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Current-induced effect on resistivity and magnetoresistance of La0.67Ba0.33MnO3 manganite
ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 320 (2008) 2741– 2745
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Current-induced effect on resistivity and magnetoresistance of La0.67Ba0.33MnO3 manganite Rajesh Kumar a,b, Ajai K. Gupta d, D.P. Singh c, Vijay Kumar a, G.L. Bhalla b, Neeraj Khare d, a
National Physical Laboratory, Dr. K.S. Krishnan Road, New Delhi 110012, India Department of Physics and Astrophysics, University of Delhi, Delhi 110007, India School of Physics and Material Science, Thaper University, Patiala 147004, India d Department of Physics, Indian Institute of Technology Delhi, New Delhi 110016, India b c
a r t i c l e in fo
abstract
Article history: Received 24 January 2008 Received in revised form 26 April 2008 Available online 10 June 2008
The influence of dc biasing current on temperature dependence of resistivity and low-field magnetoresistance (MR) of La0.67Ba0.33MnO3 bulk sample is reported. A prominent finding is the change in resistivity around the insulator-to-metal transition temperature (TIM) and the change in MR around the ferromagnetic transition temperature (TC). The decrease in MR around TC at higher biasing current indicates a strong interaction between carrier spin and spin of Mn ions resulting in a higher alignment of Mn ion spins. Change in resistivity around TIM is interpreted in the framework of percolative conduction model based on the mixed phase of itinerant electrons and localized magnetic polarons. & 2008 Published by Elsevier B.V.
Keywords: Magnetic field effect Metal–insulator transitions Percolation phenomena Magnetoresistance La0.67Ba0.33MnO3
1. Introduction Mixed valence manganese oxides with general formula R1xAxMnO3 [where R ¼ La, Nd, Pr; and A ¼ Ca, Ba, Sr, Pd] have drawn considerable attention due to rich varieties of phenomena such as colossal magnetoresistance (CMR), ferromagnetism with metallic conduction, and charge/orbital ordering [1–3]. The CMR phenomena in these systems are generally understood in terms of the double-exchange mechanism [4] combining with the local John–Teller distortions of Mn3+ ions [5]. In single crystals and epitaxial films of R1xAxMnO3 manganites CMR effect has been observed at temperatures around ferromagnetic (FM) transition temperature, whereas bulk samples and polycrystalline films show large CMR effect in relatively low magnetic field and even at temperature much lower than TC [6,7]. The observed low-field magnetoresistance (MR) at temperatures below ferromagnetic transition temperature TC has been ascribed to the spin-polarized tunneling of charge carriers across the grain boundaries [6,8]. In addition to CMR effect in the doped rare earth manganites, the closeness of free energies of various competing electronic, magnetic and orbital states causes multiphase coexistence in these systems [1,3]. Various external perturbations such as electric field (current) [9–11], magnetic field [12] and optical radiation [13] have driven the system from insulating state to conducting state. More Corresponding author. Tel./Fax: +9111 26591352.
E-mail address: [email protected] (N. Khare). 0304-8853/$ - see front matter & 2008 Published by Elsevier B.V. doi:10.1016/j.jmmm.2008.06.004
recently, there has been lot of interest in investigating the influence of electric field/current to the transport in CMR materials [14–19]. An applied dc current could lead to a transition from the insulating charge-ordered state to a FM metallic state [15]. A large resistance change by an external electric field observed in Pr1xCaxMnO3 has triggered a surge of research due to its potential for practical applications such as nonvolatile memory elements [20,21]. Lu. et al. [22] observed the electroresistance (ER) in electron-doped manganite. Gao et al. [18] reported that the resistance of La0.7Ca0.3MnO3 and La0.85Ba0.15MnO3 epitaxial thin films could be depressed significantly by a dc current, without an applied magnetic field. Most of the studies for the current-induced (ER) [14,18,19,22] have been performed on the epitaxial thin films of rare earth-doped manganites. In this paper, a study of current-induced effect on resistivity and low-field MR of a polycrystalline La0.67Ba0.33MnO3 bulk sample is reported. The application of higher biasing current reduces the resistivity of the sample around metal-to-insulator transition temperature at one hand and decreases the low-field MR around FM transition temperature at the other. A percolation model based on the mixed phase consisting of itinerant electrons and localized magnetic polarons has been proposed to explain the observed results.
