Structural, magnetic and magnetocaloric properties of Pr0.6-xErxSr0.4MnO3 \(x = 0.0, 0.1 and 0.2\)

We investigate the effect of Erbium doping on the structural, magnetic and magnetocaloric properties of Pr0.6-xErxSr0.4MnO3 (x = 0.0, 0.1 and 0.2) powder samples. Our polycrystalline samples were synthesized using the solid-state reaction at high temperature. X-ray diffraction characterizations showed that all our studied samples crystallize in the distorted orthorhombic system with Pnma space group. All our synthesized samples exhibit a paramagnetic–ferromagnetic transition with decreasing temperature. The magnetic transition temperature TC is found to decrease continuously with increasing Er concentration. It was found that the maximum value of the magnetic entropy change for Pr0.4Er0.2Sr0.4MnO3 sample reached max M S ∆ = 2.40 J kgK in a magnetic applied filed of 2T.


Introduction
Presence of well-defined crystallographic and magnetic sublattices within the same compound usually leads to very interesting phenomena in reason of the strong interplay between localized moments, itinerant electrons, magnetic order, and other important features which characterize each one of the competing phases. In the recent years manganites with general formula of Ln 1−x M x MnO 3 (Ln=rare-earth element and M=divalent metal such as Ca, Sr, Ba, etc.) have been elaborated and studied [1] in order to understand their mechanism of transport and spin ordering. The perovskite structure generally shows lattice distortion as modifications from the cubic system to rhombohedral or orthorhombic one mainly due to the Jahn-Teller (JT) effect causing the deformation of the MnO 6 octahedra. The double exchange theory and the Jahn-Teller effect were used to understand the ferromagnetic-paramagnetic (FM-PM) phase transition associated with an insulator-metal (IM) one in the manganites [2,3], thus giving rise to the well known colossal magnetoresistance (CMR) [1]. Pr 1−x Sr x MnO 3 perovskite manganite is an important member in the manganite family with intermediate one electron bandwidth [4]. The Pr-based manganites show many novel properties including the charge ordering (CO) state, FM and antiferromagnetic (AFM) coexistence and metamagnetic transition [5][6][7]. For x < 0.5, it possesses a strong ferromagnetic-paramagnetic (FM-PM) transition and a large magnetoresistance effect. As x ≥ 0.5, it shows a CO-antiferromagnetic a e-mail : abdcheikhrouhou@yahoo.fr

Experimental techniques
Polycrystalline Pr 0.6-x Er x Sr 0.4 MnO 3 samples were prepared using the conventional ceramic route at high temperature. The precursors Pr 6 O 11 , SrCO 3 , Er 2 O 3 and MnO 2 with purity more than 99% were intimately mixed in an agate mortar and then heated in air up to 1000 • C for 60 h. The obtained powders were pressed into pellets (of about 1mm thickness) and sintered at 1350 • C in air for 60 h with intermediate regrinding and repelling. Finally, these pellets were cooled slowly to room temperature in air. Phase purity, homogeneity and cell dimensions were determined by X-ray powder diffraction (XRD) at room temperature (diffractometer using Cu Kα radiation). Structural analysis was carried out using the standard Rietveld method [8,9]. Magnetization (M) versus temperature (T) in the range 20-300K under 0.05T and versus magnetic applied field (H) up to 7T at several temperatures were measured using a vibrating sample magnetometer equipped with a superconducting coil. The M(T) measurements were performed in the field cooled mode.

X-ray diffraction
The results of the X-ray diffraction (XRD) studies at room temperature of our samples indicate that our compounds were found to be single phase and crystallize in the orthorhombic structure with Pnma space group. Figure1 shows typical XRD patterns registered at room temperature for x = 0.1 and 0.2 samples. The structural parameters are refined by Rietveld's profile-filling method. The quality of the refinement is evaluated through the goodness of the fit indicator χ 2 . The structural parameters are listed in Table 1. In Figure 2 the average size of A-site cations <r A > and the mismatch size σ 2 at the A-site are plotted. This evolution can be explained in terms of average ionic radius values; in fact, the average ionic radius of Er 3+ (1.062 Å) is smaller than that of Pr 3+ (1.179Å) [10,11].    Figure 3 displays the temperature dependence of the magnetization M at a magnetic applied field of 0.05T. The M /M max (T) curves do not reveal any secondary phases, which confirm the good crystallization of our samples. In the temperature range 20-300 K, a paramagnetic to ferromagnetic transition is observed. The Curie temperature T C , which is defined as the inflection point of M (T) curve, is found to decrease from 303 K (x=0.0) to 70 K (x=0.2). Since Er 3+ has the same valence as Pr 3+ , the average manganese oxidation state remains constant by substitution of Er 3+ for Pr 3+ . The decrease of T C can be explained by the <r A > change. In order to confirm the ferromagnetic behavior at low temperatures of our samples, we have performed magnetization measurements as a function of the magnetic applied field µ 0 H up to 7T at several temperatures. Figure 4 shows isothermal magnetization curves for x=0.1 and x=0.2. We can observe, that below T C , the magnetization increases sharply with magnetic applied field for H<0.5T and tends to saturation above 1T.

Arrott curves
In order to determine the order of the magnetic phase transition for both Pr 0.

Magnetocaloric effect
Using the Maxwell's thermodynamic relation [14]: The magnetic-entropy change ∆S M , which results from the spin ordering induced by the variation of the magnetic applied field from 0 to H max is given by For magnetization measured at discrete field and temperature intervals, the magnetic entropy change defined in Eq. (2) can be approximated by Eq.(3) [15], The entropy change for both samples x=0.0 and x=0.1 calculated as a function of temperature at various magnetic fields is plotted in Figure 6. 00051-p.5  [16,17]. The relative cooling power ( RCP) or refrigerant capacity is a useful parameter which is deciding the efficiency of magnetocaloric materials based on the magnetic entropy change [18,19] [21]. These results are interesting enough, compared with materials considered as suitable for applications in magnetic refrigerators, paving a way for investigations of materials useful for magnetic refrigeration.

Conclusion
The