ساخت و مطالعه اثر آلايش و اندازه دانه ها بر خواص ساختاري ، مغناطيسي، الكتريكي و مغناطو مقاومت منگنايت هاي La1-xRxMnO3

چكيده انگليسي :

Preparation and investigation of the effect of doping and grain size on structural magnetic electrical properties and magnetoresistance of La1 xRxMnO3 manganites Shiva Shogh Email address s shogh@ph iut ac ir Date of Submission 1389 8 5 Department of Physics Isfahan University of Technology Isfahan 84156 83111 IranDegree M Sc Language FarsiSupervisor Prof Hadi Salamati Email address salamati@cc iut ac irAbstractIn this thesis structural magnetic electrical properties and low field magnetoresistance LFMR of La0 8Sr0 2MnO3 x MnFe2O4 samples with x 0 10 weight percents havebeen investigated The La0 8Sr0 2MnO3 sample has been synthesized by thermaldecomposition of acetates in citric acid new method and MnFe2O4 has been synthesizedby sol gel method Short period of ball milling and non aqueous process are the advantageof the new method Structural properties of all samples has been investigated by X raydiffraction and results show that all samples have rhombohedral structure Ac magneticsusceptibility and electrical resistance measurements of samples show that magnetictransition and metal insulator transition decrease by the increase of MnFe2O4 level Thisdecrease is due to the grain boundaries effects Also the value of magnetoresistance in 77K temperature increases for low doping level of 0 x 7 5 and then decreases at higherdoping level x 10 The spin dependent tunneling and scattering at interfaces of the grainboundaries are responsible for the increasing of LFMR at x 7 5 while reduction of LFMRat x 10 originates from the increase of the thickness of grain boundaries In the second part the influence of grain size on the structural magnetic electricalproperties and magnetoresistance of La0 9Ba0 1MnO3 samples has been studied Thesamples have been synthesized by thermal decomposition of nitrates in citric acid and havebeen sintered at 800 C and 1350 C The results of X ray diffraction show that the width ofpicks decreases and diffraction intensity increases by increase of grain size The latticeparameters and unit cell volume of samples have been obtained by FULLPROF soft ware rietveld refinement It shows that samples have rhombohedral structure and belongs toR 3C space group SEM images show that grain sizes increase by increasing of sinteringtemperature The grain size of samples that were sintered in 800 C and 1350 C are in therange of nanometer and micrometer respectively Ac magnetic susceptibilitymeasurements show that all samples have paramagnetic ferromagnetic transition and themagnitude of susceptibility increases by increase of grain size This is due to decrease ofinterface distortion with the increase of grains size Also cure temperature increases bydecreasing of grain size due to oxygen effects In addition the analysis of the susceptibilitymeasurements with different frequencies by power model and C parameter show that thecluster spin glass is formed in the 1350 C sample The values of electrical resistance athigh temperature decrease and metal insulator transition temperature increases byincreasing grain size This is also due to decrease of grain boundaries For nanosizesample insulator metal transition temperature is lower than ferromagnetic paramagnetictransition temperature This has been explained by core shell model Nonstochiometeric ofoxygen has make metal like behavior for this sample in low temperature Also magnetoresistance measurements of samples versus temperature show that LFMRdecreases and CMR increases by increasing of grain size Key words Manganite Ac susceptibility Low field magnetoresistance Doped ferrite Cluster spin glass Nano crystal

شوق، شيوا

ساخت و مطالعه اثر آلايش و اندازه دانه ها بر خواص ساختاري ، مغناطيسي، الكتريكي و مغناطو مقاومت منگنايت هاي La1-xRxMnO3

پذير فتاريac , آلايش فريت , خوشه هاي شيشه اي اسپيني , نانو ذرات