| The two-layered perovskite manganese oxide La2-2xSr1+2xMn2O7 is a special quasi- two-dimensional (2D) structured colossal magnetoresistance(CMR) material. This material has attracted great attention due to its complex characteristics such as magnetic structure, charge and orbital ordering, anisotropy of electric and magnetic properties and special Jahn-Teller effect. In this paper, the structure, electric and magnetic properties, and magnetocaloric effect of element substitution at A and B sites in bilayered perovskite manganites are investigated.In this work, two series of the samples have been prepared using the solid-state reaction method: the polycrystalline samples of La1.34Sr1.66Mn2-xCrxO7 (x=0.0, 0.1, 0.3, 0.4) with slight Cr3+ dopping at B-site and the polycrystalline samples of La1.4Sr1.6-xCaxMn2O7 (x=0.0, 0.2, 0.4, 0.8, 1.0, 1.4, 1.6) with Ca2+ dopping at A-site. The structure, electric and magnetic properties, and the magnetocaloric effect of these two series have been investigated carefully. X-ray diffraction (XRD) has been used to identify the structure and calculate the lattice parameters and cell volumes of the samples. The vibrating sample magnetometer (VSM) has been applied to measure the magnetic properties and the magnetocaloric effect of the samples. The main experimental results are as follows:1. La1.34Sr1.66Mn2-xCrxO7 (x=0.0, 0.1, 0.3, 0.4) polycrystalline samples(1) The XRD patterns indicate that all samples in this series can be indexed to a tetragonal Sr3Ti2O7 type strcture with space group I4/mmm.The cell volume V of the samples decreases monotonically with increasing Cr3+ ion concentration. This may be caused by the different ion radius of Cr3+ and Mn3+, i. e., the average radius of Cr3+ ion (0.62 ?) is smaller than that of Mn3+ ion (0.67 ?).(2) For lightly Cr3+ doped sample (x=0.1), the three-dimensional long-range ferromagnetic ordering transition temperature Tc 3 Dincreases, while the two-dimensional short-range ferromagnetic ordering transition temperature Tc 2 Ddecreases. A minimum of the c parameter, a maximum of the a parameters and a minimum of the c/a ratio were found in the x=0.1 sample. The serious lattice distortion in this sample strengthens the double-exchange-interaction of MnO2 interlayers and weakens the double-exchange-interaction of the intralayers. This may be the main cause of the increase of D Tc 3 and the decrease of Tc 2 D in this sample. For heavily Cr3+doped samples (x=0.3, and 0.4), Tc 3 Ddecreases while Tc 2 Dalmost remains stable. The experimental results indicate that the Cr3+ ion doping does not destroy the double-exchange interaction heavily as other metal ions doped at B-site. Considering the same electron configuration of Cr3+ and Mn4+ (t32g,e0g ), the double-exchange interaction may realize among Mn3+-O-Cr3+.(3) The resistivity of La1.34Sr1.66Mn2-xCrxO7 sample increases with decreasing temperature in the range of 150-300K, showing a insulating or semiconductor behaviour. The value of the resistivity increases monotonically with increasing Cr3+ doping level.2. The polycrystalline samples of La1.4Sr1.6-xCaxMn2O7 (0≤x≤1.6) with Ca2+ ion dopping at Sr2+-site.(1) The XRD patterns show that the compounds are pure Sr3Ti2O7 type tetragonal bilayered perovskites phase with space group I4/mmm for x≤0.8 samples, while they are mixture of a main orthorhombic ABO3-type perovskite and a slight cubic calcium oxide for x≥1.0.(2) For lightly Ca2+ doped samples (x≤0.8), the cell volume V decreases monotonically with increasing Ca2+ ion. This is because that the Ca2+ ion radius (0.99 ?) is smaller than that of Sr2+(1.12 ?). The minimum of a parameter, the maximum of c parameter and a maximum and the c/a ratio is found in the x=0.4 sample. The increase of the c parameter and the decreasect a parameter result in a serious Jahn–Teller distortion of MnO6 octahedra in this sample.(3) For the lightly doped sample (x=0.2), the Ca2+ is inclined to occupy the 9-coordinate rock-salt layer sites since the radius of Ca2+ is much smaller than that of La3+and Sr2+. It will become more difficult to set up a 3D-FM ordering among MnO2 bilayers. The exchange interaction along the c-axis (Jc) is depressed as Ca2+ doping content increases. Thus, the magnetization and transition temperature ( Tc 3 D) decreases in the x=0.2 sample. For the x = 0.4 sample, the serious J-T distortion may be the main cause of the higher magnetization in this sample compared to the magnetization in the x = 0.0 and x = 0.2 samples. (4) The heavily Ca2+ doped compounds (1.0≤x≤1.6) undergo a magnetic transition from a paramagnetic state at higher temperature to a ferromagnetic state at lower temperature. With increasing doping level x, the magnetization value reachs a maxmum in the x=1.4 sample. The variation in magnetization may be caused by the difference of the Sr2+/Ca2+ ratio and the CaO content in the compositions. The maximum ofΔS M upon 1T applied magnetic field in the nominal composition La1.4Sr1.6?xCaxMn2O7 for x=1.0, 1.4, 1.6 are obtained near their Curie temperatures of 320, 268 and 215K, respectively. The large magnetic entropy change of 0.84, 1.20 and 2.28 Jkg?1K?1 is obtained in x=1.0, 1.4, 1.6 samples. The large magnetocaloric effect under a low magnetic field makes it suitable for near-room temperature magnetic cooling material.
|
PDF Full Text Request