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Low field magnetoresistance in La0.67Ca0.33MnO3 and Co3O4 combined system
Journal of Alloys and Compounds 453 (2008) 423–427
Low field magnetoresistance in La0.67Ca0.33MnO3 and Co3O4 combined system Anurag Gaur, G.D. Varma ∗ Department of Physics, Indian Institute of Technology Roorkee, Roorkee 247667, India Received 13 September 2006; received in revised form 1 November 2006; accepted 17 November 2006 Available online 21 December 2006
Abstract We have studied the magneto-transport properties of the (1 − x) La0.67 Ca0.33 MnO3 /xCo3 O4 (x ∼ 0, 0.05, 0.10, 0.15 and 0.20) combined samples, prepared via sol–gel process. The powder X-ray diffraction patterns indicate no evidence of reaction between the La0.67 Ca0.33 MnO3 and Co3 O4 . A clear metal to insulator (M–I) transition is observed at 260 K in virgin LCMO sample while that vanishes in the LCMO/Co3 O4 combined samples in the measured temperature range (80–300 K). Resistivity of the combined samples increases sharply as a consequence of Co3 O4 addition, especially on the low temperatures, and magnetization decreases. An enhancement in low field magnetoresistance (LFMR) is observed at temperatures below 125 K for the composites with lower Co3 O4 content, namely, x = 0.05 and 0.10. It is argued that this improved LFMR at low temperature (T < 125 K) arises due to the enhanced spin-polarized tunneling at the grain boundaries in the combined samples. © 2006 Published by Elsevier B.V. PACS: 73.43.Qt; 71.30.+h; 75.47.Lx Keywords: Perovskite manganites; Low-field magnetoresistance; Electrical transport; Grain boundary
1. Introduction Doped rare earth manganites of type R1−x Ax MnO3 (R is a rare earth such as La, Nd, Y, etc.; A is alkaline earth such as Ca, Ba, Sr, etc.), well known for their colossal magnetoresistance (CMR) properties, have been studied extensively in the last decade [1,2]. So far, two CMR effects have been found in these manganites, that is, the intrinsic CMR and the extrinsic CMR. For most of R1−x Ax MnO3 manganites, the CMR effect is maximum near metal–insulator (M–I) transition temperature (Tp ) accompanied by a simultaneous paramagnetic to ferromagnetic (PM–FM) transition at the Curie temperature (Tc ). This is so called intrinsic CMR [3]. The intrinsic CMR, caused by double exchange (DE) mechanism proposed by Zener in 1951 [4], is useful to explain CMR phenomena mostly observed near Tc at a relatively high magnetic field. However the extrinsic CMR, which is absent in the single crystal, is related to natural and artificial grain boundaries [3,5,6] and atomic size defects at
∗
Corresponding author. Tel.: +91 1332 285353; fax: +91 1332 273560. E-mail address: [email protected] (G.D. Varma).
0925-8388/$ – see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.jallcom.2006.11.132
the film–substrate interface [7]. Spin polarized tunneling [3] or spin dependent scattering [5] among neighbouring grains seems to be responsible for this kind of CMR effects. This extrinsic effect may enhance low field magnetoresistance (LFMR) in a wide temperature range and can be more useful for practical applications to magnetic switching of recording devices. In order to reduce the required magnetic fields and widen temperature ranges of the CMR effects, many groups have attempted to synthesize CMR-insulators composites. Very recently, enhanced MR properties have been observed in some ferromagnet/insulator type two phase manganites-based composites, such as LSMO/CeO2 [8], LCMO/STO [9], LSMO/glass [10], LSMO/NiO [11], LBMO/YSZ [12] and bilayers of LCMO with Fe3 O4 [13]. In this paper, we attempt to introduce the insulating Co3 O4 as the second phase into the manganites and investigate the magneto-transport properties in La0.67 Ca0.33 MnO3 /Cobalt oxide: (LCMO)1−x (Co3 O4 )x combined system. Cobalt oxide is a very suitable candidate as second phase material because it is a nice insulator, thus it can be served as the transport barrier in the manganites matrix and can modify the LCMO grain surfaces to adjust the tunneling effect and hence the magnetoresistance.
