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Effect of sintering temperature on electrical transport of La0.67Ca0.33MnO3 granular system with 4% CuO addition
Journal of Alloys and Compounds 448 (2008) 27–31
Effect of sintering temperature on electrical transport of La0.67Ca0.33MnO3 granular system with 4% CuO addition J.H. Miao, S.L. Yuan ∗ , G.M. Ren, X. Xiao, G.Q. Yu, Y.Q. Wang, S.Y. Yin Department of Physics, Huazhong University of Science and Technology, Wuhan 430074, People’s Republic of China Received 21 July 2006; received in revised form 10 October 2006; accepted 10 October 2006 Available online 21 November 2006
Abstract The granular systems of La0.67 Ca0.33 MnO3 (LCMO) with 4% CuO addition were fabricated by three steps. Firstly, the LCMO particles were prepared and sintered at temperatures of 600 ◦ C (Ts6), 1000 ◦ C (Ts10), 1200 ◦ C (Ts12) and 1400 ◦ C (Ts14). Experimental results show that with the increment of sintering temperature, the particle size of the pre-prepared LCMO also increases, which plays a key role on electrical transport of the final composites. It is found that sample Ts14 exhibits two transition peaks both in ρ–T and MR0 –T curves, while sample Ts12 only shows two transition peaks in MR0 –T curve. Here MR0 is the abbreviation of magnetoresistance. These two transition peaks are ascribed to the intrinsic and extrinsic effects, respectively. For sample Ts6, Cu2+ ions mainly enter into the lattice of LCMO to form with La0.67 Ca0.33 Mn1−x Cux O3 . Specially, it is interesting to observe the MR0 effect of ∼90% at 0.3 T for sample Ts10 at the vicinity of insulator-metal transition temperature TIM , where considerable thermal hysteresis also appears. Such enhancement of MR effect is attributed to the field suppression of spin disordering especially at grain boundaries caused by CuO addition. © 2006 Elsevier B.V. All rights reserved. PACS: 75.30.Vn; 71.30.+h; 75.30.Kz Keywords: Perovskite manganites; Magneroresistance; Spin disordering
1. Introduction Since the discovery of colossal magnetoresistance (CMR) effect in perovskite manganites in 1993 [1], there has been a renewed interest in the research of these compounds. But CMR effect is not appealing for technological ends due to the large magnetic field of the order of several tesla [2,3]. Recently, the low-field magnetoresistance (LFMR) in polycrystalline manganites is of special interests due to its high sensitivity to small magnetic field. Very different from the origin of CMR, the LFMR is believed to originate from the spin-polarized tunneling process [4]. Enhanced LFMR response has been observed in some manganites-based two-phase composites. The second phases include insulating oxides [5–8], hard or soft magnetic materials [9,10], metal [11] and other CMR oxides [12]. In these
∗
Corresponding author. E-mail addresses: miaojh [email protected] (J.H. Miao), [email protected] (S.L. Yuan). 0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.10.033
composites, it is believed that dilution with the second phases impedes the magnetic homogeneity near the grain boundary area, which adjusts the barrier layer and, hence, influences the tunneling process. However, in those studies, the interaction and diffusion between manganites and the second phase especially at interfaces is usually ignored. On the other hand, according to Cu2+ ion (3d9 ), there is an extra electron with 1/2 spin. When CuO (Cu2+ ) was introduced to the grain boundary regions of LCMO, some spin disordering should be induced at grain boundaries. This spin disordering may affect the ferromagnetic coupling between contiguous LCMO grains. Applied magnetic field suppresses this spin disordering and may lead to an enhanced LFMR effect. Therefore, in this paper, we discuss the electrical transport behavior of LCMO granular system with 4% CuO addition. For convenience, we use their nominal composition of LCMO/xCuO (x = 4%) to name these samples. Meanwhile, we use Ts6, Ts10, Ts12, Ts14 to express the samples with pre-prepared LCMO particles sintered at different temperatures of 600, 1000, 1200 and 1400 ◦ C, respectively. The correspondingly pure LCMO
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prepared by the same experimental method is named as LC6, LC10, LC12 and LC14, respectively. 2. Experimental procedure The granular systems of LCMO/xCuO (x = 0% and 4%) were fabricated through a three-step chemical route, where x is the nominal molar fraction of CuO. Firstly, we used a sol–gel method to prepare LCMO powders according to nominal composition of La0.67 Ca0.33 MnO3 . One advantage of this method lies in a wide range for sintering temperature (Ts ) to obtain a single-phase sample with perovskite structure. In our study, the powders were sintered at temperatures of 600, 1000, 1200 and 1400 ◦ C for 6 h. Secondly, the LCMO/xCuO mixed powders were fabricated by the pre-prepared LCMO particles and Cu(NO3 )2 aqueous solution through a heterogeneous precipitation method as reported [13]. Finally, the obtained LCMO/xCuO mixed powders were ground, palletized, and then sintered at 1000 ◦ C for 2 h to get the final composites. The crystal structure of the samples was characterized by X-ray diffraction (XRD). The morphology of grain sizes was investigated by scanning electronic microscopy (SEM). The electrical transport properties were measured by the standard four-probe method in a commercial physical property measurement system (Quantum Design PPMS).
