Electron diffraction evidence of charge-ordering at room-temperature in La1−xCaxMnO3 (0.55≤x≤0.67)

Solid State Communications 137 (2006) 158–161 www.elsevier.com/locate/ssc

Electron diffraction evidence of charge-ordering at room-temperature in La1KxCaxMnO3 (0.55%x%0.67) P.R. Sagdeo, Shahid Anwar, N.P. Lalla * UGC-DAE Consortium for Scientific Research, University Campus, Khandwa Road, Indore 452017, MP, India Received 10 October 2005; accepted 27 October 2005 by A.H. MacDonald Available online 15 November 2005

Abstract Temperature dependent electron diffraction of La1KxCaxMnO3 for 0.55%x%0.67 using transmission electron microscope (TEM) has been carried out in the temperature range of 106–300 K to study the melting of charge-ordering across the transition temperature. Clear signature of charge-ordering as evident by the presence of diffuse super-lattice spots persist even at room temperature. This has been consistently observed for four different samples with compositions within the range of 0.55%x%0.67. The results indicate emergence of some degree of itinerancy in the localized charge carriers as temperature rises. q 2005 Elsevier Ltd. All rights reserved. Keywords: Manganites; Charge-ordering; Charge–density-wave; Electron diffraction

1. Introduction The mixed valent manganites are a subject of intense applied and basic research due to its spin-dependent properties [1,2]. These compounds show colossal magnetoresistance, which has immense potential for applications. These compounds also exhibit interesting physical properties of basic importance, like charge-ordering and co-existence of ferromagnetic and antiferromagnetic states [3]. Charge-ordering takes place by localization of charge carriers at MnC3 and MnC4 sites [4], which can be melted by magnetic field [5], pressure [6], electromagnetic radiation [7], and of course by heating [8]. In recent past the occurrence of incommensurate charge-ordering has also been reported [9], which indicates the possibility of itinerant character leading towards charge– density wave type ordering [10]. Usually, the occurrence of charge-ordering has been reported at low temperatures but the possibility of high temperature charge ordered fluctuations have been suggested [11]. It should also be noted that charge carriers in the manganites might get localized in paramagnetic state due to strong Jahn–Telter coupling of charge carriers with lattice [8]. All these characters of charge-ordering appear to be linked and need to be studied through some direct tool. * Corresponding author. Tel.: C91 731 2463913; fax: C91 731 2462294. E-mail address: [email protected] (N.P. Lalla).

0038-1098/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2005.10.017

Keeping this in view in the present investigation we have carried out systematic temperature dependent electron diffraction studies of the melting of charge-ordered state in La1Kx CaxMnO3 (0.55%x%0.67). The electron diffraction using TEM has unique advantage for studying weak diffraction features because electron interacts strongly with the structure than that of the X-rays. 2. Experimental Polycrystalline samples of La1KxCaxMnO3 (0.55%x%0.67) were prepared by standard solid-state reaction route using stoichiometry proportions of CaCO3, La2O3, and MnO2 as the starting ingredients and then following multiple grinding and re-sintering at 1350 8C. Structural and phase purity characterizations of the as-prepared samples were carried out using powder X-ray diffraction (XRD) on a Rigaku rotating anode X-ray source operating at 10 kW output power, and backscattered electron imaging using JEOL-SEM 5600. EDAX analysis was also carried out to check the concentrations of La, Ca and Mn in the synthesized samples. To verify whether the prepared compounds do show the reported electrical properties [4], resistance vs. temperature (R–T) measurements were carried out using Van Der Pauw four-probe resistivity measurement technique, from 300 to 85 K at temperature intervals of w0.5 K and with a temperature stability of better than 20 mK. Transmission electron microscopy between 106 and 400 K was carried out using FEI-TECNAI-20-G 2 operating at 200 K V. The TEM machine is equipped with

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Gatan double tilt liquid nitrogen holder and a CCD camera (MegaView Sys). TEM samples were prepared using Gatan precision ion polishing system after proper optimization of its operating parameters, 3 K eV, 25 mA and G38, related to the twin argon ion guns. In order to record even the weakest feature in the selected area diffraction (SAD) pattern we used a relatively large selected area aperture of (50 mm) for recording the SAD patterns.

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(a)

3. Results and discussions Fig. 1 shows the representative powder XRD data of the LCMO samples studied in the present investigation. The XRD data of all the studied samples were refined using the FULLPROF-2K Rietveld refinement program with the space group Pnma, which is shown as continuous line in Fig. 1. The typical values of goodness of fit for all the XRD data were found to be around 1.8. The absence of even a weakest unaccounted peak in the refined XRD patterns confirms the high-phase purity of the samples. The absence of any secondary contrast in the backscattered SEM image further confirms the phase purity of these samples. EDAX analysis shows that the relative concentrations of La, Ca and Mn were same to that of the starting compositions within the typical error of EDAX analysis. The detailed X-ray diffraction studies of the structural parameters of these samples may be found in Ref. [12]. Fig. 2(a) and (b) depicts the SAD patterns from xZ 0.62 and 0.67 taken at 106 and 109 K, respectively. The occurrence of sharp super-lattice spots along [100] about the fundamental spots, as indicated by arrows, can be clearly seen. For xZ0.62 sample at 106 K the modulation vector q is found to be a*/3, i.e. the observed modulation is commensurate with the fundamental lattice, whereas for xZ0.67 sample at 109 K modulation vector is not commensurate. The value of 3,

La0.38Ca0.62MnO3 Observed data Fitted data Bragg positions

Intensity (a.u.)