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The incorporation of impurity phase (Co3 O4 ) increases zero field resistivity of LCMO/Co3 O4 combined samples and metalinsulator transition disappears. This means, the combined samples show the insulating/semiconducting behaviour in whole temperature range (80–300 K). However, at low Co3 O4 concentrations, an enhanced low field magnetoresistance has been observed in this LCMO/Co3 O4 combined system. 2. Experimental procedure The La0.67 Ca0.33 MnO3 (LCMO) samples were prepared via sol–gel method. The aqueous solution of high purity nitrates of La, Ca and Mn have been taken in the desired stoichiometric proportions. An equal amount of ethylene glycol has been added to this solution with continuous stirring. This solution is then heated on a hot plate at temperature of ∼80–100 ◦ C till a dry thick brown sol is formed. This has been further decomposed in an oven at a temperature of 250 ◦ C to get the dry fluffy material. This obtained polymeric precursor is calcined at 600 ◦ C for 12 h. The calcined mixture was reground and sintered at 1000 ◦ C for 12 h. The obtained LCMO powders with single phase perovskite structure were completely mixed with a commercial Co3 O4 powder according to the nominal ratio of (LCMO)1−x /(Co3 O4 )x with x = 0, 0.05, 0.10, 0.15 and 0.20. The resulting (LCMO)1−x /(Co3 O4 )x powders were pelletized at a pressure of 5 MPa cm−2 and then sintered at 1000 ◦ C for 2 h. The low sintering temperature for small duration was chosen to avoid inter-diffusion of LCMO and Co3 O4 . By this procedure, the Co3 O4 introduced as second phase of the combined system mainly segregated at the grain boundaries and on the surfaces of the LCMO phase. The structural characterization was examined by employing X-ray diffraction (Bruker AXS D-8 advance, Cu K␣ radiation) technique at room temperature and surface morphology was investigated by scanning electron microscope (SEM Model LEO 435-VP operating at 15 kV). Resistivity as a function of temperature was measured by a standard four-probe method using Keithley instruments without or with magnetic fields (0–3 kOe). The dc magnetization measurements were done by using vibrating sample magnetometer (VSM Model 155, Princeton Applied Research).
3. Results and discussion Fig. 1 shows the room temperature XRD patterns of LCMO/Co3 O4 combined samples. The XRD spectra of the combined samples show two different sets of XRD peaks, corresponding to pseudo cubic perovskite LCMO and cubic spinel Co3 O4 structure. No extra phase is obtained indicating that the reactions between LCMO and Co3 O4 are negligible. Moreover, with increasing Co3 O4 concentrations, the most intense Co3 O4 peak (2θ = 36.82◦ ) increases almost linearly in intensity
Fig. 1. X-ray diffraction patterns of (1 − x)La0.67 Ca0.33 MnO3 /xCo3 O4 combined samples. Inset shows the intensity vs. x plot of most intense Co3 O4 peak.
as shown in the inset of Fig. 1. This also supports the above fact that there is negligible reaction between LCMO and Co3 O4 and almost all Co3 O4 introduced as second phase of the composite mainly segregates at the grain boundaries and on the surfaces of the LCMO grains. The evidence of the coexistence of two phases also comes from SEM micrographs, which clearly indicate the two different types of crystallites. The SEM micrographs of LCMO/Co3 O4 combined samples with x = 0 and 0.10 are shown in Fig. 2(a) and (b), respectively. A clear grain boundary is observed in pure LCMO as shown in Fig. 2(a). However, the grain boundaries of LCMO/Co3 O4 combined samples become ambiguous by the addition of Co3 O4 as shown in Fig. 2(b). From the SEM image (Fig. 2(b)), it is also clear that the doped Co3 O4 segregated mainly at the grain boundaries and on the surfaces of LCMO grains. In Fig. 2(b), the LCMO and Co3 O4 grains are indicated by white and black arrows, respectively, to distinguish them. The temperature dependence resistivity for the LCMO/ Co3 O4 combined samples measured by four probe technique at zero field is shown in Fig. 3. As expected, the metal like con-
Fig. 2. Scanning electron micrographs of (1 − x)La0.67 Ca0.33 MnO3 /xCo3 O4 combined samples: (a) x = 0 and (b) x = 0.10. LCMO and Co3 O4 grains are indicated by white and black arrows, respectively.
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Fig. 3. Temperature dependence of resistivity at zero field for (1 − x)La0.67 Ca0.33 MnO3 /xCo3 O4 combined samples.
