Influence of Sintering Temperature on Magnetotransport Behavior of Some Nanocrystalline Manganites

The colossal magnetoresistance (CMR) in hole doped manganese oxides, widely known as manganites with formula Ln1-xAexMnO3 (where Ln is a trivalent rare earth and Ae is a divalent alkaline earth ion; x=0-1), has been intensively studied over the last two decades. In general, these systems comprise a strong competition between charge, orbital, lattice and spin degrees of freedom all of which make them an intriguing subject of research (Dogotto, 2003; Goodenough, 2003; Ramirez, 1997; Tokura, 2000). The end-members of this system are antiferromagnetic (AFI) insulators. Partial substitution of the Ln with divalent alkaline-earth ion of the ABO3 structure introduces mixed valence Mn4+/Mn3+ on the B site. With decreasing temperature the system undergoes a transition from the paramagnetic to the ferromagnetic state accompanied by a metal-insulator transition. A large number of studies on CMR materials of different forms such as single crystals (Okuda et al., 1998; Zhou et al., 1997), thin film (Kwon et al., 1997; Rao et al., 1998; Suzuki et al., 1997) and ceramics (Hwang et al., 1995; Mahendiran et al., 1996a, 1996b) for the basic research point of view and also the possible future device applications view point. The Zener (1951) has proposed a simplest model, known as double-exchange (DE) model to explain the electrical behavior in ferromagnetic metallic region below Curie Temperature (TC). However, due to various interactions among charge, spin and lattice makes these materials more complex and DE alone cannot explain the entire electrical transport behavior. Later on various theoretical models have been proposed by considering, electron-lattice and spin lattice interaction and even today there is no comprehensive model to explain transport phenomena in manganites (Millis et al., 1995; Tokura, 2000).


Introduction
The colossal magnetoresistance (CMR) in hole doped manganese oxides, widely known as manganites with formula Ln 1-x Ae x MnO 3 (where Ln is a trivalent rare earth and Ae is a divalent alkaline earth ion; x=0-1), has been intensively studied over the last two decades. In general, these systems comprise a strong competition between charge, orbital, lattice and spin degrees of freedom all of which make them an intriguing subject of research (Dogotto, 2003;Goodenough, 2003;Ramirez, 1997;Tokura, 2000). The end-members of this system are antiferromagnetic (AFI) insulators. Partial substitution of the Ln with divalent alkaline-earth ion of the ABO 3 structure introduces mixed valence Mn 4+ /Mn 3+ on the B site. With decreasing temperature the system undergoes a transition from the paramagnetic to the ferromagnetic state accompanied by a metal-insulator transition. A large number of studies on CMR materials of different forms such as single crystals (Okuda et al., 1998;Zhou et al., 1997), thin film (Kwon et al., 1997;Rao et al., 1998;Suzuki et al., 1997) and ceramics (Hwang et al., 1995;Mahendiran et al., 1996aMahendiran et al., , 1996b for the basic research point of view and also the possible future device applications view point. The Zener (1951) has proposed a simplest model, known as double-exchange (DE) model to explain the electrical behavior in ferromagnetic metallic region below Curie Temperature (T C ). However, due to various interactions among charge, spin and lattice makes these materials more complex and DE alone cannot explain the entire electrical transport behavior. Later on various theoretical models have been proposed by considering, electron-lattice and spin lattice interaction and even today there is no comprehensive model to explain transport phenomena in manganites (Millis et al., 1995;Tokura, 2000).
These doped perovskite manganites show large magnetoresistance (MR) around metal to insulator transition temperature (T P ) at high magnetic fields and below room temperatures which restricts their applicability to hands on devices. Improved MR could be achieved for nanosized perovskite manganite samples prepared through the sol-gel process. The sol-gel process also has potential advantages over other traditional processing techniques such as better homogeneities, low processing temperature and improved material properties. Although there are several reports on the synthesis of nanoscale manganites, none of them were carried out systematically in the context of the influence of sintering temperature on the electrical, magnetic and magneto transport behavior of manganites. It is well know that when the size of the particles is reduced to a few nanometers some of the basic properties such as magnetoresistance, superparamagnetism, coercivity, Curie temperature and Saturation magnetization are effected when compared with their bulk counterparts. Several works on different nanoscale systems indicate that the surface is responsible for their apparent anomalous behavioral changes with reduced dimensions. In fact, the preparation procedure and sintering conditions determines the nature of the surface region of nanosize grains, which plays a very crucial role in electrical transport, magnetic, and magnetotransport behavior of nano dimensional systems. Therefore, it is felt that a detailed review on the influence of sintering on magnetotransport behavior of manganites is essential and important.
The senior author of the paper (P. V. Reddy) along with his group of students investigated the structural, magnetic, electrical and magnetotransport properties of a number doped manganites over a period of more than a decade and published a series of papers in a number of International Journals (Kalyana , 2009a, 2009b, 2011Kalyana Lakshmi et al., 2009;Venkataiah et al., , 2007aVenkataiah et al., , 2007bVenkataiah et al., , 2007cVenkataiah et al., , 2008Venkataiah et al., , 2010, 2009a, 2009b. A summary of the work based on the influence of sintering on various properties of the manganites substituted with both divalent and monovalent ions is presented here.

