14 6 x 6 0:17 large diamagnetism of about 0.015 emu/g is observed and their Tc ’s are well-defined at 25 K, though the SC transition was rather broad in the x ¼ 0:17 sample. In the doping region of x 6 0:12 bulk SC diamagnetism was not observed. However, we note that for the samples with x ¼ 0:10 and 0.12 small diamagnetic susceptibility <1% of that for the x ¼ 0:14 sample was detected near T 20 K. Magnetic susceptibilities for the reduced samples under FC process are shown in Fig. 2. We note that the x ¼ 0:13 sample of about 100 mg in Fig. 2 was heat-treated under a slightly different condition from the other samples (at 900 °C for 10 h). Hence the oxygen loss by the heat treatment is slightly different between the two x ¼ 0:13 samples as shown in Table 1. Susceptibility at 5 K is independent on applied magnetic field at least up to 10 Oe in the x ¼ 0:15 but not in the x ¼ 0:13 and 0.17 samples. These facts indicate the bulk and uniform superconductivity appears for the former and the inhomogeneous or granular superconductivity for the latter. The phase diagram of NCCO studied by elastic neutron scattering studies is shown in Fig. 3. TN was defined as the temperature where lCu starts increasing upon cooling (see Fig. 4). TN for the asgrown x ¼ 0:00 sample was referred from Ref. [7].
Fig. 2. Temperature dependence of susceptibility under FC process for reduced NCCO samples with (a) x ¼ 0:13, (b) x ¼ 0:15, and (c) x ¼ 0:17. The heat treatment condition for x ¼ 0:13 was different from the one shown in Fig 1.
Near the AF–SC phase boundary for reduced samples, TN remarkably decreases upon electrondoping, though the AF correlation still remains in the reduced x ¼ 0:16 sample. The effect of heat treatment on TN is clearly seen in the doping region of x > 0:10. The temperature dependence of lCu is shown in Fig. 4. In the reduced x ¼ 0:17 sample we detected magnetic intensity only at ð0:5; 0:5; 3Þ but not at
276
T. Uefuji et al. / Physica C 378–381 (2002) 273–277
Fig. 3. Magnetic-SC phase diagram of NCCO. TN for as-grown x ¼ 0:00 sample was referred from Ref. [7]. It is noted that TN is plotted against the Ce concentration x, not against the effective doping concentration. TN1 (closed triangle) is defined by lSR measurements [6]. Tc in the figure denotes the onset SC transition temperature. The inset shows the onset and the middlepoint of Tc for 0:14 6 x 6 0:17.
Fig. 5. Temperature dependences of nM along the (a) in-plane and (b) out-of-plane directions for all reduced NCCO samples.
spectively. In the region of x 6 0:13 the AF Bragg peak width is almost resolution-limited (nM > 200 ). nM also remarkably decreases upon electronA doping near the AF–SC phase boundary.
4. Discussions
Fig. 4. Temperature dependences of lCu for all reduced NCCO samples.
