The effect of sintering temperature on the magneto ...

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May 22, 2015 - P.K. Siwach, V.P.S. Awana, H. Kishan, R. Prasad, H.K. Singh, S. Balamurugan, E.T. Muromachi, O.N. Srivastava, J. Appl. Phys. 101, 073912 ...
J Mater Sci: Mater Electron DOI 10.1007/s10854-015-3235-5

The effect of sintering temperature on the magneto-transport properties of Pr0.67Sr0.332xAgxMnO3 (0 £ x £ 0.1) manganites Masroor Ahmad Bhat1 • Anchit Modi1 • N. K. Gaur1

Received: 24 March 2015 / Accepted: 22 May 2015 Ó Springer Science+Business Media New York 2015

Abstract We have synthesized Pr0.67Sr0.33-xAgxMnO3 (0 B x B 0.1) compound by conventional solid state reaction method at different sintering temperature. The structural parameters obtained using Rietveld refinement of X-ray diffraction data which showed perovskite structure with orthorhombic (Pnma) symmetry without any detectable impurity phase. The grain growth and grain connectivity extensively improved at higher sintering temperature with doping concentration. Depending upon the doping and the increment of sintering temperature, the samples showed very high magneto-resistance (MR%) and temperature coefficient of resistance (TCR). An improvement in magneto-resistance and TCR with increasing the sintering temperature i.e., 1200–1300 °C which make the compound more promising for scientific and technological applications.

1 Introduction Over the years, the pervoskite manganites R1-xAxMnO3 (R = rare earth and A = alkaline earth) have become the center juncture in advance materials science owing to the promising applications of these materials for magnetic sensors, spintronics and bolometric devices etc. [1–5]. These materials offer a high degree of chemical flexibility leading to complex interplay between structural, electronic and magnetic properties. By the partially substitution divalent/monovalent ions at the rare earth site, the resistivity & Masroor Ahmad Bhat [email protected] 1

Superconductivity Research Laboratory, Department of Physics, Barkatullah University, Bhopal 462026, India

profile exhibits a metal–insulator transition phenomenon at temperature (Tp) and these samples show paramagnetic insulator behavior above Tp and ferromagnetic metal behavior below Tp. The double exchange (DE) mechanism is used to explain the nature of attraction, metallic behavior of the material below the insulator–metal transition temperature (Tp) [6]. The resulting strong interaction among spin, charge, lattice/orbital degrees of freedom depends on which interactions dominate. It can be tuned by dopants, the application of external magnetic field, pressure, electric field, radiation etc. [7–9]. However, several studies have shown that Jahn–Teller (JT) distortion and grain boundary (GB) effects also plays an imperative function to comprehend the physics lying in these CMR materials [6]. During last decade the influence of rare earth site substitution by a divalent/trivalent element and Mn site substitution have been extensively studied [10–13]. Although very few reports are on monovalent alkali metal ion doped system. The monovalent substitution directly affect the ratio of Mn3? and Mn4? ions as compared to divalent which ultimately affects the DE mechanism and hence the magentotransport properties in a way different form that of the divalent cation substituted manganites [14]. Further, the crystal structure may be modified depending on the radius of monovalent ions [15]. Lakshmi et al. [16] successfully reported that the influence of silver doping on the electrical and magnetic behavior of La0.7Ca0.3MnO3 (LCMO) manganites. Earlier the effect of Ag? cation on La0.7Sr0.3MnO3 (LSMO) material have been reported by Koubaa et al. [17] which do not show any improvement in MR. However, Yadav et al. [18] observed the improvement in magnetoresistance (MR) and temperature coefficient of resistance (TCR) with monovalent Ag? substitution on LSMO compounds. On the other hand, the sintering temperature promotes grain growth, improves connectivity between the

