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Magnetic and magnetotransport properties of Ce doped nanocrystalline LaMnO3
Journal of Alloys and Compounds 438 (2007) 1–7
Magnetic and magnetotransport properties of Ce doped nanocrystalline LaMnO3 C. Krishnamoorthy a,∗ , K. Sethupathi a , V. Sankaranarayanan a , R. Nirmala b , S.K. Malik b a
Department of Physics, Indian Institute of Technology Madras, Chennai 600 036, India b Tata Institute of Fundamental Research, Mumbai 400 005, India Received 7 July 2006; received in revised form 19 July 2006; accepted 20 July 2006 Available online 23 August 2006
Abstract The magnetic and electrical transport properties of Ce doped LaMnO3 bulk samples have already been reported for various Ce concentrations. However there are few reports on the magnetotransport properties of these compounds. In this paper, we report the magnetic and magnetotransport properties of nanocrystalline La0.8 Ce0.2 MnO3 and La0.7 Ce0.3 MnO3 samples with different particle size. The nanocrystalline samples with different particle size were prepared through citrate-complex method by calcining the precursor at different temperatures. The transmission electron microscopy analysis of both the compounds reveal that the particle size increases with calcination temperature. Temperature dependence of magnetization of all the samples shows ferro- to paramagnetic transition. The zero field cooled and field cooled magnetization curves show irreversibility just below the magnetic transition temperature. The temperature dependence of resistivity of all the samples exhibits a metal to insulator transition without an impurity peak around 250 K, which is generally observed in multiphase samples. The results indicate that magnetoresistance (MR) increases with decreasing particle size. The observed MR will be discussed by spin dependent tunneling and enhancement of double exchange mechanism upon application of magnetic field. The physical properties suggest that the samples are single phase in nature. © 2006 Elsevier B.V. All rights reserved. Keywords: Nanostructured materials; Chemical synthesis; Magnetoresistance; Magnetization
1. Introduction In the last decade, the hole doped perovskite manganites, with a general formula of R1−x 3+ Ax 2+ Mn1−x 3+ Mnx 4+ Ox 2− (R: rare earth, A: alkali earth element), have attracted considerable interest due to their colossal magnetoresistance (CMR) property [1]. In hole doped perovskite manganites, Mn exists in both Mn3+ and Mn4+ states to maintain the charge neutrality in the compound. The Mn4+ content is proportional to the hole concentration. In Mn3+ , the three d-electrons are distributed in lower triplet t2g states and one electron in the upper doublet eg states. Similarly in Mn4+ , the existing three d-electrons are distributed in the lower t2g states, leaving the eg states vacant. When the hole concentration is close to an optimal value, the samples exhibit ferro- to paramagnetic transition concomitant with
∗
Corresponding author. Tel.: +91 44 22575867; fax: +91 44 22574852. E-mail address: [email protected] (C. Krishnamoorthy).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.07.089
metal to insulator transition (MIT) upon warming the sample. The observed magnetic and electrical concomitant transitions could be qualitatively understood by invoking double exchange mechanism (DEM) [2]. According to DEM, the spin polarized eg electron hops from Mn3+ to Mn4+ via O2− and this leads to an effective ferromagnetic metallic (FMM) interaction. The probability of hopping is maximum when t2g spins of both Mn3+ and Mn4+ are parallel to each other. Around the magnetic transition, upon application of magnetic field the magnetic disorder decreases giving rise to an increase of double exchange resulting in decrease of resistivity. Analogous to the double exchange between Mn3+ and Mn4+ ions one could also expect double exchange between Mn3+ and Mn2+ ions giving rise to ferromagnetism accompanied by MIT [3,4]. In tetravalent ion doped, called electron doped, perovskite manganites, with a general formula of R1−x 3+ Tx 4+ Mn1−x 3+ Mnx 2+ O3 2− (T: tetravalent ion), the Mn ion can exist both in Mn3+ and Mn2+ states. Mandal and Das [3] have shown that the Ce4+ doped LaMnO3 exhibits ferro- to
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paramagnetic (PM) transition with a MIT. The reported results suggest that La1−x Cex MnO3 system shows ferro- to paramagnetic transition and MIT when the Ce concentration lies between 12 and 30% [5]. However, a detailed crystal structural analysis shows that the bulk polycrystalline La1−x Cex MnO3 samples do not form a single phase when prepared through solid state reaction [3,6,7]. Even the samples prepared through wetchemical (precipitation) method show secondary phase which predominantly corresponds to CeO2 phase [6,8]. In addition, majority of researchers reported magnetic and electrical transport properties of the bulk samples [3,5,6,8,9] and few reported magnetotransport properties of these compounds [10,11]. However, the magnetotransport properties of these compounds have not been studied systematically. Furthermore, recently research on nanocrystalline materials has attracted much attention due to their interesting properties. Here we report systematic study of magnetic and magnetotransport properties of nanocrystalline La0.8 Ce0.2 MnO3 and La0.7 Ce0.3 MnO3 samples with different particle size. Recently, the synthesis of nanocrystalline materials by wetchemical methods are widely used due to their advantage and simplicity over the other methods. Particularly, Pechini method [12,13], sol–gel method [14–18], glycine nitrate method [19] and citrate-complex method [20] are widely employed to prepare manganite samples. Among many methods, the citrate-complex method is simple and may give homogeneous dispersion of metal ions in amorphous organic complex (matrix), allowing them to react only while calcination [20], thus minimizing the possibility of precipitation of individual or group of metal ions. This method may be advantageous to prepare single phase Ce doped LaMnO3 nanocrystalline materials. 2. Experimental details The La0.8 Ce0.2 MnO3 and La0.7 Ce0.3 MnO3 compounds were prepared from stoichiometric high pure La2 O3 , CeO2 , Mn(CH3 COO)2 ·4H2 O and C6 H8 O7 ·H2 O by citrate-complex method. Before weighing, La2 O3 was preheated at 950 ◦ C for several hours to decompose possible carbonates and hydroxides. La2 O3 and CeO2 were converted into their respective soluble metal nitrates by dissolving them in dil HNO3 at 50 ◦ C. The Mn(CH3 COO)2 ·4H2 O and C6 H8 O7 ·H2 O readily dissolve in deionized water. All the individual metal solutions were mixed together with citric acid solution while stirring, to form a complex. All the metal ions were homogeneously dispersed in citrate complex. The resultant solution was stirred at about 90 ◦ C for several hours to evaporate H2 O. The precursor was made into parts and each part was calcined at different temperature, viz., 700 and 900 ◦ C for 3 h, to obtain different particle size. The phase purity and crystal structure of the calcined powders were determined by powder X-ray diffraction (XRD). The morphology of the samples was determined by transmission electron microscopy (TEM). Magnetic properties of the samples were studied using vibrating sample magnetometer (VSM) or SQUID magnetometer. The temperature dependence of electrical resistivity and magnetoresistance (MR) were measured by standard four probe method using home made sample holder or physical property measurement system (PPMS).
