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3MnO3
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Physics Procedia 27 (2012) 96 – 99
ISS2011
Magnetoresistance, electroresistance and nonlinear electrical behavior in nanopolycrystalline La2/3Sr1/3MnO3 X.S. Yanga, L.Q. Yanga, C.H. Chengb, L. Lva, Y. Zhaoa,b a
Key Lab of Advanced Material Technologies (Ministry of Education), Superconductivity and New Energy R&D Center, Southwest Jiaotong University, Chengdu,610031 China b School of Materials Science and Engineering, University of New South Wales, Sydney, 2052, NSW, Australia
Abstract Nanopolycrtstalline La2/3Sr1/3MnO3 ceramics were prepared using nanosized precursor powders. Highly nonlinear current–voltage characteristics were observed at room temperature and low temperature. The nonlinearity coefficient is in the range of 1.15–53.5. The electroresistance (ER) and magnetoresistance (MR) effects have been investigated. The magnetoresistance of all the samples shows strong dependence on sintering temperature. Magnetoresistance decreases with increasing temperature.
© Selectionand/or and/orpeer-review peer-reviewunder underresponsibility responsibilityofofISS ISSProgram ProgramCommittee. Committee © 2012 2011 Published by Elsevier B.V. Ltd. Selection Keywards: Magnetoresistance; Electroresistance; Nonlinear current-voltage characteristics
1. Introduction As a strongly correlated electron system, the rare earth doped perovskite manganese, La1íxSrxMnO3, which has colossal magnetoresistance (CMR) effect has been extensively studied, due to exhibiting intriguing physical properties such as insulator-metal transition, charge ordering, orbital ordering and phase separation [1]. The optimally doped La2/3Sr1/3MnO3 (LSMO) shows highly spin polarization and a high Curie temperature (TC~370 K). Therefore, this compound appears as a promising candidate in practical application, since LSMO-based devices could operate at room temperature. Recently, electroresistance (ER) has been found in this kind of material. ER and MR are coupled, i.e., the voltage/current necessary to trigger insulator-metal transition decreases with magnetic field [2]. Most works concentrate on films [3] and single crystalline [2] in manganites. In addition, nonlinear I–V relations have been observed in La–Sr–Mn–O ceramics [4], demonstrating the effect of grain size and the influence of grain boundary region on the electronic properties. This effect, however, was only observed at very low temperature, and the nonlinear I-V effect is weak. Few reports paid attention to polycrystalline bulk. Moreover, the relationship between MR, ER and nonlinear I–V characteristics has not been studied yet. Nanopolycrystalline LSMO samples could be easily fabricated using nanoprecursor powders by sol-gel or chemical coprecipitation process. MR, ER and nonlinear I–V characteristics, which are related to grain-boundary, in nanopolycrystalline LSMO are different from those in ordinary bulk samples. In this paper, we report that nanopolycrystalline La2/3Sr1/3MnO3 ceramics have highly nonlinear I-V characteristics at room temperature. It can be an excellent candidate for low-voltage or low-current varistors above its magnetic transition temperature, even at room temperature.
Corresponding author. Tel.: +86-28-87600786 ; fax: +86-28-87600787 . E-mail address: [email protected] .
1875-3892 © 2012 Published by Elsevier B.V. Selection and/or peer-review under responsibility of ISS Program Committee doi:10.1016/j.phpro.2012.03.419
97
X.S. Yang et al. / Physics Procedia 27 (2012) 96 – 99
2. Experimental La2/3Sr1/3MnO3 precursor powders were prepared by chemical coprecipitation process and nanopolycrystalline samples were fabricate by the standard ceramic technique. Resistivity was measured by the Physical Properties Measurement System (PPMS-9, Quantum Design). Structural analysis was performed using X-ray diffraction (XRD, X’Pert Panalytical). Scanning electron microscope (SEM, FEI QUANTA200) was used for morphological study. 3. Results and discussion The SEM images of the bulk samples sintered at different temperatures are shown in Fig.1. The average grain size was determined by analyzing SEM photos using the linear intercept method, given by d=1.56L/MN, where d is the average grain size, L is the random line length on the micrograph, M is the magnification of the micrograph, and N is the number of the grain boundaries intercepted by lines. The average grain size, as shown in table 1, increases from 25 nm to 113 nm as the sintering temperature changing from 600 to 1000 oC.
