Phase separation and magnetocaloric effect in Pr0.5−xGdx Sr0.5MnO3 system
Introduction
The perovskite manganites with general formula T1−xDxMnO3 (T is a trivalent rare earth ion and D is a divalent alkaline-earth ion) have been intensively investigated in the last decades because of their potential applications in several fields. Magnetic and electrical transitions, colossal magnetoresistance (CMR) as well as magnetocaloric effect (MCE) [1], [2], [3], [4], [5], [6], [7] gave renewed interest in perovskite manganites. Double-exchange interactions between Mn3+ and Mn4+ ions were used in order to interpret the CMR effect. The MCE is an intrinsic property of magnetic materials; it originates from their heating due to the adiabatic application of magnetic field. The gadolinium is known as the best candidate for magnetic refrigeration because of its large magnetic entropy change ΔSM which is about 10.2 J kg−1 K−1 for a magnetic field change of 5 T [8].
Half doped perovskite manganites with x value equal to 0.5 possess equal amounts of Mn3+ and Mn4+ ions [1], [3], [9], [10], [11], [12], [13], [14]. The half doped compounds are generally characterized by a very important CMR effect compared to MCE. These compounds are also characterized by the complexity of the electrical and magnetic behavior as well as phase separation phenomenon consisting in the coexistence of two or more different magnetic phases. Pr0.5Sr0.5MnO3, with an average ionic radius at A-site 〈rA〉 of 1.245 Å, is one of the most studied compounds in the last years [1], [15], [16], [17], [18], [19], [20], [21]. This compound received much attention due to the discovery of the first ordered antiferromagnetic insulator-ferromagnetic metal transition under a magnetic field recorded at Néel temperature TN. This compound presents a magnetic transition from the paramagnetic (PM) state to the ferromagnetic (FM) state at TC = 260 K, this transition is followed by another one to the antiferromagnetic (AFM) state at TN = 150 K. It is also characterized by the phase separation phenomenon, with the appearance of AFM clusters just below the Curie temperature [20].
Gadolinium ions Gd3+ possess an important magnetic moment and a small ionic radius compared to praseodymium ions Pr3+. The substitution of Pr ions by Gd ones will certainly affect the ordering inside the structure. Such doping will enhance both structural and magnetic disorder. Thus, the Pr0.5−xGdxSr0.5MnO3 system seems to be very important in order to study the effect of disorder on phase separation without changing the amount of Mn3+ and Mn4+ ions. The electrical properties of Pr0.5−xGdxSr0.5MnO3 (x = 0, 0.05 and 0.1) samples have been previously reported [21]. The magnetotransport study has shown that our samples exhibit different electrical behavior as a function of temperature. The Gd doping increases the resistivity and magnetoresistance values especially at low temperature values [21]. In this work, we are interested to investigate the magnetic properties of these samples.
Section snippets
Experimental details
A series of polycrystalline Pr0.5−xGdxSr0.5MnO3 samples were prepared by the conventional solid state reaction method. High purity Pr6O11, Gd2O3, SrCO3 and MnO2 up to 99.9% were mixed in stoichiometric proportions. The mixture was first heated at 1000 °C for 24 h. After that, all the samples were repeatedly ground and heated for 20 h at 1000 °C. Then, the powders were pressed into pellets of about 2 mm of thickness and sintered at 1200 °C in air for 60 h with intermediate regrinding and
Results and discussions
The Pr0.5−xGdxSr0.5MnO3 samples were indexed in orthorhombic system with Pbnm space group. The increase of Gd content leads to the decrease of the average ionic radius of the A site 〈rA〉 as well as an increase of the mismatch effect σ2 [21]. Fig. 1 shows the temperature dependence of the field cooled (FC) and the zero field cooled (ZFC) magnetization M (T) for all our specimens in the temperature range 4–330 K with an applied magnetic field of 0.05 T. All samples present a transition from PM to
Conclusion
Magnetic properties and magnetocaloric effect of the system Pr0.5−xGdxSr0.5MnO3 with x = 0, 0.05 and 0.1 have been investigated. Competition between double exchange and super exchange interactions leads to the coexistence of different phases in this system, which can be responsible for the complex magnetic behavior. Gadolinium doping induces also the decrease of the maximum values of the entropy and the decrease of the RCP values. The magnetic entropy change can be characterized by one
Acknowledgement
This work was supported by the Tunisian Ministry of Higher Education and Scientific Research.
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2020, Journal of Solid State ChemistryCitation Excerpt :The maximum values of -ΔSM are -1.03 J kg-1 K-1, -1.53 J kg-1 K-1, -1.72 J kg-1 K-1, -1.73 J kg-1 K-1,-2.20 J kg-1 K-1, and -2.70 J kg-1 K-1 for x = 0, 0.05, 0.1,0.15, 0.2 and 0.3, respectively. The maximum of -ΔSM increase gradually with the Na content with the decrease in the σ2, similar results have been observed [16]. But we know that the TC decreases from 305 K to 235 K when the Na content increases from x = 0.2 to the x = 0.3, such results is also to the Li-doped and K-doped Pr0.5Sr0.5-xLixMnO3 system [17], it was attributed to the structure and the magnetic phase transition change with large doping contents.