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Preparation, Characterization and Magnetic Properties of Hexagonal Y<SUB>1-x</SUB>Ce<SUB>x</SUB>MnO<SUB>3</SUB>

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Indian J Rhys. 82 (2), 195-200 (2008)

Preparation, characterization and magnetic properties of hexagonal Y

1 x

Ce^Mn0

3

S K Srivastava

1

, Manoranjan Kar

2

and S Ravi

1

*

department of Physics, ^Centre for Nanotechnology

Indian Institute of Technology, Guwahati Guwahati-781 039, Assam India E-mail sravi(u ntg ernet in

Abstract Electron doped Y, ^Ce^MnO^ compounds were prepared for x = 0, 0 05, 0 10 and 0 15 by solid state route These samples were characterized by recording X-ray diffraction (XRD) pattern, Scanning Electron Micrographs and Energy Dispersive Spectrum (EDS) at room temperature All the samples are essentially in single phase form with hexagonal structure and could be refined using P63cm space group The typical lattice parameters are found to be a = b = 6 133A and c = 11 366A for Y09Ce0 ,MnOn sample Temperature variations of ac susceptibility measurements down to 20K were carried out and they show that the Ce doped samples exhibit paramagnetic to ferromagnetic transition with onset temperature at around 100K followed by low temperature peak at around 28K, indicating the presence of competing magnetic interactions

Keywords Electron doped manganites, Y, ^Ce^MnC^, Ferromagnetism, ac susceptibility PACS Nos. 75 47 Lx, 61 05 cp

1. Introduction

The rare earth perovskite manganites, (R1 O (AX) Mn03 (R = La, Rare earth elements, A = Alkaline earth, Alkali elements) have been studied extensively due to their interesting electrical and magnetic properties including colossal magnetoresistive (CMR) behaviour [1-5]. Recently there are a few reports on the electron doped rare earth manganites, where there is a possibility of CMR due to double exchange interaction between Mn2+ (t2g3 eQ2) and Mn3+ (f2g3 eg1) ions [6,7]. The La07Ce03MnO3 compounds exhibit transition from paramagnetic insulator (PMI) to ferromagnetic metallic state (FMM) along with CMR behaviour [6-11]. However it has been found that the observed CMR behaviour is mainly due to hole doping rather than electron doping, as a result of C e 02 impurity segregation and over oxidation. On the other hand, the electron doped Nd07Ce03MnO3 with Mn27Mn3+

Corresponding Author

©2008IACS

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196 SKSnvastava, Manoranjan Karand S Ravi

mixture could be prepared in single phase form, especially by depositing thin films and they exhibit FM transition around 60 K [6, 12-14]

The hexagonal YMn03 is the well known A-type antiferromagnetic (AFM) material with transition temperature around 80K [15-17] Unlike other rare earth manganites, the hole and electron doping studies in YMn03 are very limited To study the effect of electron doping and possible double exchange ferromagnetic interaction between Mn2 + and MnHf

ions, we have prepared Y1 xCexMn03 compunds for x = 0 to 0 15 As the ionic sizes of Y and Ce are comparable unlike, the case of La & Ce, it could be possible to prepare single phase materials for a wide doping range In the present article, the preparation, characterization and magnetic properties of Y1 xCexMn03 are presented

2. Experimental details

The Y1 xCexMn03 (x = 0, 005, 0 10 and 0 15) compounds were synthesized by solid state route The stoichiometric ratio of Y203, C e 02 and Mn metal powder with 99 9%

purity were weighed and mixed thoroughly under acetone The mixture was presintered at 860°C for 36 h with intermediate gnndings The sintering in pellet form was carried out at 1050°C for 24 h, followed by sintering at 1250°C for over 50h in oxygen atmosphere Then the samples were subjected to hot pressing by applying a load of 10 tonn at 950°C for 2h using a commercial hot press furnace The X-ray diffraction (XRD) patterns were recorded at room temperature using Bruker D8 Advance X-ray diffractometer by employing CuK(y

radiation Scanning Electron Micrographs were recorded at room temperature by using LEO Scanning Electron Microscope (SEM) Compositional analysis was carried out by employing SEM-EDS (energy dispersive spectrum) technique Temperature variation of ac susceptibility measurement was carried out by employing mutual inductance bridge method with an ac field amplitude of 6 0 e and at 333Hz A commercial helium closed cycle

3 1

£ 1

</>

c a>

P6qcm

JL

P6,cm

JL~-

YMnO,

* I

m &1 m ft EI

A*»mMAi •* »«„«, i„i #.>i,^^X^L^AA

I ' I ' I

10 20 30 40 50 60 70 29 (deg)

Figure 1. XRD patterns of Y M n 03 and Y0gCe01MnO3

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Preparation, characterization and magnetic properties of hexagonal Y1xCeMnO^ 197 refrigerator cryostat equipped with a Lake Shore temperature controller has been used for

temperature variation down to 20K.