3. Results and discussion The room temperature XRD spectrums of LCMO/xCuO samples with x = 0% and 4% are shown in Fig. 1(a) and (b), respectively. Results indicate that the same diffraction patterns are observed in all of the samples, and all the diffraction peaks can be indexed into a perovskite crystalline structure (as shown in Fig. 1(a)). No CuO or other phases were detected within the sensitivity of the measurement. Shifting of the most intense lines (0 0 2, 2 0 0) is shown in the inset. It can be noted that all of the pure LCMO samples almost have the same 2θ position of ∼32.96◦ . The lines (0 0 2, 2 0 0) for sample LC6 and LC10 shift a little towards higher angle due to their small grain size caused by low sintering temperature [14,15]. It can be noted from the inset of Fig. 1(b) that the diffraction lines (0 0 2, 2 0 0) for sample Ts14, Ts12 and Ts10 have the same 2θ angle of ∼32.95◦ , which is consistent with that of pure LCMO. However, the lines shift towards lower angle to 32.87◦ for sample Ts6, which indicates the expansion of unit cell of crystalline lattice. The scanning electron micrograph (SEM) images for samples Ts6, Ts10, Ts12 and Ts14 are presented in Fig. 2. Although all samples were finally sintered at the same temperature of 1000 ◦ C, their average grain sizes are quite different. It is of ∼1 and 2 m for sample Ts6 and Ts14, respectively. For sample Ts10 and Ts12, the grains are nearly spherical in shape with varying particles size from 300 to 900 nm. This discrepancy can be understood on the basis of sample fabrication procedure. With the increment of sintering temperature from 600 to 1400 ◦ C, the average particle size of pre-prepared LCMO increases [15,16]. The average grain size of sample Ts10, Ts12 and Ts14 is consistent with the particle size of pre-prepared LCMO correspondingly. However, the particle size of LCMO sintered at 600 ◦ C is of 25 ± 6 nm [16], while the sample Ts6 has an abnormal increase of grain size of ∼1 m. This phenomenon can be understood through the fabrication procedure of the samples. The LCMO particles are pre-prepared. Due to the deviation of
Fig. 1. XRD patterns for samples of (a) LC6, LC10, LC12, LC14, and (b) Ts6, Ts10, Ts12, Ts14. The inset shows the shifting of the most intense diffraction peak (0 0 2, 2 0 0).
stoichiometric composition and the termination of crystalline structure, the structure near grain boundary region should not be as tight as that inside the grains. Moreover, the pre-prepared LCMO particles sintered at 600 ◦ C are in nano-size, which makes it metastable and reactive. Therefore, the reaction between CuO and LCMO should take place unavoidably, especially at grain boundaries during the final sintering process. It is the most pronounced for sample Ts6, in which LCMO and CuO can react thoroughly during the final sintering process (1000 ◦ C, 2 h). Such reaction results in the formation of La0.67 Ca0.33 Mn1−x Cux O3 . In this case, CuO acts as a sintering aid, which insures the large grain size of Ts6. However, in case of Ts10, Ts12 and Ts14, the final sintering temperature (1000 ◦ C) is not high enough to destroy the crystalline structure of the pre-prepared LCMO grains, which has been formed by higher sintering temperature (≥1000 ◦ C). Therefore, the interaction and diffusion between LCMO and CuO can be limited at the vicinity of grain boundaries due to the large grain size and stable crystalline structure of the pre-prepared LCMO particles. And then the final grain sizes of sample Ts10, Ts12 and Ts14 are similar with that of the
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Fig. 2. SEM representative images for (a) Ts6, (b) Ts10, (c) Ts12 and (d) Ts14.