Fig. 1. SEM of La2/3Sr1/3MnO3 sintered at 600 oC (a),700 oC (b)800 oC (c), 900 oC (d) and 1000oC (e). Table 1. Grain size of the samples sintered at different temperatures. Sintering temperature(oC)
Grain size (nm)
600
25
700
31
800
49
900
62
1000
113
The I–V characteristics of the samples are shown in Fig.2. The I–V behavior is linear (ohmic behavior) at small voltage and becomes nonlinear above a certain voltage. The nonlinear coefficient Į was obtained by the formula Į=dlgI/dlgV, where V is voltage value corresponding to I. The studied samples have nonlinear coefficient Į of 17.3, 1.23 and 1.15 at a current of I=50 mA for the samples sintered at 600, 700 and 800oC, respectively. The samples sintered at 900 and 1000oC show approximately linear IV characteristics with D about 1. The values of nonlinear coefficient are given in Table 2 and Table 3. These data show that large values of D can be obtained by changing the process parameters (i.e., preparing ceramics at lower sintering temperature) and measured parameters. The sample sintered at 600oC showed the largest sensitivity to change in measured temperature. There are large number of grains and hence large area of the grain surface because of the small grain size. This results in highly nonlinear I-V characteristics. The highly nonlinear I–V characteristics indicate a tunneling type transport mechanism across a highly resistive potential barrier created by the grain boundary (or interface) and the intergrain distance [5]. Table 2. Nonlinear coefficient (Į) for La2/3Sr1/3MnO3 ceramic samples sintered at different temperatures. Sintering temperature(oC)
600
700
800
Nonlinear coefficient Į
17.3
1.23
1.15
Table 3. Nonlinear coefficient Į value for La2/3Sr1/3MnO3 ceramic sample sintered at 600oC, measured at different temperatures. Measured temperature(K)
50
300
350
Nonlinear coefficient Į
53.5
17.3
2.3
Markovich et al. studied ER effect of a single crystal La0.82Ca0.18MnO3, which was explained by assuming filamentary currents driven by local electric field gradients [6]. Zhao et al. studied of La0.67Sr0.33MnO3 films, they
98
X.S. Yang et al. / Physics Procedia 27 (2012) 96 – 99
believed that it is difficult to distinguish the current effect from electric field effect. For ER effect, electric field may play an important role [3]. These explanations may not account for nanopolycrtstalline La2/3Sr1/3MnO3 ceramics in this study, as the samples they studied were single crystal or film, not polycrystalline. In this case, there may be any association between ER and nonlinear I-V characteristics. For ER, grain size and grain boundary layer play an important role. At low applied dc current, grain boundary may be the key factor in resistivity variation; but in high applied dc current, grain itself play dominant role instead of boundary.
Fig. 2. Nonlinear I-V characteristics of La2/3Sr1/3MnO3. (a) sintered at 600,700 800, 900 and 1000oC; (b) sintered at 600oC and measured at 50K, 300K, 350K.
Fig. 3. Temperature dependence of resistance for LSMO sintered at 800oC under different dc currents. From top to bottom, I=0.001mA, 0.01mA, 0.1mA, 0.5mA, 1mA, 3mA and 5mA, respectively.