ductivity is found in pure LCMO while with increasing Co3 O4 concentration x, the zero field resistivity of LCMO/Co3 O4 combined samples increases within one or two orders of magnitude, particularly at lower temperatures, when x varies from 0 to 0.20. It can be seen from Fig. 3 that the value of resistivity at 80 K increases from 0.03 to 853 cm when x increases from x = 0 to 0.20. Meanwhile, the virgin LCMO (x = 0) shows the clear metal–insulator transition at a temperature (TP ) 260 K while the transition disappears in combined samples and they show insulating/semiconducting behaviour in the whole measured temperature range (80–300 K). The increase of resistivity and disappearance of M–I transition can be qualitatively explained by enhanced grain boundaries. Since Co3 O4 behaves as a paramagnetic insulator above 40 K, the LCMO/Co3 O4 samples are similar to the ferromagnet/insulator type composites. Large number of interfaces and boundaries between LCMO and Co3 O4 grains may act as additional barriers. This causes the increase in the carrier scattering leading to enhancement in the resistivity. The metallic transition is suppressed because of Co3 O4 induced disorders and also by the increase in the insulating Co3 O4 phase fraction in the combined system. Moreover, in pure LCMO, the electrical transport is achieved through a direct contact between LCMO grains. However, in combined samples, there are two kinds of conduction channels parallel connected to each other [14,15]. One is related to LCMO grains, which determines the transport properties of the system through direct contact between LCMO grains and other is related to Co3 O4 . The temperature dependence of magnetization measured at 5 kOe is shown in Fig. 4. The value of magnetization (M) at 80 K are 78.3, 70.12, 65.3, 60.85 and 58.1 emu/gm for x = 0.0, 0.05, 0.10, 0.15 and 0.20, respectively. These successive decreases in M with increasing Co3 O4 concentrations are due to decrease in the volume fraction of ferromagnetic LCMO phase and extra magnetic disorder caused by Co3 O4 in these combined samples. The reduction in value of magnetization for the combined samples are more as calculated by taking in account the LCMO weight fraction in the composites, which suggests the extra magnetic disorder by Co3 O4 in the combined samples. All the combined samples have almost similar behaviour of magnetization as a function of temperature and there is a
Fig. 4. Temperature dependence of magnetization at 5 kOe for (1 − x) La0.67 Ca0.33 MnO3 /xCo3 O4 combined samples.
small shift in paramagnetic (PM) to ferromagnetic (FM) phase transition temperature (Tc ), which moves towards lower value with increase in the Co3 O4 content. The transition temperatures determined from the peak in dM/dT–T curves are found to be ∼289, 281, 279, 275 and 270 K for the samples with x = 0.0, 0.05, 0.10, 0.15 and 0.20, respectively. As observed by XRD and SEM, Co3 O4 segregates into the grain boundaries and surface of LCMO grains and this grain boundary segregation of Co3 O4 affects the double exchange (DE) mechanism and this is more pronounced in the FM domains walls [16]. This consequently leads to the suppression of PM–FM transition temperature towards the lower value with increase in Co3 O4 concentration. As reduction in the magnetization also suggests the suppression of DE mechanism resulting suppression of the ferromagnetic alignment of Mn ions. This indicates the magnetic spin disorder induced by the grain boundaries in the combined system. It is also noticeable that there is a large difference between the value of metal–insulator transition temperature (TP ) and paramagnetic–ferromagnetic transition temperature (Tc ) for the combined samples. The value of TP in virgin sample (x = 0) is 260 K while it comes below 80 K in the combined samples and they show insulating/semiconducting behaviour (see Fig. 3) in whole measured temperature range (80–300 K). On the other hand, Tc decreases only marginally (289–270 K) when x increases from 0.0 to 0.20 in the combined samples. This difference is due to the fact that Tc is an intrinsic property and does not show much dependence on grain boundaries while TP is an extrinsic property that strongly depends upon the grain boundaries [15]. The temperature dependence of magnetoresistance (MR) in a field of 3 kOe for all the measured samples is shown in Fig. 5. The MR ratio is defined as MR (%) = [ρ(0,T) − ρ(H,T)]/ρ(H,T) × 100%, where ρ(0,T) and ρ(H,T) are the resistivity values for zero and applied fields, respectively. We find that LCMO/Co3 O4 combined samples having low Co3 O4 concentration, viz., x ∼ 0.05 and 0.10 exhibit a distinct enhanced MR compared with parent LCMO in low temperature region (T < 125 K). We plot the x dependence of MR in the inset
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Fig. 5. Temperature dependence of magnetoresistance (MR) in a field of 3 kOe of (1 − x)La0.67 Ca0.33 MnO3 /xCo3 O4 combined samples. Inset shows the MR vs. x curve.
