Influence Of Doping On Structural, Electrical And Magnetic Properties In Bilayered Perovskite Manganites

The structural, magnetic, magnetocaloric and magnetoresistance properties have been investigated in bilayered manganites La_1.4Sr_1.6Mn₂O₇ doping with Ca²⁺, Mg²⁺ and K+ at Sr site. The main results are as follows:1. The structure, magnetic and magnetiocaloric properties of the nominal compositions La_1.4Sr_1.6-xCa_xMn₂O₇ (x=0.0-1.6) have been investigated. The samples are tetragonal bilayered perovskite with space group I4/mmm in the 0≤x≤0.8 dopant regimes, while they are composed of a major phase of ABO3-type perovskite structure with space group Pbnm and a slight calcium oxide in the 1.0≤x≤1.6 dopant regimes. The structural transition takes place in the range of 0.8≤x≤1.0. For the x=0.2-0.8 samples, 3D FM ordering temperature decreases at first and then disappears with the increase of Ca²⁺-doping level, while the 2D FM short-range ordering remains and even increases in the x = 0.4 sample. This result implies that the exchange interaction of 3D (Jc) is depressed with increasing Ca²⁺ content. The enhancement of 2D FM short-range ordering in the x=0.4 sample is mainly derived from a sufficiently large Jahn–Teller distortion of MnO6 octahedra triggered by a variation in the nature of the orbital state of the eg electrons. A large magnetic entropy change of 2.28 J kg-1 K-1 is obtained in the nominal composition La1.4Sr1.6?xCaxMn2O7 (x = 1.6) upon 1 T applied magnetic field near its Tc=215 K. It indicates that this compound could be used as a magnetic refrigerant in sub-room temperature magnetic refrigerator.2. The doping effect of Mg²⁺ ions at Sr site in bilayered manganite La_1.4Sr_1.6Mn₂O₇ has been investigated. The Mg²⁺ ions occupy Mn sites in the perovskite structure actually, and formed La1.4Sr1.6Mn2-2yMg2yO7/La0.67Sr0.33Mn1-yMgyO3 (327/113) composites. The content of 327 phase decreases and the fraction of 113 phase increases gradually with increasing the content of Mg²⁺ ions. The practical molecular formula at x=0.4 sample is La0.6Sr0.4Mn0.83Mg0.17O3 using the Rietveld method. FM ordering and conductivity of the two phases in the composites are depressed by Mg²⁺ ions doping at Mn site. For lightly doped samples, the Neel and Curie temperature decreases with increasing the content of Mg²⁺ ions. For heavily doped samples, the magnetization exhibits the spin-glass behaveior, which may cause a insulating behavior. The resistivity in heavily doped samples is as large as four orders of that in lightly doped samples. At lower and higher temperature, the low-field magnetoresistance have been increased by doping Mg²⁺ at Mn site. For x=0.1, 0.2 samples, the MR values are as large as 40% and 39%, respectively, at 5K and 1T, and 8% and 11%, respectively, at 200 K and 1T.3. The two and three phases composites of (1-x)La_1.4Sr_1.6Mn₂O₇/xLa0.67Sr0.33MnO3 and (1-x′-y)La_1.4Sr_1.6Mn₂O₇/x′La0.67Sr0.33MnO3/yLa2O3 have been synthesized. Two magnetic transition temperatures (TC1 and TC2) at low and high temperature are observed in (1-x)La_1.4Sr_1.6Mn₂O₇/xLa0.67Sr0.33MnO3 composites because the variation of the phase content and anti-ferromagnetic coupling between two phases. The magnetization formes a plateau between TC1 and TC2, and the magnitude of magnetization increases with increasing x when TC1 >T >TC2. For the x≤0.67 samples, the magnitude of resistivity decreases gradually with increasing x, and the peak of metal-insulator transition becomes smoother and broader. For the x=0.93 and 1.0 samples, the resistivity further decreases and shows metallic behavior in the measuring temperature range. In three-phase system, the influence of La2O3 on magnetization and resistivity is quite slight above the Curie temperature of 113 phase (360K), while it is significant under the Curie temperature of 327 phase (90K). At the same time, the large magnetic entropy change of 0.45, 0.9 and 1.55J kg-1K-1 in 1, 2, and 7T is obtained in the x′=0.465 sample at 90K.