2. Experimental The bulk sample of La0.67Ba0.33MnO3 manganite was prepared by a standard solid-state reaction method. Stoichiometric
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R. Kumar et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 2741–2745
amounts of La2O3, BaCO3 and Mn O2 were taken, uniformly mixed and ground thoroughly using mortar and pestle. The mixture was calcined at 950 1C for 92 h with intermediate grinding. The resultant mixture was reground and pellets were prepared and sintered at 1300 1C for 24 h followed by a slow cooling to room temperature. X-ray diffraction studies have been done for structural characterization of the sample. The X-ray diffraction studies revealed that the sample is single-phase, cubic crystal structure with lattice parameter a ¼ 3.912 A˚. Surface morphology of the samples was determined by scanning electron microscopy studies. From the scanning electron micrograph of the sample, granular features were observed with average grain size 1–2 mm. Fourprobe technique was used for the measurement of temperature and current dependence of resistivity in the temperature range from 77 to 300 K with different biasing currents. A dc current source was used for studying the biasing current dependence of resistivity of the samples. MR of the sample was measured by applying the magnetic field to the sample by using an electromagnet. The MR ratio has been calculated using the relation ½Rð0Þ RðHÞ MR ¼ 100% (1) Rð0Þ where R(H) and R(0) were the resistance of the sample in the presence and in the absence of magnetic field, respectively. AC susceptibility studies have been done for finding the FM transition temperature of the sample.
3. Results and discussion Fig. 1 shows the temperature dependence of ac susceptibility of La0.67Ba0.33MnO3 (LBMO) sample in the temperature range from 77 to 300 K. The FM transition temperature (TC) was observed at 266 K. The variation of resistivity (r) of La0.67Ba0.33MnO3 (LBMO) bulk sample with temperature for 1 mA biasing current is shown in Fig. 2(a) both in the presence and in the absence of 1.5 kOe magnetic field. The insulator-to-metal transition temperature (TIM) occurred at 200 K, which separated its metallic and insulating behaviour. Fig. 2(b) shows the temperature dependence of resistivity of LBMO sample for 50 mA biasing current in the absence and in the presence of 1.5 kOe magnetic field. It is observed that the resistivity curves with and without magnetic field almost overlap for temperatures higher than TIM and are well separated for temperature lower than TIM. Fig. 2(c) shows
Fig. 2. (a) Variation of resistivity of La0.67Ba0.33MnO3 with temperature for 1 mA biasing current at zero and 1.5 kOe magnetic field. (b) Variation of resistivity of La0.67Ba0.33MnO3 with temperature for 50 mA biasing current at zero and at 1.5 kOe magnetic field. (c) Variation of resistivity of La0.67Ba0.33MnO3 with temperature for 1 and 50 mA biasing currents. Fig. 1. Variation of susceptibility with temperature for La0.67Ba0.33MnO3.
ARTICLE IN PRESS R. Kumar et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 2741–2745
comparison of r– T curve for 1 and 50 mA biasing currents. The application of higher biasing current resulted in the suppression of resistivity of the sample around TMI. For temperatures much higher and much lower to TMI, almost no change in resistivity was observed. Fig. 3 shows the temperature variation of MR of LBMO sample for biasing currents of 1 and 50 mA for the applied magnetic field of 1.5 kOe. The MR was decreasing with the increase in temperature. This decrease is almost linear with a small peak around 266 K (TC), for both the biasing currents. For the 50 mA biasing current, the MR value around the small peak was less as compared to 1 mA biasing current. It is evident from the plot that the effect of 50 mA biasing current on the MR was prominent only around 266 K(TC), and at low temperatures the value of MR was not affected on changing the biasing current. In the polycrystalline-doped rare earth manganites, MR at low temperature is attributed to the spin-polarized transport across grain boundaries [6,8], whereas MR at TC is the contribution from the grains and is attributed to Zener double exchange [23]. In the double-exchange mechanism, the hopping of electron from Mn3+ to Mn4+ depends upon spin alignment of Mn ions. The application of magnetic field aligns spins of the incomplete d orbital of the adjacent Mn ions, which results in the increase of the rate of hopping of electrons and therefore an increase in electrical conductivity occurs [23]. In the present case, higher biasing current has been found to affect MR around TC and not to the MR at lower temperatures. This indicates that biasing current does not affect the spin-polarized transport, but it affects the conduction of charge carriers within the grains. The decrease of MR at temperatures around TC suggests the increase of FM content on application higher biasing current in the system. In the doped rare earth manganites, the cause of the metal–insulator transition was ascribed to the competition between double-exchange mechanism and strong electron–phonon coupling mediated through Jahn–Teller effect [4,24–26]. At temperatures smaller to TMI, double-exchange mechanism dominates whereas at temperatures higher than TMI electron–phonon coupling dominates. Double-exchange mechanism considers only itinerant charge carriers in the FM state of manganite. The strong electron–phonon coupling localizes the charge carriers above TMI. Below TC such localized carriers becomes magnetic polarons due to magnetic interactions and its conduction occurs through the hopping process [27,28]. The increase of conductivity on increas-
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ing biasing current at temperatures around TMI suggests the increase of itinerant carriers in the system. In order to understand the effect of biasing current on the MR-T and r– T characteristics of polycrystalline La0.67Ba0.33MnO3 manganite, a two-phase model consisting of localized magnetic polarons and itinerant carriers is proposed. The electrical conduction occurs due to percolative transport in the mixture of itinerant electrons and localized magnetic polarons. In order to see the applicability of the percolative model, we analyzed the temperature dependence of resistivity of LBMO sample in the temperature range from 100 to 275 K. In the mixture of itinerant and localized charge carriers, the resistivity of the material can be thought as a parallel combination of resistivities due to coexisting localized and itinerant charge carriers [29,30] and is given as 1