of Fig. 5. The maximum MR ratio at 80 K is ∼20.77% and ∼24.68% for the combined samples with x = 0.05 and 0.10, respectively while ∼18.36% for pure LCMO at 3 kOe. So, the observed enhancement in MR at 80 K with respect to pure LCMO is ∼13% and ∼35% for the composites with x = 0.05 and 0.10, respectively, at 3 kOe. This increase in MR at low temperature seems to be due to enhanced tunneling across LCMO grain boundaries and additional grain boundary effects introduced by Co3 O4 grains. As proposed by Hwang et al. [3], the spin polarized tunneling between neighbouring grains of manganites is responsible for the magnetoresistance effect in polycrystalline samples. This tunneling takes place across the grain boundaries or interfaces. As Fig. 5 indicates, we get the enhancement in MR of the combined samples with x = 0.05 and 0.10 only at temperatures below 125 K which suggests that this enhanced MR basically comes from grain boundary effects in our samples because it is well known that LFMR is mainly attributed to a grain boundary effect especially at temperature T < Tc [3]. However, with further increase in Co3 O4 level from x = 0.10, the MR decreases with respect to pure LCMO even at low temperature which may result because the grain boundaries become too thick for electron tunneling. However, the MR near room temperature slightly decreases in all LCMO/Co3 O4 combined samples. The value of room temperature (300 K) MR decreases from 1.36% to 0.10% when x increases from 0 to 0.20. This slight decrease in high temperature (room temperature) MR is because of vanishing the tunneling phenomena above Tc . As, Hwang [3] proposed that tunneling phenomena occurs only at T < Tc . The magnetic field dependence MR for all the samples measured in field (0–12 kOe) at 80 K is shown in Fig. 6. Generally, there are two aspects that influence the MR by applied field. One has to do with ferromagnetic domain wall movement of LCMO as in all ferromagnets, and other has to do with grain boundaries of LCMO, which contribute substantially to the LFMR in the temperature T < Tc . Analysis of Fig. 6 shows that with increase in the magnetic field from 0 to12 kOe, the value of MR for low Co3 O4 doping (x = 0.05 and 0.10) is larger than
Fig. 6. Magnetic field dependence of magnetoresistance (MR) in magnetic field (0–12 kOe) at 80 K for (1 − x)La0.67 Ca0.33 MnO3 /xCo3 O4 combined samples.
pure LCMO (x = 0.0) and smaller for x > 0.10. It suggests that magnetic field sensitive MR can be enhanced with Co3 O4 as a secondary phase impurity at low concentration only (up to x = 0.10 in the present case). The MR increases sharply at low fields and saturate for the higher value of fields. Moreover, the percent change in MR for the composites with x = 0.05 and 0.10 with respect to pure LCMO is more at low fields (up to 3 kOe). The percentage change in MR with respect to pure LCMO at 80 K is ∼13% and ∼35% at 3 kOe while that is ∼6% and ∼12% at 12 kOe for the composites with x = 0.05 and 0.10, respectively. This indicates that large change in MR is produced at low fields and this LFMR is caused through spin disorder (as suggested by magnetization measurements) by the tunneling process at the grain boundaries and when a magnetic field is applied, the spin disorder is suppressed, resulting the high MR, especially at low field ∼3 kOe. 4. Conclusions In summary, we have studied the magnetic and magnetotransport properties of LCMO/Co3 O4 combined system. These studies show that Co3 O4 does not enter into the LCMO lattice and is found to remain at grain boundary regions and on the surfaces of LCMO grains without disturbing the stoichiometry of LCMO phase within the grain. A clear M–I transition is observed at 260 K in virgin LCMO sample while that vanishes in the LCMO/Co3 O4 combined samples in measured temperature range (80–300 K) and resistivity increases fairly as a consequence of Co3 O4 addition. A significant enhancement in MR is observed for the composites with x = 0.05 and 0.10 at temperatures below 125 K and at applied magnetic field H ∼ 3 kOe, which is encouraging with regard to the potential application. The observed value of enhanced MR with respect to pure LCMO at 80 K is ∼13% and ∼35% for the composites with x = 0.05 and 0.10, respectively at applied magnetic field H ∼ 3 kOe. It is argued that this enhanced LFMR, at low temperature (<125 K) is related to spin polarized tunneling across the grain boundaries.
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Acknowledgements The authors are grateful to Prof. O.N. Srivastava, Prof. D. Pandey (B.H.U.) and Dr. H.K. Singh (N.P.L.) for helpful discussions and encouragement. One of the authors (AG) is grateful to the University Grant Commission (UGC), New Delhi for the award of Senior Research Fellowship. References [1] R. von Helmolt, J. Wecker, B. Holzapfel, L. Schultz, K. Samwer, Phys. Rev. Lett. 71 (1993) 2331. [2] C.N.R. Rao, R. Mahesh, A.K. Raychaudhuri, R. Mahendiran, J. Phys. Chem. Solids 59 (1998) 487. [3] H.Y. Hwang, S.W. Cheong, N.P. Ong, B. Batlogg, Phys. Rev. Lett. 77 (1996) 2041. [4] C. Zener, Phys. Rev. 82 (1951) 403. [5] A. Gupta, J.Z. Sun, J. Magn. Magn. Mater. 200 (1999) 24. [6] S.P. Issac, N.D. Mathur, J.E. Evetts, M.G. Blamire, Appl. Phys. Lett. 72 (1998) 2038.
