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Muon spin relaxation study on magnetic properties of Nd2−xCexCuO4 around a boundary between the magnetically ordered and superconducting states
Physica C 357±360 (2001) 212±215
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Muon spin relaxation study on magnetic properties of Nd2 x Cex CuO4 around a boundary between the magnetically ordered and superconducting states I. Watanabe a,*, T. Uefuji b, K. Kurahashi b, M. Fujita b, K. Yamada b, K. Nagamine a,c a
Muon Science Laboratory, RIKEN (The Institute of Physical and Chemical Research), 2-1 Hirosawa, Wako, Saitama 351-0198, Japan b Institute of Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan c Meson Science Laboratory (KEK-MSL), KEK, 1-1 Oho, Tsukuba 305-0801, Japan Received 16 October 2000; accepted 22 January 2001
Abstract Zero-®eld and longitudinal-®eld muon spin relaxation (lSR) measurements have been carried out in order to investigate magnetic properties of the electron-doped high-Tc oxides of Nd2 x Cex CuO4 around a boundary between the magnetically ordered state and the superconducting one. A fast muon-spin depolarizing component by a static internal ®eld was observed in the oxygen-reduced superconducting single crystal with x 0:15 below about 10 K, indicating that a part of Cu and/or Nd spins shows a magnetically ordered state. The present lSR study suggests a possibility of a coexisting or a phase-separated state around x 0:15. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 76.75.+i; 74.72.Dn; 74.25.Ha Keywords: Muon spin relaxation; High-Tc ; Electron doped system; Nd2 - x Cex CuO4
The magnetically ordered state of the electrondoped high-Tc oxide of Nd2 x Cex CuO4 is known to be adjacent to the superconducting state [1±5]. The superconducting state appears suddenly around x 0:15 by the oxygen-reduced treatment. Although Nd2 x Cex CuO4 has been discovered soon after the discovery of the hole-doped high-Tc oxides, not so many studies have been carried out in order to clarify the relationship between the magnetically ordered state and the superconducting
* Corresponding author. Tel.: +81-48-462-1111, ext.: 3337; fax: +81-48-462-4648. E-mail address: [email protected] (I. Watanabe).
one around the phase boundary because of the diculty of the sample preparation and oxygen treatment. Recently, Kurahashi et al. [6] have developed sample-preparation conditions and have succeeded to grow high-quality single crystals of Nd2 x Cex CuO4 with a wide range of x. We carried out muon spin relaxation (lSR) measurements using these single crystals to investigate the abrupt change of the electronic state around x 0:15. Two single crystals with x 0:15 and 0.17 were prepared for the present lSR study. Conditions of the growth of single crystals and transport phenomena are reported in a separate paper [7]. After the oxygen-reduced treatment, the bulk superconductivity was con®rmed by the susceptibil-
0921-4534/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 1 ) 0 0 2 0 9 - X
I. Watanabe et al. / Physica C 357±360 (2001) 212±215
213
ity measurement. The superconducting transition temperatures were determined to be about 25 and 15 K for x 0:15 and 0.17, respectively. Zero-®eld (ZF) and longitudinal-®eld (LF) lSR measurements were carried out at the RIKENRAL Muon Facility at the Rutherford±Appleton Laboratory in the UK using the pulsed positive surface-muon beam with a momentum of 27 MeV/c. The asymmetry parameter, A(t), was de®ned as A t F t B t=F t B t, where F(t) and B(t) were total muon events counted by the forward and backward counters at a time t, respectively. Fig. 1 shows the ZF-lSR time spectra of oxygen-reduced superconducting single crystals of Nd2 x Cex CuO4 with (a) x 0:15 and (b) 0.17 obtained at various temperatures. In the case of x 0:17, all of measured time spectra were well described by the single-exponential function of A0 e k0 t , where A0 is the initial asymmetry at t 0 and k0 is the depolarization rate of the muon spin. Solid lines in Fig. 1(b) are the best ®t results. The
time spectra at each temperature did not change at all in LF of about 4 kG which is applied in the same direction of the initial muon-spin polarization. This means that the muon spin depolarizes by a dynamically ¯uctuating internal ®eld. The temperature dependence of the dynamical depolarization rate of the single crystal with x 0:17 is displayed in Fig. 2 by closed triangles, showing the linear relationship with the temperature. This result means that the ¯uctuation frequency of the internal ®eld at the muon site is proportional to the temperature which can be described by the Red®eld's theory [8]. Therefore, the origin of this dynamical depolarization is concluded to be thermally ¯uctuating Nd spins. No sign of the appearance of a statically ordered state of Cu and/or Nd spins was observed down to 4.4 K in the single crystal with x 0:17. In the case of x 0:15, in contrast with the case of x 0:17, the time spectrum started to deviate from the single exponential-type depolarization behavior below about 70 K, showing that the time spectrum consists of two depolarization components. In addition, one more component which depolarizes much faster than other components appears below about 10 K with small amplitude of the asymmetry as indicated by an arrow in the inset of Fig. 1(a). This fact means that the time spectra below about 10 K consists of at least three depolarization components. Therefore, the threeexponential function, A2 e k2 t A1 e k1 t A0 e k0 t ,
Fig. 1. Zero-®eld lSR time spectra of oxygen-reduced Nd2 x Cex CuO4 with (a) x 0:15 and (b) 0.17 at various temperatures. Insets in the ®gure show the depolarization behavior in an early time region.