3. Results and discussion The XRD profiles of the calcined powders show perovskite structure (Fig. 1). The phase purity analysis of the profiles reveals that the La0.8 Ce0.2 MnO3 sample calcined at 700 ◦ C shows pure single phase nature whereas the sample calcined at 900 ◦ C shows
Fig. 1. XRD profiles of La0.8 Ce0.2 MnO3 and La0.7 Ce0.3 MnO3 (upper) samples calcined at 700 ◦ C and 900 ◦ C for 3 h.
small amount of CeO2 segregation. The corresponding CeO2 peaks are shown at 2θ values of ∼28◦ and ∼56◦ in Fig. 1. On 700 ◦ C calcined sample, we have rescanned the XRD at 2θ = 25–34◦ for further confirmation of the absence of impurity peak. We could see no impurity peak ∼28◦ . In 900 ◦ C calcined sample, the intensity of the CeO2 impurity peak at 28◦ is ∼8% of that of the (2 0 0) peak. The CeO2 segregation is seen in La0.7 Ce0.3 MnO3 samples calcined at both 700 ◦ C and 900 ◦ C. The intensities of the CeO2 peaks are found to be ∼8% and ∼9.5% in 700 and 900 ◦ C calcined samples, respectively. We have also prepared La0.85 Ce0.15 MnO3 samples with different particle size and found that the sample calcined at 700 ◦ C exhibits single phase nature whereas the sample calcined at 900 ◦ C shows a small segregation of CeO2 . The intensity of CeO2 impurity peak in this compound is found to be ∼6%. A similar value (∼5%) of the intensity of the impurity peak at 28◦ is reported for single phase La0.7 Ce0.3 MnO3 thin film sample prepared through pulsed laser deposition [6]. On the other hand, the impurity peaks are very prominent in bulk samples of the same composition [3,6,7] and the intensity of the impurity peak, at ∼28◦ , is ∼50% [7]. The above observations clearly suggest that the samples calcined at 700 ◦ C show single phase nature and the samples calcined at higher temperature (900 ◦ C) show small CeO2 segregation. Thus we can infer that with increasing calcination temperature and Ce concentration, the probability of segregation of CeO2 increases. The reason for this could be that at higher temperatures the Ce4+ ions may have more energy to form CeO2 than at lower temperatures. In the present study the amount of CeO2 segregation is very less compared to the samples prepared through solid state reaction and precipitation method [3,7,8]. The obtained XRD profiles could be best described by orthorhombic perovskite crystal structure with a space group of Pnma, as reported for bulk samples [6,9]. The estimated average crystallite size, using Scherer’s formula, is found to be 11 and 20 nm for La0.8 Ce0.2 MnO3 samples calcined at 700 and 900 ◦ C, respectively. Similarly the crystallite size is 20 and 35 nm for
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La0.7 Ce0.3 MnO3 samples calcined at 700 and 900 ◦ C, respectively. Fig. 2 shows the TEM micrographs of La0.8 Ce0.2 MnO3 and La0.7 Ce0.3 MnO3 samples calcined at different temperatures. The micrographs show that the particles are nearly spherical in shape and are closely distributed in size. The measured average particle size is found to be 30 and 85 nm for La0.8 Ce0.2 MnO3 and 40 and 80 nm for La0.7 Ce0.3 MnO3 samples calcined at 700 and 900 ◦ C, respectively. Temperature dependence of magnetization of all the samples is shown in Figs. 3 and 4. All the samples show broad ferro- to paramagnetic transition (TC ) upon warming. The TC was deter-
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mined as the maximum rate of change of magnetization with temperature. The La0.8 Ce0.2 MnO3 samples with 30 and 85 nm show TC at 205 and 242 K, respectively. The La0.7 Ce0.3 MnO3 samples with 40 and 80 nm show TC at 247 and 249 K, respectively. These values are very close to the reported value of the TC for single phase epitaxial film of the same compositions [5]. However the transition is broad compared to the epitaxial films. This broad transition suggests a possible existence of wide distribution of exchange interactions. The bulk samples with impurity phases show TC at ∼150 K whereas the present sample show ∼250 K. In addition, Gebhardt et al. [10] reported a prominent jump (up) of magnetization at ∼45 K which is attributed
Fig. 2. TEM micrograph of La0.8 Ce0.2 MnO3 samples calcined at (a) 700 ◦ C, (b) 900 ◦ C and of La0.7 Ce0.3 MnO3 samples calcined at (c) 700 ◦ C and (d) 900 ◦ C for 3 h.
