In this communication, we are reporting the effect of electron beam (e-beam) irradiation on thermoelectric
properties of La0.8Sr0.2MnO3 manganites. The samples were prepared using solid state reaction technique.
It is observed that the lattice volume increases with increase in dosage of e-beam. With irradiation
an increase in resistivity is observed. For small irradiation dosage, we first observe a decreases in metalinsulator
transition temperature TMI; thereafter TMI increases with further increase in dosage of irradiation.
Both, the resistivity data and thermo-electric power data demonstrate that small polaron hopping
model is valid in high temperature region

Benedict Christopher, J.

Okram, G.S.

Rao, Ashok

Sanjeev, Ganesh

SocioEconomic Challenges Journal (Sumy State University)

English

Effect of Electron Beam Irradiation on Structural, Electrical and Thermo-electric Power of La0.8Sr0.2MnO3

Electronic Sumy State University Institutional Repository

JOURNAL OF NANO- AND ELECTRONIC PHYSICS ЖУРНАЛ НАНО- ТА ЕЛЕКТРОННОЇ ФІЗИКИ Vol. 7 No 1, 01001(5pp) (2015) Том 7 № 1, 01001(5cc) (2015)   2077-6772/2015/7(1)01001(5) 01001-1  2015 Sumy State University Effect of Electron Beam Irradiation on Structural, Electrical and Thermo-electric Power of La0.8Sr0.2MnO3   Ashok Rao1,*, J. Benedict Christopher1, Ganesh Sanjeev2, G.S. Okram3  1 Department of Physics, Manipal Institute of Technology, Manipal University, Manipal-576104, India 2 Microtron Centre, Mangalore University, Mangalagangotri – 574199, DK, Karnataka, India 3 UGC-DAE Consortium for Scientific Research, University Campus, Indore-452017, India  (Received 05 December 2014; published online 25 March 2015)  In this communication, we are reporting the effect of electron beam (e-beam) irradiation on thermoelec-tric properties of La0.8Sr0.2MnO3 manganites. The samples were prepared using solid state reaction tech-nique. It is observed that the lattice volume increases with increase in dosage of e-beam. With irradiation an increase in resistivity is observed. For small irradiation dosage, we first observe a decreases in metal-insulator transition temperature TMI; thereafter TMI increases with further increase in dosage of irradia-tion. Both, the resistivity data and thermo-electric power data demonstrate that small polaron hopping model is valid in high temperature region.  Keywords: Solid state reaction technique, Electrical resistivity, Thermoelectric power.   PACS numbers: 81.20.Ev, 72.15.Eb, 72.15.Jf                                                              * ashokanu_rao@rediffmail.com 1. INTRODUCTION  Ever since the discovery of colossal magneto-resistance effect (CMR) and large magneto-caloric ef-fect in rare earth manganite of the form RE1 – xAxMnO3 (RE is a rare-earth element, A is a divalent alkaline element), immense work has been done on the electri-cal, magnetic and thermal properties of these materials [1-9]. The primary aim of such studies is to understand the physics behind these properties and to explore pos-sibilities of using them in various applications [10]. These manganites show numerous interesting proper-ties like metal-insulator transition, and paramagnetic-ferromagnetic transition. Over the decades, energetic ion-beams and electron beams have been used as a vi-tal tool for modification of materials [11, 12]. Extensive work seems to have been done on various ion irradia-tions. Chattopadhyay et al. [13] have shown that La3+ ion irradiation creates disorder in La0.7Ca0.3MnO3 (LCMO) manganite sample and as a result of this an increase in room temperature resistivity is observed. Ravi Kumar et al. [14] has reported the influence of swift heavy ion irradiation on transport properties of thin films of LCMO system and have demonstrated that TC decreases with increase in fluence of the beam. On the other hand they have reported that the resistiv-ity of the films increases with fluence value. Kataria et al. [15] have reported interesting results which indicate that thickness of films of LPMO plays important role and they have demonstrated that for small thickness of 150 nm, irradiated films have lower resistivity than pristine films of same thickness. However, for films of thickness 150 nm and above, an increase in resistivity is seen when the samples are irradiated. La1 – xSrxMnO3 (LSMO) in particular has attracted