Experimental procedure
The samples with different particle sizes were prepared using sol-gel route followed by heat treatment at four different sintering temperatures (T S ) (Venkataiah et al., , 2010, 2009a, 2009bKalyana Lakshmi and Reddy 2009a). Five series of samples i.e., La 0.67 Ca 0.33 MnO 3 , La 0.67 Sr 0.33 MnO 3 , La 0.67 Ba 0.33 MnO 3 , Nd 0.67 Sr 0.33 MnO 3 and La 0.67 Na 0.33 MnO 3 were prepared to study the effect of sintering temperature on various properties. The samples are hereafter designated as LCMO, LSMO, LBMO, NSMO and LNMO respectively and the samples sintered at 800, 900, 1000 and 1100 C are designated with 8, 9, 10 and 11 numerical representations followed by compositional abbreviations. The synthesis procedure of sol-gel method is outlined in Fig. 1.
The stoichiometric amounts of oxides / carbonates were taken and all of them were converted into metal nitrates and the resultant nitrate mixture solution was converted into citrate by adding 1:1 ratio of the citric acid to the metal atoms present in the composition. To carry out the reaction at greater speed, a proper reaction environment is to be created and hence pH of the solution was adjusted between 6.5 and 7 by adding ammonia solution. Finally, the powder was pressed into pellets and sintered in air at four different temperatures i.e., 800, 900, 1000 and 1100 0 C for 4 hr. For a direct comparison of the properties of the samples as a function of sintering temperature (grain size) it is important to ensure that the Mn 4+ concentration remains similar in different samples, since it is crucial factor in controlling the transport and magnetic properties. Mn 4+ concentration has been determined by redox titrations (Vogel, 1978). The phase purity of the samples was checked by X-ray diffraction (XRD) and grain distribution was estimated by scanning electron microscopy (SEM). The electrical resistivity and magnetoresistance studies were undertaken by a Janis "supervaritemp" cryostat in applied fields of 0-7 T, over a temperature range of 77-300 K using a four-point probe method. a.c susceptibility measurements were carried out using a dual channel lock-in amplifier, while thermoelectric power studies were carried out by a two-probe differential method over a temperature range 77-300 K.

