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Electrical transport and magnetic properties of (La0.7Ca0.3MnO3)1−x(Y1Ba2Cu3O7−δ)x composite ceramics
Journal of Magnetism and Magnetic Materials 324 (2012) 1234–1238
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Electrical transport and magnetic properties of ðLa0:7 Ca0:3 MnO3 Þ1x ðY1 Ba2 Cu3 O7d Þx composite ceramics J.S. Park a, Y.P. Lee b,n, J.-H. Kang c, J. Kim d, B.W. Lee d, J.Y. Rhee a a
Institute of Basic Sciences and Department of Physics, Sungkyunkwan University, Suwon 446-740, Republic of Korea Quantum Photonic Science Research Center and Department of Physics, Hanyang University, Seoul 133-791, Republic of Korea c Department of Nano and Electronic Physics, Kookmin University, Seoul 136-702, Republic of Korea d Department of Electronic Physics, Hankuk University of Foreign Studies, Yongin, Kyungki 449-791, Republic of Korea b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 July 2011 Received in revised form 21 October 2011 Available online 22 November 2011
A cuprate/manganite composite ceramic, viz. ðLa0:7 Ca0:3 MnO3 Þ1x ðY1 Ba2 Cu3 O7d Þx with x ¼ 0, 0.10, 0.20, 0.30, and 0.50 has been synthesized, and the dc magnetization, the resistivity, and the magnetoresistance (MR) have been studied. The composite ceramic is identified as a two-phase composite consisting of ferromagnetic manganite and superconductor by x-ray diffraction and scanning electron microscopy. The temperature dependence of resistivity between 10 and 300 K shows that the transport behavior of the composite ceramic is governed by grain boundaries. With increasing the YBCO doping content, the positive MR of YBCO phase dominates the negative MR of LCMO one, which gives rise to the enhancement of magnetic inhomogeneity and the suppression of double exchange interaction. The sign of MR for the composite ceramic is observed to be dependent on magnetic field and the YBCO doping content. The tuning between positive and negative MR by means of magnetic field can be developed to be the field-sensitive tunable MR. The tunable MR is due to the coexistence of positive and negative MR, which is affected by the proximity effect between LCMO and YBCO phases below TC. & 2011 Elsevier B.V. All rights reserved.
Keywords: Manganite Superconductor Magnetoresistance Grain boundary Proximity effect
1. Introduction Recently, the colossal magnetoresistance (CMR) in hole-doped earth manganites R1 xAxMnO3 (R ¼trivalent rare-earth metal and A¼ divalent alkaline earth) has spurred significant scientific interests because of their versatile intriguing features and potential technological applications, such as magnetic read heads, field sensors and memories [1–3]. The CMR behavior can be understood in the framework of double exchange (DE) interaction, which considers the transfer of one eg electron between neighboring Mn3 þ and Mn4 þ ions through the Mn3 þ –O2 –Mn4 þ path [4]. In most cases, the CMR phenomenon is often triggered at high magnetic fields of several T and comparatively low temperatures, which hinders its practical applications [5]. Therefore, it is desirable to have the CMR materials with a high field sensitivity eventually at room temperature. Generally, the granular perovskite manganites are required owing to the fact that the extrinsic intergranular effect, resulting from the structural and the magnetic disorders in the grain boundaries (GBs), could be a major importance in the applications. The GBs lead to wide-temperature-range and low-field magnetoresistance (MR),
n
Corresponding author. Tel.: þ82 2 2281 5572; fax: þ 82 2 2281 5573. E-mail address: [email protected] (Y.P. Lee).
0304-8853/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2011.11.020
originating from the spin-polarized intergrain tunneling [6]. In essence, the intergrain tunneling process takes place between two adjacent ferromagnetic (FM) grains with misaligned spins, separated by magnetic barriers that are reduced by the magnetic-field-favoring conductivity [7]. Consequently, the field- and the temperaturedependent MR of the perovskite manganese oxides can be governed by modification of the GBs acting as the barriers for electron tunneling. In this context, various attempts have been performed to improve the low-field and the room-temperature MR through controlling the GB effects by making composites of the magnetoresistive manganites with other secondary phases for tunneling barriers [8–10]. For example, the combination of sintered granular manganite with a conducting metal gives rise to enhancements of the metal–insulator transition temperature and the room-temperature MR, due to the intrinsic intragranular effect related to the DE interaction [11,12]. By the introduction of an insulator into the FM manganite matrix, the enhanced low-field MR, ascribed to the spinpolarized intergrain tunneling, was observed at low temperatures [13–15]. Additionally, Li et al. revealed that the mixture of high-TC superconductive cuprate La1.85Sr0.15CuO4 and FM manganite La0.7Sr0.3MnO3 showed a tunable MR by varying magnetic field below TC [16]. Originally, ferromagnetism and superconductivity were mutually exclusive owing to the pair-breaking effect of magnetic moments [17]. However, it is possible that they can be compatible under certain circumstances giving rise to novel kinds of
J.S. Park et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 1234–1238
1235
combined ground states [18]. Therefore, the interest in the FM manganite/superconductor-type composite has been spurred for an application in magnetic-field-sensitive devices [19]. However, fewer studies have been carried out on the physical properties of the FM manganite/superconductor-type composites in the light of manganite. In this work, we have investigated the transport and the magnetic properties of ðLa0:7 Ca0:3 MnO3 Þ1x ðYBa2 Cu3 O7d Þx [(LCMO)1 x (YBCO)x] composite ceramic in order to clarify the physical behavior governed by the GB effects in FM manganite/superconductor-type composites. Our results reveal the characteristics of MR and the proximity effect on tunable MR of (LCMO)1 x (YBCO)x.