Fig. 4. Relative electroresistance (ER%) as a function of temperature for LSMO sample measured at different dc currents and sintered at 800oC
The temperature dependence of MR for all the studied samples measured at 3000 Oe is shown in Fig.5. All samples exhibit obvious low field magnetoresitance (LFMR) characteristics. The MR value is determined by the formula: MR(%)=[(U0-UH)/ U0]×100%, where the U0 and UH are the resistivity under applied field=0 and H, respectively. MR values sequentially increase with decreasing sintering temperature. At 50K, MR values are 18.23%, 18.11%, 16.29%,
X.S. Yang et al. / Physics Procedia 27 (2012) 96 – 99
14.36% and 13.28% for samples sintered at 600~1000oC, respectively. This phenomenon is related to the grain size effect. LFMR is ascribed to the spin-polarized tunneling between grain boundaries [7]. The grain size increases with increasing sintering temperature, so the grain boundary density decreases. This will decrease tunneling effect, resulting in low magnetoresistance. These results are consistent with other results of grain boundary induced LFMR [8]. All samples show significant MR at low field, and MR disappears at high temperature. The disappearance of the high temperature MR can be explained by weakening the double exchange (DE) mechanism around the PM–FM transition temperatures.
Fig. 5. Magnetoresistance (MR%) as a function of temperature for applied field of 3000 Oe for the samples sintered at different temperatures.
4. Conclusion In summary, we studied the effect of sintering temperature on magntoresistance, electroresistance and nonlinear electrical behavior of nanophasic La2/3Sr1/3MnO3 samples, which have been successfully synthesized by the chemical coprecipitation route. The microstructure reveals that the particle size increases from ~25 nm (sintered at 600oC) to 113 nm (sintered at 1000oC). Highly nonlinear I-V characteristics were observed in La2/3Sr1/3MnO3 ceramic at room temperature. ER were also observed at different applied dc current. Low field magnetoresistance effect has been investigated. The enhanced effect of magnetoresistance, electroresistance and nonlinear electrical behavior is related to large number of grain boundary because of small grain size for nanocrystalline samples. Acknowledgements The authors are grateful for the financial support of the National Natural Science Foundation of China (Nos 50872116, 51002125), the Specialized Research Fund for the Doctoral Program of Higher Education (200806130023), and the Fundamental Research Funds for the Central Universities (SWJTU09ZT24, SWJTU11ZT16, SWJTU11ZT29), and Fundamental Research Funds of Sichuan Province (2011JY0031). References [1] Coey JMD, Viret M, Molnar S. von. Mixed-valence manganites. Adv Phys 1999; 48:167-293. [2] Biškup N, Andrés de A, Nemes NM, García-Hernandez M, Glazyrin KV, Mukovskii YM. Colossal electroresistance without colossal magnetoresistance in La0.9Sr0.1MnO3. Appl Phys Lett 2007;90: 222502. [3] Zhao YG, Wang YH, Zhang GM, Zhang B. Universal behavior of giant electroresistance in epitaxial La0.67Ca0.33MnO3 thin films. Appl Phys Lett 2005; 86:122502. [4] Sáncheza RD, Niebieskikwiata D, et al. Nonlinear behavior of V–I curves at low temperatures in nanoparticles of La2/3B1/3MnO3 with B=Ca,S. Physica B 2002;320:115-8. [5] Debnath AK. and Lin JG. Current-induced giant electroresistance in La0.7Sr0.3MnO3 thin films. Phys Rev B 2003;67: 064412. [6] Gross R, Alff L, Büchner B, Freitag BH, Höfener C, Klein J, et al. Physics of grain boundaries in the colossal magnetoresistance manganites. J Magn Magn Mater 2000;211: 150-9. [7] Hwang HY, Cheong SW, Ong NP. Batlogg B. Spin-Polarized Intergrain Tunneling in La2/3Sr1/3MnO3. Phys Rev Lett 1996; 77: 2041-4. [8] Lȩpez-Quintela M A, Hueso L E, Rivas J and Rivadulla F. Intergranular magnetoresistance in nanomanganites. Nanotechnology 2003; 14: 212.