Structural properties
Most of the manganites crystallize in the various derivatives of the so-called perovskites structure named after the mineral perovskite, CaTiO 3 . Figure 2 illustrates the ideal cubic perovskite structure, with the general formula ABO 3 . The structure may be conceived as a close-packed array formed of O 2− anions and A cations with B cations located at the octahedral interstitial sites. The BO 6 octahedra make contact to each other by their vertices and form a three-dimensional network. In manganites the A-sites of the perovskite structure are occupied by trivalent rare earth ion while B-sites with Mn ions. The stability of the perovskite structure depends strongly on the size of the A-site and B-site ions. If there is a size mismatch between the A-site and B-site ions and the space in the lattice where they reside, the perovskite structure will become distorted. Such a lattice distortion of perovskite structure is governed by Goldschmidt's tolerance factor ( ) (Goldschmidt, 1953).
where r A and r B are the mean radii of the ions occupying the A-sites and B-sites, respectively, and r O is the ionic radius of oxygen. For an ideal cubic perovskite τ=1. If τ differs slightly from unity the atoms are displaced from their ideal positions to minimize the free energy and a distorted perovskite structure is formed. For 0.96<τ<1 the lattice structure transforms to the rhombohedral structure and then to orthorhombic structure for τ<0.96, in which B-O-B bond angle gets distorted from 180. Structure like tetragonal, hexagonal and monoclinic etc. are reported for manganites for different x values. Among the studied samples the LSMO, LBMO and LNMO systems crystallize in the rhombohedral with R3c space group (Venkataiah et al., 2007a(Venkataiah et al., , 2010Kalyana Lakshmi and Reddy 2009a) while LCMO and NSMO group of samples crystallize in to orthorhombic structure with Pbnm space group Venkataiah and Reddy 2009b). A typical XRD patterns of LSMO & NSMO samples are shown in Figure 3. The X-ray linewidths are used to estimate average crystallite size <s> values through the Scherrer's formula modified by Williamson and Hall (1953).
www.intechopen.com In this method, the experimental x-ray peak broadening is given by =tan +K/ st cos, where  is the full width at half maximum (FWHM) of the XRD peaks after subtracting FWHM of the standard sample (SiO 2 ),  is the Bragg angle, K is the grain shape factor (0.89),  is the wavelength of Cu K radiation, and  is the strain in the sample. A plot between cos versus sin is shown in Figure 3(c). The average crystallite sizes were calculated from the intercept of the straight line with the vertical axis, while the slope of the line gives the lattice strain (). The average crystallite sizes are found to increase with increasing sintering temperature and are in nanometer range. The variation of crystallite size with sintering temperature is shown in Figure 4. In fact, the computed crystallite sizes are comparable with those obtained from SEM. Figure 5 shows the representative SEM images of LSMO samples sintered at 800 and 1100ºC. The SEM observation reveals that there is a uniform distribution of grain sizes for the samples and as the sintering temperature increases, the grain size increases and porosity decreases.

Magnetic properties
Mixed-valence manganites are complex materials showing a rich variety of structural, magnetic and electronic phases. Phase transitions may be induced by changing the composition, temperature or sometimes by applying an external magnetic field. Usually www.intechopen.com structural, magnetic and electronic phase transitions occur simultaneously. The magnetic and electronic properties of lanthanum manganites, La 1-x Ae x MnO 3 (Ae = Ca, Sr, etc) are strongly related to the simultaneous presence of manganese in different valence states. The end-members, x = 0 and x = 1, containing Mn in only one valence state (for divalent ion at Asite substitution leads to Mn in 3+ and 4+ valence states), are usually antiferromagnetic insulators (AFI), but intermediate compositions, which have mixed Mn valence, may be ferromagnetic and have good conducting properties. Figure 6 illustrates the magnetic and electrical properties of La 1-x Ca x MnO 3 (x=0-1) system with change in composition and temperature (Schiffer et al., 1995, Urushibara et al., 1995. Ferromagnetic behavior starts to manifest itself when x is 0.1 and the composition upto x~0.3 have both antiferromagnetic (AFM) and ferromagnetic (FM) characteristics. The x=0.3 composition is clearly FM, while the x>0.5 compositions are AFM. The coexistence of metallic conductivity and ferromagnetic coupling in these materials at low temperature has been explained in terms of a double exchange mechanism, proposed by Zener in 1951. The double exchange mechanism involves simultaneous transfer of an electron from Mn 3+ to O 2-and from O 2-to Mn 4+ . Moreover, the properties of manganites are not only sensitive to the manganese valency, but also are strongly affected by other factors such as average cationic radius <r A > of A-site and A-site cationic mismatch quantified by 2 which effects the magnetic interaction. Due to the long history of work on these compounds, most of the studies have been performed on bulk ceramics and thin films. Apart from this it was demonstrated that the properties of manganites are strongly affected by particle size. It was found that the sintering temperature is one of the key factors which influences the crystallization and microstructure of the perovskite samples. The basic magnetic properties such as spontaneous magnetization, the Curie temperature and coercivity are strongly influenced with sintering temperature. Mahesh et al., (1996) were the first to explore the size effects in polycrystalline La 0.67 Ca 0.33 MnO 3 prepared by citrate-gel route and observed a close relationship between magnetotransport properties and microstructure.  (Schiffer et al., 1995, Urushibara et al., 1995.