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[7] S. Jin, T.H. Tiefel, M. McCormack, H.M. O’Bryan, L.H. Chen, R. Ramesh, D. Schurig, Appl. Phys. Lett. 67 (1995) 557. [8] L.I. Balcells, A.E. Carrillo, B. Martinez, J. Fontcuberta, Appl. Phys. Lett. 74 (1999) 4014. [9] D.K. Petrov, L. Krusin-Elbaum, J.Z. Sun, C. Field, P.R. Duncombe, Appl. Phys. Lett. 75 (1999) 995. [10] S. Gupta, R. Ranjit, C. Mitra, P. Raychaudhury, R. Pinto, Appl. Phys. Lett. 78 (2001) 362. [11] Anurag Gaur, G.D. Varma, Solid State Commun. 139 (2006) 310. [12] Z.C. Xia, S.L. Yuan, W. Feng, L.J. Zhang, G.H. Zhang, J. Tang, L. Liu, S. Liu, G. Peng, D.W. Niu, L. Chen, Q.H. Zheng, Z.H. Fang, C.Q. Tang, Solid State Commun. 128 (2003) 291. [13] S.N. Sadakale, R.J. Choudhary, M.S. Sahasrabhdhe, A.G. Banpurkar, K.P. Adhi, S.I. Patil, S.K. Date, J. Magn. Magn. Mater. 286 (2005) 450. [14] J. Mark Rubinstein, J. Appl. Phys. 87 (2000) 5019. [15] A. de Andres, M. Garcia-Hernandez, J.L. Martinez, C. Prieto, Appl. Phys. Lett. 74 (1999) 3884. [16] C.H. Yan, Y.H. Huang, X. Chen, C.S. Liao, Z.M. Wang, J. Phys.: Condens. Matter 14 (2002) 9607.
Low field magnetoresistance in La0.67Ca0.33MnO3 and Co3O4 combined system Anurag Gaur, G.D. Varma ∗ Department of Physics, Indian Institute of Technology Roorkee, Roorkee 247667, India Received 13 September 2006; received in revised form 1 November 2006; accepted 17 November 2006 Available online 21 December 2006
Abstract We have studied the magneto-transport properties of the (1 − x) La0.67 Ca0.33 MnO3 /xCo3 O4 (x ∼ 0, 0.05, 0.10, 0.15 and 0.20) combined samples, prepared via sol–gel process. The powder X-ray diffraction patterns indicate no evidence of reaction between the La0.67 Ca0.33 MnO3 and Co3 O4 . A clear metal to insulator (M–I) transition is observed at 260 K in virgin LCMO sample while that vanishes in the LCMO/Co3 O4 combined samples in the measured temperature range (80–300 K). Resistivity of the combined samples increases sharply as a consequence of Co3 O4 addition, especially on the low temperatures, and magnetization decreases. An enhancement in low field magnetoresistance (LFMR) is observed at temperatures below 125 K for the composites with lower Co3 O4 content, namely, x = 0.05 and 0.10. It is argued that this improved LFMR at low temperature (T < 125 K) arises due to the enhanced spin-polarized tunneling at the grain boundaries in the combined samples. © 2006 Published by Elsevier B.V. PACS: 73.43.Qt; 71.30.+h; 75.47.Lx Keywords: Perovskite manganites; Low-field magnetoresistance; Electrical transport; Grain boundary
1. Introduction Doped rare earth manganites of type R1−x Ax MnO3 (R is a rare earth such as La, Nd, Y, etc.; A is alkaline earth such as Ca, Ba, Sr, etc.), well known for their colossal magnetoresistance (CMR) properties, have been studied extensively in the last decade [1,2]. So far, two CMR effects have been found in these manganites, that is, the intrinsic CMR and the extrinsic CMR. For most of R1−x Ax MnO3 manganites, the CMR effect is maximum near metal–insulator (M–I) transition temperature (Tp ) accompanied by a simultaneous paramagnetic to ferromagnetic (PM–FM) transition at the Curie temperature (Tc ). This is so called intrinsic CMR [3]. The intrinsic CMR, caused by double exchange (DE) mechanism proposed by Zener in 1951 [4], is useful to explain CMR phenomena mostly observed near Tc at a relatively high magnetic field. However the extrinsic CMR, which is absent in the single crystal, is related to natural and artificial grain boundaries [3,5,6] and atomic size defects at
∗
Corresponding author. Tel.: +91 1332 285353; fax: +91 1332 273560. E-mail address: [email protected] (G.D. Varma).