2. Experiment The granular (LCMO)1 x(YBCO)x composite ceramics were synthesized by the conventional solid-state reaction method by two steps. First, LCMO powders were pre-prepared by annealing at 1000 1C. The final heat treatment was carried out at 1300 1C. The detailed synthesis process was described elsewhere [20]. Second, YBCO powders were made from high-purity Y2O3, BaCO3 and CuO powders as the starting materials, and then the mixture was calcined at 900 1C for 8 h in air. Subsequently, the presynthesized LCMO and YBCO powders were mixed according to the required compositions (LCMO)1 x(YBCO)x [x is the mole fraction]. Then the mixed powders were pressed into pellets, and sintered at 950 1C in a stagnant air for 12 h. Finally, all the samples were slowly cooled to room temperature in the furnace for the sufficient oxidization of YBCO phase. The phase of samples was checked by powder x-ray diffraction (XRD) with a Rigaku Miniflex diffractometer using Cu K a radiation at room temperature. The surface morphology of sample was probed by a field-emission scanning electron microscopy (FE-SEM: JEOL JSM-6330F). The resistance measurement was carried out by the conventional four-probe method in a temperature range of 10–300 K and at a magnetic field up to 5 T. During the measurement of resistivity at 10 K the magnetic field was applied perpendicularly to the transport current direction. The magnetization was measured by using a Quantum Design superconducting quantum interference device (SQUID) magnetometer. Analysis on the MR of samples was performed, based on the resistance measurements. Here, the MR is defined as [rðHÞrð0Þ=rð0Þ, where rð0Þ and rðHÞ are the resistivity at zero field and an applied magnetic field of 5 kOe, respectively.
3. Results and discussion Fig. 1 presents the room-temperature powder XRD patterns of (LCMO)1 x(YBCO)x samples with x ¼0, 0.05, 0.10, 0.20 and 0.30. As can be seen in Fig. 1, the pure parent LCMO is of single phase, indexed by the cubic lattice symmetry. For the YBCO-doped LCMO samples, the diffraction peaks centered at 2y1 ¼ 27:41 ð102Þ, 30:861 ð004Þ, and 41:741 (113) belonging to YBCO phase are clearly observed, and the peak intensity increases with the doping content. This indicates the coexistence of LCMO and YBCO phases without chemical reaction. Another noteworthy aspect in Fig. 1 is that the position of (110) peak is shifted from the pure LCMO to the YBCO peak. The local profiles of (110) peak are shown in the inset of Fig. 1. It can be inferred that the YBCO peak is more intense along with the less intense LCMO one. The XRD patterns indicate that the doped samples are the two-phase composites consisting of FM manganite LCMO and superconductor YBCO phases. The formation of coexisting two phases in the doped samples is also identified in the SEM images of Fig. 2. Small YBCO grains are
Fig. 1. XRD patterns of (LCMO)1 x(YBCO)x samples. The value of x is 0, 0.05, 0.10, 0.20 and 0.30 from bottom to top. The asterisks indicate the diffraction peaks from YBa2Cu3O7. The inset gives the local profiles of (110) reflection.
embedded between large LCMO particles, as shown in Fig. 2(a). With further doping to be x¼0.20, the small YBCO grains surrounding LCMO particles become bigger [Fig. 2(b)]. From the SEM images displayed in Fig. 2, two phases in the doped samples are clearly distinguished owing to no interdiffusion of ions between LCMO and YBCO grains. Fig. 3 is the temperature dependence of zero-field resistivity, rðTÞ, for the (LCMO)1 x(YBCO)x samples. As seen in Fig. 3(a), the resistances for 0 rx r 0.01 increase and simultaneously the metal–insulator transition disappears with increasing the YBCO content. This behavior is responsible for the spin-dependent scattering of polarized electrons at the GBs between LCMO and YBCO grains [21]. Besides, inset A of Fig. 3(a) displays the field dependence of resistivity for x¼0.01. This exhibits a decrease in the resistivity under external magnetic field, indicating the negative MR due to FM metallic LCMO phase. According to the SEM images and some other investigations, the transport behavior for a low doping level can be understood as the intergraintunneling conductance through small YBCO grain encompassing the FM metallic LCMO particles and segregated at the GBs [22]. Therefore, the GBs between two phases, serving as tunneling barriers, block the conduction channels in the whole temperature range. For 0.10 rx r 0.50, the electrical conductivity is improved without the percolative paths of superconducting YBCO as the doping content increases [Fig. 3(b)]. This behavior is due to the fact that YBCO grains connected with FM metallic LCMO particles favor a linked path for the electron transport, leading to opening of the conduction channels by contact between LCMO and YBCO phases. The field dependence of resistivity for x ¼0.50 is shown in the inset B of Fig. 3(b). It can be seen that the change in the sign of
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Fig. 2. FE-SEM images of (LCMO)1 x(YBCO)x. (a) x ¼0.03 and (b) x¼ 0.20.