99
Physics Procedia 27 (2012) 96 – 99
ISS2011
Magnetoresistance, electroresistance and nonlinear electrical behavior in nanopolycrystalline La2/3Sr1/3MnO3 X.S. Yanga, L.Q. Yanga, C.H. Chengb, L. Lva, Y. Zhaoa,b a
Key Lab of Advanced Material Technologies (Ministry of Education), Superconductivity and New Energy R&D Center, Southwest Jiaotong University, Chengdu,610031 China b School of Materials Science and Engineering, University of New South Wales, Sydney, 2052, NSW, Australia
Abstract Nanopolycrtstalline La2/3Sr1/3MnO3 ceramics were prepared using nanosized precursor powders. Highly nonlinear current–voltage characteristics were observed at room temperature and low temperature. The nonlinearity coefficient is in the range of 1.15–53.5. The electroresistance (ER) and magnetoresistance (MR) effects have been investigated. The magnetoresistance of all the samples shows strong dependence on sintering temperature. Magnetoresistance decreases with increasing temperature.
© Selectionand/or and/orpeer-review peer-reviewunder underresponsibility responsibilityofofISS ISSProgram ProgramCommittee. Committee © 2012 2011 Published by Elsevier B.V. Ltd. Selection Keywards: Magnetoresistance; Electroresistance; Nonlinear current-voltage characteristics
1. Introduction As a strongly correlated electron system, the rare earth doped perovskite manganese, La1íxSrxMnO3, which has colossal magnetoresistance (CMR) effect has been extensively studied, due to exhibiting intriguing physical properties such as insulator-metal transition, charge ordering, orbital ordering and phase separation [1]. The optimally doped La2/3Sr1/3MnO3 (LSMO) shows highly spin polarization and a high Curie temperature (TC~370 K). Therefore, this compound appears as a promising candidate in practical application, since LSMO-based devices could operate at room temperature. Recently, electroresistance (ER) has been found in this kind of material. ER and MR are coupled, i.e., the voltage/current necessary to trigger insulator-metal transition decreases with magnetic field [2]. Most works concentrate on films [3] and single crystalline [2] in manganites. In addition, nonlinear I–V relations have been observed in La–Sr–Mn–O ceramics [4], demonstrating the effect of grain size and the influence of grain boundary region on the electronic properties. This effect, however, was only observed at very low temperature, and the nonlinear I-V effect is weak. Few reports paid attention to polycrystalline bulk. Moreover, the relationship between MR, ER and nonlinear I–V characteristics has not been studied yet. Nanopolycrystalline LSMO samples could be easily fabricated using nanoprecursor powders by sol-gel or chemical coprecipitation process. MR, ER and nonlinear I–V characteristics, which are related to grain-boundary, in nanopolycrystalline LSMO are different from those in ordinary bulk samples. In this paper, we report that nanopolycrystalline La2/3Sr1/3MnO3 ceramics have highly nonlinear I-V characteristics at room temperature. It can be an excellent candidate for low-voltage or low-current varistors above its magnetic transition temperature, even at room temperature.
Corresponding author. Tel.: +86-28-87600786 ; fax: +86-28-87600787 . E-mail address: [email protected] .
1875-3892 © 2012 Published by Elsevier B.V. Selection and/or peer-review under responsibility of ISS Program Committee doi:10.1016/j.phpro.2012.03.419
97
X.S. Yang et al. / Physics Procedia 27 (2012) 96 – 99
2. Experimental La2/3Sr1/3MnO3 precursor powders were prepared by chemical coprecipitation process and nanopolycrystalline samples were fabricate by the standard ceramic technique. Resistivity was measured by the Physical Properties Measurement System (PPMS-9, Quantum Design). Structural analysis was performed using X-ray diffraction (XRD, X’Pert Panalytical). Scanning electron microscope (SEM, FEI QUANTA200) was used for morphological study. 3. Results and discussion The SEM images of the bulk samples sintered at different temperatures are shown in Fig.1. The average grain size was determined by analyzing SEM photos using the linear intercept method, given by d=1.56L/MN, where d is the average grain size, L is the random line length on the micrograph, M is the magnification of the micrograph, and N is the number of the grain boundaries intercepted by lines. The average grain size, as shown in table 1, increases from 25 nm to 113 nm as the sintering temperature changing from 600 to 1000 oC.