rðTÞ
¼
f ðTÞ
rit ðTÞ
þ
1 f ðTÞ rht ðTÞ
(2)
where rit(T) is the itinerant carrier resistivity, rht(T) is the localized carrier, i.e. magnetic polaron resistivity and f(T) is the phase fraction of the itinerant charge carriers in the system. The itinerant carriers are mobile in metallic FM domains, whereas the localized magnetic polarons are mobile in FM insulating domains at temperatures below TC and in the paramagnetic insulating domains at temperatures above TC. The resistivity due to itinerant charge carriers in FM metallic domains is given as [31]
rit ðTÞ ¼ a þ bT 2 þ cT 4:5
(3)
where the first term a represents the contribution to the resistivity due to impurities (defects, oxygen vacancies, etc.), the second term (T2) is the contribution of electron–electron scattering to the resistivity and the third term (T9/2) represents the contribution to the resistivity originated due to electron–magnon scattering. The electron–electron scattering is independent of magnetic field in Fermi-Liquid model, whereas the electron–magnon scattering is magnetic field dependent. The coefficient c is related to spin-stiffness coefficient D* as [32] !9=2 R_ ð6p2 nÞ5=3 R2 K B c¼ Dn 482 p7 e2 S2 ð0:52=3 n2=3 Þ9=2 ! n D (4) 2:52 þ 0:0017 2 n R t where R is the hopping distance of eg electrons, S is the effective spin of Mn ion, n is the hole concentration per unit cell, t* is the effective hopping integral, D* is the average spin stiffness coefficient, _( ¼ h/2p) is the Plank’s constant and KB is the Boltzman constant. The spin stiffness coefficient, D* is given by [33] Dn ¼
Fig. 3. Temperature dependence of magnetoresistance (MR) of La0.67Ba0.33MnO3 for 1 and 50 mA biasing current.
1 ZJS 3
(5)
where Z is the number of nearest-neighbour spins and J is the exchange interaction. The effect of higher biasing current is to increase the exchange interaction J and therefore the spinstiffness constant (D*) increases with the higher biasing current. The higher biasing current results in the decrease of the coefficient of electron–magnon scattering term, which in turn reduces the resistivity. The resistivity due to localized magnetic polarons in insulating domains is given as [27,28] E (6) rht ðTÞ ¼ r0 T exp A KBT where r0 is a constant and EA is the polaron activation energy.
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The phase fraction of itinerant carriers [f(T)] depends on temperature and can be given as [34] f ðTÞ ¼
1 1 þ exp½U 0 ð1 ðT=KÞÞ=K B T
(7)
where U0 and k are constants whose values determine the relative fraction of charge carriers present at a temperature T in the system. Substituting the values of rit(T) and rht(T) from Eqs. (3) and (6), we obtain the expression for temperature dependence of resistivity as 1
rðTÞ
¼
f ðTÞ a þ bT 2 þ cT 4:5
þ
1 f ðTÞ
r0 T expðEA =K B TÞ
(8)
Here 1f(T) represents the phase fraction of localized magnetic polarons. Fig. 4(a) and (b) show the simulated rT curves using Eq. (8) along with the experimentally observed rT curves for 1 and 50 mA biasing currents, respectively. The parameters for the best fit are given in Table 1. The closeness of simulated theoretical curve with the experimentally observed temperature dependence of resistivity curve for 1 and 50 mA biasing currents shows the applicability of the two-phase model. The effect of biasing current is reflected on the value of coefficient ‘c’ of electron–magnon scattering term (cT 9/2) as well as phase fraction of the carriers. The decrease in the value of c indicates the enhancement of the spin stiffness coefficient D* with the application of higher biasing current. It means that higher biasing current aligns the magnetic spins in the region around the path of the current, thereby increases the FM exchange interaction. The increase of FM exchange interaction converts some localized magnetic polarons into itinerant charge carriers and so conduction occurs via percolation in the mixture of itinerant and localized magnetic polarons. Fig. 4(c) shows the temperature dependence of simulated resistivity curves of La0.67Ba0.33MnO3 for the two biasing current values (1 and 50 mA). Solid line is for 1 mA biasing current and dotted line is for higher biasing current. The simulated curve for smaller value of ‘c’ shows the suppression of resistivity at TMI, while keeping the resistivity at the ends unchanged. It means that the application of higher biasing current changes the resistivity of the sample around the TMI. At temperatures much lower than TMI the metallic domains are already connected and at temperatures much higher than TMI there is not much metallic content to connect each other. So the effect of higher biasing current will be prominent only around TMI in decreasing the resistivity and it will not create any difference for temperatures much lower to TMI or for temperatures much higher to TMI. We have also calculated the temperature dependence of phase fraction of itinerant carriers using the best fit parameters U0 and k given in Table 1. Fig. 5 shows the temperature dependence of the phase fraction of the itinerant carriers at two biasing currents. It shows that the change in phase fraction (f) on application of biasing current is more pronounced at higher temperatures. The inset of Fig. 5 shows the temperature dependence of change of phase fraction (Df) of itinerant carriers on application of higher biasing current. The change of phase fraction of itinerant carriers on application of higher biasing current is maximum around the TMI which leads to the suppression of resistivity of LBMO sample as observed by us experimentally. The higher biasing current also strengthens the FM exchange interaction and thus increases the FM content in the system. This results in smaller value of MR around TC in the higher biased LBMO sample.