MR is observed at the superconducting transition temperature T C . It is noted that the MR becomes a positive below T C owing to the fact that the resistive vortex is formed by the magnetic field breaking the superconducting state. Since the coupling between YBCO grains surrounding the LCMO particles forms a cluster structure giving rise to the superconducting state, the two-phase composite with x¼0.50 is a system of YBCO clusters. Accordingly, it is suggested that for a high doping level, the YBCO grains tend to make the superconducting clusters with the LCMO particles keeping the intergranular connectivity. Another pertinently interesting fact in Fig. 3(b) is that the rðTÞ curves display semiconducting respond at low temperatures. The experimental resistivity curves reveal exponential rðTÞ results at low temperatures, which is clearly seen as a linear behavior in the ln rðTÞ vs. T 1=2 plot [see the inset C of Fig. 3(b)]. This feature is reminiscent of the GB-tunneling conductance mechanism for granular metals [23]. It can be deduced, therefore, that the high resistivity at low temperatures is attributed to the fact that the transport properties of two-phase composites might be governed by the GBs. Consequently, we suggest that the extrinsic intergranular effect, associated with the intergranular transport across the GBs, is enhanced according to the YBCO content. Hereafter, we mainly explore the two-phase composites with high doping contents (0.10 r x r 0.50) in order to investigate the physical properties caused by coexistence of the superconducting and the FM states. Fig. 4 exhibits the temperature dependence of MR ratio for the (LCMO)1 x(YBCO)x composite samples. It is clearly seen that the
Fig. 3. Temperature dependence of zero-field resistivity (r) for the (LCMO)1 x (YBCO)x samples. (a) x ¼0, 0.005, 0.01 and 0.10. (b) x¼ 0.20, 0.30 and 0.50. Insets A and B exhibit temperature dependence of the resistivity for x¼ 0.01 and 0.50 without (open symbols) and with (solid symbols) an applied magnetic field of 5 T, respectively. Inset C shows ln rðTÞ plotted against T 1=2 . The lines are guides to the eyes.
negative MR curves of all the samples exhibit a peak. This is due to the CMR effect related to the DE interaction in LCMO phase. In addition, the MR peak temperature T p is constant nearly except that of the sample with x¼0.50. Another characteristic feature in Fig. 4 shows that T C and T p for x¼0.50 move upwards and downwards, respectively. The higher T C is due to the enhancement of superconductivity caused by more YBCO clusters and the lower T p by the weakening of the DE interaction resulting from the reduction of FM fraction in the system with x¼0.50. On the other hand, the negative MR values of x r 0:30 at low temperatures are decreased orderly with increasing the YBCO doping level. This propensity indicates that the positive MR in YBCO grains is superimposed on the negative MR in LCMO particles. However, the MR for x¼0.50 becomes positive below T P , which reflects that the positive MR comes to be superior to the negative one. On the other hand, T C can be determined as a temperature corresponding to the occurrence of the positive MR below T P , which is indicated by an arrow in Fig. 4. Furthermore, from the result of the sample with x¼ 0.30 shown in Fig. 4, it is reasonable to suggest that T C for x¼0.30 is lower than
J.S. Park et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 1234–1238
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Fig. 4. Temperature dependence of MR ratio for the (LCMO)1 x(YBCO)x samples.
Fig. 5. Field dependence of MR ratio for the (LCMO)1 x(YBCO)x samples. Inset shows that in detail for the sample with x¼ 0.30.
that for x¼0.20. By taking into account this fact, T C for x r 0:30 is shifted to a lower temperature with increasing the YBCO content. This result is attributable to the weakening of superconductivity due to the proximity effect caused by the superconductor YBCO/ferromagnet LCMO (S/F) junction at the GBs between two phases. According to the proximity effect, the exchange field in FM metallic LCMO phase breaks Cooper pairs which penetrate into the LCMO particles from the superconductor YBCO grains [24]. From the SEM images and the transport behavior, the growth of YBCO grain according to the doping content gives rise to enhancement of the intergranular connectivity, causing improvement of the S/F junction. Consequently, T C for x r 0:30 becomes lower as the proximity effect is enhanced along with the doping content. The field dependence of MR ratio for the (LCMO)1 x(YBCO)x samples at 10 K is shown in Fig. 5. For LCMO, the field-dependent MR exhibits two regions which are the steep MR at low field and the monotonous MR at high field. This behavior is understood by the magnetic-domain rotation process [25]. However, the negative MR for x¼0.20 can be reduced significantly owing to the positive MR of YBCO phase. In addition, the inset of Fig. 5 shows that the sign of MR for x¼0.30 is observed to be dependent on magnetic field. The tuning between positive and negative MR by changing magnetic field can be applied to the field-sensitive MR to be tunable. The tunable MR is due to coexistence of positive and negative MR below T C . The field-dependant MR for x¼0.50 becomes a positive over the whole field range. This indicates that the positive MR of YBCO phase predominates over the negative MR of LCMO particles. The result implying the enhancement of superconductivity in YBCO with decrease in LCMO can be explained by an improvement of YBCO cluster structure on the strength of the above date. Fig. 6 displays the temperature dependence of magnetization MðTÞ for the (LCMO)1 x(YBCO)x samples in the zero-field-cooled (ZFC) mode at an applied magnetic field of 100 Oe. The pure parent
Fig. 6. Temperature dependence of magnetization for the (LCMO)1 x(YBCO)x samples under the ZFC condition. Inset displays the magnetic-field dependence of magnetization at 10 K for the selected two-phase composites. The lines are guides to the eyes.
