Structural , magnetic and magneto-transport properties of monovalent doped manganite Pr

Pr0.55K0. 05Sr0.4MnO3 sample have been synthesized using the conventional solid state reaction. Rietveld refinements of the X-ray diffraction patterns at room temperature confirm that the sample is single phase and crystallizes in the orthorhombic structure with Pnma space group; the crystallite size is around 70 nm. The SEM images show that grain size spreads around 1000 1200 nm. DTA analysis do not reveal any clear transition in temperature range studied. The low-temperature DSC indicates that Curie temperature is around 297 K. Magnetization measurements in a magnetic applied field of 0.01 T exhibit a paramagneticferromagnetic transition at the Curie temperature TC = 303 K. A magnetic entropy change under an applied magnetic field of 2 T is found to be 2.26 J kg−1 K−1, resulting in a large relative cooling power around 70 J/kg. Electrical resistivity measurements reveal a transition from semiconductor to metallic phase. The thermal conductivity is found to be higher than that reported for undoped and Na doped manganites [1]. Introduction Perovskite manganites with general formula R1-xTxMnO3 where R is a rare earth atom (La, Pr, Nd, .) amd T is an alkaline earth elemnt (Sr, Ca, Ba,.), have been attracted much renewed attention. Such interrest is mainly related to the discovery of the colossal magnetoresistance (CMR) in these materials. In fact, physical properties of ferromagnetic mixed-valence manganites are strongly influenced by the application of an external magnetic field resulting in a decrease of the electrical resistivity around their metal-insulator transition temperature TMI and giving rise to CMR effect [2]. The complex manganite phenomena, has been explained by using many theories, such as the double exchange theory, Jahn-Teller effect, polaronic effect and phase separation [3-5], but still complicate. In these manganites systems a simple chemical substitution in A-site leads to a mixed Mn3+/Mn4+ valence resulting the ferromagnetic interaction controlled by the hopping of eg electron between Mn3+ and Mn4+ ions via the orbital overlap 2p [6]. In fact, the physical properties of manganites are mainly controlled by the ratio between Mn3+ and Mn4+ ions, the average size and the degree of disorder at the A-site. Recently, Rocco et al. [7] confirm the direct relationship between the average ionic radius, the MCE and the Curie temperature. The materials required for magnetic refrigeration exhibit large magnetocaloric effect around their phase transition temperature. Generally, the MCE values for manganite are related to their phase transitions orders and the highest values were reported for that with first order transition [8]. Moreover, the manganites are relatively cheap and easy for technological treatments. Author postprint of the paper published in Journal of Alloys and Compounds 611 (2014) 427–432 There is a growing research activity devoted to hole doped manganites with monovalent metals [9-11]. In fact the substitution with monovalent metals converts more Mn3+ ions to Mn4+ compared to divalent metals due to valence difference. Recently, Li et al. [9] report the coexistence of large magnetoresistance and magnetocaloric effect in monovalent doped manganite La0.5Ca0.4Li0.1MnO3. Previously, we have reported with details the studies of K-doped compounds Pr0.6Sr0.4-xKxMnO3 [11] and it was found that ferromagneticparamagnetic transition temperature (TC) decreases from 310 K to 269 K with increasing K concentration from 0 up to 0.2, and the metal-insulator transition temperature TMI exhibits a slight decrease from 184 K down to 170 K, when K concentration x increases from 0 to 0.1 [12]. On contrary, for Pr0.55Na0. 05Sr0.4MnO3 manganite [1], both transition temperatures were 289 K and 104 K for TC and TMI, respectively which represents a significant decrease compared to that reported for parent sample Pr0.6Sr0.4MnO3 [12]. To our knowledge there has not been any report on physical properties of Pr0.55K0. 05Sr0.4MnO3 manganite previously. This motivates us to extend our recent studies on monovalent doped manganites and we report new investigations summarizing the effect of 5% K doping in Pr-site in Pr0.6Sr0.4MnO3 manganite on structural, magnetic, magneto caloric, electrical and thermal conductivity properties. Experimental details Pr0.55K0. 05Sr0.4MnO3 sample were prepared by using the standard conventional solid state reaction method as described previously [11, 12]. Homogeneity and cell parameters were studied by the X-ray diffraction with Cu-Kα radiation (1.54 Å) in the 2θ range of 10 100 degrees. Structural analysis was carried out using the standard Rietveld technique [13, 14]. In order to check thermal stability, simultaneous DSC and TGA analysis was performed using Q600 (TA instruments) apparatus operating under high purity argon flow of 100 ml/min. The measurements were performed during constant heating at 20 °C/min from room temperature up to 1350 °C for a sample of 47 mg. The density was measured using AccuPycII 1340 helium pycnometer. The morphology and grain size distribution were studied by SEM (Scanning Electron Microscopy). The magnetic measurements were carried out using PPMS (Physical Property Measurement System from Quantum Design) in applied magnetic field up to 2 T. The temperature variation of electrical resistivity was measured by the four probe method. The thermal conductivity was measured using the stationary heat flux method in the temperature range 5 K 300 K [1]. Structure The X-ray diffraction patterns recorded at room temperature (Fig. 1) confirm that manganite is a single phase crystallizing in the orthorhombic structure with Pnma space group. Using Rietveld refinement, lattice parameters are calculated to be equal to 5.43884(2) Å, 7.6566(2) Å and 5.48313(1) Å for a, b and c, respectively. The unit cell volume of 228.334 Å3 is found to be slightly smaller than 229.295 Å3 reported for undoped manganite Pr0.6Sr0.4MnO3 [10, 15]. Such a volume decrease can be explained by the increase of the Mn4+ content with average ionic radius (0.53 Å) on the cost of the Mn3+ content with average ionic radius (0.65 Å) [16]. This correlates with the theoretical expression: (1) The 5% K doping in Pr -site induces an increase in Mn4+ fraction from 0.4 to 0.5. The average crystallite size was evaluated from a width of diffraction peaks using Scherrer formula:    COS K CXRD * *  (2)  