0925-8388/$ – see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.jallcom.2006.11.132
the film–substrate interface [7]. Spin polarized tunneling [3] or spin dependent scattering [5] among neighbouring grains seems to be responsible for this kind of CMR effects. This extrinsic effect may enhance low field magnetoresistance (LFMR) in a wide temperature range and can be more useful for practical applications to magnetic switching of recording devices. In order to reduce the required magnetic fields and widen temperature ranges of the CMR effects, many groups have attempted to synthesize CMR-insulators composites. Very recently, enhanced MR properties have been observed in some ferromagnet/insulator type two phase manganites-based composites, such as LSMO/CeO2 [8], LCMO/STO [9], LSMO/glass [10], LSMO/NiO [11], LBMO/YSZ [12] and bilayers of LCMO with Fe3 O4 [13]. In this paper, we attempt to introduce the insulating Co3 O4 as the second phase into the manganites and investigate the magneto-transport properties in La0.67 Ca0.33 MnO3 /Cobalt oxide: (LCMO)1−x (Co3 O4 )x combined system. Cobalt oxide is a very suitable candidate as second phase material because it is a nice insulator, thus it can be served as the transport barrier in the manganites matrix and can modify the LCMO grain surfaces to adjust the tunneling effect and hence the magnetoresistance.
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The incorporation of impurity phase (Co3 O4 ) increases zero field resistivity of LCMO/Co3 O4 combined samples and metalinsulator transition disappears. This means, the combined samples show the insulating/semiconducting behaviour in whole temperature range (80–300 K). However, at low Co3 O4 concentrations, an enhanced low field magnetoresistance has been observed in this LCMO/Co3 O4 combined system. 2. Experimental procedure The La0.67 Ca0.33 MnO3 (LCMO) samples were prepared via sol–gel method. The aqueous solution of high purity nitrates of La, Ca and Mn have been taken in the desired stoichiometric proportions. An equal amount of ethylene glycol has been added to this solution with continuous stirring. This solution is then heated on a hot plate at temperature of ∼80–100 ◦ C till a dry thick brown sol is formed. This has been further decomposed in an oven at a temperature of 250 ◦ C to get the dry fluffy material. This obtained polymeric precursor is calcined at 600 ◦ C for 12 h. The calcined mixture was reground and sintered at 1000 ◦ C for 12 h. The obtained LCMO powders with single phase perovskite structure were completely mixed with a commercial Co3 O4 powder according to the nominal ratio of (LCMO)1−x /(Co3 O4 )x with x = 0, 0.05, 0.10, 0.15 and 0.20. The resulting (LCMO)1−x /(Co3 O4 )x powders were pelletized at a pressure of 5 MPa cm−2 and then sintered at 1000 ◦ C for 2 h. The low sintering temperature for small duration was chosen to avoid inter-diffusion of LCMO and Co3 O4 . By this procedure, the Co3 O4 introduced as second phase of the combined system mainly segregated at the grain boundaries and on the surfaces of the LCMO phase. The structural characterization was examined by employing X-ray diffraction (Bruker AXS D-8 advance, Cu K␣ radiation) technique at room temperature and surface morphology was investigated by scanning electron microscope (SEM Model LEO 435-VP operating at 15 kV). Resistivity as a function of temperature was measured by a standard four-probe method using Keithley instruments without or with magnetic fields (0–3 kOe). The dc magnetization measurements were done by using vibrating sample magnetometer (VSM Model 155, Princeton Applied Research).
3. Results and discussion Fig. 1 shows the room temperature XRD patterns of LCMO/Co3 O4 combined samples. The XRD spectra of the combined samples show two different sets of XRD peaks, corresponding to pseudo cubic perovskite LCMO and cubic spinel Co3 O4 structure. No extra phase is obtained indicating that the reactions between LCMO and Co3 O4 are negligible. Moreover, with increasing Co3 O4 concentrations, the most intense Co3 O4 peak (2θ = 36.82◦ ) increases almost linearly in intensity
Fig. 1. X-ray diffraction patterns of (1 − x)La0.67 Ca0.33 MnO3 /xCo3 O4 combined samples. Inset shows the intensity vs. x plot of most intense Co3 O4 peak.
as shown in the inset of Fig. 1. This also supports the above fact that there is negligible reaction between LCMO and Co3 O4 and almost all Co3 O4 introduced as second phase of the composite mainly segregates at the grain boundaries and on the surfaces of the LCMO grains. The evidence of the coexistence of two phases also comes from SEM micrographs, which clearly indicate the two different types of crystallites. The SEM micrographs of LCMO/Co3 O4 combined samples with x = 0 and 0.10 are shown in Fig. 2(a) and (b), respectively. A clear grain boundary is observed in pure LCMO as shown in Fig. 2(a). However, the grain boundaries of LCMO/Co3 O4 combined samples become ambiguous by the addition of Co3 O4 as shown in Fig. 2(b). From the SEM image (Fig. 2(b)), it is also clear that the doped Co3 O4 segregated mainly at the grain boundaries and on the surfaces of LCMO grains. In Fig. 2(b), the LCMO and Co3 O4 grains are indicated by white and black arrows, respectively, to distinguish them. The temperature dependence resistivity for the LCMO/ Co3 O4 combined samples measured by four probe technique at zero field is shown in Fig. 3. As expected, the metal like con-
Fig. 2. Scanning electron micrographs of (1 − x)La0.67 Ca0.33 MnO3 /xCo3 O4 combined samples: (a) x = 0 and (b) x = 0.10. LCMO and Co3 O4 grains are indicated by white and black arrows, respectively.
