| As a typical Ruddlesden-Popper layered perovskite solid solution, CaMnO3 and its doping oxides Ca1-xAxMnO3 has a variety of properties in crystal structure, magnetic phase transition, magnetic frustration and electrical transport properties. In the past few years, the research on CaMnO3 was attracted an extensive interest by the researchers because of its complex properties. They were more likely to study the formation mechanism, trend and principle of the change on these samples. They expected to develop a variety of magnetic materials for human life through these studies. Considering that the previous reports are mostly on electron doping such as La3+, Th4+, Bi3+ ions, in this paper, we investigated the structure, the valence state and the magnetic behavior of the self-doped Ca3-xMn2O7-d compounds and the Bi3+ doped Ca3-xBixMn2O7 (x=0.05,0.1,0.2 and 0.3) compounds.The sample of the Ca3-xMn2O7-d(x=0.04,0.08,0.12) were synthesized by the conventional solid-state reaction method with high-temperature sintering. In the synthesis of these self-doped samples, the amount of Ca defects in the sample was controlled by the evaporation of Pb elements in the sample. The crystal structure of the power were characterized by X-ray diffraction (XRD). The Rietveld method indicated that the crystal structure changed from the tetragonal to an orthorhombic system with the Ca deficiency appeared. The space group changed from I4/mmm toCmcm. This change is because of the Jahn-Teller effect which perhaps induced by Mn3+ appears. Analogously, the sample of the Ca3-xBixMn2O7 (x=0.05,0.1,0.2 and 0.3) were synthesized by the conventional solid-state reaction method with high-temperature sintering. The X-ray diffraction patterns show that the compounds with the nominal composition x≤0.3 crystallize in tetragonal structure with space group I4/mmm. The lattice parameter a becomes longer and c becomes shorter as the Bi3+ ion increases.The XPS spectrum shows that the Mn3+ appeared with the Ca deficiency appeared. And the Mn ions in the Ca deficient compounds are at the mix-valent Mn3+/Mn4+ state. And there was a certain amount of Ca defect in the sample, which was in agreement with the expected value.The magnetization was measured with a commercial Quantum Design SQUID magnetometer in the temperature range of 5-400 K. The measurement results show that The Ca deficiency induces a conspicuous ferromagnetic component in Ca2.96Mn2O7-d below 350 K. And In the Ca deficient Ca2.92Mn2O7-d compound, there is a conspicuous FM-like transition at 345 K. However, there is nothing can be in Ca2.88Mn2O7-d compound. These special magnetic phenomena can be attributed to the double exchange mechanism and super exchange mechanism. As a typical layered perovskite, the pseudo thin film structure leads to the low dimensional magnetic properties in the sample. The exchange integral Jo obtained by calculation is 25.4 K, 22.6 K and 19.7 K respectively. A canted AFM transition is observed below around 110 K, and a low dimensional peak is observed at around 200 K. The divergence between the ZFC and FCC (and FCW) below about 350 K for Ca2.96 and Ca2.92, and below about 111 K for Ca2.88 proposes the existence of the magnetic frustration in the compounds. For the Bi3+-doped Ca3-xBirMn2O7 (x=0.05,0.1,0.2 and 0.3) compounds, at both ZFC and FC states, a magnetic transition is observed at about 130 K for x=0.05 and 0.1, and 120 K for x=0.2 and 0.3. The long range magnetic ordering is induced in Ca3-xBiYMn2O7, and takes the place of weak ferromagnetic interaction in Ca3Mn2O7. M(H) is not saturated when the applied field is as high as 7 T. |
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