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J Mater Sci: Mater Electron

grains also plays a major role in deciding the electronic and magnetic properties in polycrystalline CMR materials. The grain boundaries can be altered with sintering temperature which influences electrical and magnetic properties [19– 21]. Chang et al. [22] have paid much attention to investigate the effect of sintering temperature on the electrical and wide range magnetoresistance which shows improvement in magnetoresistance, suppression of resistivity at larger instant. Similarly, Yadav et al. [23] obtained the large enhancement in magnetoresistance as function of sintering temperature in La0.7Ca0.3-xAgxMnO3 system. In the present paper, we have synthesized Pr0.67 Sr0.33-xAgxMnO3 (0 B x B 0.1) compounds by conventional solid state reaction method at different sintering temperature 1200 and 1300 °C to understand the influence of sintering temperature on doping of Ag? cation on magneto-resistance (MR) behavior and temperature coefficient of resistance (TCR). We observed very high wide range magnetoresistance and temperature coefficient of resistance on increasing the sintering temperature in these samples which from the application point of view can be used to tune the sensing mechanism in bolometric sensors.

2 Experimental procedure We have synthesized the polycrystalline samples Pr0.67 Sr0.33-xAgxMnO3 (0 B x B 0.1) by conventional solid state reaction technique from high purity (C99.99 % Aldrich) stoichiometric of Pr6O11, SrCo3, Ag2O and MnO2 quantities. These quantities were mixed in agate mortar pastel and grinded for several hours. The calcinations were performed twice at 950 °C for 12 h with their respective intermediate grindings. After calcinations the resulting powders were again re-grounded and palletized by applying hydraulic pressure of 8–10 tons. The two separate batches of pallets of size 10 mm diameter and approximately 1–2 mm thickness were finally sintered at 1200 and 1300 °C respectively for 24 h in air ambient atmosphere. All the samples slowly cool down to room temperature to maximize the oxygen content. The samples in the present investigation were subjected to X-ray powder diffraction (XRD) technique by using Bruker D8 Advance diffractrometer with Cu-Ka (k = ˚ ) radiation operating at 40 kV/100 mA. The data 1.56406 A collected from 20° to 80° in 2h range with steps size 0.02° and a counting time 15 s/step. The XRD data was analyzed using Rietvled refinement techniques via Fullprof program [24] to confirm the phase formation as well as to obtain the lattice parameters, space group and crystal structural. The surface morphological analysis of the samples investigated by scanning electron microscopy (SEM) at room temperature on a JSM-6400 apparatus working at 20 kV. The

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temperature dependence of resistivity measurements were performed on a rectangular sample cut from the sintered pellet by four-probe resistivity measurement setup using superconducting magnet. The electrical contacts were made by attaching four fine copper wires to the samples using indium soldering.

3 Results and discussion 3.1 Structural analysis The X-ray diffraction patterns of Pr0.67Sr0.33-xAgxMnO3 (0 B x B 0.1) samples sintered at 1200 and 1300 °C with corresponding fit using Rietveld refinement for all nominal composition are shown in Fig. 1. The data fitted over 20°–80°. All the peaks have indexed satisfactorily as is evident from the bars representing the peak position. There is no new peak which suggest that all the samples are single phase without any detectable impurity or any additional phase. The lattice parameters and all relevant parameters of interest are recapitulated in Table 1. The crystal structure for all the compositions are indexed to the class of orthorhombic structure having space group (Pnma). To examine the structure stability by calculating pffiffiffi the tolerance factor t ¼ ðrA þ rB Þ 2ðrB þ ro Þ which satisfies the condition for stable perovskite phase (i.e., t \ 1). It is observed that the substitution of Ag ? cation slightly increase the lattice parameter and unit cell vol˚) ume. It might be due to larger ionic radii for Ag? (1.28 A 2? ˚ than Sr (1.26 A) [25]. Further, with increasing the sintering temperature from 1200 to 1300 °C the lattice parameters and unit cell volume also increases. The sintering temperature melts the silver through dissociative evaporation in a sharp manner and increased the Mn–O bond length and unit cell volume. 3.2 Microstructural study To inspect to microstructure information of Pr0.67Sr0.33-x AgxMnO3 (0 B x B 0.1) samples as revealed through scanning electron microscopy images sintered at 1200 and 1300 °C is shown in Fig. 2. It is clearly seen that the grain size improved dramatically with increasing Ag concentration and sintering temperature. The high sintering temperature gives birth to grain growth and cause little shrinkage in the specimen. The sample had flake/porous like grains with good connectivity of size of the order of 5 lm. Thus, sintering temperature had enhanced conducting channels among the grains that is the reason for sharp slope in electrical behavior and resulting high MR value.