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Fig. 3. Temperature dependence ZFC and FC magnetization of La0.8 Ce0.2 MnO3 with an average particle size of (a) 30 nm and (b) 85 nm.
to the presence of MnO2 impurity in the samples. In the present samples we could not see any such anomaly in the M–T curve (Fig. 4). This further confirms that the samples are single phase nature. The temperature dependence of zero field cooled (ZFC) and field cooled (FC) magnetization curves show irreversibility just below TC . The FC magnetization increases with decreasing temperature below irreversibility temperature (Tirr ). This suggests that the samples have short range magnetic interactions. These characteristics resemble cluster glass behaviour rather than spin glass behaviour. In the spin glass system, the irreversibility is observed below spin freezing temperature (Tf ) with FC curve almost flat below irreversibility and Tf is observed far below TC . The magnetic field dependent isothermal magnetization of all the samples exhibits typical ferromagnetic behaviour at low temperatures and paramagnetic behaviour at room temperature. The saturation magnetization (Ms ) decreases
with particle size. The La0.7 Ce0.3 MnO3 samples show Ms of 60.4 and 70 emu/g at 5 K and 5 T for 40 and 80 nm samples, respectively (Figs. 4 and 5). These values are higher than the reported value of ∼55 emu/g for the bulk samples of the same composition [9,10]. The observed Ms values in these nanocrystalline samples are far below the theoretically estimated Ms value of 99.1 emu/g for La0.7 Ce0.3 MnO3 . The above value is obtained by assuming spin only moment of Mn ions existing in Mn3+ and Mn2+ states (0.7 × 4 B + 0.3 × 5 B = 4.3 B ). The decrease of Ms value in bulk samples are attributed to existence of noncollinear spins at intra-grain and the presence of small amount of unreacted MnO2 and CeO2 [9,10]. The present studies reveal that the Ms value of ∼15 emu/g is higher than that of the bulk samples. In addition, the XRD profiles show single phase nature or less amount of CeO2 segregation. By comparing the XRD and magnetization data one can infer that the high solubility of Ce in LaMnO3 in these nanocrystalline
Fig. 4. Temperature dependence ZFC and FC magnetization of La0.3 Ce0.7 MnO3 with an average particle size of (a) 40 nm and (b) 80 nm. The inset of figure (b) shows magnetic field dependence of magnetization for 80 nm sample at 5 and 300 K.
C. Krishnamoorthy et al. / Journal of Alloys and Compounds 438 (2007) 1–7
Fig. 5. The magnetic field dependence of magnetization of La0.7 Ce0.3 MnO3 sample with 40 nm particle size at 5 K and 300 K.
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samples may give rise to higher value of Ms than the bulk samples. Temperature dependence of resistivity of all the samples shows broad MIT upon warming the samples (Figs. 6 and 7). The transition broadness decreases with increasing particle size. The magnitude of resistivity decreases with increasing particle size. The present samples do not show impurity peak in the temperature–resistivity curve whereas the bulk samples of the same composition show impurity peak at ∼250 K, which is attributed to the presence of CeO2 segregation [3,6]. The present observed resistivity behaviour is similar to the one reported by Mitra et al. [6] for single phase thin film sample. This further suggest that the samples are close to single phase. The temperature dependence of MR of La0.8 Ce0.2 MnO3 sample with 30 nm particle size shows almost a linear decrease with increasing temperature in 0.5 and 1.5 T (inset of Fig. 6a). On the other hand, the 85 nm particle size sample shows a linear decrease of MR with a broad hump around MIT (Fig. 6c). The
Fig. 6. Temperature dependence of electrical resistivity, in different applied magnetic fields, of La0.8 Ce0.2 MnO3 sample with (a) 30 nm and (b) 85 nm. (c) The temperature dependence of MR of 85 nm samples in 0.5 and 1.5 T. The inset of (a) shows temperature dependence of MR of 30 nm sample.
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Fig. 7. Temperature dependence of electrical resistivity and MR (right hand scale) of La0.7 Ce0.3 MnO3 sample with (a) 40 nm and (b) 80 nm.