lot of attention as they are considered to be prototypical and reference ma-terials [16, 17]. They are promising materials since they show ferromagnetic-paramagnetic transition and also exhibit significant magneto-resistance (MR) value  [18, 19] which essentially make them promising candi-dates for applications. Sahasrabudhe et al. [20] have done a detailed investigation on the effect of heavy ion irradiation on the surface morphology of highly orient-ed thin films of LSMO and concluded using XRD that irradiation induces strain in the films. Less work seems to have been done on effect of electron beam irradiation on manganites. Samoilenko et al. [21] have demonos-trated that when thin films of LSMO are irradiated by KrF laser emitting in the UV range, the metallic phase of the film is observed to increase. Pallecchi et al. [22] have investigated the effect of focused ion beam (Ga ion) on LSMO films and have shown that resistivity increas-es with increases in dosage of irradiation. To the best of our knowledge, no work seems to have been done electron beam irradiation on LSMO system. Keeping this in mind, we have investigated the effect of electron beam irradiation on structural, elec-trical and thermo-electric properties of La0.8Sr0.2MnO3 compounds. We have focused the present studies on thermoelectric power as it is sensitive to the nature of charge carriers. The Seebeck coefficient (S) of mang-nites exhibits a complex behavior indicating both posi-tive and negative signs which depends on temperature as well as on the nature and degree of substitution. In the low temperature regime S is related to the changes in electronic structure and scattering processes, where-as in the insulating region S provides information on energy dependent parameters which follow small po-laron hopping mechanism. Thus it would be interesting to see the effect of electron beam irradiation on ther-moelectric power of La0.8Sr0.2MnO3 compounds.  2. EXPERIMENTAL  In this work, single phase polycrystalline stronti-um-substituted lanthanum manganite (La0.8Sr0.2MnO3) compounds were prepared by conventional solid state reaction technique. Stoichiometric amounts of lantha- ASHOK RAO, J. BENEDICT CHRISTOPHER, ET AL. J. NANO- ELECTRON. PHYS. 7, 01001 (2015)   01001-2 num oxide (La2O3), strontium carbonate (SrCO3), and manganese oxide (MnO2) were weighed and mixed thoroughly. The finely ground stoichiometric mixture of the starting materials was calcined in air several times at 1200 C for 24 hours with a final calcination at 1250 C for 24 hours. The obtained powder after calci-nation was uni-axially pressed into pellets (rectangular pellets of thickness  1.5 mm) by applying pressure  80 kg/cm2. Finally the samples were sintered at 1350 C for 24 hours. All the samples were cooled to room temperature at a cooling rate of about 3 C/min.  In order to ascertain the purity and to determine the lattice dimensions, X-ray diffraction (XRD) tech-nique was used (D8 Advanced X-ray Diffractometer). The data taken from XRD was analyzed using Rietveld refinement method. Then the samples were irradiated by electron beam irradiation. It may be mentioned that all the samples were prepared under identical condi-tions. We have carried out electron beam irradiation using an 8 MeV electron accelerator which is able to provide energetic electrons, intense bremsstrahlung radiation and neutrons of moderate flux. The accelera-tor provides pulsed electron beam current of 50 mA (peak) with a pulse width of ~ 2.5 s at a maximum pulse repetition rate of 100 Hz. Electrical resistivity measurements on the rectangular shaped samples were carried out in a temperature range of 10-380 K in a cryostat (Closed Cycle Refrigerator by JANIS). The resistivity of the samples was measured using conven-tional four probe technique employing a Keithley cur-rent source and a Keithley nano-voltmeter. The See-beck coefficient measurement was performed in the temperature range of 5-340 K using differential dc method in a closed cycle refrigerator.  