Influence of sintering temperature on resistivity, magnetoresistance and thermopower of La0.67Ca0.33MnO3
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Influence of sintering temperature on resistivity, magnetoresistance and thermopower of La Ca MnO G Venkataiah , YK Lakshmi and PV Reddy* Address: Department of Physics, Osmania University, Hyderabad 500 007, India and Department of Physics, National Cheng KungUniversity, Tainan 701, Taiwan, Republic of China ROCEmail: G Venkataiah - [email protected]; YK Lakshmi - [email protected]; PV Reddy* - [email protected]* Corresponding author
Abstract
A systematic investigation of La Ca MnO manganites has been undertaken, mainly tounderstand the influence of varying crystallite size (nano meter range) on electricalresistivity, magnetic susceptibility and thermoelectric power. The materials were preparedby the sol-gel method of sintering at four different temperatures between 800–1100°C.The samples were characterized by X-ray diffraction and data were analyzed using Rietveldrefinement. The metal-insulator transition temperatures (T P ) are found to increase withincreasing sintering temperatures, while the magnetic transition temperatures (T C )decrease.The electrical resistivity and thermoelectric power data at low temperatures (T < T P ) havebeen analyzed by considering various scattering phenomena, while the high temperature(T > T P ) data were analyzed with Mott's small polaron hopping conduction mechanisms. PACS Codes:
1. Introduction
In recent years, nanocrystalline materials have attracted the attention of the scientific communitybecause of the rich Physics involved as well as their potential use in device applications [1-6]. Thepresence of a large amount of grain boundaries and/or the broad distribution of interatomicspacings in the grain boundaries give rise to the unusual properties of nanocrystalline materialswhen compared to conventional polycrystals or single crystals with the same chemical composi-tion. Magnetic nanoparticles with smaller grain sizes exhibit richer electronic and magnetic prop-erties arising from structural and magnetic disorders in the grain surfaces. Recently, modificationof the properties of nanosized perovskites has aroused much interest [7-10]. Doped manganiteswith strongly correlated electrons exhibit fascinating properties originating from the strong inter-
Published: 12 March 2008
PMC Physics B MC Physics B (page number not for citation purposes) play between charge, spin, orbital, and lattice degrees of freedom. Doped manganites R A x MnO (R = rare earth trivalent cation and A = divalent alkaline earth cation) undergo a para-magnetic insulator (PMI) to ferromagnetic metal (FMM) transition and exhibit colossal magne-toresistance (CMR) phenomena in the vicinity of their transition temperature. The observedcorrelation between the metallicity and ferromagnetism in manganites has been explainedwithin the framework of the double exchange (DE) mechanism, which describes the electronichopping between neighboring Mn (e g ) orbitals. However, the DE mechanism cannot explain theentire observed phase diagram. In addition to DE, the polaron effect due to a very strong electronphonon coupling coming from the Jahn-Teller (JT) lattice distortion of the Mn is very helpfulin explaining the resistivity and magnetoresistance of these compounds [11]. Among variousaspects of transport studies in these materials, thermopower has attracted much attention,because it is a very sensitive physical property and depends on the nature of charge carriers andtheir interaction with spins. Although a lot of work on nanocrystalline manganites, based ontheir resistivity and magnetoresistance, is available in the literature, data on thermopower ofthese materials are scanty. In view of these facts, a systematic investigation of electrical resistivity,magnetoresistance and thermopower studies of La Ca MnO manganites have been carriedout and the results of such an investigation are presented here.