MR is calculated using, MR% = [ρ(H,T) − ρ(0,T)]×100/ρ(0,T); where ρ(0,T) and ρ(H,T) are resistivities in the absence and presence of applied magnetic field H, respectively. The temperature dependence of MR of La0.7 Ce0.3 MnO3 in 5 T shows a weak temperature dependence below MIT and a smooth decrease with increasing temperature above MIT (Fig. 7). The same behavior was observed both in 40 and 80 nm samples. The low temperature MR increases with decreasing particle size. The observed MR in these nanocrystals is higher than the MR observed in the bulk samples [11]. The reported MR for bulk sample at 5 K and in 7.7 T is 54% whereas in the present nanocrystalline samples the MR is 58% at the same value of field and temperature in La0.7 Ce0.3 MnO3 (Fig. 9a ). The qualitative understanding of MR will be discussed in the following paragraphs. The magnetic field dependence of isothermal MR of all the samples show sharp increase of MR at low fields below TC (Figs. 8 and 9). The low field MR (LFMR) increases with decreasing particle size. The La0.8 Ce0.2 MnO3 samples with particle size of 30 nm and 85 nm exhibit MR of ∼21% and ∼13% at
80 K and 0.5 T, respectively. Similarly the MR at 5 K and 0.5 T is ∼28% and ∼26% for La0.7 Ce0.3 MnO3 samples with particle size of 40 and 80 nm, respectively. The high field MR (HFMR) changes almost linearly with applied magnetic field below TC as observed in hole doped nanocrystalline manganites [21]. The MR at 80 K and 2.1 T is ∼46% and ∼25% for La0.8 Ce0.2 MnO3 samples with 30 and 85 nm respectively. The MR at 5 K and 9 T is ∼61% and ∼50% for La0.7 Ce0.3 MnO3 samples with particle size of 40 and 80 nm, respectively. The observed MR could be best understood by invoking spin dependent tunneling mechanism, as in hole doped manganites [21]. This model assumes that (i) the core of the nanoparticle is perfect ferromagnetic and its magnetic moment is equal to bulk magnetization. (ii) The surface (shell) of the nanoparticle is magnetically disordered due to the loss of lattice symmetry and magnetic coordination. Thus the shell is assumed as an amorphous layer with zero magnetization and is electrically insulator [22]. In this situation the itinerant electron can move freely within the core and to move to another particle, the electron has to tunnel across the electrical barrier formed
Fig. 8. Magnetic field dependence of isothermal MR of La0.8 Ce0.2 MnO3 samples with (a) 30 nm and (b) 85 nm at different temperatures.
C. Krishnamoorthy et al. / Journal of Alloys and Compounds 438 (2007) 1–7
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Fig. 9. Magnetic field dependence of isothermal MR of La0.7 Ce0.3 MnO3 samples with (a) 40 nm and (b) 80 nm.
by shell of the nanoparticle. In the absence of applied magnetic field, individual magnetic moments of the particles are oriented along their own direction and this results in maximum magnetic exchange energy. This gives rise to minimum spin dependent electron tunneling. Upon application of magnetic field, the neighbouring grains align parallel to each other at low fields and this leads to minimum magnetic exchange energy resulting in increase of spin dependent electron tunneling [23]. Thus the sharp increase of MR below 0.5 T may be attributed to the increase of spin dependent electron tunneling across the grain boundaries. After certain field strength all the grains/particles align magnetically parallel to each other and this leads to saturation of electron tunneling. The almost linear increase of high field MR (HFMR), above 0.5 T, could be attributed to the decrease of spin canting at the intra-grains and/or grain boundaries [24]. This decrease of spin canting leads to an increase of double exchange interaction resulting in increase of MR with applied field. Thus one can understand qualitatively the observed LFMR and HFMR by spin dependent inter grain tunneling and decrease of canted spins upon application of magnetic field, respectively. The present studies are the first reports, to the best of our knowledge, on detailed MR studies on Ce doped LaMnO3 . 4. Conclusions Nanocrystalline La0.8 Ce0.2 MnO3 and La0.7 Ce0.3 MnO3 samples have been synthesised by citrate-complex method. The estimated average crystallite/particle size is found to increase with calcination temperature. All the samples show ferro- paramagnetic transition and metal-insulator transition upon warming. The saturation magnetization decreases with particle size. The temperature dependence of resistivity shows no impurity peak near 250 K. The temperature dependence of magnetoresistance shows linear decrease upon warming in low magnetic fields. The low field as well as high field MR increases with decreasing particle size. The observed magnetoresistance behaviour could be best understood by invoking spin dependent tunneling model