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425
Fig. 3. Temperature dependence of resistivity at zero field for (1 − x)La0.67 Ca0.33 MnO3 /xCo3 O4 combined samples.
ductivity is found in pure LCMO while with increasing Co3 O4 concentration x, the zero field resistivity of LCMO/Co3 O4 combined samples increases within one or two orders of magnitude, particularly at lower temperatures, when x varies from 0 to 0.20. It can be seen from Fig. 3 that the value of resistivity at 80 K increases from 0.03 to 853 cm when x increases from x = 0 to 0.20. Meanwhile, the virgin LCMO (x = 0) shows the clear metal–insulator transition at a temperature (TP ) 260 K while the transition disappears in combined samples and they show insulating/semiconducting behaviour in the whole measured temperature range (80–300 K). The increase of resistivity and disappearance of M–I transition can be qualitatively explained by enhanced grain boundaries. Since Co3 O4 behaves as a paramagnetic insulator above 40 K, the LCMO/Co3 O4 samples are similar to the ferromagnet/insulator type composites. Large number of interfaces and boundaries between LCMO and Co3 O4 grains may act as additional barriers. This causes the increase in the carrier scattering leading to enhancement in the resistivity. The metallic transition is suppressed because of Co3 O4 induced disorders and also by the increase in the insulating Co3 O4 phase fraction in the combined system. Moreover, in pure LCMO, the electrical transport is achieved through a direct contact between LCMO grains. However, in combined samples, there are two kinds of conduction channels parallel connected to each other [14,15]. One is related to LCMO grains, which determines the transport properties of the system through direct contact between LCMO grains and other is related to Co3 O4 . The temperature dependence of magnetization measured at 5 kOe is shown in Fig. 4. The value of magnetization (M) at 80 K are 78.3, 70.12, 65.3, 60.85 and 58.1 emu/gm for x = 0.0, 0.05, 0.10, 0.15 and 0.20, respectively. These successive decreases in M with increasing Co3 O4 concentrations are due to decrease in the volume fraction of ferromagnetic LCMO phase and extra magnetic disorder caused by Co3 O4 in these combined samples. The reduction in value of magnetization for the combined samples are more as calculated by taking in account the LCMO weight fraction in the composites, which suggests the extra magnetic disorder by Co3 O4 in the combined samples. All the combined samples have almost similar behaviour of magnetization as a function of temperature and there is a
Fig. 4. Temperature dependence of magnetization at 5 kOe for (1 − x) La0.67 Ca0.33 MnO3 /xCo3 O4 combined samples.
small shift in paramagnetic (PM) to ferromagnetic (FM) phase transition temperature (Tc ), which moves towards lower value with increase in the Co3 O4 content. The transition temperatures determined from the peak in dM/dT–T curves are found to be ∼289, 281, 279, 275 and 270 K for the samples with x = 0.0, 0.05, 0.10, 0.15 and 0.20, respectively. As observed by XRD and SEM, Co3 O4 segregates into the grain boundaries and surface of LCMO grains and this grain boundary segregation of Co3 O4 affects the double exchange (DE) mechanism and this is more pronounced in the FM domains walls [16]. This consequently leads to the suppression of PM–FM transition temperature towards the lower value with increase in Co3 O4 concentration. As reduction in the magnetization also suggests the suppression of DE mechanism resulting suppression of the ferromagnetic alignment of Mn ions. This indicates the magnetic spin disorder induced by the grain boundaries in the combined system. It is also noticeable that there is a large difference between the value of metal–insulator transition temperature (TP ) and paramagnetic–ferromagnetic transition temperature (Tc ) for the combined samples. The value of TP in virgin sample (x = 0) is 260 K while it comes below 80 K in the combined samples and they show insulating/semiconducting behaviour (see Fig. 3) in whole measured temperature range (80–300 K). On the other hand, Tc decreases only marginally (289–270 K) when x increases from 0.0 to 0.20 in the combined samples. This difference is due to the fact that Tc is an intrinsic property and does not show much dependence on grain boundaries while TP is an extrinsic property that strongly depends upon the grain boundaries [15]. The temperature dependence of magnetoresistance (MR) in a field of 3 kOe for all the measured samples is shown in Fig. 5. The MR ratio is defined as MR (%) = [ρ(0,T) − ρ(H,T)]/ρ(H,T) × 100%, where ρ(0,T) and ρ(H,T) are the resistivity values for zero and applied fields, respectively. We find that LCMO/Co3 O4 combined samples having low Co3 O4 concentration, viz., x ∼ 0.05 and 0.10 exhibit a distinct enhanced MR compared with parent LCMO in low temperature region (T < 125 K). We plot the x dependence of MR in the inset
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Fig. 5. Temperature dependence of magnetoresistance (MR) in a field of 3 kOe of (1 − x)La0.67 Ca0.33 MnO3 /xCo3 O4 combined samples. Inset shows the MR vs. x curve.