Fig. 4. (a) Variation of resistivity of La0.67Ba0.33MnO3 with temperature: circles represent the experimental data for 1 mA biasing current and the solid line represents the simulated curve. (b) Variation of resistivity of La0.67Ba0.33MnO3 with temperature: circles represent the experimental data for 50 mA biasing current and the solid line represents the simulated curve. (c) Temperature dependence of simulated resistivity of La0.67Ba0.33MnO3. Dotted line is for 1 mA biasing current and solid line is for 50 mA biasing current.
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Table 1 Values of fitting parameters a, b, c, U0, k, ro and EA for different biasing currents obtained from fitting of experimental r– T curves for LBMO sample Biasing current (mA)
1 50
Percolation model (1/r(T)) ¼ (f(T)/a+bT2+cT4.5)+(1f(T)/r0T exp(EA/KBT)) a (O cm)
b (O cm K2)
c (O cm K4.5)
U0 (meV)
k (K)
r0 (O cm K1)
EA (meV)
0.14314 0.14314
7.87 106 7.87 106
1.54 1012 6.96 1013
84.74 86.09
266 267
0.00003 0.00003
68.17 67.16
New Delhi, India. Rajesh Kumar and Ajai K. Gupta are thankful to CSIR, New Delhi, India for the award of Senior Research Fellowship and Research Associateship, respectively. References
Fig. 5. Temperature dependence of the phase fraction (f) of the itinerant carriers at two biasing currents 1 and 50 mA. Inset of the figure shows the temperature dependence of the change of phase fraction (Df) of the itinerant carriers on application of higher biasing current.
4. Conclusion Current-induced changes in the resistivity and MR of bulk sample of the La0.67Ba0.33MnO3 manganite were studied. The higher biasing current reduces the resistivity around TIM and decreases the MR around TC. The application of higher biasing current strengthens the FM exchange interaction between Mn ions and thus increases the FM content in the system, which in turn results into the smaller value of MR around TC. The decrease in resistivity around TIM is explained in the framework of model based on the percolative transport in the mixed phases of conducting itinerant carriers and insulating localized magnetic polarons in the LBMO sample.
Acknowledgments The authors gratefully acknowledge the support and encouragement received from Prof. Vikram Kumar, Director, NPL,
[1] C.N.R. Rao, B. Raveau (Eds.), Colossal Magnetoresistance, Charge ordering and Related Properties of Manganese Oxides, World Scientific, Singapore, 1998. [2] A.M. Haghiri-Gosnet, J.P. Renard, J. Phys. D: Appl. Phys. 36 (2003) R127. [3] E. Dagotto, N. J. Phys. 7 (2005) 67. [4] C. Zener, Phys. Rev. 82 (1951) 403. [5] A.J. Millis, P.B. Littlewood, B.I. Shraiman, Phys. Rev. Lett. 74 (1995) 5144. [6] H.Y. Hwang, S.W. Cheong, N.P. Ong, B. Batlogg, Phys. Rev. Lett. 77 (1996) 2041. [7] X.L. Wang, S.X. Dou, H.K. Liu, M. Lonescu, B. Zeimetz, Appl. Phys. Lett. 73 (1998) 396. [8] N. Khare, U.P. Moharil, B. Singh, A.K. Gupta, Sci. Lett. 26 (2003) 214. [9] A. Asamitsu, Y. Tomioka, H. Kuwahara, Y. Tokura, Nature 388 (1997) 50. [10] A. Guha, A.K. Raychaudhuri, A.R. Raju, C.N.R. Rao, Phys. Rev. B 62 (2000) 5320. [11] P. Padhan, W. Prellier, C. Simon, R.C. Budhani, Phys. Rev. B 70 (2004) 134403. [12] H. Kuwahara, Y. Tomioka, A. Asamitsu, Y. Moritomo, Y. Tokura, Science 270 (1995) 961. [13] M. Fiebig, K. Miyano, Y. Tomioka, Y. Tokura, Science 280 (1998) 1925. [14] T. Wu, S.B. Ogale, J.E. Garrison, B. Nagaraj, A. Biswas, Z. Chen, R.L. Greene, R. Ramesh, T. Venkatesan, A.J. Millis, Phys. Rev. Lett. 86 (2001) 5998. [15] V. Ponnambalam, S. Parashar, A.R. Raju, C.N.R. Rao, Appl. Phys. Lett. 74 (1999) 206. [16] Y. Yuzhelevski, V. Markovich, V. Dikovsky, E. Rozenberg, G. Gorodetsky, G. Jung, D.A. Shulyatev, Y.M. Mukovskii, Phys. Rev. B 64 (2001) 224428. [17] V. Markovich, G. Jung, Y. Yuzhelevski, G. Gorodetsky, A. Szewczyk, M. Gutowska, D.A. Shulyatev, Y.M. Mukovskii, Phys. Rev. B 70 (2004) 064414. [18] J. Gao, S.Q. Shen, T.K. Li, J.R. Sun, Appl. Phys. Lett. 82 (2003) 4732. [19] F.X. Hu, J. Gao, Phys. Rev. B 69 (2004) 212413. [20] S.Q. Liu, N.J. Wu, A. Ignatiev, Appl. Phys. Lett. 76 (2000) 2749. [21] A. Baikalov, Y.Q. Wang, B. Shen, B. Lorenz, S. Tsui, Y.Y. Sun, Y.Y. Xue, C.W. Chu, Appl. Phys. Lett. 83 (2003) 957. [22] W.J. Lu, Y.P. Sun, B.C. Zhao, X.B. Zhu, W.H. Song, Solid State Commun. 