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LCMO displays the steep magnetic transition, indicating an intensive DE interaction in a homogeneous system. However, with increasing the doping content, the magnetic transition tends to be broadened owing to the enhancement of magnetic inhomogeneity resulting from the existence of YBCO phase in the two-phase composites. Hence, the introduction of YBCO serving as secondary phase into the LCMO matrix is expected to induce the spin disorder ascribed to the magnetic surface effect at the GBs. The inset of Fig. 6 presents the evolution of the magnetization as a function of an applied magnetic field at 10 K for the selected two-phase composites. The magnetization curves show that a saturation cannot be reached in a field up to 5 T, indicating the spin-glass-like state due to the magnetic disorder at the GBs. However, the influence of structural and magnetic disorder in GBs on the MR of two-phase composites can be neglected because of no observation of the low-field MR in the field-dependent MR data (Fig. 5) [26]. From the magnetization dependences of the two-phase composites in Fig. 6, it is reasonable to suggest that the magnetic exchange between LCMO particles is interrupted by the YBCO grains. Consequently, the YBCO clusters weaken the DE interaction, and thereby the T M for x¼0.50 shifts downwards. 4. Conclusions We have investigated the effects of YBCO on the transport and the magnetic properties of (LCMO)1 x(YBCO)x composite ceramic consisting of FM manganite and superconductor. The transport behavior of the two-phase composite is governed by the GBs between LCMO and YBCO grains. The positive MR of YBCO phase dominates the negative MR of LCMO one with increasing the doping content, which results in enhancement of the magnetic inhomogeneity and suppression of the DE interaction. The MR measurement reveals that the sign of MR is dependent on magnetic field and the YBCO doping level. The tuning between positive and negative MR can be applied for the field-sensitive MR to be tunable. Moreover, it is found that the proximity effect in the FM manganite/superconductor-type composite controls the operating temperature of field-sensitive tunable MR below T C . Acknowledgements This work was supported by Priority Research Centers Program through the NRF funded by the MEST (2010-0029700) and by
Basic Science Research Program through the National Research Foundation of Korea (NRF) granted by the MEST (q-Psi).
References [1] Y. Tokura (Ed.), Colossal Magnetoresistance Oxides, Gordon and Breach, London, 1999; E. Dagotto, T. Hotta, A. Moreo, Physics Reports 344 (2001) 1. [2] S. Jin, T.H. Tiefel, M. McCormack, R.A. Fastnacht, R. Ramesh, L.H. Chen, Science 264 (1994) 413. [3] V.G. Prokhorov, S.Y. Park, Y.H. Hyun, J.S. Park, G.H. Kim, Y.P. Lee, J.-H. Kang, V.L. Svetchnikov, Journal of the Korean Physical Society 53 (2008) 1534. [4] C. Zener, Physical Review 82 (1951) 403. [5] J.S. Park, C.O. Kim, Y.P. Lee, V.G. Prokhorov, G.G. Kaminsky, V.A. Komashko, H.C. Ri, Journal of the Korean Physical Society 42 (2003) L309. [6] R. Mahesh, R. Mahendiran, A.K. Raychaudhuri, C.N. Rao, Applied Physics Letters 68 (1996) 2291. [7] J.S. Helman, B. Abeles, Physical Review Letters 37 (1976) 1429. [8] C.-H. Yan, Z.-G. Xu, T. Zhu, Z.-M. Wang, F.-X. Cheng, Y.-H. Huang, C.-S. Liao, Journal of Applied Physics 87 (2000) 5588. [9] X.-B. Yuan, Y.-H. Liu, C.-J. Wang, L.-M. Mei, Applied Physics Letters 88 (2006) 042508. [10] S. Taran, S. Karmakar, S. Chatterjee, B.K. Chaudhuri, C.P. Sun, C.L. Huang, H.D. Yang, Journal of Applied Physics 99 (2006) 073703. [11] X.-B. Yuan, Y.-H. Liu, B.-X. Huang, C.-J. Wang, L.-M. Mei, Solid State Communications 135 (2005) 170. [12] Y.-H. Huang, C.-H. Yan, F. Luo, W. Song, Z.-M. Wang, C.-S. Liao, Journal of Applied Physics 81 (2002) 76. [13] C.-H. Yan, Z.-G. Xu, T. Zhu, Z.-M. Wang, F.-X. Cheng, Y.-H. Huang, C.-S. Liao, Journal of Applied Physics 87 (2000) 5588. [14] S. Gupta, R. Ranjit, C. Mitra, P. Raychaudhuri, R. Pinto, Applied Physics Letters 78 (2001) 362. [15] O.A. Shlyakhtin, K.H. Shin, Y.-J. Oh, Journal of Applied Physics 91 (2002) 7403. [16] X.-H. Li, Y.-H. Huang, Z.-M. Wang, C.-H. Yan, Applied Physics Letters 81 (2002) 307. [17] A.A. Abrikosov, L.P. Gor’kov, Soviet Physics—JETP 12 (1961) 1243. [18] L.N. Bulaevskii, A.I. Buzdin, M.L. Kulic, S.V. Panjukov, Advances in Physics 34 (1985) 175. [19] A. Grigorenko, S. Bending, T. Tamegai, S. Ooi, M. Henini, Nature (London) 414 (2001) 728. [20] J.S. Park, Y.P. Lee, Y.S. Lee, J.