Difference plot

25

50

75

100

2θ Fig. 1. A representative powder X-ray diffraction pattern of the La1KxCaxMnO3 (0.55%x%0.67) samples refined using Rietveld refinement program with space group Pnma.

(b)

Fig. 2. The selected area electron diffraction patterns of (a) La0.38Ca0.62MnO3 taken at 106 K and (b) of La0.33Ca0.67MnO3 taken at 109 K.

a measure of in-commensuration and defined as (3Z(1/3)Kq/ a*) is found to be 0.057. The occurrence of these super-lattice spots have been attributed to the charge-ordering in these mixed valent manganites [9]. To study in detail the melting of charge-ordering with increasing temperature various such SAD patterns were recorded at temperatures ranging from 106 K to room temperature, 295 K. Fig. 3(a) and (b) shows a sequence of a sections of SAD patterns taken at representative temperatures, as indicated, for xZ0.62 and 0.67 samples. Data exists at many intermediate temperatures but are not shown. It can be seen that as the temperature is increased the sharp super-lattice spots gradually become diffuse and shift towards the fundamental spot. This means that in direct-space the superlattice modulation vector increases but its coherence (structural correlation length) decrease with increasing temperature. At low temperatures, below 200 K, the rate of shift of the superlattice spots with temperature is rather slow, but above 200 K it increases and becomes maximum about the transition temperature TCO established based on R–T measurement. A plot of these shifts for xZ0.62 and 0.67, in terms of incommensuration as a function of temperature, is shown in Fig. 4(a). Fig. 4(b) shows the corresponding R–T variation. It can be clearly seen that the rate of change of shift with respect to temperature is maximum about the kink observed in R–T

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(a)

0.14 (a)

152 K

210 K

0.12

Incommensuration (ε)

185 K

La0.33Ca0.67MnO3 La0.38Ca0.62MnO3

0.10 0.08 0.06 0.04 0.02

223 K

0.00

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246 K

La0.38Ca0.62MnO3 La0.33Ca0.67MnO3

260 K 275 K

285 K

Resistivity (Ω-Cm)

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1000 100

10 10

La0.38Ca0.62MnO3 1

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La0.33Ca0.67MnO3 0.1

0.1 0.01 0.01

295K 1E-3 50

(b) 150 K

180 K

200 K 220 K

240 K

250 K

260 K

270 K

290 K

Fig. 3. Sequence of sections of selected area electron diffraction patterns of (a) La0.38Ca0.62MnO3 and (b) La0.33Ca0.67MnO3 taken at various temperatures.

data. It should be noted that such a kink in the R–T data has been attributed to the CO transition [4]. This comparison clearly shows that the rise in the in-commensuration is related with the same phenomena, which is also responsible for the kink in the R–T data. An unusual feature observed in the present investigations, see Fig. 3(a) and (b), is that the super lattice spots due to charge-ordering do not vanish even much

100

150

200

250

300

Temperature(K) Fig. 4. (a) A plot of incommensuration for La0.33Ca0.67MnO3 and La0.38Ca0.62MnO3 as a function of temperature. (b) Resistivity-vs.-temperature data for La0.33Ca0.67MnO3 and La0.38Ca0.62MnO3.

above the ordering temperature TCO. The presence of diffuse super-lattice spots can be seen clearly even at room temperature. To check whether these are just reminiscence of charge-ordering taken place while cooled below TCO, we heated the sample up to 100 8C for an hour and again cooled it back to room temperature, but these diffuse spots were still present. The presence of these diffuse charge-ordering superlattice spots at room temperature were found even in samples, which were never ever cooled to liquid nitrogen temperature. These observations tell that the presence of charge-ordering at high temperature is intrinsic to these samples. This feature is observed for all the studied samples with xZ0.55, 0.60, 0.62 and 0.67. A comparison of room temperature SAD patterns from these four samples is shown in Fig. 5. The occurrence of charge-ordering in the mixed valent LCMO compounds has been attributed to the ordering of MnC3 and MnC4 ions at their fixed crystallographic site along [100] of its room temperature Pnma structure [4]. The chargeordering is known to cause a rise in the R–T variation accompanied by the antiferromagnetic ordering of MnC3 and MnC4 ions. Hence, disappearance of antiferromagnetism and the kink in the R–T variation above the transition temperature TCO, have been concluded as the disappearance of chargeordering also. But the present results of systematic temperature dependent electron diffraction study across the TCO clearly shows that, although diffuse, but clear signature of CO is present even at room temperature. In our study, we found that,