and the decrease of spin canting at the intra-grain upon application of magnetic field. References [1] S. Jin, T.H. Tiefel, M. McCormack, R.A. Fastnacht, R. Ramesh, L.H. Chen, Science 264 (1994) 413–415. [2] C. Zener, Phys. Rev. 82 (1951) 403–405. [3] P. Mandal, S. Das, Phys. Rev. B 56 (1997) 15073–15080. [4] G. Tan, X. Zhang, Z. Chen, J. Appl. Phys. 95 (2004) 6322–6324. [5] P. Raychauduri, C. Mitra, P.D.A. Mann, S. Wirth, J. Appl. Phys. 93 (2003) 8328–8330. [6] C. Mitra, P. Raychaudhuri, J. John, S.K. Dhar, A.K. Nigam, R. Pinto, J. Appl. Phys. 89 (2001) 524–530. [7] R. Ganguly, I.K. Gopalakrishnan, J.V. Yakhmi, J. Phys.: Condens. Matter. 12 (2000) L719–L722. [8] J. Philip, T.R.N. Kutty, J. Phys.: Condens. Matter 11 (1999) 8537–8546. [9] S. Das, A. Poddar, B. Roy, S. Giri, J. Alloy. Compd. 365 (2004) 94–101. [10] J.R. Gebhardt, S. Roy, N. Ali, J. Appl. Phys. 85 (1999) 5390–5392. [11] S. Das, P. Mandal, Z. Phys. B 104 (1997) 7–9. [12] M.P. Pechini, US Patent No. 3330697 (1967). [13] L. Malavasi, M.C. Mozzati, S. Polizzi, C.B. Azzoni, G. Flor, Chem. Mater. 15 (2003) 5036–5043. [14] P.K. Siwach, U.K. Goutam, P. Srivastava, H.K. Singh, R.S. Tiwari, O.N. Srivastava, J. Phys. D: Appl. Phys. 39 (2006) 14–20. [15] M.A. L´opez-Quintela, L.E. Hueso, J. Rivas, F. Rivadulla, Nanotechnology 14 (2003) 212–219. [16] X.-H. Li, Y.-H. Huang, C.H. Yan, Z.-M. Wang, C.-S. Liao, J. Phys.: Condens. Matter. 14 (2002) L177–L183. [17] A. Dutta, N. Gayathri, R. Ranganathan, Phys. Rev. B 68 (2003) 054432. [18] S. Karmakar, S. Taran, B.K. Chaudhuri, Phys. Stat. Sol. (b) 241 (2004) 3563–3571. [19] L.A. Chick, L.R. Pederson, G.D. Maupin, J.L. Bates, L.E. Thomas, G.J. Exarhos, Mater. Lett. 10 (1990) 6–12. [20] P.H. Courty, H. Ajot, C.H. Marcilly, B. Delmon, Powder Technol. 7 (1973) 21–38. [21] H.Y. Hwang, S.-W. Cheong, N.P. Ong, B. Batlogg, Phys. Rev. Lett. 77 (1996) 2041–2044. [22] A.E. Berkowitz, W.J. Schuele, P.J. Flanders, J. Appl. Phys. 39 (1968) 1261–1263. [23] B. Abeles, P. Sheng, M.D. Coutts, Y. Arie, Adv. Phys. 24 (1975) 407–461. [24] P. Raychaudhuri, K. Seshadri, P. Taneja, S. Bandyopadhyay, P. Ayyub, A.K. Nigam, R. Pinto, S. Chaudhary, S.B. Roy, Phys. Rev. B 59 (1999) 13919–13926.
Magnetic and magnetotransport properties of Ce doped nanocrystalline LaMnO3 C. Krishnamoorthy a,∗ , K. Sethupathi a , V. Sankaranarayanan a , R. Nirmala b , S.K. Malik b a
Department of Physics, Indian Institute of Technology Madras, Chennai 600 036, India b Tata Institute of Fundamental Research, Mumbai 400 005, India Received 7 July 2006; received in revised form 19 July 2006; accepted 20 July 2006 Available online 23 August 2006
Abstract The magnetic and electrical transport properties of Ce doped LaMnO3 bulk samples have already been reported for various Ce concentrations. However there are few reports on the magnetotransport properties of these compounds. In this paper, we report the magnetic and magnetotransport properties of nanocrystalline La0.8 Ce0.2 MnO3 and La0.7 Ce0.3 MnO3 samples with different particle size. The nanocrystalline samples with different particle size were prepared through citrate-complex method by calcining the precursor at different temperatures. The transmission electron microscopy analysis of both the compounds reveal that the particle size increases with calcination temperature. Temperature dependence of magnetization of all the samples shows ferro- to paramagnetic transition. The zero field cooled and field cooled magnetization curves show irreversibility just below the magnetic transition temperature. The temperature dependence of resistivity of all the samples exhibits a metal to insulator transition without an impurity peak around 250 K, which is generally observed in multiphase samples. The results indicate that magnetoresistance (MR) increases with decreasing particle size. The observed MR will be discussed by spin dependent tunneling and enhancement of double exchange mechanism upon application of magnetic field. The physical properties suggest that the samples are single phase in nature. © 2006 Elsevier B.V. All rights reserved. Keywords: Nanostructured materials; Chemical synthesis; Magnetoresistance; Magnetization
1. Introduction In the last decade, the hole doped perovskite manganites, with a general formula of R1−x 3+ Ax 2+ Mn1−x 3+ Mnx 4+ Ox 2− (R: rare earth, A: alkali earth element), have attracted considerable interest due to their colossal magnetoresistance (CMR) property [1]. In hole doped perovskite manganites, Mn exists in both Mn3+ and Mn4+ states to maintain the charge neutrality in the compound. The Mn4+ content is proportional to the hole concentration. In Mn3+ , the three d-electrons are distributed in lower triplet t2g states and one electron in the upper doublet eg states. Similarly in Mn4+ , the existing three d-electrons are distributed in the lower t2g states, leaving the eg states vacant. When the hole concentration is close to an optimal value, the samples exhibit ferro- to paramagnetic transition concomitant with
∗