Fig. 2. Temperature dependences of the dynamical muonspin depolarization rates of oxygen-reduced superconducting Nd2 x Cex CuO4 with x 0:15 and 0.17. Broken lines in the ®gure were the best ®t results showing the relationship of k / T .
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I. Watanabe et al. / Physica C 357±360 (2001) 212±215
Fig. 3. Temperature dependence of the muon-spin depolarization rates of the oxygen-reduced superconducting Nd2 x Cex CuO4 with x 0:15.
Fig. 4. Longitudinal-®eld dependence of the lSR time spectrum of the oxygen-reduced Nd2 x Cex CuO4 with x 0:15 at 5.1 K.
was applied for convenient analyses. The best ®t results are shown in Fig. 1(a) as the solid lines. Fig. 3 shows the temperature dependences of the initial asymmetry of A2 , A1 and A0 , respectively. The depolarization behavior of the main component described by A0 e k0 t and the second component described by A1 e k1 t did not change under the LF of about 4 kG at 5 K. This result shows that these depolarizing components are led by dynamically ¯uctuating internal ®elds at the muon sites. The temperature dependence of k0 is proportional to the temperature above about 10 K as shown in Fig. 2 by open circles, meaning that the muon-spin depolarization of the main component is derived from the thermally ¯uctuating Nd spins as well as observed in the case of the single crystal with x 0:17. The second component described by A1 e k1 t has a bit faster depolarization rate than the main component, so that the second component is expected to re¯ect an additional eect from the Cu-spin ¯uctuations which slow down with decreasing temperature. On the other hand, the fast depolarizing component described by A2 e k2 t , which was indicated by an arrow in Fig. 4, was recovered by the application of LF of 500 G. This fact means that the origin of this fast depolarization is a static magnetic ®eld at the muon site, indicating the appearance of a static magnetically ordered state of Cu and/or Nd spins. Since no coherent precession of the muon spin was observed in ZF, the mag-
netically ordered state is expected to be not a coherent long-range ordered state but a disordered one like a spin-glass state. It has been pointed out from the neutron scattering experiment that a magnetically ordered state of the Nd spins which is induced by the ordered Cu spins appears below about 10 K [6]. Our present result seems to be consistent with this scheme. Although it is hard for the lSR to clarify whether the Cu spins also order with Nd spins or not below about 10 K, the volume fraction of the magnetically ordered state can be obtained from the comparison of the initial asymmetries of each depolarizing component. It was estimated to be about 17(10)% of the total volume of the sample at 4.4 K. Other Cu and Nd spins are still dynamically ¯uctuating. Taking into account that all of time spectra are well described by the combination of three exponential functions, a phase-separated state of a magnetically ordered state and nonmagnetically ordered state is expected to appear in the single crystal with x 0:15 below about 10 K. Since the bulk superconductivity was con®rmed in this single crystal [7], a coexisting state of the superconducting state and the magnetically ordered state of the Cu spins may be expected in this electron-doped Nd2 x Cex CuO4 with x 0:15. In order to clarify this matter, more careful investigations of oxygen treatment conditions, measurements of the volume fraction of the superconducting state and other experiments are needed.
I. Watanabe et al. / Physica C 357±360 (2001) 212±215
References [1] [2] [3] [4]
Y. Tokura, et al., Nature (London) 377 (1989) 345. H. Takagi, et al., Phys. Rev. Lett. 62 (1989) 1197. G.M. Luke, et al., Physica C 162±164 (1989) 825. G.M. Luke, et al., Phys. Rev. B 42 (1990) 7981.