pre-prepared LCMO particles sintered at the correspondingly temperatures. As shown in Fig. 1, the shift of diffraction lines towards lower angle for sample Ts6 can be interpreted in the same way. Since Cu2+ ion has a larger ionic size (0.074 nm), as compared to Mn3+ and Mn4+ (0.066 and 0.060 nm), with the formation of La0.67 Ca0.33 Mn1−x Cux O3 , the substitution of Cu for Mn should cause an expansion of unit cell. The shift of diffraction lines towards lower angle is corresponding to the expansion of unit cell. Therefore, it is believed that Cu2+ ions are mainly substituted for Mn ions in sample Ts6. However, for sample Ts10, Ts12 and Ts14, the interaction and diffusion is limited at the vicinity of grain boundary. Its effect on the crystalline structure of LCMO matrix is neglectable. In this case, direct XRD control of the interaction between LCMO and CuO at interfaces is difficult due to the iso-structure of LCMO and the reaction product La0.67 Ca0.33 Mn1−x Cux O3 . Fig. 3 presents the resistivity as a function of temperature and magnetic field for the composites of (a) Ts6, (b) Ts10, (c) Ts12 and (d) Ts14, respectively. Some special experimental results can be found in this figure. (1) All the composites exhibit an obvious transition characterized by a peak from metallic state to insulator both at zero and applied magnetic field. (2) The transition temperature TIM at zero field is about 147.8, 105.0, 127.8 and 151.8 K for the sample of Ts6, Ts10, Ts12 and Ts14, respectively. The corresponding peak resistivity (ρmax ) at TIM is of about 37.6, 360.6, 37.7 and 3.73 cm, respectively. It is noted that the sample Ts10 exhibits the lowest TIM and the largest ρmax .
(3) It is interesting to observe another transition peak in the ρ–T curve for sample Ts14 occurring at ∼258 K (as marked asterisk in Fig. 3(d)). Such double peak behavior has been observed earlier in LCMO/Fe3 O4 [17] composites, which is attributed to the intrinsic and extrinsic effects, respectively. (4) With an application of magnetic field, the resistivity of all the samples decreases sharply and TIM shifts towards higher temperature. It is worth comparing the electrical transport behavior for sample Ts6 and Ts10 with that of La0.67 Ca0.33 Mn1−x Cux O3 (x = 4%) [18]. Cu2+ has a larger ions radius comparing with Mn ion. The substitution of Cu for Mn induced stress at local regions. In order to relax the stress, Cu2+ ions at the vicinity of grain boundary tend to segregate towards surfaces, which is more pronounced with small grain size. As reported, if sintered at 1100 ◦ C, Cu2+ ions largely segregate towards surfaces and form some Cu dependent materials at grain boundary regions, while Cu2+ ions are mainly substituted for Mn with high sintering temperature of 1300 ◦ C. As shown in Fig. 3(a) and (b), the sample Ts6 and Ts10 exhibit similar electrical transport behavior with that of La0.67 Ca0.33 Mn1−x Cux O3 (x = 4%) sintered at high (1300 ◦ C) and low (1100 ◦ C) temperature [18], respectively. This comparison is another indication of the different structure for sample Ts6 and Ts10. The temperature dependence of magnetoresistance (MR0 ) has been measured in an applied magnetic field of 0.3 T for all samples and the results are shown in Fig. 4. Here MR0 is defined as MR0 % = [ρ(0, T) − ρ(0.3 T, T)]/ρ(0, T) × 100% , where ρ(0, T) and ρ(0.3 T, T) are resistivity values for zero and applied field
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Fig. 3. The temperature and magnetic field dependence of resistivity for samples of (a) Ts6, (b) Ts10, (c) Ts12 and (d) Ts14, respectively. The arrows were drawn for the guide of eyes. The asterisk in (d) denotes the other insulator-metal transition for sample Ts14.
of 0.3 T, respectively. It can be found that all the samples stand out for a sharp peak at temperatures near TIM in the MR0 –T curve. The MR0 value for sample Ts10 can reach as high as ∼90%. Similar to the two transition peaks observed in the ρ–T curve for sample Ts14, there are two transition peaks in the MR0 –T curve for sample Ts14 and Ts12 (as shown inset). The presence of first peak at higher temperature regime should be indicative of the alignment of Mn spins within the LCMO grains, that is, the intrinsic property of manganites. The other one occurs at low temperature is usually attributed to the influence of grain boundaries, which is an extrinsic behavior.
Fig. 4. Temperature dependence of MR0 measured under 0.3 T for samples of Ts6, Ts10, Ts12 and Ts14. Inset: temperature dependence of MR0 at temperature range of 170–200 K for sample Ts12.