J Mater Sci: Mater Electron Fig. 1 Rietveld plot of Pr0.67Sr0.33-xAgxMnO3 (0 B x B 0.10) at 1200 and 1300 °C

Table 1 Lattice and structural parameters obtained at room temperature

Samples sintered at 1200 °C

Samples sintered at 1300 °C

Sample code

x = 0.0

x = 0.1

x = 0.0

x = 0.1

˚) a (A ˚) b (A ˚) c (A

5.4498 7.7002

5.4780 7.7568

5.4561 7.7090

5.4582 7.7298

5.4777

5.4734

5.4882

5.4587

˚ 3) Volume (A

229.86

232.57

230.83

230.30

Space group

Pnma

Pnma

Pnma

Pnma

v2

1.332

2.047

1.484

1.518

Bragg factor

6.84

11.19

8.95

5.80

3.3 Electrical resistivity The variation of electrical resistivity (q) as a function of temperature for Pr0.67Sr0.33-xAgxMnO3 (0 B x B 0.1) at zero, 5 and 8 T magnetic field at different sintering temperature are shown in Fig. 3. On inspection of Fig. 3 it is found that the sample x = 0 exhibit metal–insulator transition below 300 K at low sintering atmosphere while TP is shifted beyond room temperature with a kink at *272 K and TP * 297 K at 0 T with increasing sintering temperature while in Ag? (x = 0.10) substituted samples, the sharp incensement in TP is observed at higher sintering atmosphere with lesser resistivity as compared to sample sintered at lower sintering atmosphere. The TP values are found to increase with increasing sintering temperature and

Ag? cation substitution resulting decrease in resistivity. The manganites are very sensitive to dopant concentration, oxygen content, synthesis conditions, temperature etc. The applicable reason behind this process is that the grain boundaries can be altered by varying sintering temperature which restrains the magnetic disorder thereby decreasing the number of GB defects [26, 27]. The samples sintered at 1300 °C leads more shrinkage, promotes grain growth/ improves grain connectivity because sintering reduces the total pore surface and volume effects which enhances the concentration of hopping mobility of carriers between Mn4?/Mn3? ratio. This enhances the conducting channels and drives the TP of the system towards room temperature. It can be seen that the variation in TP at higher sintering atmosphere may be attributed to increase the ratio of

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Fig. 2 Scanning electron microscopy image of Pr0.67Sr33-xAgxMnO3 (0 B x B 0.1) sintered at 1200 °C (a 0.0, b 0.1) and 1300 °C (c 0.0, d 0.1) Fig. 3 Temperature dependent resistivity of Pr0.67Sr0.33-xAgxMnO3 (0 B x B 0.1) samples sintered at a 1200 °C and b 1300 °C without and with magnetic field of 5 and 8 T

Mn4?/Mn3? which enhances the number of charge carriers in eg band. The decrease in resistivity at higher fields might be due to the reason that at x = 0.1 amount of Ag ions, there will be (0.3 ? x) amount of increase in Mn4? ions result of a small amount of sintering agent, which create large number of charge carriers suppressing the resistivity and shifts TP towards room temperature for practical application purposes.