-H. Kang, Journal of the Korean Physical Society 51 (2007) 1564. [21] X.W. Li, A. Gupta, G. Xiao, G.Q. Gong, Applied Physics Letters 71 (1997) 1124. [22] C.L. Yuan, S.G. Wang, W.H. Song, T. Yu, J.M. Dai, S.L. Ye, Y.P. Sun, Applied Physics Letters 75 (1999) 3853. [23] P. Sheng, B. Abeles, Y. Arie, Physical Review Letters 31 (1973) 44. ˜ a, J.E. Villegas, M. Varela, P. Prieto, C. Leo´n, [24] Z. Sefrioui, D. Arias, V. Pen J.L. Martinez, J. Santamarial, Physical Review B 67 (2003) 214511. [25] C. Kwon, Y. Gim, Y. Fan, M.F. Hundley, J.M. Roper, P.N. Arendt, Q.X. Jia, Applied Physics Letters 73 (1998) 695. [26] Y.-H. Huang, C.-H. Yan, Z.-M. Wang, C.-S. Liao, G.-X. Xu, Journal of Alloys and Compounds 349 (2003) 224.
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Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Electrical transport and magnetic properties of ðLa0:7 Ca0:3 MnO3 Þ1x ðY1 Ba2 Cu3 O7d Þx composite ceramics J.S. Park a, Y.P. Lee b,n, J.-H. Kang c, J. Kim d, B.W. Lee d, J.Y. Rhee a a
Institute of Basic Sciences and Department of Physics, Sungkyunkwan University, Suwon 446-740, Republic of Korea Quantum Photonic Science Research Center and Department of Physics, Hanyang University, Seoul 133-791, Republic of Korea c Department of Nano and Electronic Physics, Kookmin University, Seoul 136-702, Republic of Korea d Department of Electronic Physics, Hankuk University of Foreign Studies, Yongin, Kyungki 449-791, Republic of Korea b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 July 2011 Received in revised form 21 October 2011 Available online 22 November 2011
A cuprate/manganite composite ceramic, viz. ðLa0:7 Ca0:3 MnO3 Þ1x ðY1 Ba2 Cu3 O7d Þx with x ¼ 0, 0.10, 0.20, 0.30, and 0.50 has been synthesized, and the dc magnetization, the resistivity, and the magnetoresistance (MR) have been studied. The composite ceramic is identified as a two-phase composite consisting of ferromagnetic manganite and superconductor by x-ray diffraction and scanning electron microscopy. The temperature dependence of resistivity between 10 and 300 K shows that the transport behavior of the composite ceramic is governed by grain boundaries. With increasing the YBCO doping content, the positive MR of YBCO phase dominates the negative MR of LCMO one, which gives rise to the enhancement of magnetic inhomogeneity and the suppression of double exchange interaction. The sign of MR for the composite ceramic is observed to be dependent on magnetic field and the YBCO doping content. The tuning between positive and negative MR by means of magnetic field can be developed to be the field-sensitive tunable MR. The tunable MR is due to the coexistence of positive and negative MR, which is affected by the proximity effect between LCMO and YBCO phases below TC. & 2011 Elsevier B.V. All rights reserved.
Keywords: Manganite Superconductor Magnetoresistance Grain boundary Proximity effect
1. Introduction Recently, the colossal magnetoresistance (CMR) in hole-doped earth manganites R1 xAxMnO3 (R ¼trivalent rare-earth metal and A¼ divalent alkaline earth) has spurred significant scientific interests because of their versatile intriguing features and potential technological applications, such as magnetic read heads, field sensors and memories [1–3]. The CMR behavior can be understood in the framework of double exchange (DE) interaction, which considers the transfer of one eg electron between neighboring Mn3 þ and Mn4 þ ions through the Mn3 þ –O2 –Mn4 þ path [4]. In most cases, the CMR phenomenon is often triggered at high magnetic fields of several T and comparatively low temperatures, which hinders its practical applications [5]. Therefore, it is desirable to have the CMR materials with a high field sensitivity eventually at room temperature. Generally, the granular perovskite manganites are required owing to the fact that the extrinsic intergranular effect, resulting from the structural and the magnetic disorders in the grain boundaries (GBs), could be a major importance in the applications. The GBs lead to wide-temperature-range and low-field magnetoresistance (MR),
n
Corresponding author. Tel.: þ82 2 2281 5572; fax: þ 82 2 2281 5573. E-mail address: [email protected] (Y.P. Lee).
