J Mater Sci: Mater Electron (2014) 25:3795–3800 DOI 10.1007/s10854-014-2091-z

Comparative study on multifunctional behaviour of rare earth manganites with micro and nano grain size Hilal Ahmad Reshi • Shreeja Pillai Vilas Shelke

•

Received: 16 April 2014 / Accepted: 11 June 2014 / Published online: 29 June 2014 Ó Springer Science+Business Media New York 2014

Abstract We have investigated electrical and magnetotransport properties of La0.7Sr0.3MnO3 samples with grain size in micro to nanometric regime. The structural parameters obtained by Rietveld refinement of X-ray diffraction data revealed perovskite structure with orthorhombic (Pnma) and rhombohedral (R3C) symmetry for nano and bulk samples respectively. The average particle size was 22 nm and 2 lm for the two representative samples. A metal–insulator transition and substantial increase in electrical resistivity was observed in nanomaterials. An enhanced magnetoresistance observed in nanomaterials samples, makes them more promising for advanced device applications. The magnetization study showed signature of ferromagnetic cluster behavior and higher temperature coefficient of magnetization in nanoparticle samples.

1 Introduction Doped perovskite manganites, exhibiting colossal magnetoresistance (CMR) effect have attracted considerable attention not only for fundamental research, but also for potential applications [1–3]. The magnetic property of ultrafine granular systems is an interesting subject of research. Considerable difference in magnetization has been observed by varying particle size and has been attributed to the presence of nonmagnetic or magnetically dead surface layer or disordered spin orientation in the surface [4]. The nature of interplay between the crystal structure, magnetic and transport properties of manganites H. A. Reshi
S. Pillai
V. Shelke (&) Novel Materials Research Laboratory, Department of Physics, Barkatullah University, Bhopal 462026, India e-mail: [email protected]

is still a matter of discussion in spite of numerous investigations. The size and shape of particle, particle size distribution, finite size effect and numerous exchange interactions between the particles strongly influence the properties of manganites [5, 6]. As a result, the materials exhibit variety of physical properties in terms of magnetic, electric, magneto-electric, charge ordering, and orbital/ lattice degrees of freedom [7]. The phenomenon of metal–insulator transition and electronic phase diagram of doped manganite with nanoparticle samples has not been satisfactorily explained despite extensive efforts [3]. Most of the investigations show that the ferromagnetic (FM) Curie temperature and the metal–insulator transition temperatures in such materials are strongly dependent on composition and synthesis condition. The magnetic field induced modulation of resistance, has been widely studied in the manganites [8]. The composition La0.7Sr0.3MnO3 (LSMO) shows a variety of structural, electrical and magnetic transitions over a range of temperatures [9]. The physical properties of these materials are usually dependent on their preparation routes [10]. We have reported high and wide range of magnetoresistance (MR) in bulk manganite through combination of substitution and processing parameters [11, 12]. A ceramic material consisting of small manganite particles can show a large grain boundary effect at low temperatures. It is important to study comparatively the structure and physical properties of manganites in micron to nanoscale range. The samples with smaller grain sizes possibly show richer electronic and magnetic properties, due to the influence of the structural and magnetic disorders at the grain interfaces. Different techniques have been used to investigate the structural, magnetic and transport properties on manganites like Pr0.7Sr0.3MnO3 [13] and La0.85K0.15MnO3 [14]. However, detailed study on structural and magnetotransport behaviour of LSMO

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samples synthesized through different techniques has not been reported. In this report, an attempt has been made to compare the structural, electrical and magnetic properties of micro and nano grain size LSMO compound. The nanoparticle system shows distinct behavior in terms of crystal structure, electrical transport and magnetization. We observed metal– insulator transition arising from particle size reduction. Higher values of electrical resistance, MR, magnetic moment and temperature coefficient of magnetization (TCM) were observed in nanoparticle samples as compared to microparticle samples. The multifunctional features like structural transformation, electrical transition and magnetic ordering are attributed to the finite size effect.

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without and with magnetic field respectively. The magnetic measurement was carried by Quantum Design (MPMS XL5) SQUID magnetometer with 50 Oe magnetic field in the temperature range 10 K B T B 300 K. The TCM was quantified as TCM = (1/M)(dM/dT) 9 100 %, where M is field-cooled (FC) magnetization in the unit emu/gm.

3 Results and discussion Figure 1 shows grain structure for the polycrystalline samples synthesized by solid state reaction and sol–gel