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The results obtained by the authors of the present investigation on five series of samples sintered at various temperatures are presented in this section. Out of these samples, the a.c. susceptibility () versus temperature plots of two representative samples viz., LSMO and LBMO sintered at different temperatures are displayed in Figure 7. All the five groups of samples exhibit para to ferromagnetic transition with decreasing temperature and the width of transition broadens on lowering sintering temperature indicating magnetic inhomogeneity in the samples. The variation of T C with sintering temperature (T S ) for the samples is shown in Figure 8. T C values do not vary much in the case of LSMO and LBMO systems (Venkataiah et al., 2007a(Venkataiah et al., , 2010 while after an initial increase they remain almost constant with further increase in sintering temperature for La 0.67 Na 0.33 MnO 3 manganites (Kalyana Lakshmi & Reddy 2009a). The observed behavior may be explained qualitatively as follows. In general variations in T C values take place whenever there is a change in Mn-O -M n b o n d a n g l e a s w e l l a s M n -O b o n d l e ngth, thereby indicating the presence of magnetic inhomogeneity in the sample as it approaches the transition temperature. In contrast to these arguments, perhaps due to magnetic homogeneity especially as sample approaches the phase transition, T C might be remaining constant, thereby giving an impression that T C might be independent of grain size. In fact a similar conclusion was arrived at earlier in the case of La 0.7 Sr 0.3 MnO 3 (Gaur & Verma 2006) and La 0.8 Ca 0.2 MnO 3 manganites (Balcells et al., 1998;Fu 2000;Rivas et al., 2000).   In contrast to the above results, T C values decrease with increasing sintering temperature in the case of LCMO and NSMO manganites . The observed reduction of ferromagnetic transition temperature (T C ) with increasing particle size is related to increase in unit cell volume. The increased unit cell volume enhances Mn-O bond length thereby deviating Mn-O-Mn bond angle from 180 0 , which in turn decreases the overlap integral decreasing the bandwidth. Thus the lattice effects arising from increasing particle size are expected to decrease in bandwidth, which in turn decreases T C . In addition, the dependence of T C on <s> (T C  1/ <s>) can also be rationalized if the size reduction can be thought as producing an internal pressure (P I ) such as the hydrostatic pressure (P E ): Size reduction due to interface stress can lead to an internal pressure ΔP I  A g / V g , where A g and V g are the surface area and the volume of a nanocrystal respectively. For a nearly spherical particle, the relationship between A g and V g is given by a relation, A g / V g  1/ <s>. This gives ΔP I  1/ <s>. As per the external, hydrostatic pressure, dT C = dP I  constant, one can write as ΔT C  1/ <s> (Shankar et al., 2004;Venkataiah and Reddy, 2009b).