0304-8853/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2011.11.020
originating from the spin-polarized intergrain tunneling [6]. In essence, the intergrain tunneling process takes place between two adjacent ferromagnetic (FM) grains with misaligned spins, separated by magnetic barriers that are reduced by the magnetic-field-favoring conductivity [7]. Consequently, the field- and the temperaturedependent MR of the perovskite manganese oxides can be governed by modification of the GBs acting as the barriers for electron tunneling. In this context, various attempts have been performed to improve the low-field and the room-temperature MR through controlling the GB effects by making composites of the magnetoresistive manganites with other secondary phases for tunneling barriers [8–10]. For example, the combination of sintered granular manganite with a conducting metal gives rise to enhancements of the metal–insulator transition temperature and the room-temperature MR, due to the intrinsic intragranular effect related to the DE interaction [11,12]. By the introduction of an insulator into the FM manganite matrix, the enhanced low-field MR, ascribed to the spinpolarized intergrain tunneling, was observed at low temperatures [13–15]. Additionally, Li et al. revealed that the mixture of high-TC superconductive cuprate La1.85Sr0.15CuO4 and FM manganite La0.7Sr0.3MnO3 showed a tunable MR by varying magnetic field below TC [16]. Originally, ferromagnetism and superconductivity were mutually exclusive owing to the pair-breaking effect of magnetic moments [17]. However, it is possible that they can be compatible under certain circumstances giving rise to novel kinds of
J.S. Park et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 1234–1238
1235
combined ground states [18]. Therefore, the interest in the FM manganite/superconductor-type composite has been spurred for an application in magnetic-field-sensitive devices [19]. However, fewer studies have been carried out on the physical properties of the FM manganite/superconductor-type composites in the light of manganite. In this work, we have investigated the transport and the magnetic properties of ðLa0:7 Ca0:3 MnO3 Þ1x ðYBa2 Cu3 O7d Þx [(LCMO)1 x (YBCO)x] composite ceramic in order to clarify the physical behavior governed by the GB effects in FM manganite/superconductor-type composites. Our results reveal the characteristics of MR and the proximity effect on tunable MR of (LCMO)1 x (YBCO)x.
2. Experiment The granular (LCMO)1 x(YBCO)x composite ceramics were synthesized by the conventional solid-state reaction method by two steps. First, LCMO powders were pre-prepared by annealing at 1000 1C. The final heat treatment was carried out at 1300 1C. The detailed synthesis process was described elsewhere [20]. Second, YBCO powders were made from high-purity Y2O3, BaCO3 and CuO powders as the starting materials, and then the mixture was calcined at 900 1C for 8 h in air. Subsequently, the presynthesized LCMO and YBCO powders were mixed according to the required compositions (LCMO)1 x(YBCO)x [x is the mole fraction]. Then the mixed powders were pressed into pellets, and sintered at 950 1C in a stagnant air for 12 h. Finally, all the samples were slowly cooled to room temperature in the furnace for the sufficient oxidization of YBCO phase. The phase of samples was checked by powder x-ray diffraction (XRD) with a Rigaku Miniflex diffractometer using Cu K a radiation at room temperature. The surface morphology of sample was probed by a field-emission scanning electron microscopy (FE-SEM: JEOL JSM-6330F). The resistance measurement was carried out by the conventional four-probe method in a temperature range of 10–300 K and at a magnetic field up to 5 T. During the measurement of resistivity at 10 K the magnetic field was applied perpendicularly to the transport current direction. The magnetization was measured by using a Quantum Design superconducting quantum interference device (SQUID) magnetometer. Analysis on the MR of samples was performed, based on the resistance measurements. Here, the MR is defined as [rðHÞrð0Þ=rð0Þ, where rð0Þ and rðHÞ are the resistivity at zero field and an applied magnetic field of 5 kOe, respectively.