215
[5] M. Matsuda, et al., Phys. Rev. B 45 (1992) 12548. [6] K. Kurahashi, et al., Doctor Thesis, 2000. [7] T. Uefuji, et al., Physica C 357±360 (2001), these Proceedings. [8] C.P. Slichter, Principles of Magnetic Resonance, Springer, Berlin, 1978.
www.elsevier.com/locate/physc
Muon spin relaxation study on magnetic properties of Nd2 x Cex CuO4 around a boundary between the magnetically ordered and superconducting states I. Watanabe a,*, T. Uefuji b, K. Kurahashi b, M. Fujita b, K. Yamada b, K. Nagamine a,c a
Muon Science Laboratory, RIKEN (The Institute of Physical and Chemical Research), 2-1 Hirosawa, Wako, Saitama 351-0198, Japan b Institute of Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan c Meson Science Laboratory (KEK-MSL), KEK, 1-1 Oho, Tsukuba 305-0801, Japan Received 16 October 2000; accepted 22 January 2001
Abstract Zero-®eld and longitudinal-®eld muon spin relaxation (lSR) measurements have been carried out in order to investigate magnetic properties of the electron-doped high-Tc oxides of Nd2 x Cex CuO4 around a boundary between the magnetically ordered state and the superconducting one. A fast muon-spin depolarizing component by a static internal ®eld was observed in the oxygen-reduced superconducting single crystal with x 0:15 below about 10 K, indicating that a part of Cu and/or Nd spins shows a magnetically ordered state. The present lSR study suggests a possibility of a coexisting or a phase-separated state around x 0:15. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 76.75.+i; 74.72.Dn; 74.25.Ha Keywords: Muon spin relaxation; High-Tc ; Electron doped system; Nd2 - x Cex CuO4
The magnetically ordered state of the electrondoped high-Tc oxide of Nd2 x Cex CuO4 is known to be adjacent to the superconducting state [1±5]. The superconducting state appears suddenly around x 0:15 by the oxygen-reduced treatment. Although Nd2 x Cex CuO4 has been discovered soon after the discovery of the hole-doped high-Tc oxides, not so many studies have been carried out in order to clarify the relationship between the magnetically ordered state and the superconducting
* Corresponding author. Tel.: +81-48-462-1111, ext.: 3337; fax: +81-48-462-4648. E-mail address: [email protected] (I. Watanabe).
one around the phase boundary because of the diculty of the sample preparation and oxygen treatment. Recently, Kurahashi et al. [6] have developed sample-preparation conditions and have succeeded to grow high-quality single crystals of Nd2 x Cex CuO4 with a wide range of x. We carried out muon spin relaxation (lSR) measurements using these single crystals to investigate the abrupt change of the electronic state around x 0:15. Two single crystals with x 0:15 and 0.17 were prepared for the present lSR study. Conditions of the growth of single crystals and transport phenomena are reported in a separate paper [7]. After the oxygen-reduced treatment, the bulk superconductivity was con®rmed by the susceptibil-
0921-4534/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 1 ) 0 0 2 0 9 - X
I. Watanabe et al. / Physica C 357±360 (2001) 212±215
213
ity measurement. The superconducting transition temperatures were determined to be about 25 and 15 K for x 0:15 and 0.17, respectively. Zero-®eld (ZF) and longitudinal-®eld (LF) lSR measurements were carried out at the RIKENRAL Muon Facility at the Rutherford±Appleton Laboratory in the UK using the pulsed positive surface-muon beam with a momentum of 27 MeV/c. The asymmetry parameter, A(t), was de®ned as A t F t B t=F t B t, where F(t) and B(t) were total muon events counted by the forward and backward counters at a time t, respectively. Fig. 1 shows the ZF-lSR time spectra of oxygen-reduced superconducting single crystals of Nd2 x Cex CuO4 with (a) x 0:15 and (b) 0.17 obtained at various temperatures. In the case of x 0:17, all of measured time spectra were well described by the single-exponential function of A0 e k0 t , where A0 is the initial asymmetry at t 0 and k0 is the depolarization rate of the muon spin. Solid lines in Fig. 1(b) are the best ®t results. The
time spectra at each temperature did not change at all in LF of about 4 kG which is applied in the same direction of the initial muon-spin polarization. This means that the muon spin depolarizes by a dynamically ¯uctuating internal ®eld. The temperature dependence of the dynamical depolarization rate of the single crystal with x 0:17 is displayed in Fig. 2 by closed triangles, showing the linear relationship with the temperature. This result means that the ¯uctuation frequency of the internal ®eld at the muon site is proportional to the temperature which can be described by the Red®eld's theory [8]. Therefore, the origin of this dynamical depolarization is concluded to be thermally ¯uctuating Nd spins. No sign of the appearance of a statically ordered state of Cu and/or Nd spins was observed down to 4.4 K in the single crystal with x 0:17. In the case of x 0:15, in contrast with the case of x 0:17, the time spectrum started to deviate from the single exponential-type depolarization behavior below about 70 K, showing that the time spectrum consists of two depolarization components. In addition, one more component which depolarizes much faster than other components appears below about 10 K with small amplitude of the asymmetry as indicated by an arrow in the inset of Fig. 1(a). This fact means that the time spectra below about 10 K consists of at least three depolarization components. Therefore, the threeexponential function, A2 e k2 t A1 e k1 t A0 e k0 t ,
Fig. 1. Zero-®eld lSR time spectra of oxygen-reduced Nd2 x Cex CuO4 with (a) x 0:15 and (b) 0.17 at various temperatures. Insets in the ®gure show the depolarization behavior in an early time region.