Electrical studies
A fundamental characteristic of mixed-valence manganites is the close relationship between electronic transport and magnetism. Most of the La 1-x Ae x MnO 3 compositions are paramagnetic insulators at room temperature and exhibit an increase in electrical resistivity with a decrease in temperature. Compositions that are ferromagnetic show insulating (dρ/dT< 0) behavior above T C , but the resistivity decreases with decreasing temperature, as in metals (dρ/dT > 0), when they are cooled below T C . This insulator-metal transition is therefore associated with a peak in resistivity at a temperature, T P .
www.intechopen.com is shown in Figure 9. All the samples show metal to insulator transition at T P and their variation with sintering temperature for the investigated samples is shown in Figure 10 (a). The metal to insulator transitions shifts towards low temperature side and the magnitude of resistivity increases on reducing the sintering temperature or particle size.These observations are in good agreements with those reported earlier Gaur & Verma 2006). This behaviour can be explained by the core-shell model proposed by Zhang et al., (1997). In the core-shell structure the inner part of the grain, i.e. the core, would have the same properties as the bulk manganite, whereas the outer layer, i.e. the shell (thickness t), would contain most of the oxygen defects and crystallographic imperfections, which would lead to a magnetically disordered dead layer. As the surface to volume ratio becomes larger, i.e., the grain size is reduced, the shell thickness (t) increases. Basically, the net intercore barrier thickness (s = 2t + d), namely the total shell thickness (2t) of two neighboring grains together with the intergranular distance (d), increases with the reduction of grain size. Further, with the decrease in grain size, core separation (s ~ 2t) increases significantly with the thickness of the shell (t ), even if we consider the grains to be in intimate contact (d = 0) for all grain size samples. Another important fact is that in the absence of magnetic field the contributory portion of each individual grain to the magnetization is the core and not www.intechopen.com the shell, in the absence of applied magnetic field the net magnetization of the shell is considered to be zero. Since the surface would contain most of the oxygen defects and faults in the crystallographic structure that will lead to a magnetically disordered state, which may lead to formation of spin canting or antiferromagnetic state at the manganite grain surface, thereby decreasing T P values and increasing the magnitude of resistivity (Figure 11). It can also be noticed from the Figure 10(b) (in the case of LSMO system) that there exists a large difference between T C and T P and the difference increases with decreasing sintering temperature. The large difference in the magnetic and electrical transitions for all the samples is thought to be due to the existence of the disorder and is in fact a common feature of the polycrystalline manganites (Gaur & Verma 2006). The T C being an intrinsic characteristic does not show significant change as a function of sintering temperature. On the other hand T P is an extrinsic property that strongly depends on the synthesis conditions and microstructure (e.g. grain boundary density). The suppression of the T P as compared to T C is caused by the induced disorders and also by the increase in the non-magnetic phase fraction, which is due to enhanced grain boundary densities as a consequence of lower sintering temperature. This also causes the increase in the carrier scattering leading to a corresponding enhancement in the resistivity. Thus lowering of sintering temperature reduces the metallic transition temperature and hence the concomitant increase in resistivity. www.intechopen.com In contrast to the above results, it has been observed that T P values of sodium doped samples, are found to decrease with increasing grain size except in the case of the sample sintered at 1100ºC (Kalyana Lakshmi & Reddy, 2009a). The observed variation may be interpreted on the basis of oxygen deficiency. According to Malavasi et al., 2002Malavasi et al., , 2003aMalavasi et al., , 2003b) the presence of oxygen vacancies deeply affect the transport properties resulting in both Mn 4+ reduction and point defect creation within the structure thereby shifting T P towards low temperature side. This behavior is in close agreement with the results reported in the case of LCMO samples (Hueso et al., 1998) where T C remained constant while T P decreased with oxygen deficiency. This suggests that the combined effect of crystallite size and oxygen deficiency might be responsible for the observed resistivity behavior (Hueso et al., 1998;Malavasi et al., 2002Malavasi et al., , 2003aMalavasi et al., , 2003b.