Corresponding author. Tel.: +91 44 22575867; fax: +91 44 22574852. E-mail address: [email protected] (C. Krishnamoorthy).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.07.089
metal to insulator transition (MIT) upon warming the sample. The observed magnetic and electrical concomitant transitions could be qualitatively understood by invoking double exchange mechanism (DEM) [2]. According to DEM, the spin polarized eg electron hops from Mn3+ to Mn4+ via O2− and this leads to an effective ferromagnetic metallic (FMM) interaction. The probability of hopping is maximum when t2g spins of both Mn3+ and Mn4+ are parallel to each other. Around the magnetic transition, upon application of magnetic field the magnetic disorder decreases giving rise to an increase of double exchange resulting in decrease of resistivity. Analogous to the double exchange between Mn3+ and Mn4+ ions one could also expect double exchange between Mn3+ and Mn2+ ions giving rise to ferromagnetism accompanied by MIT [3,4]. In tetravalent ion doped, called electron doped, perovskite manganites, with a general formula of R1−x 3+ Tx 4+ Mn1−x 3+ Mnx 2+ O3 2− (T: tetravalent ion), the Mn ion can exist both in Mn3+ and Mn2+ states. Mandal and Das [3] have shown that the Ce4+ doped LaMnO3 exhibits ferro- to
2
C. Krishnamoorthy et al. / Journal of Alloys and Compounds 438 (2007) 1–7
paramagnetic (PM) transition with a MIT. The reported results suggest that La1−x Cex MnO3 system shows ferro- to paramagnetic transition and MIT when the Ce concentration lies between 12 and 30% [5]. However, a detailed crystal structural analysis shows that the bulk polycrystalline La1−x Cex MnO3 samples do not form a single phase when prepared through solid state reaction [3,6,7]. Even the samples prepared through wetchemical (precipitation) method show secondary phase which predominantly corresponds to CeO2 phase [6,8]. In addition, majority of researchers reported magnetic and electrical transport properties of the bulk samples [3,5,6,8,9] and few reported magnetotransport properties of these compounds [10,11]. However, the magnetotransport properties of these compounds have not been studied systematically. Furthermore, recently research on nanocrystalline materials has attracted much attention due to their interesting properties. Here we report systematic study of magnetic and magnetotransport properties of nanocrystalline La0.8 Ce0.2 MnO3 and La0.7 Ce0.3 MnO3 samples with different particle size. Recently, the synthesis of nanocrystalline materials by wetchemical methods are widely used due to their advantage and simplicity over the other methods. Particularly, Pechini method [12,13], sol–gel method [14–18], glycine nitrate method [19] and citrate-complex method [20] are widely employed to prepare manganite samples. Among many methods, the citrate-complex method is simple and may give homogeneous dispersion of metal ions in amorphous organic complex (matrix), allowing them to react only while calcination [20], thus minimizing the possibility of precipitation of individual or group of metal ions. This method may be advantageous to prepare single phase Ce doped LaMnO3 nanocrystalline materials. 2. Experimental details The La0.8 Ce0.2 MnO3 and La0.7 Ce0.3 MnO3 compounds were prepared from stoichiometric high pure La2 O3 , CeO2 , Mn(CH3 COO)2 ·4H2 O and C6 H8 O7 ·H2 O by citrate-complex method. Before weighing, La2 O3 was preheated at 950 ◦ C for several hours to decompose possible carbonates and hydroxides. La2 O3 and CeO2 were converted into their respective soluble metal nitrates by dissolving them in dil HNO3 at 50 ◦ C. The Mn(CH3 COO)2 ·4H2 O and C6 H8 O7 ·H2 O readily dissolve in deionized water. All the individual metal solutions were mixed together with citric acid solution while stirring, to form a complex. All the metal ions were homogeneously dispersed in citrate complex. The resultant solution was stirred at about 90 ◦ C for several hours to evaporate H2 O. The precursor was made into parts and each part was calcined at different temperature, viz., 700 and 900 ◦ C for 3 h, to obtain different particle size. The phase purity and crystal structure of the calcined powders were determined by powder X-ray diffraction (XRD). The morphology of the samples was determined by transmission electron microscopy (TEM). Magnetic properties of the samples were studied using vibrating sample magnetometer (VSM) or SQUID magnetometer. The temperature dependence of electrical resistivity and magnetoresistance (MR) were measured by standard four probe method using home made sample holder or physical property measurement system (PPMS).
3. Results and discussion The XRD profiles of the calcined powders show perovskite structure (Fig. 1). The phase purity analysis of the profiles reveals that the La0.8 Ce0.2 MnO3 sample calcined at 700 ◦ C shows pure single phase nature whereas the sample calcined at 900 ◦ C shows
Fig. 1. XRD profiles of La0.8 Ce0.2 MnO3 and La0.7 Ce0.3 MnO3 (upper) samples calcined at 700 ◦ C and 900 ◦ C for 3 h.