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3.3.1 Magnetoresistance behaviour The change in resistance under applied magnetic field is termed as magnetoresistance (MR) and its variation with temperature is derived by using following relation as; q  qH MR% ¼ O  100 ð1Þ qO

J Mater Sci: Mater Electron

where q0 and qH are the zero and high field resistivity respectively. It is clearly seen from the Fig. 4 that MR% is maximum *55 % at low temperatures for pure sample at 8 T and a kink at higher sintering temperature is observed which is a typical characteristic of granular system. It reflects the spin-dependent electron tunneling or scattering process takes place at grain boundaries which is an extrinsic property of granular manganites [28]. It is evident from the Fig. 4. that temperature dependent magnetoresistance shows maximum MR% *91 % at a broad transition temperature Tc * 243 K at 1200 °C and a sharp MR% *93 % at a transition temperature Tc * 233 K at 1300 °C which is much more as compared to previously reported [22]. The MR percentage increases with increase of magnetic field and sintering temperature. It might be due to suppression of the magnetic spins scattering with the application of magnetic field, causing the local ordering of the magnetic spins. This ordering suppresses the paramagnetic insulating state and dominates the ferromagnetic metallic state. Therefore conduction of electrons will get polarized completely inside the magnetic domain area resulting in the smooth transfer between Mn3? and Mn4? via oxygen [28]. It indicates that samples sintered at high temperature improves the grain growth having spiral pattern along a favorable direction giving rise birth to high MR. Another possible reason is that the grains are agglomerated and microstructure densification is occurred via silver doping at higher sintering temperature. The increase in average grain size will improve the connectivity among grains which enlarges high magnetoresistance value suitable for applications [29].

3.3.2 Temperature coefficient of resistivity (TCR) The metal to insulator transition nature of the polycrystalline samples can be manifested from temperature coefficient of resistivity (TCR). The TCR% can be derived by using the formula: TCR% ¼

  1 dq  100 q0 dt

ð2Þ

The clear representation of TCR value for Ag substituted (Ag = 0.10) sintered at 1200 and 1300 °C are displayed in Fig. 5. The sample exhibited TCR value of 11 % with broad transition at low temperature while a sharp TCR% *45 % occurs at similar composition with high sintering temperature which is quite high as compared to reported earlier. Recently, it was observed that TCR value of 11 % in Ag doped LSMO annealed in air at high sintering temperature [18]. The improvement in TCR was considered due to enhancement of grain connectivity at higher sintering temperature and conducting channels by silver addition. The reason for the high TCR value originates from the sharp electrical transition. The clear depiction for the increase in high TCR at higher sintering temperature in our case is perhaps the crystalline quality becomes better which removes pores and voids and improves the conducting channels across the grains at a large scale. Since manganites are very much popular have close packed perovskite structure and therefore no interstitial site is available for excessive oxygen. It is very tedious to form cationic vacancies if it has to accommodate excess oxygen. Our system sintered at higher temperature synthesized in air ambient with the nominal composition received proper

Fig. 4 Temperature dependent magnetoresistance behavior of Pr0.67Sr0.33-xAgxMnO3 (0 B x B 0.1) samples sintered at a 1200 °C and b 1300 °C without and with magnetic field of 5 and 8 T

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J Mater Sci: Mater Electron Fig. 5 Temperature dependent TCR value of Pr0.67Sr0.33-xAgxMnO3 (0 B x B 0.1) samples sintered at a 1200 °C and b 1300 °C

insitu oxygenation composition and process parameters resulting in sharp transition and high TCR value.

4 Conclusion The single phase polycrystalline samples of Pr0.67Sr0.33-x AgxMnO3 (0 B x B 0.10) have been synthesized successfully by solid state reaction method. The structural analysis (XRD) for all composition suggests the lattice parameter and unit cell volume increase on substitution of Ag? cation due to larger ionic radii for Ag? than Sr2?. However no change is observed in unit cell structure and space group. We demonstrate that the sharp transition resulted in narrow range has huge MR value and conducting channels among the grains has been found to get enhanced with increasing sintering temperature. The higher MR% of *93 % and TCR% value of about *45 % observed in the present investigation resulted due to higher sintering temperature could serve as a potential candidate for infrared detector applications. Acknowledgments The authors are thankful to University Grant Commission (UGC), New Delhi for providing the financial support. Authors are admiringly acknowledged Dr. Rajeev Rawat, Dr. Mukul Gupta of UGC-DAE CSR Indore for providing the measurement facility.