3. Results and discussion Fig. 1 presents the room-temperature powder XRD patterns of (LCMO)1 x(YBCO)x samples with x ¼0, 0.05, 0.10, 0.20 and 0.30. As can be seen in Fig. 1, the pure parent LCMO is of single phase, indexed by the cubic lattice symmetry. For the YBCO-doped LCMO samples, the diffraction peaks centered at 2y1 ¼ 27:41 ð102Þ, 30:861 ð004Þ, and 41:741 (113) belonging to YBCO phase are clearly observed, and the peak intensity increases with the doping content. This indicates the coexistence of LCMO and YBCO phases without chemical reaction. Another noteworthy aspect in Fig. 1 is that the position of (110) peak is shifted from the pure LCMO to the YBCO peak. The local profiles of (110) peak are shown in the inset of Fig. 1. It can be inferred that the YBCO peak is more intense along with the less intense LCMO one. The XRD patterns indicate that the doped samples are the two-phase composites consisting of FM manganite LCMO and superconductor YBCO phases. The formation of coexisting two phases in the doped samples is also identified in the SEM images of Fig. 2. Small YBCO grains are
Fig. 1. XRD patterns of (LCMO)1 x(YBCO)x samples. The value of x is 0, 0.05, 0.10, 0.20 and 0.30 from bottom to top. The asterisks indicate the diffraction peaks from YBa2Cu3O7. The inset gives the local profiles of (110) reflection.
embedded between large LCMO particles, as shown in Fig. 2(a). With further doping to be x¼0.20, the small YBCO grains surrounding LCMO particles become bigger [Fig. 2(b)]. From the SEM images displayed in Fig. 2, two phases in the doped samples are clearly distinguished owing to no interdiffusion of ions between LCMO and YBCO grains. Fig. 3 is the temperature dependence of zero-field resistivity, rðTÞ, for the (LCMO)1 x(YBCO)x samples. As seen in Fig. 3(a), the resistances for 0 rx r 0.01 increase and simultaneously the metal–insulator transition disappears with increasing the YBCO content. This behavior is responsible for the spin-dependent scattering of polarized electrons at the GBs between LCMO and YBCO grains [21]. Besides, inset A of Fig. 3(a) displays the field dependence of resistivity for x¼0.01. This exhibits a decrease in the resistivity under external magnetic field, indicating the negative MR due to FM metallic LCMO phase. According to the SEM images and some other investigations, the transport behavior for a low doping level can be understood as the intergraintunneling conductance through small YBCO grain encompassing the FM metallic LCMO particles and segregated at the GBs [22]. Therefore, the GBs between two phases, serving as tunneling barriers, block the conduction channels in the whole temperature range. For 0.10 rx r 0.50, the electrical conductivity is improved without the percolative paths of superconducting YBCO as the doping content increases [Fig. 3(b)]. This behavior is due to the fact that YBCO grains connected with FM metallic LCMO particles favor a linked path for the electron transport, leading to opening of the conduction channels by contact between LCMO and YBCO phases. The field dependence of resistivity for x ¼0.50 is shown in the inset B of Fig. 3(b). It can be seen that the change in the sign of
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Fig. 2. FE-SEM images of (LCMO)1 x(YBCO)x. (a) x ¼0.03 and (b) x¼ 0.20.
MR is observed at the superconducting transition temperature T C . It is noted that the MR becomes a positive below T C owing to the fact that the resistive vortex is formed by the magnetic field breaking the superconducting state. Since the coupling between YBCO grains surrounding the LCMO particles forms a cluster structure giving rise to the superconducting state, the two-phase composite with x¼0.50 is a system of YBCO clusters. Accordingly, it is suggested that for a high doping level, the YBCO grains tend to make the superconducting clusters with the LCMO particles keeping the intergranular connectivity. Another pertinently interesting fact in Fig. 3(b) is that the rðTÞ curves display semiconducting respond at low temperatures. The experimental resistivity curves reveal exponential rðTÞ results at low temperatures, which is clearly seen as a linear behavior in the ln rðTÞ vs. T 1=2 plot [see the inset C of Fig. 3(b)]. This feature is reminiscent of the GB-tunneling conductance mechanism for granular metals [23]. It can be deduced, therefore, that the high resistivity at low temperatures is attributed to the fact that the transport properties of two-phase composites might be governed by the GBs. Consequently, we suggest that the extrinsic intergranular effect, associated with the intergranular transport across the GBs, is enhanced according to the YBCO content. Hereafter, we mainly explore the two-phase composites with high doping contents (0.10 r x r 0.50) in order to investigate the physical properties caused by coexistence of the superconducting and the FM states. Fig. 4 exhibits the temperature dependence of MR ratio for the (LCMO)1 x(YBCO)x composite samples. It is clearly seen that the
Fig. 3. Temperature dependence of zero-field resistivity (r) for the (LCMO)1 x (YBCO)x samples. (a) x ¼0, 0.005, 0.01 and 0.10. (b) x¼ 0.20, 0.30 and 0.50. Insets A and B exhibit temperature dependence of the resistivity for x¼ 0.01 and 0.50 without (open symbols) and with (solid symbols) an applied magnetic field of 5 T, respectively. Inset C shows ln rðTÞ plotted against T 1=2 . The lines are guides to the eyes.
negative MR curves of all the samples exhibit a peak. This is due to the CMR effect related to the DE interaction in LCMO phase. In addition, the MR peak temperature T p is constant nearly except that of the sample with x¼0.50. Another characteristic feature in Fig. 4 shows that T C and T p for x¼0.50 move upwards and downwards, respectively. The higher T C is due to the enhancement of superconductivity caused by more YBCO clusters and the lower T p by the weakening of the DE interaction resulting from the reduction of FM fraction in the system with x¼0.50. On the other hand, the negative MR values of x r 0:30 at low temperatures are decreased orderly with increasing the YBCO doping level. This propensity indicates that the positive MR in YBCO grains is superimposed on the negative MR in LCMO particles. However, the MR for x¼0.50 becomes positive below T P , which reflects that the positive MR comes to be superior to the negative one. On the other hand, T C can be determined as a temperature corresponding to the occurrence of the positive MR below T P , which is indicated by an arrow in Fig. 4. Furthermore, from the result of the sample with x¼ 0.30 shown in Fig. 4, it is reasonable to suggest that T C for x¼0.30 is lower than
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Fig. 4. Temperature dependence of MR ratio for the (LCMO)1 x(YBCO)x samples.