Fig. 1. SEM of La2/3Sr1/3MnO3 sintered at 600 oC (a),700 oC (b)800 oC (c), 900 oC (d) and 1000oC (e). Table 1. Grain size of the samples sintered at different temperatures. Sintering temperature(oC)
Grain size (nm)
600
25
700
31
800
49
900
62
1000
113
The I–V characteristics of the samples are shown in Fig.2. The I–V behavior is linear (ohmic behavior) at small voltage and becomes nonlinear above a certain voltage. The nonlinear coefficient Į was obtained by the formula Į=dlgI/dlgV, where V is voltage value corresponding to I. The studied samples have nonlinear coefficient Į of 17.3, 1.23 and 1.15 at a current of I=50 mA for the samples sintered at 600, 700 and 800oC, respectively. The samples sintered at 900 and 1000oC show approximately linear IV characteristics with D about 1. The values of nonlinear coefficient are given in Table 2 and Table 3. These data show that large values of D can be obtained by changing the process parameters (i.e., preparing ceramics at lower sintering temperature) and measured parameters. The sample sintered at 600oC showed the largest sensitivity to change in measured temperature. There are large number of grains and hence large area of the grain surface because of the small grain size. This results in highly nonlinear I-V characteristics. The highly nonlinear I–V characteristics indicate a tunneling type transport mechanism across a highly resistive potential barrier created by the grain boundary (or interface) and the intergrain distance [5]. Table 2. Nonlinear coefficient (Į) for La2/3Sr1/3MnO3 ceramic samples sintered at different temperatures. Sintering temperature(oC)
600
700
800
Nonlinear coefficient Į
17.3
1.23
1.15
Table 3. Nonlinear coefficient Į value for La2/3Sr1/3MnO3 ceramic sample sintered at 600oC, measured at different temperatures. Measured temperature(K)
50
300
350
Nonlinear coefficient Į
53.5
17.3
2.3
Markovich et al. studied ER effect of a single crystal La0.82Ca0.18MnO3, which was explained by assuming filamentary currents driven by local electric field gradients [6]. Zhao et al. studied of La0.67Sr0.33MnO3 films, they
98
X.S. Yang et al. / Physics Procedia 27 (2012) 96 – 99
believed that it is difficult to distinguish the current effect from electric field effect. For ER effect, electric field may play an important role [3]. These explanations may not account for nanopolycrtstalline La2/3Sr1/3MnO3 ceramics in this study, as the samples they studied were single crystal or film, not polycrystalline. In this case, there may be any association between ER and nonlinear I-V characteristics. For ER, grain size and grain boundary layer play an important role. At low applied dc current, grain boundary may be the key factor in resistivity variation; but in high applied dc current, grain itself play dominant role instead of boundary.
Fig. 2. Nonlinear I-V characteristics of La2/3Sr1/3MnO3. (a) sintered at 600,700 800, 900 and 1000oC; (b) sintered at 600oC and measured at 50K, 300K, 350K.
Fig. 3. Temperature dependence of resistance for LSMO sintered at 800oC under different dc currents. From top to bottom, I=0.001mA, 0.01mA, 0.1mA, 0.5mA, 1mA, 3mA and 5mA, respectively.