of Fig. 5. The maximum MR ratio at 80 K is ∼20.77% and ∼24.68% for the combined samples with x = 0.05 and 0.10, respectively while ∼18.36% for pure LCMO at 3 kOe. So, the observed enhancement in MR at 80 K with respect to pure LCMO is ∼13% and ∼35% for the composites with x = 0.05 and 0.10, respectively, at 3 kOe. This increase in MR at low temperature seems to be due to enhanced tunneling across LCMO grain boundaries and additional grain boundary effects introduced by Co3 O4 grains. As proposed by Hwang et al. [3], the spin polarized tunneling between neighbouring grains of manganites is responsible for the magnetoresistance effect in polycrystalline samples. This tunneling takes place across the grain boundaries or interfaces. As Fig. 5 indicates, we get the enhancement in MR of the combined samples with x = 0.05 and 0.10 only at temperatures below 125 K which suggests that this enhanced MR basically comes from grain boundary effects in our samples because it is well known that LFMR is mainly attributed to a grain boundary effect especially at temperature T < Tc [3]. However, with further increase in Co3 O4 level from x = 0.10, the MR decreases with respect to pure LCMO even at low temperature which may result because the grain boundaries become too thick for electron tunneling. However, the MR near room temperature slightly decreases in all LCMO/Co3 O4 combined samples. The value of room temperature (300 K) MR decreases from 1.36% to 0.10% when x increases from 0 to 0.20. This slight decrease in high temperature (room temperature) MR is because of vanishing the tunneling phenomena above Tc . As, Hwang [3] proposed that tunneling phenomena occurs only at T < Tc . The magnetic field dependence MR for all the samples measured in field (0–12 kOe) at 80 K is shown in Fig. 6. Generally, there are two aspects that influence the MR by applied field. One has to do with ferromagnetic domain wall movement of LCMO as in all ferromagnets, and other has to do with grain boundaries of LCMO, which contribute substantially to the LFMR in the temperature T < Tc . Analysis of Fig. 6 shows that with increase in the magnetic field from 0 to12 kOe, the value of MR for low Co3 O4 doping (x = 0.05 and 0.10) is larger than
Fig. 6. Magnetic field dependence of magnetoresistance (MR) in magnetic field (0–12 kOe) at 80 K for (1 − x)La0.67 Ca0.33 MnO3 /xCo3 O4 combined samples.
pure LCMO (x = 0.0) and smaller for x > 0.10. It suggests that magnetic field sensitive MR can be enhanced with Co3 O4 as a secondary phase impurity at low concentration only (up to x = 0.10 in the present case). The MR increases sharply at low fields and saturate for the higher value of fields. Moreover, the percent change in MR for the composites with x = 0.05 and 0.10 with respect to pure LCMO is more at low fields (up to 3 kOe). The percentage change in MR with respect to pure LCMO at 80 K is ∼13% and ∼35% at 3 kOe while that is ∼6% and ∼12% at 12 kOe for the composites with x = 0.05 and 0.10, respectively. This indicates that large change in MR is produced at low fields and this LFMR is caused through spin disorder (as suggested by magnetization measurements) by the tunneling process at the grain boundaries and when a magnetic field is applied, the spin disorder is suppressed, resulting the high MR, especially at low field ∼3 kOe. 4. Conclusions In summary, we have studied the magnetic and magnetotransport properties of LCMO/Co3 O4 combined system. These studies show that Co3 O4 does not enter into the LCMO lattice and is found to remain at grain boundary regions and on the surfaces of LCMO grains without disturbing the stoichiometry of LCMO phase within the grain. A clear M–I transition is observed at 260 K in virgin LCMO sample while that vanishes in the LCMO/Co3 O4 combined samples in measured temperature range (80–300 K) and resistivity increases fairly as a consequence of Co3 O4 addition. A significant enhancement in MR is observed for the composites with x = 0.05 and 0.10 at temperatures below 125 K and at applied magnetic field H ∼ 3 kOe, which is encouraging with regard to the potential application. The observed value of enhanced MR with respect to pure LCMO at 80 K is ∼13% and ∼35% for the composites with x = 0.05 and 0.10, respectively at applied magnetic field H ∼ 3 kOe. It is argued that this enhanced LFMR, at low temperature (<125 K) is related to spin polarized tunneling across the grain boundaries.
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