137 (2006) 288. [23] J. Fontcuberta, Phys. World 35 (1999) 33. [24] H. Roder, J. Zang, A.R. Bishop, Phys. Rev. Lett. 76 (1996) 1356. [25] A.J. Millis, R. Mueller, I. Shraiman, Phys. Rev. B 54 (1996) 5405. [26] J.D. Lee, B.I. Min, Phys. Rev. B 55 (1997) 12454. [27] D. Emin, P.G. Le Comber, N.F. Mott (Eds.), Electronic Structure Properties of Amorphous Semiconductors, Academic Press, New York, 1973. [28] D. Emin, T. Holestein, Phys. Rev. B 13 (1976) 647. [29] M. Jaime, P. Lin, S.H. Chun, M.B. Salamon, P. Dorsey, M. Rubinstein, Phys. Rev. B 60 (1999) 1028. [30] M. Rubinstein, J. Appl. Phys. 87 (2000) 5019. [31] X. Liu, H. Zhu, Y. Zhang, Phys. Rev. B 65 (2001) 024412. [32] K. Kubo, N. Ohata, J. Phys. Soc. Jpn. 33 (1972) 21. [33] S.V. Vonsovsky, M.I. Katsnelson, Springer Series in Solid-State Sciences, vol. 73, Springer, Berlin, 1989. [34] G. Li, H.D. Zhou, S.J. Feng, X.J. Fan, X.G. Li, Z.D. Wang, J. Appl. Phys. 92 (2002) 1406.
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Current-induced effect on resistivity and magnetoresistance of La0.67Ba0.33MnO3 manganite Rajesh Kumar a,b, Ajai K. Gupta d, D.P. Singh c, Vijay Kumar a, G.L. Bhalla b, Neeraj Khare d, a
National Physical Laboratory, Dr. K.S. Krishnan Road, New Delhi 110012, India Department of Physics and Astrophysics, University of Delhi, Delhi 110007, India School of Physics and Material Science, Thaper University, Patiala 147004, India d Department of Physics, Indian Institute of Technology Delhi, New Delhi 110016, India b c
a r t i c l e in fo
abstract
Article history: Received 24 January 2008 Received in revised form 26 April 2008 Available online 10 June 2008
The influence of dc biasing current on temperature dependence of resistivity and low-field magnetoresistance (MR) of La0.67Ba0.33MnO3 bulk sample is reported. A prominent finding is the change in resistivity around the insulator-to-metal transition temperature (TIM) and the change in MR around the ferromagnetic transition temperature (TC). The decrease in MR around TC at higher biasing current indicates a strong interaction between carrier spin and spin of Mn ions resulting in a higher alignment of Mn ion spins. Change in resistivity around TIM is interpreted in the framework of percolative conduction model based on the mixed phase of itinerant electrons and localized magnetic polarons. & 2008 Published by Elsevier B.V.
Keywords: Magnetic field effect Metal–insulator transitions Percolation phenomena Magnetoresistance La0.67Ba0.33MnO3
1. Introduction Mixed valence manganese oxides with general formula R1xAxMnO3 [where R ¼ La, Nd, Pr; and A ¼ Ca, Ba, Sr, Pd] have drawn considerable attention due to rich varieties of phenomena such as colossal magnetoresistance (CMR), ferromagnetism with metallic conduction, and charge/orbital ordering [1–3]. The CMR phenomena in these systems are generally understood in terms of the double-exchange mechanism [4] combining with the local John–Teller distortions of Mn3+ ions [5]. In single crystals and epitaxial films of R1xAxMnO3 manganites CMR effect has been observed at temperatures around ferromagnetic (FM) transition temperature, whereas bulk samples and polycrystalline films show large CMR effect in relatively low magnetic field and even at temperature much lower than TC [6,7]. The observed low-field magnetoresistance (MR) at temperatures below ferromagnetic transition temperature TC has been ascribed to the spin-polarized tunneling of charge carriers across the grain boundaries [6,8]. In addition to CMR effect in the doped rare earth manganites, the closeness of free energies of various competing electronic, magnetic and orbital states causes multiphase coexistence in these systems [1,3]. Various external perturbations such as electric field (current) [9–11], magnetic field [12] and optical radiation [13] have driven the system from insulating state to conducting state. More Corresponding author. Tel./Fax: +9111 26591352.
