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La_ (1/2) (Sr_ (1-x) Ca_x) _ (3/2) MnO_4 and Nd_ (2-2x) Ca_ (1 2x) Mn_2O_7 electromagnetic properties and structure

Author: ZhanChaoShi
Tutor: XuSheng
School: Qingdao University
Course: Condensed Matter Physics
Keywords: Charge ordering Spin glass state Curie temperature Perovskite manganite Colossal magnetoresistance
CLC: O482
Type: Master's thesis
Year: 2009
Downloads: 54
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The chapter one, two and three: Reviewed the physical characteristics of the ABO3 type manganese oxide. Summarized the current research results and the recent progress of this system. And the anticipation and existence solution’s question has been given the explanation. Introduced the sample preparation process, experimental methods, the principle of Rietveld method and the basic steps of fitting.The chapter four and five: Polycrystals of single-layered perovskite La1/2(Sr1-xCax)3/2MnO4 and Nd2-2xCa1+2xMn2O7 were prepared by solid-state reactions. With the Rietveld-Method, the samples’ XRD pattern was fitted by the program FULLPROF. The compounds show the space group I4/mmm at the room temperature. The temperature dependence of the electrical transport properties and magnetic properties were measured. The results show:The resistivities of La1/2Sr1-xCax)3/2MnO4 samples increase with the decrease of temperature. It shows the insulator behavior within testing temperature range. The resistivities of samples decrease with increase of doping level. Transport properties below TCO can be explained quite well based on the Variable Range Hopping Model. In the high temperature region, the series of samples show the paramagnetism. The Charge -Orbital- Spin order transition happens near 200 - 235K, and the COS order has been extended to low temperature. The susceptibility of sample x = 0.3 increases abruptly due to the spin-glass transition.The sample of Nd1.2Ca1.8Mn2O7 shows the ferromagnetism in the low temperature. Magnetization is 1.4μB. With the rise of temperature, the sample undergoes the FM - PM transition near 120 K. The resistivity of the Nd1.2Ca1.8Mn2O7 becomes lower with the rise of temperature. It shows the insulator behavior within testing temperature range. The sample of NdCa2Mn2O7 shows the antiferromagnetism in the low temperature range because it exists the orbital order and charge order. With the rise of the temperature, the orbital order and charge order were broken. The sample shows the AFM - PM transition near 125 K. The activation energy Eac[≡d(lnρ)/d(1/T)] of NdCa2Mn2O7 was calculated with the temperature dependence of resistivity. The critical temperature TCO for the charge ordering transition was 125 K.

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