As discussed above, it is believed that the composites with 4% CuO addition have a heterostructure with Cu2+ ions mainly dispersing at grain boundaries except for the sample Ts6, in which Cu2+ ions largely enter into the lattice of LCMO. There are four possible conditions. (1) If the grain size of pre-prepared LCMO is too small, such as sample Ts6, Cu2+ ions largely enter into LCMO lattice to form La0.67 Ca0.33 Mn1−x Cux O3 . (2) If the LCMO grain size is large enough, such as sample Ts14, Cu2+ disperses mainly at grain boundaries and surfaces. This is consistent with the observed two transition peaks related to LCMO grains and the modified grain boundaries, respectively. (3) For the sample Ts10, the reaction product layer at grain boundaries has certain thickness. It has a strong effect on the electrical transport behavior, which covers up the intrinsic transition peak of LCMO intra-grains both in ρ–T and MR0 –T curves. (4) For sample Ts12, the pre-prepared LCMO has grain size between that of Ts14 and Ts10. Therefore, sample Ts12 exhibits two transition peaks in the MR0 –T curve, but one peak in ρ–T curve due to the sensitivity of the intrinsic transition peak to applied magnetic field. Now we emphasize our attention on the sample of Ts10, in which substantial enhancement of LFMR effect was observed. Cu2+ (3d9 ) has an extra electron carrying 1/2 spin. With the formation of Cu2+ dependent reaction product layer at grain boundaries, some spin disordering is created. This local spin disordering should suppress the parallel alignment of the neighboring magnetic moments, and therefore lead to a lower value in TIM and a large increase in the peak resistivity (ρmax ). When a magnetic field is applied, the field decreases the random distribution of the local spins, which results in
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with 4% CuO addition was investigated. It is of interests to observe two transition peaks in ρ–T or MR0 –T curves for sample Ts14 and Ts12. For sample Ts6, Cu2+ ions mainly enter into the lattice of LCMO to form with La0.67 Ca0.33 Mn1−x Cux O3 . The sample Ts10 exhibits LFMR effect as large as ∼90% at an applied field of 0.3 T. At the same temperature region where abnormal MR effect appears, considerable thermal hysteresis is observed, which indicates the same underlying of physical origin for them. Therefore, we suggest that such experimental procedure of introduction extra spin disorder especially at grain boundaries by addition CuO to LCMO granular system may be a significant way to enhance the LFMR effect especially at the temperatures near TIM . Acknowledgements Fig. 5. The resistivity versus temperature for sample Ts10 taken in cooling and warming runs under zero field. The arrows were drawn for the guide of eyes.
the FM correlation between the contiguous grain moments to occur at higher temperature and a large decrease of resistivity. It means that the sharp MR peak observed near ∼TIM is due to the field induced ordering of local spin disordering especially at grain boundaries, which can be realized at a low magnetic field. Similar observations were reported in the composites of La0.67 Ca0.33 Mn1−x Cux O3 (x = 4%) sintered at low temperature [18], in which some Cu-dependent materials with spin disorder were supposed to form surrounding the LCMO grains. To gain some more information for the understanding above, we perform measurements of resistivity for temperature cycling of the composite Ts10 in zero magnetic field (shown in Fig. 5). The observed considerable thermal hysteresis is another indication of local spin disordering especially at interfaces. It is noted that the ρ data obtained in the cooling mode is larger than that in the warming-up mode and the thermal hysteresis becomes the most remarkable near the temperature of TIM . In cooling process, the spin ordering of Mn ions occurs within grains due to the double exchange effect. The magnetic moment within grains may affect the local spin disordering at grain boundaries and align them towards the same direction. In the warming process, the local spins at grain boundaries remain their initial orientation due to the strong affect of initial magnetic moments within grains. This alignment of spins at grain boundaries was not destroyed until the further increase of temperature. The spin alignment at grain boundaries is benefit to FM coupling of contiguous grains, which corresponds to low resistivty state. Therefore, the ρ at warming-mode is much smaller than that of cooling process especially at the vicinity of transition temperature TIM . 4. Conclusion The effect of sintering temperature of pre-prepared LCMO particles on electrical transport behavior of the granular system