E-mail address: [email protected] (N. Khare). 0304-8853/$ - see front matter & 2008 Published by Elsevier B.V. doi:10.1016/j.jmmm.2008.06.004
recently, there has been lot of interest in investigating the influence of electric field/current to the transport in CMR materials [14–19]. An applied dc current could lead to a transition from the insulating charge-ordered state to a FM metallic state [15]. A large resistance change by an external electric field observed in Pr1xCaxMnO3 has triggered a surge of research due to its potential for practical applications such as nonvolatile memory elements [20,21]. Lu. et al. [22] observed the electroresistance (ER) in electron-doped manganite. Gao et al. [18] reported that the resistance of La0.7Ca0.3MnO3 and La0.85Ba0.15MnO3 epitaxial thin films could be depressed significantly by a dc current, without an applied magnetic field. Most of the studies for the current-induced (ER) [14,18,19,22] have been performed on the epitaxial thin films of rare earth-doped manganites. In this paper, a study of current-induced effect on resistivity and low-field MR of a polycrystalline La0.67Ba0.33MnO3 bulk sample is reported. The application of higher biasing current reduces the resistivity of the sample around metal-to-insulator transition temperature at one hand and decreases the low-field MR around FM transition temperature at the other. A percolation model based on the mixed phase consisting of itinerant electrons and localized magnetic polarons has been proposed to explain the observed results.
2. Experimental The bulk sample of La0.67Ba0.33MnO3 manganite was prepared by a standard solid-state reaction method. Stoichiometric
ARTICLE IN PRESS 2742
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amounts of La2O3, BaCO3 and Mn O2 were taken, uniformly mixed and ground thoroughly using mortar and pestle. The mixture was calcined at 950 1C for 92 h with intermediate grinding. The resultant mixture was reground and pellets were prepared and sintered at 1300 1C for 24 h followed by a slow cooling to room temperature. X-ray diffraction studies have been done for structural characterization of the sample. The X-ray diffraction studies revealed that the sample is single-phase, cubic crystal structure with lattice parameter a ¼ 3.912 A˚. Surface morphology of the samples was determined by scanning electron microscopy studies. From the scanning electron micrograph of the sample, granular features were observed with average grain size 1–2 mm. Fourprobe technique was used for the measurement of temperature and current dependence of resistivity in the temperature range from 77 to 300 K with different biasing currents. A dc current source was used for studying the biasing current dependence of resistivity of the samples. MR of the sample was measured by applying the magnetic field to the sample by using an electromagnet. The MR ratio has been calculated using the relation ½Rð0Þ RðHÞ MR ¼ 100% (1) Rð0Þ where R(H) and R(0) were the resistance of the sample in the presence and in the absence of magnetic field, respectively. AC susceptibility studies have been done for finding the FM transition temperature of the sample.
3. Results and discussion Fig. 1 shows the temperature dependence of ac susceptibility of La0.67Ba0.33MnO3 (LBMO) sample in the temperature range from 77 to 300 K. The FM transition temperature (TC) was observed at 266 K. The variation of resistivity (r) of La0.67Ba0.33MnO3 (LBMO) bulk sample with temperature for 1 mA biasing current is shown in Fig. 2(a) both in the presence and in the absence of 1.5 kOe magnetic field. The insulator-to-metal transition temperature (TIM) occurred at 200 K, which separated its metallic and insulating behaviour. Fig. 2(b) shows the temperature dependence of resistivity of LBMO sample for 50 mA biasing current in the absence and in the presence of 1.5 kOe magnetic field. It is observed that the resistivity curves with and without magnetic field almost overlap for temperatures higher than TIM and are well separated for temperature lower than TIM. Fig. 2(c) shows
Fig. 2. (a) Variation of resistivity of La0.67Ba0.33MnO3 with temperature for 1 mA biasing current at zero and 1.5 kOe magnetic field. (b) Variation of resistivity of La0.67Ba0.33MnO3 with temperature for 50 mA biasing current at zero and at 1.5 kOe magnetic field. (c) Variation of resistivity of La0.67Ba0.33MnO3 with temperature for 1 and 50 mA biasing currents. Fig. 1. Variation of susceptibility with temperature for La0.67Ba0.33MnO3.
