Study of the percolative nature of thermoelectric power and resistivity in Pr 0.66 R 0.04 Sr 0.3 MnO 3 (R = Tb, Y, Ho an
- PDF / 80,489 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 30 Downloads / 179 Views
FF9.22.1
Study of the percolative nature of thermoelectric power and resistivity in Pr0.66R0.04Sr0.3MnO3 (R = Tb, Y, Ho and Er ) manganites. N. Rama1,2, V. Sankaranarayanan1 and M.S.R. Rao1,2* 1
Department of Physics and 2Materials Science Research Centre, Indian Institute of Technology (IIT) Madras, Chennai – 600 036. India. * Corresponding author: [email protected] ABSTRACT:
We have studied the resistivity and thermoelectric power of Pr0.66R0.04Sr0.3MnO3 where R = Tb, Y, Ho and Er and have analyzed it within a percolative framework. Both resistivity and thermoelectric power quantities were found to show a strong dependence on the disorder (2) with the high temperature activation energy in both increasing as 2 is increased, implying that as the disorder increases, carriers get more localized. The percolative nature of the thermopower was analyzed assuming that the lattice has coexisting metallic (Smet) and insulating (Sins) components above and below the Curie temperature (TC) which are independent of each other. Hence total S(T) = pSmet(T) + (1-p)Sins with the volume metallic fraction p=1/(1+exp(Uo(1-T/TC)/kBT) [1]. A similar analysis was done using the resistivity data. It was seen that the TC from both thermopower and resistivity fits were nearly the same indicating that they have a common origin. This analysis clearly proves that the metal insulator transition in manganites is percolative in nature and that the transport properties show a strong percolation tendency. INTRODUCTION: Ln1-xAxMnO3, where Ln is a lanthanide or Y3+ and A is divalent alkaline earth ion, exhibits colossal magnetoresistance (CMR) near the Curie temperature, TC. At low temperatures (TTC) in the paramagnetic state a thermally activated hopping or an adiabatic polaron hopping behaviour [1] is observed. The paramagnetic to ferromagnetic and the concomitant metal to insulator transition were explained within the framework of the Zener double exchange (ZDE) model [2] in which the mobile eg electrons (from Mn3+ ions) couple the core t2g spins ferromagnetically. However, calculations of resistivity within this framework did not yield satisfactory results and a localization mechanism not considered by the double exchange model was necessary to be invoked to explain the large field and temperature dependent changes [3]. There is now evidence from both experimental [4] and theoretical [5] calculations that polaron formation due to a strong Jahn-Teller (JT) interaction plays a major role in the physics of these materials. Several structural factors control the paramagnetic to ferromagnetic transitions in these systems. The first is the hole doping at the A-site by the alkaline earth ion. The second factor is the average ionic radius of the A-site ion , which influences the Curie temperature. The Curie temperature increases as increases [6]. The last factor is the ionic size mismatch between the various A-site ions and is represented by the variance = Σyir2i -2 where ri is the ionic radii size of the A-site ion of concentration yi [7]. It is seen
Data Loading...