3. RESULTS AND DISCUSSION  Fig. 1 shows the room temperature X-ray diffraction patterns of bulk samples of La0.8Sr0.2MnO3 at different dosage levels of electron beam in the range of 0-50 kGy. The diffraction data was refined using Rietveld refine-ment technique and Rietveld refined plots are shown in the Fig. 2. The open circles show the actual data, the calculated curve is superimposed on them, the solid line represents the refined data and the vertical lines represent the Bragg peak. We observe that there is good agreement between the observed and refinement data which confirms that the prepared samples are free from secondary or impurity phase. From the refine-ment procedure we observe that the samples are rhom-bohedral in structure with R3c space group and the structure does not change with irradiation. The lattice parameters and structure estimated in the present work match well with that reported in literature [23]. We now discuss the effect of electron beam irradia-tion on structural properties. From table 1 it is evident that both a and c parameters increase with dosage lev-el of electron beam thereby volume increases with EB dosage. It is also seen that there is an increase in Mn-O-Mn bond length which may be attributed to intro-duction of magnetic inhomogeneity due to EB irradia-tion [24]. The temperature dependence of the resistivity () for the samples of La0.8Sr0.2MnO3 over a temperature range, 100-380 K is illustrated in Fig. 3. It is evident from the ρ-T plot, that all the samples undergo metal to insulator transition (MI) around the characteristic temperature TMI. For the pristine sample the value of TMI is about 314 K. For the sample irradiated with dos-age of 15 kGy, a decrease in TMI is observed. However, with further increase in electron beam dosage, TMI  increases with dosage. It is evidently from Fig. 3 that there is continuous suppression in peak resistivity with increase dosage of electron irradiation. The observed drop in resistivity with electron beam irradiation may be attributed to relaxation of strain in the lattice via irradiation. This is consistent with the observed in-crease in cell volume of the lattice. Similar results have been reported by Chattopadhyay et al. [25] for ion beam    Fig. 1 – XRD patterns of pristine as well as electron beam irradiated samples of La0.8Sr0.2MnO3    Fig. 2 – Rietveld refined XRD plots for the pristine and irra-diated samples La0.8Sr0.2MnO3  EFFECT OF ELECTRON BEAM IRRADIATION ON STRUCTURAL… J. NANO- ELECTRON. PHYS. 7, 01001 (2015)   01001-3    Fig. 3 – Resistivity versus temperature behavior of pristine as well as electron beam irradiated samples of La0.8Sr0.2MnO3    irradiated samples of Li0.7Pb0.3MnO3 which has been attributed to radiation induced strain relaxation and ordering in the lattice. In order to understand the conduction phenomenon at low temperatures, the electrical resistivity in the low temperature region (ferromagnetic metallic phase) be-low TMI is fitted using the equation,   ρ(T)  ρ0 + ρ2T2 + ρ4.5T4.5 (1)  where ρ0 is the residual resistivity due to grain bound-ary scattering, ρ2 is arising from electron-electron scat-tering (e–-e–), and ρ4.5 is the resistivity factor due to various effects of e–-e–, e–-magnon and e–-phonon scat-tering processes. In the present work we have used the experimental data of resistivity and have fitted tis us-ing Eq. 1. It is satisfactory to note that the experi-mental data matches well with the above equation. The fitting parameters are presented in Table 1. We ob-serve that ρ0  ρ2 and ρ0  ρ4.5 which indicates that the electron-electron scattering is the dominating fac-tor for the conduction phenomenon in the low tempera-ture region.  Table 1 – Best fit parameters for low temperature region  Dosage 0 2 4.5 0 9.6  10 – 3 1.7  10 – 6 – 2.4  10 – 13 15 9.1  10 – 3 1.7  10 – 7 – 6.6  10 – 15 30 3.6  10 – 3 3.8  10 – 7 –2.6  10 – 14 50 2.1  10 – 3 2.2  10 – 7 –3.7  10 – 14  We also observe that ρ0 decreases with dosage of electron beam. This implies that electron beam irradia-tion is reducing the electron-electron scattering which would mean that there should be reduction in resistivi-ty of the samples when they are irradiated with elec-tron beam. This indeed is observed which is evident from Fig. 3. We now analyze the high temperature resistivity data using small polaron hopping (SPH) using the equation,   ρ(T)  ρ0 T exp(Eρ / kBT) (2)  We have used the high temperature resistivity data (above TMI) and fitted the data using Eq. 2. It is satis-factory to note that the present results in the high temperature regime follow SPH model. Using this model we have estimated activation energy Eρ for the pristine as well as irradiated samples of La0.8Sr0.2MnO3. The es-timated parameters are tabulated in Table 2.     