ð0:5; 0:5; 1Þ within the experimental sensitivity and evaluated the staggered moments from Eq. (1) lCu lNd ð0:006 0:004ÞlB at 7 K. The temperature dependences of nM along the in-plane and the out-of-plane directions for reduced samples are shown in Fig. 5(a) and (b), re-
The results obtained in the present study clearly demonstrate that the AF correlations remarkably degrade upon electron-doping near the AF–SC phase boundary. In reduced samples with x smaller than 0.12, bulk superconductivity does not appear, though the SC property is very sensitive to the heat treatment condition [8]. These results suggest a competitive relationship between antiferromagnetism and superconductivity in the electron-doped system. However, the magnetic order with a finite value of nM is observable even in the optimally doped (x ¼ 0:15) SC sample. The value for x P 0:15 suggests the exof nM ¼ 50 100 A
T. Uefuji et al. / Physica C 378–381 (2002) 273–277
istence of isolated AF regions in the SC background. We note again that in inhomogeneous system the calculated value of lCu does not simply correspond to the intrinsic value of staggered moment but to the averaged value over the sample volume. Then the decrease of lCu near phase boundary upon doping originates in the decrease of staggered moment and/or AF volume fraction. According to our recent lSR measurements AF volume fraction at 5 K changes abruptly near the phase boundary from (80 5%) in the samples showing only fractional superconductivity to (15 10%) in the bulk SC samples [5]. Therefore, the effect of volume fraction on the evaluated value of lCu possibly dominates near the phase boundary. However, whether the AF and SC phases coexist or phase separate cannot be concluded at present. As shown in Fig. 2(c), over-doping degrades both AF order and superconductivity. Our recent lSR measurements on NCCO [6] defined two characteristic temperatures TN1 and TN2 ðTN1 > TN2 Þ related with the magnetic order of this system: upon cooling below TN1 a fast exponential type relaxation firstly appears in the time spectrum in addition to the slow relaxation due to the fluctuation of Nd spins and below TN2 an oscillation in the time spectrum or an another faster exponential type relaxation additionally appears. As shown in Fig. 3, TN determined by the neutron scattering measurements nearly corresponds to TN1 by lSR measurements. From lSR measurements under a longitudinal magnetic field [6], we concluded that the AF order between TN1 and TN2 is quasi-static and the one below TN2 is static, while the magnetic order below TN TN1 is static when seen by neutron scattering. The effect of the heat treatment on TN seems to be different between samples with x < 0:10 and with x > 0:10. Appreciable effect is seen in the latter region. According to the recent resistivity measurement on NCCO thin films, superconductivity is observed in x down to 0.10 [9]. Although the present result does not confirm bulk superconductivity in x 6 0:12, the finite heat treatment
277
effect on the magnetic properties for x > 0:10 may connect with the onset of superconductivity.
Acknowledgements We greatly acknowledge K. Nemoto and S. Watanabe for technical support for the experiments in JAERI. This research was in part supported by the Japanese Ministry of Education, Culture, Sports, Science and Technology, Grantin-Aid for Scientific Research on Priority Areas (Novel Quantum Phenomena in Transition Metal Oxides), 12046239, 2000, for Scientific Research (A), 10304026, 2000, for Encouragement of Young Scientists, 13740216, 2001 and for Creative Scientific Research (13NP0201) ‘‘Collaboratory on Electron Correlations––Toward a New Research Network between Physics and Chemistry––’’ and by the Japan Science and Technology Corporation, the Core Research for Evolutional Science and Technology Project (CREST).
References [1] G.M. Luke, L.P. Le, B.J. Sternlieb, Y.J. Uemura, J.H. Brewer, R.F. Kiefl, S.R. Kreitzman, T.M. Riseman, C.E. Stronach, M.R. Davis, et al., Phys. Rev. B 42 (1990) 7981. [2] Ch. Niedermayer, C. Bernhard, T. Blasius, A. Golnik, A. Moodenbaugh, J.I. Budnick, Phys. Rev. Lett. 80 (1998) 3843. [3] M. Matsuda, Y. Endoh, K. Yamada, H. Kojima, I. Tanaka, R.J. Birgeneau, M.A. Kastner, G. Shirane, Phys. Rev. B 45 (1992) 12548. [4] S. Skanthakumar, J.W. Lynn, J.L. Peng, Z.Y. Li, Phys. Rev. B 47 (1993) 6173. [5] T. Uefuji, T. Kubo, K. Yamada, M. Fujita, K. Kurahashi, I. Watanabe, K. Nagamine, Physica C 357–360 (2001) 208. [6] T. Kubo, T. Uefuji, M. Fujita, K. Yamada, I. Watanabe, K. Nagamine, in: Proceedings of ISS2001, 2001. [7] M. Matsuda, K. Yamada, K. Kakurai, H. Kadowaki, T.R. Thurston, Y. Endoh, Y. Hidaka, R.J. Birgeneau, M.A. Kastner, P.M. Gehring, et al., Phys. Rev. B 42 (1990) 10098. [8] K. Kurahashi, Doctor Thesis, Tohoku University, 1999. [9] B. Prijamboedi, S. Kashiwaya, A. Sawa, Y. Tanaka, 19aTE8, in: Proc. Annual Meeting of Phys. Soc. Japan, Tokushima, 2001.