2. Experimental details
Nanosized polycrystalline samples with compositional formula, La Ca MnO were synthe-sized by the sol-gel route, taking corresponding metal nitrates as starting materials in a stoichio-metric ratio. Later, these solutions were converted into citrates and the p H was adjusted between6.5 and 7. After getting a sol on slow evaporation, a gelating reagent ethylene glycol was addedand heated on a hot plate between 160 and 180°C to get a gel. Finally, the resulting powder waspressed into circular pellets, which were sintered in air atmosphere for 4 hrs at 800, 900, 1000and 1100°C to produce samples of different particle sizes. All the samples were characterized byX-ray diffraction (XRD) (Philips xpert diffractometer). The XRD data were analyzed usingRietveld refinement technique [12] and the average crystallite sizes were estimated using peakbroadening. The electrical resistivity and magnetoresistance (MR) measurements were performedusing JANI'S 'supervaritemp' cryostat in an applied magnetic field of 0,1,3,5 and 7T over a tem-perature range 77–300 K and magnetic transition temperatures (T C ) were determined by meas-uring AC susceptibility ( χ ) in the temperature range 77–300 K by using the mutual inductanceprinciple. Finally, a dynamic two probe differential method was employed for the measurementof the thermopower [13]. The samples were attached with silver paint between two copper elec-trodes with an adjustable temperature gradient and were monitored using a copper-constantanthermocouple. The assembly was placed in a closed liquid nitrogen cryostat and nitrogen atmos-phere was used as an exchange, to maintain uniform temperature and avoid condensation ofmoisture. The data were collected in heating mode. The measured thermopower data were cor-rected by subtracting thermopower values of copper, so as to obtain the absolute thermopowervalues of the samples. MC Physics B (page number not for citation purposes)
3. Results and discussion
The XRD pattern of La Ca MnO samples, sintered at different temperatures are shown inFig. 1. The XRD data have been analyzed with the Rietveld refinement technique and the mate-rials were found to crystallize in Pbnm space group. It is also clear from the analysis that the sam-ples are single phase with no detectable impurity. The Mn-O-Mn bond angle and Mn-O bondlength obtained from the Rietveld refinement are presented in Table 1. The average particle sizesof the materials were estimated using peak broadening [10] through the Scherrer formula, = K λ / β cos θ , where average crysttallite size in Å, K is a constant (shape factor; 0.89), λ isthe Cu K α wavelength and β is the corrected full-width-half-maxima of XRD peaks of the sample.SiO was used to correct the intrinsic width associated with the equipment. The calculated aver-age crystallite size values are given in Table 1. It is clear from the table that the crystallite sizes arefound to increase with increasing sintering temperature. AC susceptibility measurements of all the samples have been carried out as a function of temper-ature (Fig. 2) and based on these results, the ferro to paramagnetic transition temperatures (T C )were obtained and are given in Table 2. It is clear from the table that T C values decrease withincreasing sintering temperature thereby indicating that T C is decreasing with increasing particlesize. The observed behavior may be explained following the work by Dutta et al. [14]; According X-ray diffraction patterns of La Ca MnO manganites Figure 1
X-ray diffraction patterns of La Ca MnO manganites.
20 30 40 50 60 70 80
LC - 11LC - 10LC - 9 I n t en s i t y ( a . u . ) θ ( Degree ) LC - 8 MC Physics B (page number not for citation purposes) to these authors, the magnetic and transport properties of the perovskite manganites are stronglycoupled and are very sensitive to Mn-O-Mn bond angle and Mn-O bond length. It has beenfound that a decrease in magnetization and an increase in resistivity occur as we decrease the par-ticle size, due to broken Mn-O-Mn bonds at the surface of smaller particles that hamper exchangeinteraction and degrade connectivity for electron conduction, but in this case it seems that thespin interaction increases and the connectivity improves as we decrease the particle size. Thedecrease in particle size results increase in Mn-O-Mn bond angle and decrease in Mn-O bondlength. Therefore magnetization increases and T C enhances with decreasing particle size. It is alsoclearly evident from Table 1 that the bond angles are found to be almost constant, while the bondlengths decrease continuously with decreasing particle size of the material. In view of these argu-ments and explanations, it is reasonable to understand that there is a continuous decrease in thevalues of T C with a continuous increasing sintering temperature. Temperature dependence of AC susceptibility of La Ca MnO samples Figure 2
Temperature dependence of AC susceptibility of La Ca MnO samples.