Thermopower studies
Measurement of emf (electromotive force) induced by a temperature gradient across a sample provides complementary information to the resisitivty. The Seebeck coefficient (S) is defined as ∆V/∆T, the thermoelectric voltage per degree of temperature difference. Among various transport properties, thermopower is a simple and sensitive one for detecting the scattering mechanism that dominates the electronic conduction and also provides insight into changes of band structure near the metal-insulator transition. The thermoelectric power (TEP) of manganites shows strong temperature dependence and it can have positive or negative sign depending on the temperature and degree of substitution of divalent/monovalent cation that determines the carrier density in these systems. This behaviour suggests that either both types of carriers (electrons and hole) may be involved in the charge transport mechanism or additional scattering phenomenon may contribute to thermopower. Because no current flows, the thermopower does not depend on the connectivity of conducting regions, and the thermopower of individual grains are additive. There have been many measurements of the thermopower of samples with x ≈ 0.3 which exhibit a metal-insulator transition. These data, on ceramics, thin films and single crystals, are not very consistent and results are sensitive to details of sample preparation and composition (Jaime etal., 1996;Mahendiran et al., 1996b;Mandal 2000).
In order to get further insight into the effect of particle size on thermopower, the variations of thermopower as a function of sintering temperature on various manganites systems were carried out. This section describes thermopower results obtained by the authors of the present investigation on various manganite systems doped with both divalent and monovalent ions sintered at different temperatures. Figure 12 shows the variation of seebeck coefficient (S) with temperature in LCMO and NSMO manganites. It has been observed that the magnitude of S is found to increase with decreasing particle size and positive throughout the temperature range of investigation, thereby representing that the charge carriers are holes for LSMO, LBMO, NSMO, LNMO, LCMO-8, LCMO-9 and LCMO-10. In contrast to this, the sign of S for LCMO-11 changes from positive to negative value indicating the coexistence of two types of carriers. Banerjee et al., (2003) has also found a similar variation of S with varying particle size in lead doped manganites. One can notice from the figure that as the temperature of the samples is decreased from room temperature to liquid nitrogen temperature, S values increase continuously attaining a maximum value with a sharp peak in the vicinity of TC. The peak observed at T C may be attributed to enhancement in spin polarization caused by the magnetic transition (Das et al., 2004). Apart from this, except LCMO-8 and LCMO-9 all are found to exhibit a broad peak around 120 K and similar behavior was reported earlier (Das et al., 2004;Urushibara et al., 1995). It was reported earlier that (Banerjee et al., 2003) phonon drag (S P ) and magnon drag (S M ) contributions are present in the low temperature region and that these are proportional to their respective specific heat contribution so that S P  T 3 and S M  T 3/2 . This suggests that in the low temperature ferromagnetic region, a magnon drag effect is produced due to the presence of electron -magnon interaction along with the phonon drag due to electron -phonon interaction. In general, to understand the influence of various contributions to TEP in ferromagnetic region, the following equation was used earlier (Mandal 2000) 3/2 3/2 4 04 where S 0 term has no physical origin and is inserted to account for the problem of truncating the low temperature thermopower data. The second term of the equation represents the single-magnon scattering process. Although the origin of S 4 T 4 term is still not clear, it is generally believed that it may be due to spin wave fluctuation. It has been observed that the eq. (2) doesn't fit the TEP data in the entire ferromagnetic region. Therefore, the above equation has been modified by taking into account the diffusion and phonon drag contributions and the equation is as follows, (Kim et al., 2008), 3/2 3 4 01 3 / 23 4

SS S TS T S T S T
Here, the second and fourth terms represents the diffusion and phonon drag contributions to TEP respectively. The experimental data in the low temperature region were fitted to the above equation and the solid curve in Figure 13 (a) (in the case of NSMO) represents the best fit to the experimental data in FMM region and the best fit parameters for NSMO sample is given in Table 1.  All the fitting parameters decrease with increasing particle size. This might be due to the magnetic domain and grain boundary scattering mechanism discussed previously in analyzing resistivity data of the samples.  In order to explain the origin of peak at low temperatures, the phonon and magnon drag contributions to the TEP were investigated using eq. (3). The variation of phonon drag component with T 3 for all the samples is shown in Figure 14 (a). It is clear from the figure that the phonon drag contribution (S 3 T 3 ) is found to vary linearly up to a certain temperature and deviates (indicated by arrow in the figure) below 250K. As the phonon drag contribution deviates at 250K, it has been concluded that the origin of the low temperature peak might not be due to the phonon drag effect. It is interesting to note from Fig. 14(a) that the extrapolated phonon drag component is approaching zero as the sample approaches to T = 0 indicating that the phonon drag effect might be disappearing probably due to depleting number of phonons at T = 0. It can also be seen from the figure that the deviation temperatures occurring from phonon and magnon drag are almost equal in the case of NSMO-8 and NSMO-9 samples indicating that the magnon and phonon drag effects are occurring in different temperature regions. The magnon drag component was calculated from eq (3) and is shown in Figure 14 (b). It is interesting to note that the magnon drag component fits well below 200K, indicating that the low temperature peak might have arisen due to magnon drag effect (Venkataiah & Reddy, 2009b).

High temperature behavior (T>T P )
The charge carriers in the insulating region are not itinerant and the transport properties are governed by thermally activated carriers because the effect of Jahn-Teller distortions in manganites results in strong electron-phonon coupling and hence the formation of polarons. Therefore, the thermoelectric power data of the present samples in the insulating regime are fitted to Mott's polaron hopping equation, where e is the electronic charge; E S is the activation energy obtained from TEP data and α is a constant. α<1 implies the applicability of small polaron hopping model whereas α>2 is for large polaron hopping (Das et al., 2004). It has been found that the above equation fits well with the experimental data of the present investigation. The best fit curves of S versus 1/T of NSMO samples are shown in Figure 13 (b). The calculated values of  for the samples of the present investigation are less than one, therefore it may be concluded that the small polaron hopping mechanism is more appropriate to explain the thermopower data in the hightemperature regime.