This work was supported by the National Science Foundation of China (Grant Nos. 10374032 and 10574049). References [1] R. von Helmolt, J. Wecker, B. Holzapfel, L. Schultz, K. Samwer, Phys. Rev. Lett. 71 (1993) 2331. [2] K. Charhara, T. Ohno, M. Kasai, Y. Kozono, Appl. Phys. Lett. 63 (1993) 1999. [3] S. Jin, T.H. Tiefel, M. Mccormack, R.A. fastnacht, R. Ramesh, L.H. Chen, Science 264 (1994) 413. [4] H.Y. Hwang, S.-W. Cheong, N.P. Ong, B. Batlogg, Phys. Rev. Lett. 77 (1996) 2041. [5] D. Das, A. Saha, S.E. Russek, R. Raj, D. Bahadur, J. Appl. Phys. 93 (2003) 8301. [6] D.K. Petrov, L. Krusin-Elbaum, J.Z. Sun, C. Field, P.R. Duncombe, Appl. Phys. Lett. 75 (1999) 995. [7] D. Das, C.M. Srivastava, D. Bahadur, A.K. Nigam, S.K. Malik, J. Phys.: Condens. Matter 16 (2004) 4089. [8] P. Kameli, H. Salamati, M. Eshraghi, M.R. Mohammadizadeh, J. Appl. Phys. 98 (2005) 043908. [9] Q. Huang, J. Li, X.J. Huang, C.K. Ong, X.S. Gao, J. Appl. Phys. 90 (2001) 2924. [10] C.H. Yan, Z.G. Xu, T. Zhu, Z.M. Wang, F.X. Cheng, Y.H. Huang, C.S. Liao, J. Appl. Phys. 87 (2000) 5588. [11] T. Tang, S.Y. Zhang, R.S. Huang, Y.W. Du, J. Alloy Compd. 353 (2003) 91. [12] J.M. Liu, G.L. Yuan, H. Sang, Z.C. Wu, X.Y. Chen, Z.G. Liu, Y.W. Du, Q. Huang, C.K. Ong, Appl. Phys. Lett. 78 (2001) 1110. [13] J.H. Miao, L. Yuan, Y.Q. Wang, J.L. Shang, G.Q. Yu, G.M. Ren, X. Xiao, S.L. Yuan, Mater. Lett. 60 (2006) 2214. [14] J. Yang, B.C. Zhao, R.L. Zhang, Y.Q. Ma, Z.G. Sheng, W.H. Song, Y.P. Sun, Solid State Commun. 132 (2004) 83. [15] R.D. S´anchez, J. Rivas, C. V´azquez-V´azquez, A. L´opez-Quintela, M.T. Causa, M. Tovar, S. Oseroff, Appl. Phys. Lett. 68 (1996) 134. [16] L.E. Hueso, J. Rivas, F. Rivadula, M.A. Lopez-Quintela, J. Appl. Phys. 86 (1999) 3881. [17] Z.C. Xia, S.L. Yuan, W. Feng, et al., Solid State Commun. 126 (2003) 567. [18] S.L. Yuan, L.J. Zhang, Z.C. Xia, et al., Phys. Rev. B 68 (2003) 172408.
Effect of sintering temperature on electrical transport of La0.67Ca0.33MnO3 granular system with 4% CuO addition J.H. Miao, S.L. Yuan ∗ , G.M. Ren, X. Xiao, G.Q. Yu, Y.Q. Wang, S.Y. Yin Department of Physics, Huazhong University of Science and Technology, Wuhan 430074, People’s Republic of China Received 21 July 2006; received in revised form 10 October 2006; accepted 10 October 2006 Available online 21 November 2006
Abstract The granular systems of La0.67 Ca0.33 MnO3 (LCMO) with 4% CuO addition were fabricated by three steps. Firstly, the LCMO particles were prepared and sintered at temperatures of 600 ◦ C (Ts6), 1000 ◦ C (Ts10), 1200 ◦ C (Ts12) and 1400 ◦ C (Ts14). Experimental results show that with the increment of sintering temperature, the particle size of the pre-prepared LCMO also increases, which plays a key role on electrical transport of the final composites. It is found that sample Ts14 exhibits two transition peaks both in ρ–T and MR0 –T curves, while sample Ts12 only shows two transition peaks in MR0 –T curve. Here MR0 is the abbreviation of magnetoresistance. These two transition peaks are ascribed to the intrinsic and extrinsic effects, respectively. For sample Ts6, Cu2+ ions mainly enter into the lattice of LCMO to form with La0.67 Ca0.33 Mn1−x Cux O3 . Specially, it is interesting to observe the MR0 effect of ∼90% at 0.3 T for sample Ts10 at the vicinity of insulator-metal transition temperature TIM , where considerable thermal hysteresis also appears. Such enhancement of MR effect is attributed to the field suppression of spin disordering especially at grain boundaries caused by CuO addition. © 2006 Elsevier B.V. All rights reserved. PACS: 75.30.Vn; 71.30.+h; 75.30.Kz Keywords: Perovskite manganites; Magneroresistance; Spin disordering
1. Introduction Since the discovery of colossal magnetoresistance (CMR) effect in perovskite manganites in 1993 [1], there has been a renewed interest in the research of these compounds. But CMR effect is not appealing for technological ends due to the large magnetic field of the order of several tesla [2,3]. Recently, the low-field magnetoresistance (LFMR) in polycrystalline manganites is of special interests due to its high sensitivity to small magnetic field. Very different from the origin of CMR, the LFMR is believed to originate from the spin-polarized tunneling process [4]. Enhanced LFMR response has been observed in some manganites-based two-phase composites. The second phases include insulating oxides [5–8], hard or soft magnetic materials [9,10], metal [11] and other CMR oxides [12]. In these
∗
Corresponding author. E-mail addresses: miaojh [email protected] (J.H. Miao), [email protected] (S.L. Yuan). 0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.10.033