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comparison of r– T curve for 1 and 50 mA biasing currents. The application of higher biasing current resulted in the suppression of resistivity of the sample around TMI. For temperatures much higher and much lower to TMI, almost no change in resistivity was observed. Fig. 3 shows the temperature variation of MR of LBMO sample for biasing currents of 1 and 50 mA for the applied magnetic field of 1.5 kOe. The MR was decreasing with the increase in temperature. This decrease is almost linear with a small peak around 266 K (TC), for both the biasing currents. For the 50 mA biasing current, the MR value around the small peak was less as compared to 1 mA biasing current. It is evident from the plot that the effect of 50 mA biasing current on the MR was prominent only around 266 K(TC), and at low temperatures the value of MR was not affected on changing the biasing current. In the polycrystalline-doped rare earth manganites, MR at low temperature is attributed to the spin-polarized transport across grain boundaries [6,8], whereas MR at TC is the contribution from the grains and is attributed to Zener double exchange [23]. In the double-exchange mechanism, the hopping of electron from Mn3+ to Mn4+ depends upon spin alignment of Mn ions. The application of magnetic field aligns spins of the incomplete d orbital of the adjacent Mn ions, which results in the increase of the rate of hopping of electrons and therefore an increase in electrical conductivity occurs [23]. In the present case, higher biasing current has been found to affect MR around TC and not to the MR at lower temperatures. This indicates that biasing current does not affect the spin-polarized transport, but it affects the conduction of charge carriers within the grains. The decrease of MR at temperatures around TC suggests the increase of FM content on application higher biasing current in the system. In the doped rare earth manganites, the cause of the metal–insulator transition was ascribed to the competition between double-exchange mechanism and strong electron–phonon coupling mediated through Jahn–Teller effect [4,24–26]. At temperatures smaller to TMI, double-exchange mechanism dominates whereas at temperatures higher than TMI electron–phonon coupling dominates. Double-exchange mechanism considers only itinerant charge carriers in the FM state of manganite. The strong electron–phonon coupling localizes the charge carriers above TMI. Below TC such localized carriers becomes magnetic polarons due to magnetic interactions and its conduction occurs through the hopping process [27,28]. The increase of conductivity on increas-
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ing biasing current at temperatures around TMI suggests the increase of itinerant carriers in the system. In order to understand the effect of biasing current on the MR-T and r– T characteristics of polycrystalline La0.67Ba0.33MnO3 manganite, a two-phase model consisting of localized magnetic polarons and itinerant carriers is proposed. The electrical conduction occurs due to percolative transport in the mixture of itinerant electrons and localized magnetic polarons. In order to see the applicability of the percolative model, we analyzed the temperature dependence of resistivity of LBMO sample in the temperature range from 100 to 275 K. In the mixture of itinerant and localized charge carriers, the resistivity of the material can be thought as a parallel combination of resistivities due to coexisting localized and itinerant charge carriers [29,30] and is given as 1
rðTÞ
¼
f ðTÞ
rit ðTÞ
þ
1 f ðTÞ rht ðTÞ
(2)
where rit(T) is the itinerant carrier resistivity, rht(T) is the localized carrier, i.e. magnetic polaron resistivity and f(T) is the phase fraction of the itinerant charge carriers in the system. The itinerant carriers are mobile in metallic FM domains, whereas the localized magnetic polarons are mobile in FM insulating domains at temperatures below TC and in the paramagnetic insulating domains at temperatures above TC. The resistivity due to itinerant charge carriers in FM metallic domains is given as [31]
rit ðTÞ ¼ a þ bT 2 þ cT 4:5
(3)
where the first term a represents the contribution to the resistivity due to impurities (defects, oxygen vacancies, etc.), the second term (T2) is the contribution of electron–electron scattering to the resistivity and the third term (T9/2) represents the contribution to the resistivity originated due to electron–magnon scattering. The electron–electron scattering is independent of magnetic field in Fermi-Liquid model, whereas the electron–magnon scattering is magnetic field dependent. The coefficient c is related to spin-stiffness coefficient D* as [32] !9=2 R_ ð6p2 nÞ5=3 R2 K B c¼ Dn 482 p7 e2 S2 ð0:52=3 n2=3 Þ9=2 ! n D (4) 2:52 þ 0:0017 2 n R t where R is the hopping distance of eg electrons, S is the effective spin of Mn ion, n is the hole concentration per unit cell, t* is the effective hopping integral, D* is the average spin stiffness coefficient, _( ¼ h/2p) is the Plank’s constant and KB is the Boltzman constant. The spin stiffness coefficient, D* is given by [33] Dn ¼
Fig. 3. Temperature dependence of magnetoresistance (MR) of La0.67Ba0.33MnO3 for 1 and 50 mA biasing current.
1 ZJS 3
(5)
where Z is the number of nearest-neighbour spins and J is the exchange interaction. The effect of higher biasing current is to increase the exchange interaction J and therefore the spinstiffness constant (D*) increases with the higher biasing current. The higher biasing current results in the decrease of the coefficient of electron–magnon scattering term, which in turn reduces the resistivity. The resistivity due to localized magnetic polarons in insulating domains is given as [27,28] E (6) rht ðTÞ ¼ r0 T exp A KBT where r0 is a constant and EA is the polaron activation energy.
