科研之友
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摘要
CaCu2.5Mn4.5O12 perovskite has been prepared in polycrystalline form under moderate pressure conditions of 20 kbar, in the presence of KClO4 as oxidizing agent. This material has been studied by X-ray and neutron powder diffraction (NPD), magnetic, magnetotransport, and thermopower measurements. The crystal structure is cubic, space group Im (3) over bar (No. 204), with a = 7.2279(l) Angstrom at room temperature (RT). In the ABO(3) perovskite superstructure, the A positions are occupied by Ca2+ and (Cu2.52+Mn0.53+), ordered in a 1:3 arrangement giving rise to the body-centering of the unit cell. At the B positions, Mn adopts a mixed oxidation state of 3.875+; MnO6 octahedra are considerably tilted by 19degrees, due to the relatively small size of the A-type cations. The Curie temperature is 345 K. Low temperature (2 K) NPD data show evidence for a ferrimagnetic coupling between Mn3.875+ and (Cu2.52+Mn0.53+) spins, with ordered magnetic moments of 2.32(5) and -0.54(3) mu(B), respectively. An additional canting effect of the Mn moments at B positions has been detected below 70 K, breaking down the body centered symmetry. We have developed a microscopic model that explains both the saturation magnetization values and the refined magnetic moments at A and B positions. Our results show that Cu2+ and Mn3+ spins at the A substructure are almost perpendicular to each other. A magnetoresistance (MR) of 34% has been observed at 5 K for H = 9 T; the low field MR at RT is as high as 2% and shows an appreciable temperature stability.
- 出版日期2003-6-3