50 100 150 200 250 300 35002040608010050 100 150 200 250 300 3501.01.21.41.61.82.0 50 100 150 200 250 300 3501.01.52.02.53.03.54.050 100 150 200 250 300 3501.001.051.101.151.20 (d) (cid:70) (cid:3) ( a . u . ) LC - 11 T(K) (cid:70) (cid:3) ( a . u . ) (cid:70) (cid:3) ( a . u . ) (a) T(K)LC - 8 (b)
T(K)LC - 9 (cid:70) (cid:3) ( a . u . ) (c) LC - 10 T(K)
Table 1: Experimental data of LCMO manganites
Compositional formula Sample Code Sintering temp. (°C) (nm) a(Å) b(Å) c(Å) Mn-O-Mn (Degree) Mn-O (Å)La Ca MnO LC-8 800 20 5.466(3) 5.449(2) 7.727(5) 148.5 1.82205La Ca MnO LC-9 900 30 5.468(3) 5.446(2) 7.742(3) 148.4 1.82345La Ca MnO LC-10 1000 35 5.475(1) 5.462(1) 7.732(3) 148.5 1.82430La Ca MnO LC-11 1100 40 5.480(2) 5.462(2) 7.741(6) 148.5 1.82550
MC Physics B (page number not for citation purposes) The metal insulator transition temperature (T P ) and peak resistivity ( ρ Peak ) obtained from elec-trical resistivity measurements are given in Table 2. It is interesting to note that T P values arefound to increase with increasing sintering temperature from 145 to 195 K, while ρ Peak values aredecreasing. In view of these observations, one can understand that there is a tremendous influ-ence of crystallite size on various properties mentioned, and that the observed behavior may beexplained on the basis of a qualitative model. According to this model, it has been assumed thatin the case of sol-gel prepared samples of the present investigation, when the grain size of thematerial is decreased, a non-magnetic surface layer having nanocrystalline size would be createdaround the grain. This may increase the residual resistivity of the material, which in turndecreases the density of Ferromagnetic Metallic (FMM) particles. Therefore, lowering of T P andenhancement of electrical resistivity at a given temperature ( ρ T ) are expected. Finally, the shiftingof T P towards the low temperature region could be due to the loss of long-range ferromagneticorder in the sample [15].It can also be seen that a difference between T C and T P is observed and it is has been calculated ∆ T (T C ~ T P ) for each material and given in Table 2. The ∆ T values are found to decrease from alarge value of 108 to 24 K as the sintering temperature increases continuously and in fact, a sim-ilar difference between T C and T P has been reported earlier [14]. The observed difference betweenthe two transition temperatures may be explained as outlined here. It is well known that two con-tributions are responsible for the transport properties among CMR materials. One of them isintrinsic and might have originated from the double exchange (DE) interaction between theneighboring Mn ions, while the other one is extrinsic and is due to spin-polarized tunnelingbetween ferromagnetic grains through an insulating grain boundary (GB) barrier. According toDE model, the metal-insulator transition always occurs in the vicinity of T C . However, in the caseof granular samples with a large number of GBs, the influence of interfaces and boundariesshould be taken into account. Further, as the GB is similar to the amorphous state, the magneticconfiguration on the grain surface is more disordered than in the core. In such a situation, theoccurrence of anti-ferromagnetic insulating regions on the grain boundary may not modify themagnetic transition temperature, T C . However, the phenomenon may influence the metal-insu-lator (electrical) transition, T P thereby shifting it to lower temperatures [16]. Table 2: Electrical and magnetic data of LCMO manganites.
Sample Code T C (K) T P (K) T S (K) ∆ T = T C -T P (K) ρ Peak ( Ω cm) MR% (7T)LC-8 253 145 229 108 969 63LC-9 248 160 236 88 429 71LC-10 246 180 257 66 176 70LC-11 219 195 266 24 112 - MC Physics B (page number not for citation purposes) Magnetoresistance measurements were carried out in the presence of different magnetic fieldsviz; 1, 3, 5 and 7 T. A typical plot of variation of electrical resistivity with temperature in the caseof LC-9 at different field runs is shown in Fig 3. It can be seen from the figure that the resistivityis found to decrease with increasing magnetic field and that T P shifts towards higher temperaturesand that as a matter of fact, T P values are found to change from 160–185 K when the field changesfrom 0–7 T. This may be due to the fact that the applied magnetic field induces delocalization ofcharge carriers, which in turn might suppress the resistivity and also cause local ordering of themagnetic spins. Due to this ordering, the ferromagnetic metallic (FMM) state may suppress theparamagnetic insulating (PMI) regime. As a result, the conduction electrons (e ) are completelypolarized inside the magnetic domains and are easily transferred between the pairs of Mn (t e : S = 2) and Mn (t e og : S = 3/2) via oxygen and hence the peak temperature (T P ) shifts tohigh temperature side with application of magnetic field [17]. In fact a similar explanation wasgiven earlier [18].Further, the percentage of MR of all the materials (except LC-11) of the present investigationhas been calculated in the vicinity of T C by using the well-known relation,MR% = {[ ρ (0) - ρ (H) ]/ ρ (0) } × 100 (1) Resistivity versus absolute temperature of LC-9, at different magnetic fields
Figure 3
Resistivity versus absolute temperature of LC-9, at different magnetic fields.
