Panwar, Sunil; Kumar, Vijay and Singh, Ishwar
The magnetothermal properties of hole doped CMR manganites Re1-x AxMnO3 (x=0.1 to 0.5) have been studied as a function of temperature (0 to 500K) by means of Variational method. We have used two band (ℓ-b) model Hamiltonian for manganites in the strong electron- lattice Jahn- Teller (JT) coupling regime to study the temperature variation of the electronic specific heat (Cv) and linear coefficient of the electronic specific heat (Cv/T) in these compounds. We have already used this variational method to study the zero field electrical resistivity ρ (T) & magnetic susceptibility of doped CMR manganites. We have also observed the role of the model parameters e.g. local Coulomb repulsion U, strong ferromagnetic Hund’s Rule coupling JH between eg & t2g spins & hybridization Vk between ℓ– polarons & d – electrons of the same spins on Cv (T). We find from our results that as the temperature is lowered below a critical temperature Tc (~50K) , there is an anomaly (sharp peak) in both Cv & Cv /T resembling with the key feature of many CMR compounds like La1-x Sr x MnO3. The low temperature peak in Cv for a particular value of magnetic parameters h & m becomes broader & shifts towards the high temperature region on increasing Vk or JH or doping x value. With increase in magnetic field, the specific heat decreases and the peak at Tc in Cv /T shifts to higher temperatures. The electronic specific heat values (Cv) & linear coefficient of the electronic specific heat (Cv / T) obtained by us are in reasonably good agreement with the available experimental data for some concentrations (x) of La1-x Srx MnO3.
Keywords
Colossal Magnetoresistive manganites | Electronic specific heat | Variational method
PACS No: 75.47.Lx; 75.47.Gk; 71.27. +a; 71.30.+h; 71.38.-k
Introduction
Colossal magnetoresistive (CMR) manganites have attracted considerable attention worldwide due to their peculiar physical properties & potential applications (Helmolt et al., 1993; Jin et al., 1999; Xu et al., 2001). They revealed very interesting features like complex interplay of charge, spin and orbital degrees freedom (Tokura, 2000; Salamon and Jaime, 2001). Below Curie temperature (Tc) manganites behave as ferromagnetic metal (FM) and above Tc as paramagnetic insulator (PI). The ferromagnetism in manganites arises due to the double exchange (DE) interaction (Zener, 1951). Millis et al. (1996) have reported that besides DE, other mechanisms like Jahn Teller (JT) distortion and charge / orbital ordering (CO / OO) are required to explain the CMR property. In these compounds, electrical resistivity decreases by orders of magnitudes upon applying a magnetic field (Mamatha et al., 2014). The low temperature specific heat of peroskite manganites has been intensively studied (De Teresa et al., 1997; Gordan et al., 1999, Gaur and Srivastava, 2010). The specific heat anomaly (in the form of a peak) near the ferromagnetic- paramagnetic transition temperature (Tc), however, has not been dealt with in detail. The observed thermal transport properties of manganites can be reasonably understood from the viewpoint of polaronic transport.