2 Experimental technique Powder samples with composition LSMO were prepared by traditional solid state reaction and sol–gel techniques. In solid state reaction method, the stoichiometric amounts of high purity chemicals La2O3, SrCO3 and MnO2 were mixed in an agate mortar-pestle for several hours. In order to improve the reaction rate and ensure complete phase formation, intermediate grindings were performed. The mix was calcined at 950 °C for 24 h. The well-calcined mass was pressed into pellets of 12 mm diameter and then sintered at 1,100 °C for 24 h in an ambient air. In sol–gel process, (CH3COO)3 La
XH2O, (CH3COO)2Sr and C4H6MnO4
4H2O were used as starting reagents. The precursors were dissolved in triple distilled water containing 30 ml acetic acid. The resulting transparent solution was thoroughly mixed with 100 ml ethylene glycol and then 20 % of ammonium acetate solution was added. The complete solution was heated on a thermal plate under constant stirring at *80 °C to eliminate excess water. A homogeneous brown gel was achieved by heating at 110 °C and finally brown-black powder was formed. The powder was ground for 2 h and then calcined at 500 °C for 12 h. The calcined powder was pressed in the form of pellets and was sintered at 600 °C for 12 h. Scanning electron microscope (SEM) and transmission electron microscope (TEM) were used to obtain the grain structure and size. The phase formation and crystal structure parameters of the powder samples were identified by X-ray diffraction (D8 Advance Bruker) using CuKa radiation in the range 10° B 2h B 60° with step size of 0.02°. The X-ray diffraction data was analyzed by Rietveld refinement using FULLPROF package. We performed electrical resistivity measurement by standard four probe method using a commercial cryostat (Oxford Instruments Inc., UK) with zero and 5 tesla magnetic field in the temperature range 10 K B T B 300 K. The MR was defined as MR = [(RoRH)/Ro] 9 100 %, where Ro and RH are the resistances

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Fig. 1 Scanning electron micrographs for a solid state reaction, b sol–gel samples and c transmission electron micrograph of sol–gel sample

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3797 Table 1 The lattice parameters, wyckoff positions and R-factors of refined XRD patterns for micro and nanocrystalline LSMO samples Parameters

Microcrystalline

˚) a (A ˚) b (A

5.498

5.446

–

7.712

˚) c (A ˚ 3) V (A

13.338 349.220

Nanocrystalline

5.487 230.518

La?3 x

0.0000

0.0148

Y Z

0.0000 0.7439

0.2500 0.0114

x

0.0000

0.0000

y

0.0000

0.0000

Z

-0.0042

0.5000

x

0.4353

0.5041

y

0.4781

0.2500

z

0.2397

-0.0090

Mn?3

O-2 1

O-2 2

Fig. 2 Rietveld fitted X-ray diffraction patterns of LSMO samples prepared by a sol–gel (nanocrystalline) and b solid state reaction (microcrystalline) methods

techniques. The SEM image for the sample prepared by solid state reaction method (Fig. 1a), revealed spherical grains with average grain size around 2 lm. The SEM image for sol–gel prepared samples (Fig. 1b) showed agglomeration of nanoparticles. A TEM image of this sample shown in Fig. 1c) indicated clear picture of nanoparticles with average size around 22 nm. Generally, the grain size increases as the sintering temperature is increased and at temperature above 1,000 °C, rapid grain growth results in micron size particles. In solid state reaction method the constituents have micron grain size and after sintering at elevated temperature larger grain size is expected. On the other hand, chemical route like sol–gel, with low sintering temperature (600 °C) results in nanometer grain size. The Rietveld refined diffraction patterns for the samples with nominal composition LSMO sintered at 600 °C (sol– gel) and 1,100 °C (solid state reaction) are shown in Fig. 2a, b respectively. All the prominent peaks indicate the presence of single phase compound with orthorhombic (Pnma) and rhombohedral (R3C) lattice structures for nanocrystalline and microcrystalline LSMO samples

x

–

0.2009

y

–

0.0221

z

–

0.6819

Rwp

19.6

4.16

Rp Re

16.9 17.93

5.36 13.83

RBragg

11.02

7.91

v2

1.20

1.54

respectively. The refined values of lattice parameters obtained through Rietveld analysis are shown in Table 1. The XRD pattern of nanocrystalline sample indicates that the sample is fully crystallized without any kind of impurity. The grain size obtained through Scherer’s formula for nano sample was 14 nm in reasonable agreement with TEM result. The synthesis of samples through solid state reaction method needs higher and long sintering time to obtain homogeneous composition and desired structures. The microcrystalline sample showed rhombohedral structure which agree with earlier reports [15, 16]. The structural transition from rhombohedral to orthorhombic has been observed by replacing Sr content around 10–15 % by Ca [17]. However, we have observed orthorhombic structure in nanocrystalline material. The rhombohedral-toorthorhombic phase transformation can be understood as a cooperative Jahn–Teller distortion of MnO6 octahedra. One long and two short Mn–O bonds with reduced bond angle can impart orthorhombic structure in nanosized LSMO compound [18, 19]. The grain size induced strain can impart reasonable structural distortion.