small amount of CeO2 segregation. The corresponding CeO2 peaks are shown at 2θ values of ∼28◦ and ∼56◦ in Fig. 1. On 700 ◦ C calcined sample, we have rescanned the XRD at 2θ = 25–34◦ for further confirmation of the absence of impurity peak. We could see no impurity peak ∼28◦ . In 900 ◦ C calcined sample, the intensity of the CeO2 impurity peak at 28◦ is ∼8% of that of the (2 0 0) peak. The CeO2 segregation is seen in La0.7 Ce0.3 MnO3 samples calcined at both 700 ◦ C and 900 ◦ C. The intensities of the CeO2 peaks are found to be ∼8% and ∼9.5% in 700 and 900 ◦ C calcined samples, respectively. We have also prepared La0.85 Ce0.15 MnO3 samples with different particle size and found that the sample calcined at 700 ◦ C exhibits single phase nature whereas the sample calcined at 900 ◦ C shows a small segregation of CeO2 . The intensity of CeO2 impurity peak in this compound is found to be ∼6%. A similar value (∼5%) of the intensity of the impurity peak at 28◦ is reported for single phase La0.7 Ce0.3 MnO3 thin film sample prepared through pulsed laser deposition [6]. On the other hand, the impurity peaks are very prominent in bulk samples of the same composition [3,6,7] and the intensity of the impurity peak, at ∼28◦ , is ∼50% [7]. The above observations clearly suggest that the samples calcined at 700 ◦ C show single phase nature and the samples calcined at higher temperature (900 ◦ C) show small CeO2 segregation. Thus we can infer that with increasing calcination temperature and Ce concentration, the probability of segregation of CeO2 increases. The reason for this could be that at higher temperatures the Ce4+ ions may have more energy to form CeO2 than at lower temperatures. In the present study the amount of CeO2 segregation is very less compared to the samples prepared through solid state reaction and precipitation method [3,7,8]. The obtained XRD profiles could be best described by orthorhombic perovskite crystal structure with a space group of Pnma, as reported for bulk samples [6,9]. The estimated average crystallite size, using Scherer’s formula, is found to be 11 and 20 nm for La0.8 Ce0.2 MnO3 samples calcined at 700 and 900 ◦ C, respectively. Similarly the crystallite size is 20 and 35 nm for
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La0.7 Ce0.3 MnO3 samples calcined at 700 and 900 ◦ C, respectively. Fig. 2 shows the TEM micrographs of La0.8 Ce0.2 MnO3 and La0.7 Ce0.3 MnO3 samples calcined at different temperatures. The micrographs show that the particles are nearly spherical in shape and are closely distributed in size. The measured average particle size is found to be 30 and 85 nm for La0.8 Ce0.2 MnO3 and 40 and 80 nm for La0.7 Ce0.3 MnO3 samples calcined at 700 and 900 ◦ C, respectively. Temperature dependence of magnetization of all the samples is shown in Figs. 3 and 4. All the samples show broad ferro- to paramagnetic transition (TC ) upon warming. The TC was deter-
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mined as the maximum rate of change of magnetization with temperature. The La0.8 Ce0.2 MnO3 samples with 30 and 85 nm show TC at 205 and 242 K, respectively. The La0.7 Ce0.3 MnO3 samples with 40 and 80 nm show TC at 247 and 249 K, respectively. These values are very close to the reported value of the TC for single phase epitaxial film of the same compositions [5]. However the transition is broad compared to the epitaxial films. This broad transition suggests a possible existence of wide distribution of exchange interactions. The bulk samples with impurity phases show TC at ∼150 K whereas the present sample show ∼250 K. In addition, Gebhardt et al. [10] reported a prominent jump (up) of magnetization at ∼45 K which is attributed
Fig. 2. TEM micrograph of La0.8 Ce0.2 MnO3 samples calcined at (a) 700 ◦ C, (b) 900 ◦ C and of La0.7 Ce0.3 MnO3 samples calcined at (c) 700 ◦ C and (d) 900 ◦ C for 3 h.
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Fig. 3. Temperature dependence ZFC and FC magnetization of La0.8 Ce0.2 MnO3 with an average particle size of (a) 30 nm and (b) 85 nm.
to the presence of MnO2 impurity in the samples. In the present samples we could not see any such anomaly in the M–T curve (Fig. 4). This further confirms that the samples are single phase nature. The temperature dependence of zero field cooled (ZFC) and field cooled (FC) magnetization curves show irreversibility just below TC . The FC magnetization increases with decreasing temperature below irreversibility temperature (Tirr ). This suggests that the samples have short range magnetic interactions. These characteristics resemble cluster glass behaviour rather than spin glass behaviour. In the spin glass system, the irreversibility is observed below spin freezing temperature (Tf ) with FC curve almost flat below irreversibility and Tf is observed far below TC . The magnetic field dependent isothermal magnetization of all the samples exhibits typical ferromagnetic behaviour at low temperatures and paramagnetic behaviour at room temperature. The saturation magnetization (Ms ) decreases
with particle size. The La0.7 Ce0.3 MnO3 samples show Ms of 60.4 and 70 emu/g at 5 K and 5 T for 40 and 80 nm samples, respectively (Figs. 4 and 5). These values are higher than the reported value of ∼55 emu/g for the bulk samples of the same composition [9,10]. The observed Ms values in these nanocrystalline samples are far below the theoretically estimated Ms value of 99.1 emu/g for La0.7 Ce0.3 MnO3 . The above value is obtained by assuming spin only moment of Mn ions existing in Mn3+ and Mn2+ states (0.7 × 4 B + 0.3 × 5 B = 4.3 B ). The decrease of Ms value in bulk samples are attributed to existence of noncollinear spins at intra-grain and the presence of small amount of unreacted MnO2 and CeO2 [9,10]. The present studies reveal that the Ms value of ∼15 emu/g is higher than that of the bulk samples. In addition, the XRD profiles show single phase nature or less amount of CeO2 segregation. By comparing the XRD and magnetization data one can infer that the high solubility of Ce in LaMnO3 in these nanocrystalline
Fig. 4. Temperature dependence ZFC and FC magnetization of La0.3 Ce0.7 MnO3 with an average particle size of (a) 40 nm and (b) 80 nm. The inset of figure (b) shows magnetic field dependence of magnetization for 80 nm sample at 5 and 300 K.
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Fig. 5. The magnetic field dependence of magnetization of La0.7 Ce0.3 MnO3 sample with 40 nm particle size at 5 K and 300 K.