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(a)

(b)

(c)

(d)

Fig. 5. A comparison of selected area electron diffraction patterns of La1Kx CaxMnO3 (a) xZ0.67, (b) xZ0.62, (c) xZ0.60 and (d) xZ0.55 samples exhibiting the presence of diffuse super-lattice spots due to charge-ordered state at room temperature.

as the temperature is increased, the charge-ordering modulation vector in direct-space increases, but its long-range order decreases. Hence, we argue that the kink observed in the R–T data is not itself the very first signature of charge-ordering, but it corresponds to the increase in the structural correlation length of the already existing precursor of charge-ordered state at room temperature. If we compare the magnetization data for similar samples as reported by Cheong et al. [4] with the resistivity transition temperature, then it becomes clear that the kink in the resistivity data corresponds to the emergence of antiferromagnetic ordering betweens the spins of neighboring MnC3 and MnC4. The charge transfer integral t for these systems is defined [13] as tZtij cos(qij/2), where qij is the classical angle between spins of neighboring MnC3 and MnC4. From the above expression it is obvious that localization of charge carriers should increases in the antiferromagnetic state. The continuous increase of charge-ordering modulation vector, which is incommensurate with the fundamental lattice, indicates that the wave function of the hopping charge carriers is no more confined only between Mn–O–Mn bond length, but is extended beyond this limit, i.e. some degree of itinerancy has started developing. The concept of bond-charge–density wave in which the charge is no more localized only to Mn site has been proposed [14]. As the sample is cooled below the transition temperature the increasing antiferromagnetic interactions forces the charge carriers to localize close to the crystallographic sites of Mn, hence the modulation vector will decrease and may saturate to an integral multiple of fundamental unit-cell parameter, i.e. the modulation vector may become commensurate. That is what has been observed in the present investigations.

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4. Conclusions Based on the above discussions of results of systematic lowtemperature electron diffraction studies it can be concluded that charge-ordered state in La1KxCaxMnO3 (0.55%x%0.67) compound exists even at room temperature. The structural coherence of the order is rather poor at room temperature but it gets enhanced at lower temperature, probably due to the antiferromagnetic ordering between MnC3 and MnC4 ions. Hence, it may be argued that the kink observed in the R–T data is not itself the very first signature of charge-ordering, but it corresponds to the increase in the structural correlation length of the already existing precursor of charge-ordered state at room temperature. Acknowledgements Authors would like to thank Dr P. Chaddah, Director. Prof A. Gupta, Center-Director of UGC-DAE-CSR Indore, Prof D. D. Sarma, IISc Bangalore, and Dr S. Sunder IGCAR Kalpakkam, for their interest in the work and helpful discussions. References [1] G.A. Prinz, Phys. Today 48 (1995) 58. [2] E. Dagotto (Ed.), Nanoscale Phase Separation and Colossal Magnetoresistance, Springer Series in Solid-State Sciences, vol. 136. [3] J.C. Loudon, N.D. Mathur, P.A. Midgley, Nature (London) 420 (2002) 797. [4] C.N.R. Rao, B. Raveau (Eds.), Colossal Magnetoresistance Charge Ordering and Related Properties of Manganese Oxides, World Scientific, Singapore, 1998. [5] Y. Tomioka, A. Asamitsu, Y. Moritomo, H. Kuwahara, Y. Tokura, Phys. Rev. Lett. 74 (1995) 5108. [6] Y. Moritomo, H. Kuwahara, Y. Tomioka, Y. Tokura, Phys. Rev. B 55 (1997) 7549. [7] V. Kiryukhin, D. Casa, J.P. Hill, B. Keimer, A. Vigliante, Y. Tomioka, Y. Tokura, Nature (London) 386 (1997) 813. [8] J.M. Zuo, J. Tao, Phys. Rev. B 63 (2001) 060407. [9] C.H. Chen, S.-W. Cheong, Phys. Rev. Lett. 76 (1996) 4042. [10] J.C. Loudon, S. Cox, A.J. Williams, J.P. Attfield, P.B. Littlewood, P.A. Midgley, N.D. Mathur, Phys. Rev. Lett. 94 (2005) 097202. [11] K.H. Kim, M. Uehara, S.-W. Cheong, Phys. Rev. B 62 (2000) R11945. [12] P.R. Sagdeo, S. Anwar, N.P. Lalla, Powder Diffr. Submitted for publication. [13] Y. Tokura, in: Y. Tokura (Ed.), Colossal Magneto-resistive Oxides, Gordon and Breach Science Publisher, New York, 2000. [14] V. Ferrari, M. Towler, P.B. Littlewood, Phys. Rev. Lett. 91 (2003) 227202.