Fig. 4. Relative electroresistance (ER%) as a function of temperature for LSMO sample measured at different dc currents and sintered at 800oC
The temperature dependence of MR for all the studied samples measured at 3000 Oe is shown in Fig.5. All samples exhibit obvious low field magnetoresitance (LFMR) characteristics. The MR value is determined by the formula: MR(%)=[(U0-UH)/ U0]×100%, where the U0 and UH are the resistivity under applied field=0 and H, respectively. MR values sequentially increase with decreasing sintering temperature. At 50K, MR values are 18.23%, 18.11%, 16.29%,
X.S. Yang et al. / Physics Procedia 27 (2012) 96 – 99
14.36% and 13.28% for samples sintered at 600~1000oC, respectively. This phenomenon is related to the grain size effect. LFMR is ascribed to the spin-polarized tunneling between grain boundaries [7]. The grain size increases with increasing sintering temperature, so the grain boundary density decreases. This will decrease tunneling effect, resulting in low magnetoresistance. These results are consistent with other results of grain boundary induced LFMR [8]. All samples show significant MR at low field, and MR disappears at high temperature. The disappearance of the high temperature MR can be explained by weakening the double exchange (DE) mechanism around the PM–FM transition temperatures.
Fig. 5. Magnetoresistance (MR%) as a function of temperature for applied field of 3000 Oe for the samples sintered at different temperatures.
4. Conclusion In summary, we studied the effect of sintering temperature on magntoresistance, electroresistance and nonlinear electrical behavior of nanophasic La2/3Sr1/3MnO3 samples, which have been successfully synthesized by the chemical coprecipitation route. The microstructure reveals that the particle size increases from ~25 nm (sintered at 600oC) to 113 nm (sintered at 1000oC). Highly nonlinear I-V characteristics were observed in La2/3Sr1/3MnO3 ceramic at room temperature. ER were also observed at different applied dc current. Low field magnetoresistance effect has been investigated. The enhanced effect of magnetoresistance, electroresistance and nonlinear electrical behavior is related to large number of grain boundary because of small grain size for nanocrystalline samples. Acknowledgements The authors are grateful for the financial support of the National Natural Science Foundation of China (Nos 50872116, 51002125), the Specialized Research Fund for the Doctoral Program of Higher Education (200806130023), and the Fundamental Research Funds for the Central Universities (SWJTU09ZT24, SWJTU11ZT16, SWJTU11ZT29), and Fundamental Research Funds of Sichuan Province (2011JY0031). References [1] Coey JMD, Viret M, Molnar S. von. Mixed-valence manganites. Adv Phys 1999; 48:167-293. [2] Biškup N, Andrés de A, Nemes NM, García-Hernandez M, Glazyrin KV, Mukovskii YM. Colossal electroresistance without colossal magnetoresistance in La0.9Sr0.1MnO3. Appl Phys Lett 2007;90: 222502. [3] Zhao YG, Wang YH, Zhang GM, Zhang B. Universal behavior of giant electroresistance in epitaxial La0.67Ca0.33MnO3 thin films. Appl Phys Lett 2005; 86:122502. [4] Sáncheza RD, Niebieskikwiata D, et al. Nonlinear behavior of V–I curves at low temperatures in nanoparticles of La2/3B1/3MnO3 with B=Ca,S. Physica B 2002;320:115-8. [5] Debnath AK. and Lin JG. Current-induced giant electroresistance in La0.7Sr0.3MnO3 thin films. Phys Rev B 2003;67: 064412. [6] Gross R, Alff L, Büchner B, Freitag BH, Höfener C, Klein J, et al. Physics of grain boundaries in the colossal magnetoresistance manganites. J Magn Magn Mater 2000;211: 150-9. [7] Hwang HY, Cheong SW, Ong NP. Batlogg B. Spin-Polarized Intergrain Tunneling in La2/3Sr1/3MnO3. Phys Rev Lett 1996; 77: 2041-4. [8] Lȩpez-Quintela M A, Hueso L E, Rivas J and Rivadulla F. Intergranular magnetoresistance in nanomanganites. Nanotechnology 2003; 14: 212.
99