composites, it is believed that dilution with the second phases impedes the magnetic homogeneity near the grain boundary area, which adjusts the barrier layer and, hence, influences the tunneling process. However, in those studies, the interaction and diffusion between manganites and the second phase especially at interfaces is usually ignored. On the other hand, according to Cu2+ ion (3d9 ), there is an extra electron with 1/2 spin. When CuO (Cu2+ ) was introduced to the grain boundary regions of LCMO, some spin disordering should be induced at grain boundaries. This spin disordering may affect the ferromagnetic coupling between contiguous LCMO grains. Applied magnetic field suppresses this spin disordering and may lead to an enhanced LFMR effect. Therefore, in this paper, we discuss the electrical transport behavior of LCMO granular system with 4% CuO addition. For convenience, we use their nominal composition of LCMO/xCuO (x = 4%) to name these samples. Meanwhile, we use Ts6, Ts10, Ts12, Ts14 to express the samples with pre-prepared LCMO particles sintered at different temperatures of 600, 1000, 1200 and 1400 ◦ C, respectively. The correspondingly pure LCMO
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prepared by the same experimental method is named as LC6, LC10, LC12 and LC14, respectively. 2. Experimental procedure The granular systems of LCMO/xCuO (x = 0% and 4%) were fabricated through a three-step chemical route, where x is the nominal molar fraction of CuO. Firstly, we used a sol–gel method to prepare LCMO powders according to nominal composition of La0.67 Ca0.33 MnO3 . One advantage of this method lies in a wide range for sintering temperature (Ts ) to obtain a single-phase sample with perovskite structure. In our study, the powders were sintered at temperatures of 600, 1000, 1200 and 1400 ◦ C for 6 h. Secondly, the LCMO/xCuO mixed powders were fabricated by the pre-prepared LCMO particles and Cu(NO3 )2 aqueous solution through a heterogeneous precipitation method as reported [13]. Finally, the obtained LCMO/xCuO mixed powders were ground, palletized, and then sintered at 1000 ◦ C for 2 h to get the final composites. The crystal structure of the samples was characterized by X-ray diffraction (XRD). The morphology of grain sizes was investigated by scanning electronic microscopy (SEM). The electrical transport properties were measured by the standard four-probe method in a commercial physical property measurement system (Quantum Design PPMS).
3. Results and discussion The room temperature XRD spectrums of LCMO/xCuO samples with x = 0% and 4% are shown in Fig. 1(a) and (b), respectively. Results indicate that the same diffraction patterns are observed in all of the samples, and all the diffraction peaks can be indexed into a perovskite crystalline structure (as shown in Fig. 1(a)). No CuO or other phases were detected within the sensitivity of the measurement. Shifting of the most intense lines (0 0 2, 2 0 0) is shown in the inset. It can be noted that all of the pure LCMO samples almost have the same 2θ position of ∼32.96◦ . The lines (0 0 2, 2 0 0) for sample LC6 and LC10 shift a little towards higher angle due to their small grain size caused by low sintering temperature [14,15]. It can be noted from the inset of Fig. 1(b) that the diffraction lines (0 0 2, 2 0 0) for sample Ts14, Ts12 and Ts10 have the same 2θ angle of ∼32.95◦ , which is consistent with that of pure LCMO. However, the lines shift towards lower angle to 32.87◦ for sample Ts6, which indicates the expansion of unit cell of crystalline lattice. The scanning electron micrograph (SEM) images for samples Ts6, Ts10, Ts12 and Ts14 are presented in Fig. 2. Although all samples were finally sintered at the same temperature of 1000 ◦ C, their average grain sizes are quite different. It is of ∼1 and 2 m for sample Ts6 and Ts14, respectively. For sample Ts10 and Ts12, the grains are nearly spherical in shape with varying particles size from 300 to 900 nm. This discrepancy can be understood on the basis of sample fabrication procedure. With the increment of sintering temperature from 600 to 1400 ◦ C, the average particle size of pre-prepared LCMO increases [15,16]. The average grain size of sample Ts10, Ts12 and Ts14 is consistent with the particle size of pre-prepared LCMO correspondingly. However, the particle size of LCMO sintered at 600 ◦ C is of 25 ± 6 nm [16], while the sample Ts6 has an abnormal increase of grain size of ∼1 m. This phenomenon can be understood through the fabrication procedure of the samples. The LCMO particles are pre-prepared. Due to the deviation of
Fig. 1. XRD patterns for samples of (a) LC6, LC10, LC12, LC14, and (b) Ts6, Ts10, Ts12, Ts14. The inset shows the shifting of the most intense diffraction peak (0 0 2, 2 0 0).