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samples may give rise to higher value of Ms than the bulk samples. Temperature dependence of resistivity of all the samples shows broad MIT upon warming the samples (Figs. 6 and 7). The transition broadness decreases with increasing particle size. The magnitude of resistivity decreases with increasing particle size. The present samples do not show impurity peak in the temperature–resistivity curve whereas the bulk samples of the same composition show impurity peak at ∼250 K, which is attributed to the presence of CeO2 segregation [3,6]. The present observed resistivity behaviour is similar to the one reported by Mitra et al. [6] for single phase thin film sample. This further suggest that the samples are close to single phase. The temperature dependence of MR of La0.8 Ce0.2 MnO3 sample with 30 nm particle size shows almost a linear decrease with increasing temperature in 0.5 and 1.5 T (inset of Fig. 6a). On the other hand, the 85 nm particle size sample shows a linear decrease of MR with a broad hump around MIT (Fig. 6c). The
Fig. 6. Temperature dependence of electrical resistivity, in different applied magnetic fields, of La0.8 Ce0.2 MnO3 sample with (a) 30 nm and (b) 85 nm. (c) The temperature dependence of MR of 85 nm samples in 0.5 and 1.5 T. The inset of (a) shows temperature dependence of MR of 30 nm sample.
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Fig. 7. Temperature dependence of electrical resistivity and MR (right hand scale) of La0.7 Ce0.3 MnO3 sample with (a) 40 nm and (b) 80 nm.
MR is calculated using, MR% = [ρ(H,T) − ρ(0,T)]×100/ρ(0,T); where ρ(0,T) and ρ(H,T) are resistivities in the absence and presence of applied magnetic field H, respectively. The temperature dependence of MR of La0.7 Ce0.3 MnO3 in 5 T shows a weak temperature dependence below MIT and a smooth decrease with increasing temperature above MIT (Fig. 7). The same behavior was observed both in 40 and 80 nm samples. The low temperature MR increases with decreasing particle size. The observed MR in these nanocrystals is higher than the MR observed in the bulk samples [11]. The reported MR for bulk sample at 5 K and in 7.7 T is 54% whereas in the present nanocrystalline samples the MR is 58% at the same value of field and temperature in La0.7 Ce0.3 MnO3 (Fig. 9a ). The qualitative understanding of MR will be discussed in the following paragraphs. The magnetic field dependence of isothermal MR of all the samples show sharp increase of MR at low fields below TC (Figs. 8 and 9). The low field MR (LFMR) increases with decreasing particle size. The La0.8 Ce0.2 MnO3 samples with particle size of 30 nm and 85 nm exhibit MR of ∼21% and ∼13% at
80 K and 0.5 T, respectively. Similarly the MR at 5 K and 0.5 T is ∼28% and ∼26% for La0.7 Ce0.3 MnO3 samples with particle size of 40 and 80 nm, respectively. The high field MR (HFMR) changes almost linearly with applied magnetic field below TC as observed in hole doped nanocrystalline manganites [21]. The MR at 80 K and 2.1 T is ∼46% and ∼25% for La0.8 Ce0.2 MnO3 samples with 30 and 85 nm respectively. The MR at 5 K and 9 T is ∼61% and ∼50% for La0.7 Ce0.3 MnO3 samples with particle size of 40 and 80 nm, respectively. The observed MR could be best understood by invoking spin dependent tunneling mechanism, as in hole doped manganites [21]. This model assumes that (i) the core of the nanoparticle is perfect ferromagnetic and its magnetic moment is equal to bulk magnetization. (ii) The surface (shell) of the nanoparticle is magnetically disordered due to the loss of lattice symmetry and magnetic coordination. Thus the shell is assumed as an amorphous layer with zero magnetization and is electrically insulator [22]. In this situation the itinerant electron can move freely within the core and to move to another particle, the electron has to tunnel across the electrical barrier formed
Fig. 8. Magnetic field dependence of isothermal MR of La0.8 Ce0.2 MnO3 samples with (a) 30 nm and (b) 85 nm at different temperatures.
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Fig. 9. Magnetic field dependence of isothermal MR of La0.7 Ce0.3 MnO3 samples with (a) 40 nm and (b) 80 nm.
by shell of the nanoparticle. In the absence of applied magnetic field, individual magnetic moments of the particles are oriented along their own direction and this results in maximum magnetic exchange energy. This gives rise to minimum spin dependent electron tunneling. Upon application of magnetic field, the neighbouring grains align parallel to each other at low fields and this leads to minimum magnetic exchange energy resulting in increase of spin dependent electron tunneling [23]. Thus the sharp increase of MR below 0.5 T may be attributed to the increase of spin dependent electron tunneling across the grain boundaries. After certain field strength all the grains/particles align magnetically parallel to each other and this leads to saturation of electron tunneling. The almost linear increase of high field MR (HFMR), above 0.5 T, could be attributed to the decrease of spin canting at the intra-grains and/or grain boundaries [24]. This decrease of spin canting leads to an increase of double exchange interaction resulting in increase of MR with applied field. Thus one can understand qualitatively the observed LFMR and HFMR by spin dependent inter grain tunneling and decrease of canted spins upon application of magnetic field, respectively. The present studies are the first reports, to the best of our knowledge, on detailed MR studies on Ce doped LaMnO3 . 4. Conclusions Nanocrystalline La0.8 Ce0.2 MnO3 and La0.7 Ce0.3 MnO3 samples have been synthesised by citrate-complex method. The estimated average crystallite/particle size is found to increase with calcination temperature. All the samples show ferro- paramagnetic transition and metal-insulator transition upon warming. The saturation magnetization decreases with particle size. The temperature dependence of resistivity shows no impurity peak near 250 K. The temperature dependence of magnetoresistance shows linear decrease upon warming in low magnetic fields. The low field as well as high field MR increases with decreasing particle size. The observed magnetoresistance behaviour could be best understood by invoking spin dependent tunneling model
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