100 150 200 250 300 01020304050607080
75 100 125 150 175 200 225 250 275 300 325 MR M R % T(K)
LC - 9 (cid:85) ( (cid:58) c m ) T ( K )
MC Physics B (page number not for citation purposes) The calculated MR values are found to remain constant (Table 2) with increasing sinteringtemperature. The variation percentage of MR of LC-9 with temperature is also shown in the insetof the Fig. 3 and one may note from the figure that the percentage of MR values is found to bevery high in the vicinity of T C .The electrical resistivity data (T
Table 3: Best fit parameters obtained from of thermopower and electrical resistivity of LCMO manganites.
Sample Code S ( µ VK -1 ) S ( µ VK -5/2 ) S ( µ VK -5 ) E P (meV) E S (meV) W H = E P -E S (meV) α 'LC-8 -3.5850 0.0084 -3.0890 × 10 -9 -8 -8 -8 MC Physics B (page number not for citation purposes) value, while those with higher crystallite size (45 nm) exhibit both negative and positive values.The observed behaviour may be explained as follows. The positive sign exhibited by LC-8, LC-9and LC-10 materials might be attributed to holes which are excited from the valence band (VB)into the impurity band, while in the case of LC-11 sample, the change in sign may be attributedto the orbital degeneracy of the e g band. According to this model, the orbital degree of freedomof the e g band may play an important role. It is well known that e g band consists of degenerate 3 d orbital (i.e., d and d x2-y2 ) and may split into upper and lower bands by an order of J H [19]. Ifthe lower (spin-up) band, splits further into two bands in the FM state, then the lowest band isfilled, the dopants may introduce holes thereby showing positive S values while the lowest bandis empty; it shows negative (electron-like) values. Therefore, one may conclude that there may bea possibility of changing sign from positive to negative with varying particle size [20].Close observation of S versus T plots shows that as the temperature decreases from 300 to 77K, the values of S increases thereby attaining a maximum value in the vicinity of magnetic tran-sition temperature (T C ) designated as T S . The obtained T S values from S vs. T plots are includedin the Table 2. Further, T S values and the magnitude of S at T S increase with increasing particlesize. In the ferromagnetic metallic part (T < T P ), a broad peak is found to develop and increasewith increased particle size. In fact, similar behaviour was reported earlier in Pr-based manganites[21,22]. The observed broad peak may be explained on the basis of the spin-wave theory. Accord-ing to this theory, in ferromagnets and antiferromagnets, electrons are scattered by spin waves, Variation of S with T of LCMO manganites
Figure 4
Variation of S with T of LCMO manganites.
100 150 200 250 300 -5 LC - 8LC - 9LC - 10LC - 11 S ( V / K ) T(K)
MC Physics B (page number not for citation purposes) giving rise to the electron-magnon scattering effect. In a manner similar to the scattering of pho-nons resulting in phonon drag effects, the electron-magnon interaction also produces a magnondrag effect. As the magnon drag effect is approximately proportional to the magnon specific heat,one may expect the variation of S with temperature as T for ferromagnetic materials [22]. Inview of these arguments, one may conclude that the observed broad peaks may be attributed tothe magnon drag effect which increases with increase in particle size. P ) behaviour Similarly to the electrical conduction, several factors, namely impurity, complicated band struc-ture, electron-electron, magnon scattering, etc, affect the TEP data at low temperatures (T < T P ).Therefore, the ferromagnetic metallic part of thermopower data was fitted to an equation:S = S + S T + S T (4)where S is a constant that accounts for the low temperature TEP data, while the S term arisesfrom the electron magnon scattering contribution and the origin of the S term, which is domi-nant in the high temperature region especially near the transition temperature, is still not clear. Variation of S with T of LCMO manganites at (T 100 125 150 175 200-7.0-3.50.03.57.010.514.0 LC - 8 LC - 9 LC - 10 LC - 11 S ( (cid:80) V / K ) T ( K ) MC Physics B (page number not for citation purposes) The best-fit parameters obtained from equation (4) are given in Table 3 and correspondingplots