stoichiometric composition and the termination of crystalline structure, the structure near grain boundary region should not be as tight as that inside the grains. Moreover, the pre-prepared LCMO particles sintered at 600 ◦ C are in nano-size, which makes it metastable and reactive. Therefore, the reaction between CuO and LCMO should take place unavoidably, especially at grain boundaries during the final sintering process. It is the most pronounced for sample Ts6, in which LCMO and CuO can react thoroughly during the final sintering process (1000 ◦ C, 2 h). Such reaction results in the formation of La0.67 Ca0.33 Mn1−x Cux O3 . In this case, CuO acts as a sintering aid, which insures the large grain size of Ts6. However, in case of Ts10, Ts12 and Ts14, the final sintering temperature (1000 ◦ C) is not high enough to destroy the crystalline structure of the pre-prepared LCMO grains, which has been formed by higher sintering temperature (≥1000 ◦ C). Therefore, the interaction and diffusion between LCMO and CuO can be limited at the vicinity of grain boundaries due to the large grain size and stable crystalline structure of the pre-prepared LCMO particles. And then the final grain sizes of sample Ts10, Ts12 and Ts14 are similar with that of the
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Fig. 2. SEM representative images for (a) Ts6, (b) Ts10, (c) Ts12 and (d) Ts14.
pre-prepared LCMO particles sintered at the correspondingly temperatures. As shown in Fig. 1, the shift of diffraction lines towards lower angle for sample Ts6 can be interpreted in the same way. Since Cu2+ ion has a larger ionic size (0.074 nm), as compared to Mn3+ and Mn4+ (0.066 and 0.060 nm), with the formation of La0.67 Ca0.33 Mn1−x Cux O3 , the substitution of Cu for Mn should cause an expansion of unit cell. The shift of diffraction lines towards lower angle is corresponding to the expansion of unit cell. Therefore, it is believed that Cu2+ ions are mainly substituted for Mn ions in sample Ts6. However, for sample Ts10, Ts12 and Ts14, the interaction and diffusion is limited at the vicinity of grain boundary. Its effect on the crystalline structure of LCMO matrix is neglectable. In this case, direct XRD control of the interaction between LCMO and CuO at interfaces is difficult due to the iso-structure of LCMO and the reaction product La0.67 Ca0.33 Mn1−x Cux O3 . Fig. 3 presents the resistivity as a function of temperature and magnetic field for the composites of (a) Ts6, (b) Ts10, (c) Ts12 and (d) Ts14, respectively. Some special experimental results can be found in this figure. (1) All the composites exhibit an obvious transition characterized by a peak from metallic state to insulator both at zero and applied magnetic field. (2) The transition temperature TIM at zero field is about 147.8, 105.0, 127.8 and 151.8 K for the sample of Ts6, Ts10, Ts12 and Ts14, respectively. The corresponding peak resistivity (ρmax ) at TIM is of about 37.6, 360.6, 37.7 and 3.73 cm, respectively. It is noted that the sample Ts10 exhibits the lowest TIM and the largest ρmax .
(3) It is interesting to observe another transition peak in the ρ–T curve for sample Ts14 occurring at ∼258 K (as marked asterisk in Fig. 3(d)). Such double peak behavior has been observed earlier in LCMO/Fe3 O4 [17] composites, which is attributed to the intrinsic and extrinsic effects, respectively. (4) With an application of magnetic field, the resistivity of all the samples decreases sharply and TIM shifts towards higher temperature. It is worth comparing the electrical transport behavior for sample Ts6 and Ts10 with that of La0.67 Ca0.33 Mn1−x Cux O3 (x = 4%) [18]. Cu2+ has a larger ions radius comparing with Mn ion. The substitution of Cu for Mn induced stress at local regions. In order to relax the stress, Cu2+ ions at the vicinity of grain boundary tend to segregate towards surfaces, which is more pronounced with small grain size. As reported, if sintered at 1100 ◦ C, Cu2+ ions largely segregate towards surfaces and form some Cu dependent materials at grain boundary regions, while Cu2+ ions are mainly substituted for Mn with high sintering temperature of 1300 ◦ C. As shown in Fig. 3(a) and (b), the sample Ts6 and Ts10 exhibit similar electrical transport behavior with that of La0.67 Ca0.33 Mn1−x Cux O3 (x = 4%) sintered at high (1300 ◦ C) and low (1100 ◦ C) temperature [18], respectively. This comparison is another indication of the different structure for sample Ts6 and Ts10. The temperature dependence of magnetoresistance (MR0 ) has been measured in an applied magnetic field of 0.3 T for all samples and the results are shown in Fig. 4. Here MR0 is defined as MR0 % = [ρ(0, T) − ρ(0.3 T, T)]/ρ(0, T) × 100% , where ρ(0, T) and ρ(0.3 T, T) are resistivity values for zero and applied field
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Fig. 3. The temperature and magnetic field dependence of resistivity for samples of (a) Ts6, (b) Ts10, (c) Ts12 and (d) Ts14, respectively. The arrows were drawn for the guide of eyes. The asterisk in (d) denotes the other insulator-metal transition for sample Ts14.
of 0.3 T, respectively. It can be found that all the samples stand out for a sharp peak at temperatures near TIM in the MR0 –T curve. The MR0 value for sample Ts10 can reach as high as ∼90%. Similar to the two transition peaks observed in the ρ–T curve for sample Ts14, there are two transition peaks in the MR0 –T curve for sample Ts14 and Ts12 (as shown inset). The presence of first peak at higher temperature regime should be indicative of the alignment of Mn spins within the LCMO grains, that is, the intrinsic property of manganites. The other one occurs at low temperature is usually attributed to the influence of grain boundaries, which is an extrinsic behavior.
