International Conference on Ceramics, Bikaner, India International Journal of Modern Physics: Conference Series Vol. 22 (2013) 626–629 World Scientific Publishing Company DOI: 10.1142/S2010194513010763
Int. J. Mod. Phys. Conf. Ser. 2013.22:626-629. Downloaded from www.worldscientific.com by 14.139.240.218 on 05/27/13. For personal use only.
ELECTRONIC TRANSPORT MECHANISM IN YMn0.8Ru0.2O3 COMPOUND RAJESH K. THAKUR*, 1, RASNA THAKUR1, ANCHIT MODI1, N. KAURAV2, G. S. OKRAM3, N.K. GAUR1 2
1 Department of Physics, Barkatullah University, Bhopal 462026, India Department of Physics, Government Holkar Science College, A. B. Road, Indore 452001, India 3 UGC-DAE Consortium for Scientific Research, Indore 452001, India *
[email protected]
A single phase polycrystalline hexagonal YMn0.8Ru0.2O3 compound with space group P63cm (25-1079) have been synthesized by using solid state reaction method at sintering temperature 12800C. The detailed low temperature electrical properties were evaluated over a broad range of temperature..The temperature dependence of the dc resistivity at low temperature reveals the semiconducting behavior and favored the variable range hopping conduction model. The obtained experimental data in the temperature range of our study can be described by the equation ρ(T) = ρ0exp[(T*/T)1/4]. The fitting results are used for the calculation of the temperature scale T* ~ 1.05 x 106 K and finally the density of state at Fermi level N(EF) is calculated to be ~ 5.6x1019 eV-1 cm-3. Keywords: XRD; magnetic transition; density of state.
1. Introduction Recently multiferroic materials have been the subject of intense study both because of the interesting physics involved and their promising practical applications [1-4]. Due to the interactions between the magnetic and electric orders, multiferroic materials also have additional functionalities such as the magnetoelectric effect. It is the induction of a magnetization by an electric field, or of a polarization by a magnetic field [5-8]. Singlephase multiferroic materials in the nature are rare, and YMnO3 is one of them. It belongs to the space group of P63cm, with a high ferroelectric transition temperature (TC~900 K) and a low antiferromagnetic transition temperature (TN~70 K) [9]. Several studies on the effect of doping in yttrium-based manganites have been reported, however most of these studies focused on the doping at the yttrium site, and relatively few studies have been reported on the effect of doping at the Mn site. Substitution of approximately 22 at% of Ca in the A site of YMnO3 results in a structural phase transition from the hexagonal to the well-known CaMnO3 structure. A similar type of phase transition was also observed when the Mn site was substituted by small divalent cations, Cu2+, Ni2+, and Co2+. In the literature there are very few reports on the substitution of Ru at the Mn site. Park et al. [10] studied the YMn0.9X0.1O3 (X=Al, Ru and Zn) system for its magnetic structure and thermodynamic behavior. Here we report the structural and electrical properties of the YMn0.8Ru0.2O3 compound. 626
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2. Experimental Techniques We have synthesized the polycrystalline sample of YMn0.8Ru0.2O3 using Y2O3, Mn2O3 and Ru2O3 of 99.99% purity (analytical reagent grade) by standard solid-state reaction method [11]. All the starting materials were mixed and ground several times in order to produce a homogeneous mixture. The resulting powder was then subsequently pulverized, compressed into pellets of 8mm diameter and approximately 1-2mm thickness under a hydraulic pressure of 8 - 10 tons. Final sintering was made at 12800C for 30 hours in order to ensure complete reaction, homogeneity and required compactness in pellets for planned measurements. Subsequent x-ray diffraction (XRD) measurements with high resolution Bruker D-8 powder diffractometer with Cu-Kα radiation (λ=1.54Å) operating at 40kV/100mA showed that prepared samples have no trace of impurity phases, and their measured patterns can be indexed according to the P63cm space group of the hexagonal manganites. The dc electrical measurement was carried by using the ring probe technique and high resistance electrometer (G-Ohm range). 3. Results and Discussions The XRD analysis carried out by a PowderX program reveals the single phase formation of YMn0.8Ru0.2O3 compound in the hexagonal structure with space group P63cm. The obtained XRD pattern is as shown in the fig.1 and the calculated values of the lattice parameters are a = b = 6.1457 Å, c = 11.4235Å.
Fig. 1. X-ray diffractions pattern of YMn0.8Ru0.2MnO3 compound, along with the respective hkl values.
The pointed kinks in the XRD pattern shows the better crystalline nature of he prepared compound and observed systematic variation in the lattice parameter with reference to parent counterpart of the reported compound confirms the uniform replacement Mn3+ by the Ru3+ ions. Plotted in Fig. 2 is the temperature dependence of the resistivity of an YMn0.8Ru0.2O3 pellet measured upon cooling.