Magnetoresistance
One of the important properties of the doped perovskite manganites is their mangetoresistace (MR).The percentage of MR is defined as the relative change in the electrical resistivity of a material on the application of an external magnetic field and is given by a relation, where ρ H and ρ 0 are the resistivities at a given temperature in the applied and zero magnetic fields, respectively. MR can be positive or negative depending on increase or decrease in the resistivity values. The magneto resistance studies were undertaken on all the five groups of samples of present investigation. The variation of electrical resisitivity with temperature at different fields for LBMO sample sintered at two different sintering temperatures (T S = 800 & 1100ºC) is shown in Figure 15. It is also clear from the figures that the resistivity value decrease with increasing magnetic field and T P shifts towards high temperature side. This may be due to the fact that the applied magnetic field induces delocalization of charge carriers, which in turn might suppress the resistivity and also causes the local ordering of the magnetic spins. Due to this ordering, the ferromagnetic metallic (FMM) state may suppress the paramagnetic insulating (PMI) regime. As a result value of T P shifts to high temperature with application of magnetic field. The variation of MR% with temperature in the case of LBMO system is shown in Figure 16.The measurements of MR reveal that the maximum MR is observed for the samples sintered at the lowest temperature. All the samples are exhibiting high MR% at low magnetic fields and also remain high in low temperature regime and then decrease with increasing temperature. This is a clear signature of low field magnetoresistance (LFMR) behavior. T (K)  0 T 1 T 3 T 7 T

T (K)
cm  www.intechopen.com The observed behavior may be explained on the basis of a qualitative model. According to this model, magnetoresistance may be classified into two categories (Dutta et al., 2003;Hwang et al., 1996;Mandal et al., 1998). The first one is the intrinsic MR, which arises due to the suppression of spin fluctuations by aligning the spins on the application of magnetic field. This MR has highest value near the ferromagnetic transition temperature and is generally observed among single crystalline bulk as well as thin films. In the polycrystalline samples, there is an additional MR, which is extrinsic in nature; arises due to inter-grain spin-polarized tunneling (ISPT), across the grain boundaries (GBs). This MR contribution increases as the temperature decreases and is usually found in nanocrystalline materials. Rivas et al. (2000) pointed out that the sol-gel prepared samples sintered at low temperatures (900C) with particle size less than 150nm show more extrinsic nature of magnetoresistance than the intrinsic one. Until recently, it was believed that the former mechanism is responsible for the MR at high fields and the later one at low fields. But recent experiments have shown that high field response is also due to the existence of GB. The nature of GB is a key ingredient in the mechanism of electrical transport, as it constitutes the barrier through which carriers cross or tunnel (Hsu et al., 2006). In view of this, one may conclude that the samples of the present investigation, might be exhibiting extrinsic nature of magnetoresistance arising due to inter-grain spin-polarized tunneling.

Conduction mechanism
Inspite of extensive experimental and theoretical work to understand the conduction mechanism of CMR materials in general and rare-earth manganites in particular, the present scenario is more confusing than earlier. Therefore, an effort has been made to understand the conduction mechanism of these materials by analyzing the experimental data of both the ferro-as well as the paramagnetic regions using various theoretical models.