Fig. 5. Field dependence of MR ratio for the (LCMO)1 x(YBCO)x samples. Inset shows that in detail for the sample with x¼ 0.30.
that for x¼0.20. By taking into account this fact, T C for x r 0:30 is shifted to a lower temperature with increasing the YBCO content. This result is attributable to the weakening of superconductivity due to the proximity effect caused by the superconductor YBCO/ferromagnet LCMO (S/F) junction at the GBs between two phases. According to the proximity effect, the exchange field in FM metallic LCMO phase breaks Cooper pairs which penetrate into the LCMO particles from the superconductor YBCO grains [24]. From the SEM images and the transport behavior, the growth of YBCO grain according to the doping content gives rise to enhancement of the intergranular connectivity, causing improvement of the S/F junction. Consequently, T C for x r 0:30 becomes lower as the proximity effect is enhanced along with the doping content. The field dependence of MR ratio for the (LCMO)1 x(YBCO)x samples at 10 K is shown in Fig. 5. For LCMO, the field-dependent MR exhibits two regions which are the steep MR at low field and the monotonous MR at high field. This behavior is understood by the magnetic-domain rotation process [25]. However, the negative MR for x¼0.20 can be reduced significantly owing to the positive MR of YBCO phase. In addition, the inset of Fig. 5 shows that the sign of MR for x¼0.30 is observed to be dependent on magnetic field. The tuning between positive and negative MR by changing magnetic field can be applied to the field-sensitive MR to be tunable. The tunable MR is due to coexistence of positive and negative MR below T C . The field-dependant MR for x¼0.50 becomes a positive over the whole field range. This indicates that the positive MR of YBCO phase predominates over the negative MR of LCMO particles. The result implying the enhancement of superconductivity in YBCO with decrease in LCMO can be explained by an improvement of YBCO cluster structure on the strength of the above date. Fig. 6 displays the temperature dependence of magnetization MðTÞ for the (LCMO)1 x(YBCO)x samples in the zero-field-cooled (ZFC) mode at an applied magnetic field of 100 Oe. The pure parent
Fig. 6. Temperature dependence of magnetization for the (LCMO)1 x(YBCO)x samples under the ZFC condition. Inset displays the magnetic-field dependence of magnetization at 10 K for the selected two-phase composites. The lines are guides to the eyes.
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LCMO displays the steep magnetic transition, indicating an intensive DE interaction in a homogeneous system. However, with increasing the doping content, the magnetic transition tends to be broadened owing to the enhancement of magnetic inhomogeneity resulting from the existence of YBCO phase in the two-phase composites. Hence, the introduction of YBCO serving as secondary phase into the LCMO matrix is expected to induce the spin disorder ascribed to the magnetic surface effect at the GBs. The inset of Fig. 6 presents the evolution of the magnetization as a function of an applied magnetic field at 10 K for the selected two-phase composites. The magnetization curves show that a saturation cannot be reached in a field up to 5 T, indicating the spin-glass-like state due to the magnetic disorder at the GBs. However, the influence of structural and magnetic disorder in GBs on the MR of two-phase composites can be neglected because of no observation of the low-field MR in the field-dependent MR data (Fig. 5) [26]. From the magnetization dependences of the two-phase composites in Fig. 6, it is reasonable to suggest that the magnetic exchange between LCMO particles is interrupted by the YBCO grains. Consequently, the YBCO clusters weaken the DE interaction, and thereby the T M for x¼0.50 shifts downwards. 4. Conclusions We have investigated the effects of YBCO on the transport and the magnetic properties of (LCMO)1 x(YBCO)x composite ceramic consisting of FM manganite and superconductor. The transport behavior of the two-phase composite is governed by the GBs between LCMO and YBCO grains. The positive MR of YBCO phase dominates the negative MR of LCMO one with increasing the doping content, which results in enhancement of the magnetic inhomogeneity and suppression of the DE interaction. The MR measurement reveals that the sign of MR is dependent on magnetic field and the YBCO doping level. The tuning between positive and negative MR can be applied for the field-sensitive MR to be tunable. Moreover, it is found that the proximity effect in the FM manganite/superconductor-type composite controls the operating temperature of field-sensitive tunable MR below T C . Acknowledgements This work was supported by Priority Research Centers Program through the NRF funded by the MEST (2010-0029700) and by
Basic Science Research Program through the National Research Foundation of Korea (NRF) granted by the MEST (q-Psi).
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