Fig. 2. Temperature dependences of the dynamical muonspin depolarization rates of oxygen-reduced superconducting Nd2 x Cex CuO4 with x 0:15 and 0.17. Broken lines in the ®gure were the best ®t results showing the relationship of k / T .
214
I. Watanabe et al. / Physica C 357±360 (2001) 212±215
Fig. 3. Temperature dependence of the muon-spin depolarization rates of the oxygen-reduced superconducting Nd2 x Cex CuO4 with x 0:15.
Fig. 4. Longitudinal-®eld dependence of the lSR time spectrum of the oxygen-reduced Nd2 x Cex CuO4 with x 0:15 at 5.1 K.
was applied for convenient analyses. The best ®t results are shown in Fig. 1(a) as the solid lines. Fig. 3 shows the temperature dependences of the initial asymmetry of A2 , A1 and A0 , respectively. The depolarization behavior of the main component described by A0 e k0 t and the second component described by A1 e k1 t did not change under the LF of about 4 kG at 5 K. This result shows that these depolarizing components are led by dynamically ¯uctuating internal ®elds at the muon sites. The temperature dependence of k0 is proportional to the temperature above about 10 K as shown in Fig. 2 by open circles, meaning that the muon-spin depolarization of the main component is derived from the thermally ¯uctuating Nd spins as well as observed in the case of the single crystal with x 0:17. The second component described by A1 e k1 t has a bit faster depolarization rate than the main component, so that the second component is expected to re¯ect an additional eect from the Cu-spin ¯uctuations which slow down with decreasing temperature. On the other hand, the fast depolarizing component described by A2 e k2 t , which was indicated by an arrow in Fig. 4, was recovered by the application of LF of 500 G. This fact means that the origin of this fast depolarization is a static magnetic ®eld at the muon site, indicating the appearance of a static magnetically ordered state of Cu and/or Nd spins. Since no coherent precession of the muon spin was observed in ZF, the mag-
netically ordered state is expected to be not a coherent long-range ordered state but a disordered one like a spin-glass state. It has been pointed out from the neutron scattering experiment that a magnetically ordered state of the Nd spins which is induced by the ordered Cu spins appears below about 10 K [6]. Our present result seems to be consistent with this scheme. Although it is hard for the lSR to clarify whether the Cu spins also order with Nd spins or not below about 10 K, the volume fraction of the magnetically ordered state can be obtained from the comparison of the initial asymmetries of each depolarizing component. It was estimated to be about 17(10)% of the total volume of the sample at 4.4 K. Other Cu and Nd spins are still dynamically ¯uctuating. Taking into account that all of time spectra are well described by the combination of three exponential functions, a phase-separated state of a magnetically ordered state and nonmagnetically ordered state is expected to appear in the single crystal with x 0:15 below about 10 K. Since the bulk superconductivity was con®rmed in this single crystal [7], a coexisting state of the superconducting state and the magnetically ordered state of the Cu spins may be expected in this electron-doped Nd2 x Cex CuO4 with x 0:15. In order to clarify this matter, more careful investigations of oxygen treatment conditions, measurements of the volume fraction of the superconducting state and other experiments are needed.
I. Watanabe et al. / Physica C 357±360 (2001) 212±215
References [1] [2] [3] [4]
Y. Tokura, et al., Nature (London) 377 (1989) 345. H. Takagi, et al., Phys. Rev. Lett. 62 (1989) 1197. G.M. Luke, et al., Physica C 162±164 (1989) 825. G.M. Luke, et al., Phys. Rev. B 42 (1990) 7981.
215
[5] M. Matsuda, et al., Phys. Rev. B 45 (1992) 12548. [6] K. Kurahashi, et al., Doctor Thesis, 2000. [7] T. Uefuji, et al., Physica C 357±360 (2001), these Proceedings. [8] C.P. Slichter, Principles of Magnetic Resonance, Springer, Berlin, 1978.