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Fig. 4 Temperature dependent variation of magnetoresistance of LSMO nano and microcrystalline samples

Fig. 3 Temperature dependence of resistivity under zero and high fields for Nano and microcrystalline LSMO samples

Figure 3 shows the temperature dependence of zero and high field resistivity for LSMO nano and microcrystalline samples. The electrical transport properties depend on grain size and the porosity of the pellets. The resistivity value also appeared to be dependent to a large extent on the synthesis conditions. The sample prepared by solid state reaction showed linear metallic behavior without metal–insulator transition throughout the measured range. The metal–insulator transition has been reported in composites [20]. However, the sample prepared by sol–gel route showed metal– insulator transition around 250 K. It indicated that the transition has a strong relation with reduced grain size. A low temperature resistivity upturn appears in the nanocrystalline sample, as a contribution of Coulomb blockade or Kondo effect [21, 22]. These features are predominantly observed in nanomaterials with a possible existence of new interaction by strong correlation characteristics between the charge carriers. The overall resistivity increased several orders of magnitude (milliohm to ohm scale) as we move from micro to nano grain size. This increase in resistivity occurs due to enhanced scattering of the charge carriers at grain boundaries with smaller grain size. There is more suppression of resistivity in nanocrystalline sample as compared to microcrystalline on application of high magnetic field. In solid state reaction method, increase in grain size leads to decrease in grain boundaries, thereby decreasing the electrical resistivity. The resistivity for the samples with smaller grain size shows the predominance of the grain boundary in the transport process. The effect of external magnetic field on electrical behavior measured under 0 and 5 T magnetic field is shown in Fig. 4. Both nano and microcrystalline samples

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show MR value linearly increasing with decreasing temperature. The nanocrystalline sample showed higher MR value than microcrystalline sample. Both nano and microcrystalline samples reveal the monotonous increase in MR down to low temperature. Several efforts are under way to diversify the compounds having better microstructural properties for making appreciably large MR near room temperature under low magnetic field [23]. We have recently reported that low field MR can be improved in nanostructured polycrystalline samples for promising device applications [24]. The decreasing grain size leads to the enhancement in low field MR at lower temperatures while MR is suppressed at higher temperature [25]. The low temperature enhanced MR contribution originates from magnetic field dependent intergrain spin-polarized tunneling of conduction electrons between the adjacent grain boundaries [8, 26, 27]. The higher surface to volume ratio with decreasing grain size resulted in larger contribution from surface region to different physical properties. In nanosized materials, a huge fraction of atoms residing at grain boundaries have different temperature dependent relaxation processes which directly affect the transport properties. 3.1 Magnetic behavior The temperature dependent field cooled (FC) and zero field cooled (ZFC) magnetizations behaviour for nanograin (FC60, ZFC60) and micrograin (FC11, ZFC11) samples is shown in Fig. 5a. The paramagnetic to FM transition may be well above room temperature, which was beyond the range of our measurement. One interesting feature is the huge bifurcation between FC and ZFC magnetization in both micro and nanocrystalline samples below irreversibility temperature. The magnetic irreversibility temperature (Tirr) in bulk sample is around 300 K, however it is

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TCM signifies rate of change of magnetic moment with temperature. In nanograin sample the values of magnetic moment are higher at low temperature and lower at room temperature than those of micrograin samples. This results in higher TCM value for nanograin sample.

4 Conclusions We synthesized LSMO samples by solid state reaction and sol–gel methods. The samples have pure LSMO phase with orthorhombic (Pnma) and rhombohedral (R3C) unit cells for nano and micron grain size samples respectively. The metal–insulator transition appeared in finite sized sample is a characteristic feature, which is not observed in bulk sample with composition LSMO. An enhanced MR has been observed in nanocrystalline samples with MR value up to 52 %. The nanocrystalline sample also exhibited the FM cluster like features indicated by wide divergence of FC and ZFC magnetization curves. This sample also showed higher magnetization and TCM values than the bulk sample.

Fig. 5 a Zero-field cooled (ZFC) and field cooled (FC) magnetization and b TCM for micro (ZFC11, FC11, TCM11) and nano (ZFC60, FC60, TCM60) grain samples

reduced to 240 K for nanocrystalline sample. Also the magnetization value is higher in nanocrystalline sample compared to bulk one. The surface to volume ratio becomes large enough with reduced grain size and the FM clusters near the surface can form a percolation path. If the grains are connected compactly to each other, the FMclusters near the particle surface can easily form percolation path and accordingly the magnetism increases. In ZFC, the moment of FM-clusters freezes into random orientations so that the magnetization at low temperature is low. The frozen clusters gradually melt with increasing temperature and magnetization rises. When the temperature reaches near to blocking temperature (Tp), the moments of FM-clusters are being disturbed by thermal fluctuations and the magnetization starts to decrease leading to the appearance of defreezing clusters. The shifting of Tp is possible by the variation of the Mn?3/Mn?4 ratios [28] or by surface to volume ratio due to decreasing particle size. The decrease of Tirr and Tp is a clear indication of frozen moment of FM-clusters in nanoparticle samples. The variation of TCM with temperature is shown in Fig. 5 b. The micrograin sample (TCM11) showed negative value \1 % whereas in nanograin sample (TCM60) the value is 3.5 % at room temperature. Recently, Staruch et al. [29], reported similar behaviour in LSMO thin films. Basically,

Acknowledgments We are thankful to M. P. Council of Science and Technology, Bhopal and University Grants Commission, New Delhi for providing financial support. We are grateful to Dr. Alok Banerjee, Dr. Rajiv Rawat and Dr. Mukul Gupta, UGC-DAE Consortium for Scientific Research, Indore for providing experimental facilities.

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