Fig. 4. Temperature dependence of MR0 measured under 0.3 T for samples of Ts6, Ts10, Ts12 and Ts14. Inset: temperature dependence of MR0 at temperature range of 170–200 K for sample Ts12.
As discussed above, it is believed that the composites with 4% CuO addition have a heterostructure with Cu2+ ions mainly dispersing at grain boundaries except for the sample Ts6, in which Cu2+ ions largely enter into the lattice of LCMO. There are four possible conditions. (1) If the grain size of pre-prepared LCMO is too small, such as sample Ts6, Cu2+ ions largely enter into LCMO lattice to form La0.67 Ca0.33 Mn1−x Cux O3 . (2) If the LCMO grain size is large enough, such as sample Ts14, Cu2+ disperses mainly at grain boundaries and surfaces. This is consistent with the observed two transition peaks related to LCMO grains and the modified grain boundaries, respectively. (3) For the sample Ts10, the reaction product layer at grain boundaries has certain thickness. It has a strong effect on the electrical transport behavior, which covers up the intrinsic transition peak of LCMO intra-grains both in ρ–T and MR0 –T curves. (4) For sample Ts12, the pre-prepared LCMO has grain size between that of Ts14 and Ts10. Therefore, sample Ts12 exhibits two transition peaks in the MR0 –T curve, but one peak in ρ–T curve due to the sensitivity of the intrinsic transition peak to applied magnetic field. Now we emphasize our attention on the sample of Ts10, in which substantial enhancement of LFMR effect was observed. Cu2+ (3d9 ) has an extra electron carrying 1/2 spin. With the formation of Cu2+ dependent reaction product layer at grain boundaries, some spin disordering is created. This local spin disordering should suppress the parallel alignment of the neighboring magnetic moments, and therefore lead to a lower value in TIM and a large increase in the peak resistivity (ρmax ). When a magnetic field is applied, the field decreases the random distribution of the local spins, which results in
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with 4% CuO addition was investigated. It is of interests to observe two transition peaks in ρ–T or MR0 –T curves for sample Ts14 and Ts12. For sample Ts6, Cu2+ ions mainly enter into the lattice of LCMO to form with La0.67 Ca0.33 Mn1−x Cux O3 . The sample Ts10 exhibits LFMR effect as large as ∼90% at an applied field of 0.3 T. At the same temperature region where abnormal MR effect appears, considerable thermal hysteresis is observed, which indicates the same underlying of physical origin for them. Therefore, we suggest that such experimental procedure of introduction extra spin disorder especially at grain boundaries by addition CuO to LCMO granular system may be a significant way to enhance the LFMR effect especially at the temperatures near TIM . Acknowledgements Fig. 5. The resistivity versus temperature for sample Ts10 taken in cooling and warming runs under zero field. The arrows were drawn for the guide of eyes.
the FM correlation between the contiguous grain moments to occur at higher temperature and a large decrease of resistivity. It means that the sharp MR peak observed near ∼TIM is due to the field induced ordering of local spin disordering especially at grain boundaries, which can be realized at a low magnetic field. Similar observations were reported in the composites of La0.67 Ca0.33 Mn1−x Cux O3 (x = 4%) sintered at low temperature [18], in which some Cu-dependent materials with spin disorder were supposed to form surrounding the LCMO grains. To gain some more information for the understanding above, we perform measurements of resistivity for temperature cycling of the composite Ts10 in zero magnetic field (shown in Fig. 5). The observed considerable thermal hysteresis is another indication of local spin disordering especially at interfaces. It is noted that the ρ data obtained in the cooling mode is larger than that in the warming-up mode and the thermal hysteresis becomes the most remarkable near the temperature of TIM . In cooling process, the spin ordering of Mn ions occurs within grains due to the double exchange effect. The magnetic moment within grains may affect the local spin disordering at grain boundaries and align them towards the same direction. In the warming process, the local spins at grain boundaries remain their initial orientation due to the strong affect of initial magnetic moments within grains. This alignment of spins at grain boundaries was not destroyed until the further increase of temperature. The spin alignment at grain boundaries is benefit to FM coupling of contiguous grains, which corresponds to low resistivty state. Therefore, the ρ at warming-mode is much smaller than that of cooling process especially at the vicinity of transition temperature TIM . 4. Conclusion The effect of sintering temperature of pre-prepared LCMO particles on electrical transport behavior of the granular system
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