3. Results and discussions

Typical XRD patterns for x = 0 and x = 0.10 are shown in Figure 1. All the observed peaks could be indexed to P63cm space group and hence the samples are essentially in single phase form. All the XPD patterns were analyzed using Fullprof program by employing Rietveld refinement technique [18]. Typical XRD pattern along with Rietveld refinement is shown in Figure 2 for x = 0.05. Here the experimental data are shown as crosses (+) and

3200 2700 2200 5 1700

3-

£1200 c jz 700

200 -300 -600

16 23 30 37 44 51 58 65 72 2()(°)

Figure 2. XRD pattern along with refined data for the sample Y095Ce0 05MnO3. Experimental data are shown as crosses (+) and calculated intensities are shown as solid lines. The bottom line represents the difference between measured and calculated intensities.

the calculated intensities are shown as solid lines. The bottom line represents the difference between measured and calculated intensities. The obtained lattice parameters are given in Table-1 and are comparable to those reported by Wood et al [16].

In polycrystalline materials, tiny crystals are arranged in random direction, the size of the crystals can be estimated from the width of the XRD patterns. The average crystal size depends upon the annealing temperature and doping concentrations and, under optimum preparation conditions; larger crystallite sizes are generally formed. In the present work, average crystallite size (Sc) has been calculated using the Scherrer's formula [19]

S

c

=/d//?cos0 (1)

where, constant 'k' depends upon the shape of the grain size (= 089, assuming the

circular grain), P is full width at half maximum (FWHM) of intensity vs. 20 profile, is the wavelength of the CuKa radiation and 0 is the Bragg's diffraction angle. The values are

i » i i | v * * v | i r t i 1 ' M ' ' » M " i r ' T > t ' t f i

li I I I I I ' III I l l i l l U I II Hill II III'

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198 S KSrivastava, Manoranjan KarandS Ravi

Table 1. Parameters obtained from XRD, SEM & ac susceptibility measurements for the samples Y1 xCexMnOj (x = 0 0, 0 05 010 and 0 15) Sc is the crystallite size and SG is the grain size ()c is Curie temperature

Sample x = 0 X = 005 x = 0 10 x = 0 1 5 a(A) 6 132 6 156 6 133 6 157 C(A) 11368 11293 11366 11273 Chr* 5 70 4 81 4 45 6 35

Volume (A1) 370 2 370 6 370 1 370 2

S( (nm) 55 6 53 6 50 3 48 5

S( j (^m) 1 23 1 1 4 1 09 1 03

0t (k) - 1 2 7 20 1 33 6

listed in Table 1 and it is found to decrease with increase in Ce concentration The crystallite sizes are found to decrease systematically with increase in Ce doping and it could be mainly due to minor mismatch is ionic size of doped element

The typical SEM micrographs for x - 00 and x = 0 10 with the magnification of 7000 are shown in Figures 3 and 4 respectively. We can see that the morphology of the samples is almost uniform and it suggests the monophasic nature of the sample The

Figure 3. SEM photograph (magnification 7000) of the Figure 4. SEM photograph (magnification 7000) of the YMn03 sample with average grain size of 1 23 nm Y 09C eo iM n° 3 sample with average grain size of 1 09

nm

average grain size for all the samples are given in Table 1. The grains are generally formed due to the agglomeration of several crystals and as a result, the size of grains is higher than that of crystals. The grain size is found to decrease with increase in 'Ce' concentration and it is in correlation with the trend of decrease in crystallite size.

Typical SEM EDS spectra for x = 0.10 sample is shown in Figure 5. One can see that all the elements are present and their composition ratio is comparable to the nominal starting composition and it is found to be Y0 93Ce0 08Mn1 0 1O2 g 8 for x = 0 10 sample.

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preparation, characterization and magnetic properties of hexagonal Y1xCexMnO,< 199 Figure 6. shows the typical plots of X versus temperature (T) for the samples x =

0.05 and 0.10. We can see that the samples exhibit upturn in susceptibility with onset temperature around 100K followed by sharp increase in susceptibility at low temperature.

