| Summary: | 碩士 === 國立成功大學 === 物理學系碩博士班 === 101 === Manganites have attracted renewed attention because of both their intrinsic scientific interest and prospective applications in novel magnetoelectric and magneto-optical devices. Bivalent rare earth elements such as Sr, Ca etc doped La1-xAxMnO3 are extensively studied by researchers. Electrical conductivity of manganites is due to the mixed valency of the manganese ions. Substitution of the ‘La3+’ and ‘Dy3+’ site with isovalent ‘Bi3+’ in LaMnO3 and DyMnO3 has not been explored so widely. The purpose of this dissertation is to explore isovalent Bi3+ doped Lanthanum manganese oxide (LaMnO3) and dysprosium manganese oxide
(DyMnO3) materials. The Bi3+ is stable and non-magnetic which does not introduce any holes into the system; instead it induces a very small A-site cationic mismatch.The effect of “chemical pressure” introduced by substitution may lead to an increase in the distortion of the perovskite structure of the studied system. Hence substitution of ‘La3+’ and ‘Dy3+’ site of LaMnO3 and DyMnO3 by isovalent
non-magnetic ‘Bi3+’ without changing the effective Mn valency and/or introducing any magnetic exchange interaction will be interesting to explore for their structural and magnetic properties.Doping of ‘Bi3+’ in LaMnO3 induced structural transition from orthorhombic to
cubic phase for x≥ 0.3. The two magnetic transitions in the ZFC-FC plots decreased upon doping with ‘Bi’ which is correlated to the distortion induced by ‘Bi3+’ doping.Spin glass like feature was found to appear upon doping with ‘Bi’ within the orthorhombic phase and vanished for x≥ 0.3 in the cubic symmetry. At 10 K,coercivity decreased in orthorhombic phase, whereas it increased marginally in the cubic phase. Thus isovalent ‘Bi3+’ doping in LaMnO3 was found to induce structural change from orthorhombic to cubic which also reflected in the magnetic properties as a change over from hard to soft magnetic behavior.No structural change was observed in ‘Bi3+’ doped DyMnO3. It decreased the ‘a’ and ‘c’ parameters and increased the ‘b’ parameter. The overall effect is a decrease in the cell volume due to ‘Bi3+’ doping. The ZFC-FC plots become irreversible upon doping with ‘Bi’. Ferrimagnetic ordering of the Dy atoms decreased for x=0 to 0.10 from 8 K to 2 K. At 2 K, spin flop like transition observed in pure DyMnO3 gradually decreased and tend towards a ferromagnetic behavior upon doping with ‘Bi3+’. For x= 0.15 and 0.2, the ferrimagnetic Neel transition nearly disappear and a relatively sharp transitions emerge around 40 K. At 2 K,increase in coercivity and remnant magnetization with ‘Bi3+’ content imply change from soft to hard magnetic behavior contrary to the observation in ‘Bi3+’ doped LaMnO3. The specific heat capacity data imply that the ferroelectric transition Tlock vanished upon doping with ‘Bi’ and the ferrimagnetic transition (8 K) of the Dy ions and the antiferromagnetic transition at 40 K to shift to low and high temperature respectively.
keyword:LaMnO3;DyMnO3
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