Fig. 4 – Fitting of low temperature resistivity data of pristine and irradiated samples of La0.8Sr0.2MnO3  Table 2 – Best fit parameters for high temperature region  Dosage E (meV) 0 73.3 15 75.7 30 80.3 50 90.3  Fig. 5 depicts the high temperature resistivity data along with the fitted curves for pristine and irradiated samples. We notice two features. Firstly the experi-mental data match well with the theoretical fitting. Secondly the estimated value of activation energy is observed to increase in dosage of electron beam.    Fig. 5 – Fitting of high temperature resistivity data of pris-tine and irradiated samples of La0.8Sr0.2MnO3   ASHOK RAO, J. BENEDICT CHRISTOPHER, ET AL. J. NANO- ELECTRON. PHYS. 7, 01001 (2015)   01001-4 In view to elucidate other evident features of the pristine as well as irradiated samples, Seebeck coefficient (S) was measured in the temperature range of 5-340 K. The plots of the measurement are presented in Fig. 6.     Fig. 6 – Thermo-electric power versus temperature behavior of pristine as well as electron beam irradiated samples of La0.8Sr0.2MnO3  We observe that for the pristine sample, S(T) is negative for high temperature region. As the tempera-ture is reduced we observe a change in sign of S(T). This negative to positive crossover indicates that the nature of charge carriers is changing from electron to holes as the samples are becoming more conducting with electron beam irradiation. This is consistent with electrical resistivity data. We further see that S(T) de-creases with reduction in temperature. At a particular temperature, a peak value in S(T) is observed. Thereaf-ter, with further decrease in temperature S(T) is seen to decrease. The negative S(T) at the high temperature regime is due to electrons which are excited into the conduction band from the valence band. This trend is seen in pristine as well as irradiated samples. Akin to the electrical resistivity, many scattering mechanisms are responsible for the total S(T) in differ-ent temperature ranges. Hence we analyze the contri-bution of different scattering mechanisms to the tem-perature dependent S(T) data in the metallic and insu-lating regimes. First we analyze the thermo-electric power data in the metallic region. For this region we have utilized the following well-known equation,   S(T)  S0 + S3/2T3/2 + S4T4 (3)  where S0 is a constant, S3/2T3/2 arises due to electron-magnon scattering and S4T4 arises due to spin wave fluctuations in the FM region. We observe that the thermo-electric power data in metallic region matches well with Eq. 3. Fig. 7 shows the experimental S(T) data with theoretical values and demonstrates a good match between the two. The fitting parameters are presented in Table 3. The value of S3/2 is observed to be much larger compared to S4 thereby demonstrating that the e–-magnon scattering process is dominant mechanism in the low temperature metallic region. For the present studies, S(T) data in the insulating regime is analyzed within the framework of small po-laron hopping model [35]. Figure 8 shows a plot of S versus 1 / T and it is satisfactory to see that the present data fits well with SPH model.     Fig. 7 – Fitting of S(T) in the metallic region for the  pristine and irradiated samples of La0.8Sr0.2MnO3  Table 1 – Best fit parameters for low temperature region  Dosage S0 S3/2T3/2 S4T4 0 1.25 1.5  10 – 3 1.1  10 – 9 15 2.72 7.7  10 – 4 1.8  10 – 10 30 2.67 3.8  10 – 4 3.0  10 – 10 50 2.22 1.2  10 – 4 4.7  10 – 10    Fig. 8 – Fitting of S(T) in the insulating for the pristine and irradiated samples of La0.8Sr0.2MnO3  4. SUMMARY  The structural, electrical and thermo-electric proper-ties of pristine and irradiated samples of La0.8Sr0.2MnO3 are reported in this communication. We observe that there is no change in structure of the sample upon elec-tron beam irradiation. However, it is seen that the volume of the lattice increases with increase in dosage of irradia-tion which means that irradiation reduces strain in the lattice. 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Effect of Electron Beam Irradiation on Structural, Electrical and Thermo-electric Power of La0.8Sr0.2MnO3

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