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The phase fraction of itinerant carriers [f(T)] depends on temperature and can be given as [34] f ðTÞ ¼
1 1 þ exp½U 0 ð1 ðT=KÞÞ=K B T
(7)
where U0 and k are constants whose values determine the relative fraction of charge carriers present at a temperature T in the system. Substituting the values of rit(T) and rht(T) from Eqs. (3) and (6), we obtain the expression for temperature dependence of resistivity as 1
rðTÞ
¼
f ðTÞ a þ bT 2 þ cT 4:5
þ
1 f ðTÞ
r0 T expðEA =K B TÞ
(8)
Here 1f(T) represents the phase fraction of localized magnetic polarons. Fig. 4(a) and (b) show the simulated rT curves using Eq. (8) along with the experimentally observed rT curves for 1 and 50 mA biasing currents, respectively. The parameters for the best fit are given in Table 1. The closeness of simulated theoretical curve with the experimentally observed temperature dependence of resistivity curve for 1 and 50 mA biasing currents shows the applicability of the two-phase model. The effect of biasing current is reflected on the value of coefficient ‘c’ of electron–magnon scattering term (cT 9/2) as well as phase fraction of the carriers. The decrease in the value of c indicates the enhancement of the spin stiffness coefficient D* with the application of higher biasing current. It means that higher biasing current aligns the magnetic spins in the region around the path of the current, thereby increases the FM exchange interaction. The increase of FM exchange interaction converts some localized magnetic polarons into itinerant charge carriers and so conduction occurs via percolation in the mixture of itinerant and localized magnetic polarons. Fig. 4(c) shows the temperature dependence of simulated resistivity curves of La0.67Ba0.33MnO3 for the two biasing current values (1 and 50 mA). Solid line is for 1 mA biasing current and dotted line is for higher biasing current. The simulated curve for smaller value of ‘c’ shows the suppression of resistivity at TMI, while keeping the resistivity at the ends unchanged. It means that the application of higher biasing current changes the resistivity of the sample around the TMI. At temperatures much lower than TMI the metallic domains are already connected and at temperatures much higher than TMI there is not much metallic content to connect each other. So the effect of higher biasing current will be prominent only around TMI in decreasing the resistivity and it will not create any difference for temperatures much lower to TMI or for temperatures much higher to TMI. We have also calculated the temperature dependence of phase fraction of itinerant carriers using the best fit parameters U0 and k given in Table 1. Fig. 5 shows the temperature dependence of the phase fraction of the itinerant carriers at two biasing currents. It shows that the change in phase fraction (f) on application of biasing current is more pronounced at higher temperatures. The inset of Fig. 5 shows the temperature dependence of change of phase fraction (Df) of itinerant carriers on application of higher biasing current. The change of phase fraction of itinerant carriers on application of higher biasing current is maximum around the TMI which leads to the suppression of resistivity of LBMO sample as observed by us experimentally. The higher biasing current also strengthens the FM exchange interaction and thus increases the FM content in the system. This results in smaller value of MR around TC in the higher biased LBMO sample.
Fig. 4. (a) Variation of resistivity of La0.67Ba0.33MnO3 with temperature: circles represent the experimental data for 1 mA biasing current and the solid line represents the simulated curve. (b) Variation of resistivity of La0.67Ba0.33MnO3 with temperature: circles represent the experimental data for 50 mA biasing current and the solid line represents the simulated curve. (c) Temperature dependence of simulated resistivity of La0.67Ba0.33MnO3. Dotted line is for 1 mA biasing current and solid line is for 50 mA biasing current.
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Table 1 Values of fitting parameters a, b, c, U0, k, ro and EA for different biasing currents obtained from fitting of experimental r– T curves for LBMO sample Biasing current (mA)
1 50
Percolation model (1/r(T)) ¼ (f(T)/a+bT2+cT4.5)+(1f(T)/r0T exp(EA/KBT)) a (O cm)
b (O cm K2)
c (O cm K4.5)
U0 (meV)
k (K)
r0 (O cm K1)
EA (meV)
0.14314 0.14314
7.87 106 7.87 106
1.54 1012 6.96 1013
84.74 86.09
266 267
0.00003 0.00003
68.17 67.16
New Delhi, India. Rajesh Kumar and Ajai K. Gupta are thankful to CSIR, New Delhi, India for the award of Senior Research Fellowship and Research Associateship, respectively. References
Fig. 5. Temperature dependence of the phase fraction (f) of the itinerant carriers at two biasing currents 1 and 50 mA. Inset of the figure shows the temperature dependence of the change of phase fraction (Df) of the itinerant carriers on application of higher biasing current.
4. Conclusion Current-induced changes in the resistivity and MR of bulk sample of the La0.67Ba0.33MnO3 manganite were studied. The higher biasing current reduces the resistivity around TIM and decreases the MR around TC. The application of higher biasing current strengthens the FM exchange interaction between Mn ions and thus increases the FM content in the system, which in turn results into the smaller value of MR around TC. The decrease in resistivity around TIM is explained in the framework of model based on the percolative transport in the mixed phases of conducting itinerant carriers and insulating localized magnetic polarons in the LBMO sample.
Acknowledgments The authors gratefully acknowledge the support and encouragement received from Prof. Vikram Kumar, Director, NPL,
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