R. K. Thakur et al.
T (K) Fig. 2. Temperature dependence of electrical resistivity (ρ) measured in zero magnetic fields. Inset: shows the variable range hopping (VRH) fit to the experimental data.
The semiconductor-like character of the material is vividly recognized in this plot. Interestingly, no anomaly, generally associated with structural phase transitions is observed in the measured temperature-range.
Rav (nm)
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ρ (MΩ Ω cm)
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T (K) Fig. 3. Calculated Rav along the c-axis direction as a function of temperature for YMn0.8Ru0.2O3 compound.
The electronic transport mechanism was tested by using Mott’s VRH model which depicts the temperature dependence of the electrical resistivity as shown in the inset (fig.2). As per the Mott’s VRH model in which the dependence of the resistivity on the temperature is given by ρ(T) = ρ0exp(T*/T)1/n here, ρ0, T* and n are constants. The quantity 1/n may take the values1/2, 1/3, or 1/4, depending on the dimensionality D and, in some compounds, on the temperature range. From the graphical analysis it can be concluded that, the resistivity data measured on the reported compound seemed to obey the VRH model when n = 1/4. The fitting procedure yielded a temperature scale
Int. J. Mod. Phys. Conf. Ser. 2013.22:626-629. Downloaded from www.worldscientific.com by 14.139.240.218 on 05/27/13. For personal use only.
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T* = 1.05x106 K (R2 = 0.998; slope = 35.16; standard error = 0.04). The encountered value is the same order of magnitude that that determined for other transition metal oxides [12]. The characteristic temperature T* is related to the parameter x through the expression kBT*= 24/πN(EF)ξ3 being N(EF) the density of states at the Fermi level [13]. By taking ξ = c = 11.4235Å the estimated density of state N(EF) is 5.6x1019 eV-1 cm-3. As it was stated above, the VRH transport comes to pass by hopping over a certain distance in the bulk of the dielectric that is controlled by the thermally available energy window that restricts the selection of possible hopping targets. The resulting average hopping distance (Rav) is then given by Rav= ξ(T*/T)1/4/2. The typical Rav decreases with increasing temperature as shown in fig. 3. Therefore it can be concluded that the reported YMn0.8Ru0.2O3 compound can be obtained in single phase by using conventional solid state reaction method with sintering temperature of 12800C. Further the Mott’s VRH model can better explain the electronic transport mechanism for the broader temperature region and yields that the hopping distance increases with increasing temperature. Acknowledgments RKT would like to thank the UGC-DAE Consortium for scientific research, India for providing the measurement facilities. References 1. X. Qi, J. Dho, Rumen Tomov, Mark G. Blamire and Judith L. MacManus-Driscoll, Appl. Phys. Lett. 86, 062903 (2005) 2. Z. J. Huang, Y. Cao, Y. Y. Sun, Y. Y. Xue and C. W. Chu, Phys. Rev. B 56, 2623 (1997). 3. N. A. Hill, J. Phys. Chem. B 104, 6694 (2000). 4. T. Yoshimura, N. Fujimura and T. Ito, Appl. Phys. Lett. 73, 414 (1998). 5. T. Thomson, G. Hu and B. D. Terris, Phys. Rev. Lett. 96, 257204 (2006) 6. A. Filippetti and N. A. Hill, Phys. Rev. B 65, 195120 (2002). 7. H. Sugie, N. Iwata and K. Kohn, J. Phys. Soc. Jpn. 71, 1558 (2002). 8. Th. Lonkai, D. G. Tomuta, U. Amann, J. Ihringer, R. W. A. Hendrikx, D. M. Tobbens and J. A. Mydosh, Phys. Rev. B 69, 134108 (2004). 9. Bas B. Van Aken, Thomas T.M. Palstra, Alessio Filippetti and Nicola A. Spaldin, Nat. Mater. 3, 164 (2004). 10. J. Park, M. Kang, J. Kim, S. Lee, K.-H. Jang, A. Pirogov, J.-G. Park, C. Lee, S.-H. Park, and H. C. Kim, Phys. Rev. B 79, 064417 (2009). 11. Rajesh K. Thakur, Rasna Thakur, S. Shanmukhrao Samatham , N. Kaurav , N. K. Gaur, V. Ganesan and G.S. Okram, J. Appl. Phys. 112, 104115 (2012). 12. Sir Nevill Mott, Conduction in Non-Crystalline Materials. Clarendon Press, Oxford, (1993). 13. G.J. Snyder, C.H. Booth, F. Bridges, R. Hiskes, S. Dicarolis, M.R. Beasley and T.H. Geballe, Phys. Rev. B 55, 6453 (1997).