References 1. E. Dagotto, Nanoscale Phase Separation and Colossal Magnetoresistance, Solid State Physics, vol. 136 (Springer, Berlin, 2003) 2. Y. Tokura, Rep. Prog. Phys. 69, 797 (2006) 3. G. Catalan, J.F. Scott, Adv. Mater. 21, 2463 (2009) 4. R. Mahendiran, A. Maignan, S. Hebert, C. Martin, M. Hervieu, B. Raveau, J.F. Mitchell, P. Schiffer, Phys. Rev. Lett. 89, 286602 (2002)

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5. K. Dorr, J. Phys. D Appl. Phys. 39, R131 (2006) 6. P.K. Siwach, V.P.S. Awana, H. Kishan, R. Prasad, H.K. Singh, S. Balamurugan, E.T. Muromachi, O.N. Srivastava, J. Appl. Phys. 101, 073912 (2007) 7. Y. Tokura, N. Nagaosa, Science 288, 462 (2000) 8. L. Hu, Y.P. Sun, B. Wang, Z.G. Sheng, X. Luo, X.B. Zhu, Z.R. Yang, W.H. Song, J.M. Dai, Z.Z. Yin, W.B. Wu, J. Appl. Phys. 106, 083903 (2009) 9. A. Antonakos, M. Filippi, P. Auban-Senzier, D. Lampakis, C.R. Pasquier, W. Prellier, E. Liarokapis, Phys. Status Solidi B 246, 622 (2009) 10. M.D. Daivajna, A. Rao, G.S. Okram, J. Alloys Compd. 617, 345351 (2014) 11. R. Choithrani, M.A. Bhat, N.K. Gaur, Phys. B 441, 320322 (2014) 12. A. Modi, N.K. Gaur, J. Alloys Compd. 644, 575 (2015) 13. G. Venkataiah, P.V. Reddy, J. Mater. Sci. 43, 4760 (2008) 14. S. Bhattacharya, R.K. Mukherjee, B.K. Chaudhuri, Appl. Phys. Lett. 23, 4101 (2003) 15. S. Roy, Y.Q. Guo, S. Venkatesh, N. Ali, J. Phys. Condens. Matter 13, 9547 (2001) 16. Y.K. Lakshmi, P.V. Reddy, Solid. State Science 12, 1731 (2010) 17. W.C. Koubaa, M. Koubaa, A. Chiekhrouhou, J. Alloys Compd. 453, 42 (2008) 18. R. Yadav, A. Anshul, V. Shelke, J. Mater. Sci. Mater. Electron. 22, 1173 (2011) 19. C.S. Kumara, M.B. Bellakki, A.S.P. Prakash, N.Y. Vasanthacharya, J. Am. Ceram. Soc. 90, 3852 (2007) 20. R. Tripathi, A. Dogra, A.K. Srivastava, V.P.S. Awana, R.K. Kontala, G.L. Bhalla, H. Kishan, J. Phys. D Appl. Phys. 42, 025003 (2009) 21. L.W. Lei, Z.Y. Fu, J.Y. Zhang, Mater. Lett. 60, 970 (2006) 22. Y.L. Chang, C.K. Ong, J. Phys. Condens. Matter 16, 3711 (2004) 23. R. Yadav, V. Shelke, J. Mater. Sci. Mater. Electron. 24, 1141 (2013) 24. H.M. Rietveld, J. Appl. Crystallogr. 2, 65 (1969) 25. R.D. Shanon, Acta Crystallogr. A 32, 751 (1976) 26. C. Shivakumara, M.B. Bellakki, A.S. Prakash, N.Y. Vasanthacharya, J. Am. Ceram. Soc. 90, 3852 (2007) 27. L.W. Lei, Z.Y. Fu, J.Y. Zhang, Mater. Lett. 60, 970 (2006) 28. G. Venkataiah, V. Prasad, P.V. Reddy, J. Alloys Compd. 429, 1 (2007) 29. S. Qixiang, N. Xiaofei, W. Guiying, T. Yonggang, C. Zhirang, P. Zhensheng, Rare Met. 29, 2 (2010)