Colossal Magnetoresistance in New Manganites

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rmed by Asano et al. [5]. In the Lal 2Ca . 8Mn 2O7 layered compound, they observed a ferromagnetic transition at Tc = 240 K and a higher MR ratio than those of n = 3 and n = oo materials. However, the properties of the solid solution of Sr 2' and Ca 2' resulting in La 1.2(Sr1 8 .Cax)Mn 2 O7 have not been systematically studied. In the n - oo compounds, the gradual replacement of Ca2 +by Sr2' in La0.75(Ca0.25_.Srj)MnO 3 results in an increase of Tc from - 225 K (x = 0) to - 340 K (x = 0.25) [6]. The phase transformation from orthorhombic to rhombohedral has been observed in the La0. 75(Ca0.25-.Sr.)MnO 3 system with fixed Mn valence, which is difficult to be explained as the effect of increasing the Mn valence. Therefore, a detailed study on the phase transformation is very informative. In this paper, we demonstrate the evolution of the structural, electrical and magnetic properties with the isovalent chemical substitution of Ca2+ into the Sr2+ sites in series of twodimensional

Lal 2 (Srl,_.•Ca.)Mn 2 O 7

compounds

(x

=

0

-

1.8)

and

three-dimensional

La0.6(Sr0 .4-.Ca)MnO3 compounds (x = 0 - 0.4). EXPERIMENT High purity powders of La 20 3 , SrCO 3, CaCO 3 and MnO 2 were weighted in appropriate proportions to obtain the nominal compositions of La,.2(Sr,.8-xCax)Mn 2O7 (0 < x < 1.8) [7, 8] and LaO.6(Sr 0 4_.Ca)MnO 3 ( x = 0 - 0.4). The mixtures were calcined in air for 24 h at 1200 'C and 900'C, respectively. Then, the samples were sintered in air at 1400 - 1500 'C for 24 h with intermediate grinding after each heating step. X-ray powder diffraction (XRD) measurements were carried out with a SCINTAG (XI) diffractometer (Cu Ka radiation, / = 1.5406 A) at 40 kV and 30 mA. The program of GSAS [9] was used for the Rietveld refinement in order to obtain the information of crystal structures of Lan.n(Sr,Ca)j~nxMnO3n+• (n = 2 and n = cc). The highresolution transmission electron microscopy (HRTEM) was carried out using a JEOL 4000EX electron microscope operated at 400 kV. Magnetization data were taken from a superconducting quantum interference device (SQUID) magnetometer (Quantum Design). RESULTS In Fig. 1 we show the XRD patterns of the series samples of La1 2(Srl.8.- Cax)Mn 2O7 (x = 0 1.8). All of the samples are single phase. The samples can be indexed to the Sr 3Ti 20O-type structure with tetragonal unit cell (space group: 14/mmm). Base on our XRD refinements, both lattice constants (a and c) and cell volume (V) decrease as the Ca content increases. The 2 structural changes are simply due to a manifestation of the size effect between Ca ' [1.18 A for 2 C.N. (coordination number) = 9] and Sr ' (1.31 A for C.N. = 9) [10]. The inset of Fig. 2 shows the ideal crystal structure of Lal 2(Srl. 4Ca04)Mn 2O 7 (n = 2) which consists of double perovskite layers, and each layer is made up of a two dimensional network of MnO 6 octahedra. The unit cell is shown with a solid line. The arrangement of MnO 6 octahedra is shaded. This model is also supported by HRTEM observations. Fig. 2 shows HRTEM image with the incident electron be