are shown in Fig. 5. It is clear from the table that the S values increase with increasing par-ticle size for first three samples (LC-8, LC-9 and LC-10) and decrease in the case of the LC-11 sam-ple, while the S values increase continuously. The S values do not vary systematically withvarying particle size of the materials. The increase in S values with increasing particle sizeshows a continuous increase in ferromagnetic magnon strength in these materials. Similarly, S represents the temperature independent thermopower. P ) behaviour There is a strong experimental evidence for the presence of small polarons at high temperaturesin the case of manganites [23]. In fact, pair-density function (PDF) analysis [24,25] of powderneutron scattering data of manganites clearly indicates that the doped holes are likely to be local- Plots of S versus T -1 of LCMO manganites Figure 6Plots of S versus T -1 of LCMO manganites . The solid line gives the best fit of the equation S = K B /e(E S /K B T + α '). S ( (cid:80) V / K ) T -1 (K -1 ) LC - 8 T -1 ( K -1 ) S ( (cid:80) (cid:3) V / K ) LC - 9 S ( (cid:80) (cid:3) V / K ) T -1 ( K -1 ) LC -10 S ( (cid:80) (cid:3) V / K ) T -1 ( K -1 ) LC - 11 MC Physics B (page number not for citation purposes) ized within one octahedron as a Mn ion, forming a single-site polaron (small polarons). Thesesmall polarons in the paramagnetic insulating region are in good agreement with the theoreticalprediction of entropic localizations. Therefore, it has been concluded that the small polaronsmight be responsible for the high temperature (T > T P ) paramagnetic conduction process in man-ganites. In view of this, the thermopower data of the present investigation have been analyzedusing small polaron hopping model described by the following equation [26],S = k B /e [E S /k B T+ α '] (5)where, k B is Boltzmann's constant, e is electron's charge, E S is the activation energy obtained fromTEP data and the α ' is a sample dependent constant, which is associated with the spin and themixing entropy. In addition, α ' < 1 suggests the conduction mechanism is due to small polarons,while α ' > 2 represents the conduction may be due to large polarons [27].The high temperature TEP data are found to fit well with equation (5) and from the fittingparameters, the E S and α ' values are obtained and given in Table 3. Plots of S versus 1/T of all thesamples are shown in Fig. 6. The calculated values show that both the activation energies (E P andE S ) decrease with increasing sintering temperature. Moreover, the magnitude of E S is muchsmaller than that of E P , which is a characteristic property of small polaron conduction [27]. Thedifference between the activation energies, measured from resistivity and TEP studies is thepolaron hopping energy W H = E P -E S . The decrease in the E P , E S and W H may be explained as fol-lows: It is known that with increasing grain size, the interconnectivity between grains increases,which in turn enhances the possibility of conduction electron to hop the neighboring sites [28],thereby conduction bandwidth increases and as a result the value of E P , E S and W H valuesdecreases. Therefore one may conclude that the conduction bandwidth may be tuned by varyingthe particle size of the material. References Magnetic Properties of Fine Particles. Edited by: Dormann JL, Fiorani D. North-Holland, Amsterdam; 1992. 2. Nanostructures and Mesoscopic Systems. Edited by: Kirk WP, Reed MA. Academic, New york; 1992. 3. Leslie-Pelecky DL, Rieke RD: Chem Mater J Phys: Condens Matter R841.5. Alivisatos AP: Science Science Appl Phys Lett Appl Phys Lett J Magn Magn Mater J Nanoscience and Nanotech Phys Rev Lett The Rietveld Method Oxford University Press, New York; 1993. MC Physics B (page number not for citation purposes) 13. Hejtmanek J, Jirak Z, Sedmidubsky D, Maignan A, Simon Ch, Caignaert V, Martin C, Raveau B: Phys Rev B Phys Rev B Solid State Commun Solid State Commun J Chem Phys Solid State Commun Phys Rev B R2952.20. Ang R, Sun YP, Yang J, Zhu XB, Song WH: J Appl Phys J Phys D: Appl Phys Phys Rev B Phys Rev B R8475.24. Egami T, Billinge SJL: Prog Mater Sci Acta Crystallogr Sect A Electronic Process in Noncrystalline Materials Clarendon, Oxford; 1971. 27. Ang R, Lu WJ, Zhang RL, Zhao BC, Zhu XB, Song WH, Sun YP: Phys Rev B J Appl Phys91: