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Study of Structure and Magnetism for La0.75Sr0.25 Mn1-yO3 and La0.6Sr0.1CuxMnO3 Materials

Author: JiDengHui
Tutor: TangGuiDe
School: Hebei Normal
Course: Condensed Matter Physics
Keywords: perovskite structure Cu doping vacancy doping curie temperature crystal structure magnetic propertiy
CLC: O482.5
Type: Master's thesis
Year: 2010
Downloads: 45
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Abstract


ABO3 perovskite manganites Ln1-xMxMnO3 (Ln=rare earth ion, M=Ca, Sr, Ba or Pb) have attracted much attention due to the observation of colossal magnetoresistance (CMR) in these materials. The electromagnetic characteristics of this kind of compound can be explained by the double-exchange mechanism (DE).In this paper, two problems were discussed on the based of theoretical calculations and experimental results, and the satisfactory explanation was given.1. The vacancy content at B sites in the perovskite phase is not only related to the nominal content, but also related to the calcination temperature of the samples. Compound samples with nominal composition La0.75Sr0.25Mn1-xO3 (x=0.00, 0.03, 0.05, 0.08, 0.10, 0.13, 0.15, 0.17) have been prepared by the sol-gel method, with the calcination temperature TCal = 973K, 1073K, 1273K and 1473K. The structure and magnetic property of the samples have been investigated. For the perovskite phase in samples, assuming that the content of vacancy at B sites changes with the calcination temperature, the ion ratios of at A, B and O sites were calculated. The ion ratios were used to the Rietveld fitting for XRD spectra, obtained error parameters Rp, Rwp and s were acceptable. The structrure of the samples was analysed by using of the data of X-ray diffraction, which show: (i) the samples with x≤0.08 and TCal = 1073K, possess single perovskite phase and the maximum Curie temperature, other samples possess lanthanum mangnites as the second phase; (ii)there is a maximum value zm of the vacancy content at B sites in ABO3 perovskite phase, and the value of that zm changes as the calcination temperature. (iii) A shell-nucleus modle about crystal grain has been employed to explain the effect of the calcination temperature on the magnetic property and the vacancy content. (vi) The grain size increases with the calcination temperature increasing, but the second phase has an inhibiting effect on the growth of grain of the main phase; (v) It is only about 8% that the maximum value of the content of vacancy at B sites. Because the vacancy space of the A sites is larger than B sites and the distance ratio of AO and BO is 2 , the experiment may be as an evidence that there is no vacancy at A sites, and even shows that the content of vacancy at A sites should be far less than 8%.2. There are not vacancy and Cu ion at A sites of the perovskite phase in the samples of nominal compositon La0.6Sr0.1CuxMnO3. A serie of polycrystalline La0.6Sr0.1CuxMnO3 (x= 0.00, 0.05, 0.10, 0.15, 0.20, 0.25, 0.30) samples were prepared by conventional sol-gel method. Based on the X-ray diffraction studies of the samples, it was found that: (i)all samples possess phases with the R3c perovskite being the dominant phase; (ii)when x≤0.20, the samples possess a small amount of Mn3O4 phase which the content increases with the Cu-doped content x increase; (iii)when x≥0.15, the samples possess a small amount of (CuyMnz)3O4 phase which increases with the Cu-doped content x increase; (iv)the Curie temperature(TC) of the perovskite phases decreases monotonously with x increase, the ranging from 330K(x=0.0) to 232K(x = 0.30); (v)the Curie temperature(TC) of the (CuyMnz)3O4 phases decrease monotonously with x increases, the ranging from 75K when x =0.15 to 50K when x = 0.30. All the experimental results indicate a part of Cu3+ ions were doped into the B sites not A sites and A sites do not exist the cation vacancy in the perovskite phase of the samples.

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CLC: > Mathematical sciences and chemical > Physics > Solid State Physics > Solid nature of the > Magnetic properties
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