Spectrum 6H

1 2 3 4 5 6

Ful Scale 990 els Cursor, 9 466 keV (4 els) keV

Figure 5. Typical SEM-EDS spectra for Yo gCe0 , M n 03 sample

In addition to that, they exhibit a sharp peak at 24K and 28 K for x = 0.05 and x = 0.10 samples respectively. The observed behaviour is comparable to that reported by Chen et al [20] on YMn1+x03 compound. The above observed FM behaviour can be explained on the basis of double exchange interaction between Mn2+ and Mn3+ ions. It is clear from the above observation that Ce plays the role of electron doping by creating a mixture of Mn2+ and Mn3+ ions. The appearance of low temperature peak indicates the presence of spin glass like behaviour due to competing magnetic interactions. The data in the high temperature region were fitted to the Curie Weiss law,

X = T - 0 , (2)

0 09-

0 06-

0 0 3 -

0 0 0 - 1

h

K

r — i — i » i—

Y^.Ce.MnOa x = 0.05 - - x = 0.10

1 I 1 1 1 I ' 1

50 100 150 200 250 Temperature (K)

300

f i g u r e 6. Temperature variation of inphase ac susceptibility ( x ' ) of Y1 0 (CexMn03 samples for x = 0.05 and 0.10.

150

100 150 Temperature (K)

200

Figure 7. M x versus Temperature for the sample Y0 95Ce0 05MnO3. Solid line represents the fit to the Curie- Weiss law.

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200 S K Snvastava, Manoranjan Kar and S Ravi

Here 0C is the Curie temperature, C is the Curie constant and X is the measured susceptibility. The typical MX versus T plot for x = 0.05 is shown in Figure 7 and it () exhibits a linear behaviour The fitted data are shown as solid line The Curie temperature obtained from the above fit is given in Table 1. The value of 0 x = 0 sample is found to be comparable to that reported by Munoz et al [21] The Curie temperature for Ce doped compounds is found to be positive, which reveals the presence of ferromagnetic interaction 4. Conclusions

Y1 xCexMn03 compounds have been prepared essentially in single phase form for x = 0 to 0 15 All the samples are found to be crystallized in hexagonal structure with P6^cm space group Increase in susceptibility magnitude, possitive Curie temperature and FM like transition have been observed in Ce doped samples The Curie temperature is found to be -ve for the parent YMn03 The observed magnetic behaviour suggest that there is a possibility of double exchange interaction between Mn24 and Mn3+ ions due to electron doping They also show the presence of spin glass like behaviour

Acknowledgment

The authors are thankful to DST, New Delhi for financial support

References

[1] R von Helmolt, J Wecker, B Holzapfel, L Scultz and K Samwer Phys Rev Lett 71 2331 (1993) [2] S Jin T H Tiefel, M McCormack, R A Fastnacht, R Ramesh and L H Chen Science 264 413 (1994) [3] A Asamitsu, Y Montomo, Y Tomioka, T Anma and Y Tokura Nature 373 407 (1995)

[4] R Mahesh, R Mahendiran, A K Raychudhun and C N R Rao J Solid State Chem 114 297 (1995) [5] Colossal Magneto-Resistance, Charge Ordering and Related Properties of Manganese Oxides

(ed) C N R Rao and B Raveau (Singapore World Scientific) (1998) [6] P Mandal and S Das Phys Rev B56 15073 (1997)

[7] J R Gebhardt S Roy and N Ah J Appl Phys 85 5390 (1999)

[8] C Mitra, P Raychaudhury, J John, S K Dhar A K Nigam and R Pinto J Appl Phys 89 524 (200D [9] R Ganguly, I K Gopalknshnan and J V Yakhmi Physica B275 308 (2004)

[10] Y G Zhao, R C Snvastava, P Fournier, V Smolyaninova, M Rajeswan, T Wu, Z Y Li, R L Greene and T Venkatesan J Magn Magnet Mat 220 161 (2000)

[11] T Yanagida, T Kanki, B Vilquin, H Tanaka and T Kawal J Appl Phys 97 033905 (2005) [12] S Z Hang, S Tan, W Tong and Y Zhang Phys Rev B72 014453 (2005)

[13] T Yanagida, T Kanki, B Vilquin H Tanaka and T Kawal J Appl Phys 99 053908 (2006)

[14] T Yanagida H Tanaka, T Kawal E Ikenaga, M Kobata, J J Kim and K Kobayashi Phys Rev B73 132503 (2006)

[15] W C Koehler, H L Yakel, E O Wollan and J W Cable Phys Lett 9 93 (1964)

[16] V E Wood, A E Austin, E W Collmgs and K C Brog J Phys Chem Solids 34 859 (1973) [17] Z J Huang, Y Cao, Y Y Sun, Y Y Xue and C W Chu Phys Rev B56 2623 (1997)

[18] R A Young "The Rtetveld Method" International Union of Crystallography (New York Oxford University Press) (1996)

[19] A Taylor X-ray Metallography (New York Wiley) (1961)

[20] W R Chen, F C Zhang, J Miao, B Xu, L X Cao, X G Qiu and B R Zhao J Phys Condens Matter 17 8029 (2005)

[21] A Munoz, J A Alnso, M T Castas, M J Martinez-Lope, J L Martinez and M T Fernandez J Phys Condes Matt 14 3285 (2002)

References

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