Low temperature resistivity (T<T P )
In order to understand the relative strengths of different scattering mechanisms originating from different contributions both as a function of particle size and magnetic field, the FMM (T<T P ) part of electrical resistivity data of the samples of the present investigation has been fitted to various empirical equations (De Teresa et al., 1996;Kubo & Ohata, 1972;Schiffer et al., 1995;Snyder et al., 1996;Urushibara et al., 1995 where ρ 0 arises due to the grain or domain boundaries, ρ 2 T 2 indicates the electron-electron scattering, while ρ 4.5 T 4.5 is attributed to two magnon scattering process. The two magnon process is more favorable in half-metallic band structure materials such as manganites. The best fit curve for a representative sample viz., LBMO sintered at 800 & 1100 C various sintering temperatures is shown in Figure 17 (a, b) and fitting parameters are given in Table  2. On the other hand, in the case of LCMO system resistivity in metallic region is well fitted to an equation =+ 2.5 T 2.5 , the term  2.5 T 2.5 the resistivity due to single magnon scattering mechanism. It has been observed that the temperature independent residual resistivity term for all the samples is larger than that obtained for single crystals.
Further, the fitting parameters are found to decrease continuously with increasing particle size as well as the magnetic field. One of the reasons may be due to decrease in the grain boundary size with increasing sintering temperature. The second reason may be due to the spins present in the domain wall align in the direction of applied magnetic field resulting in the enlargement of magnetic domains. Therefore, the parallel configuration of the spins present in the domain, suppresses various scattering contributions and as a result  0 ,  2 ,  2.5 and  4.5 are decreasing with the application of the magnetic field and sintering temperature. (Banerjee et al., 2001).

Insulating region (T>T P )
The pair-density function (PDF) analysis (Toby & Egami, 1992;Mott & Davis 1971) of powder neutron scattering data on manganites shows that the high temperature semiconducting region (T>T P ) is mainly due to lattice polarons. The analysis clearly indicates that the doped holes in manganites are likely to be localized within one octahedron of Mn 4+ ion, forming a single-site polaron (small polarons). These small polarons in the high temperature semi-conducting regime of electrical resistivity data are in good agreement with the theoretical prediction of entropic localizations. Therefore, an attempt has been made to fit the high temperature resistivity data of the samples of present investigation to small polaron hopping (SPH) model explained by the equation (Emin & Holstein 1969), where ρ α is a pre-factor and E  is the activation energy. ρ α = 2k B /3ne 2 a 2 ν, where k B is Boltzmann's constant, e is the electronic charge, n is the number of charge carriers, a is siteto -site hopping distance and ν is the longitudinal optical phonon frequency.
The high temperature data of the samples of the present study is found to fit well with the SPH model. A typical plot of ln (/T) versus T -1 in the case of LBMO-8 & 11 is shown in Figure 17 (c, d). From the best fit parameters the activation energy (E) values have been estimated and the values are found to decrease continuously with increasing particle size and magnetic field. The observed behavior may be due to the fact that the increasing particle size may increase the interconnectivity between grains, which in turn enhances the possibility www.intechopen.com of conduction electron to hop to the neighboring sites, thereby conduction bandwidth increases and as a result the value of E decreases. Therefore, the conduction bandwidth of the materials may be modified by proper tuning of their particle size. The increase in E in the presence of magnetic field is mostly attributed to the increase of charge localization (Banerjee et al., 2002).

Summary and outlook
For over more than a decade, studies on manganites have been focused on both various details covering the underlying fundamental Physics and on applied aspects; it is the former which has received most of the attention. This chapter embodies the influence of sintering temperature on physical properties in doped manganites. Five series of doped manganties i.e., and are found to decrease on lowering the sintering temperature. The ferro to paramagnetic transition temperatures (T C ) remains unaltered, while the metal to insulator transitions (T P ) decrease with decreasing sintering temperature. The behavior was explained using coreshell model. The percentage of magentoresistance is high at low temperatures and varies linearly with decreasing temperature and the behavior is explained based on inter grain spin polarized tunneling mechanism. The Seebeck coefficient decreases with increasing sintering temperature and remains positive throughout the temperature range of investigation indicating that the charge carriers are holes. The broad peak observed in the low temperature thermopower data was analyzed using phonon and magnon drag terms and it has been concluded that magnon drag contribution dominates in the low temperature region. The resistivity data in the low temperature region were analyzed using various theoretical models. The contribution from various scattering mechanism are found to depend both on grain size and applied magnetic field. Small polaron hopping mechanism dominates both the resistivity and thermopower data in the high temperature region.
The trends of sintering effects on various physical properties, particularly the electric and magnetic transition temperatures are encouraging. The authors envisage that further work will be useful in achieving one of the challenging aspects such as the above room temperature ferromagnetism in these highly spin polarized half metallic manganites for future commercial spintronic devices.