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چكيده انگليسي :

Preparation and investigation of structural electrical and magnetic properties of La0 8Sr0 2Mn1 xTixO3 0 00 x 0 075 manganite Student name Bagher Aslibeiki Email address b aslibeiki@ph iut ac ir Date of submission 1387 11 7 Department of Physics Isfahan University of Technology Isfahan 84156 83111 Iran Degree M Sc Language Farsi Supervisor Dr Parviz Kameli kameli@cc iut ac irAbstract In this thesis we have investigated the effect of Ti4 substitution and preparation conditions on the structural magnetic and electrical properties of La0 8Sr0 2Mn1 xTixO3 0 00 x 0 075 manganite The samples have beenprepared by the standard solid state reaction method In order to investigate the effect of sintering temperature onvarious properties of samples the samples divided into two series and sintered at 1200 LT and 1400 HT centigrade The analysis of the XRD pattern showed that all samples have rhombohedral structure and Ti4 substitution has no considerable effect on the samples structure but it just changed the peaks positions which showsthe variation of unit cell volume and the lattice parameters In the HT samples the lattice parameters and the unitcell volume were increased with increasing of substitution content Since the ionic radii of Ti4 Mn4 0 605 0 54 so the substitution of Ti4 will increase the bond length lattice parameters and the volumes In the case ofLT samples the lattice parameter and volume remain nearly constant with increasing Ti concentration Since theionic radius of Ti4 0 605 ion is in between the ionic radii of Mn4 0 54 and Mn3 0 64 therefore thereis a possibility that a fraction of Ti4 ions substitute Mn3 sites in addition to the Mn4 sites and for that reason theaverage lattice parameter and volume remain nearly constant for low temperature sintered samples The SEMimages show that the magnitude of grain size in HT samples is large compared to LT samples The AC magneticsusceptibility measurements showed that the Curie temperature decreases with increasing of the Ti4 content andthis means that the double exchange diminishes by titanium doping into the samples In the x 0 05 and x 0 075 ofHT samples the ferromagnetic long range ordering is no longer stable after paramagnetic ferromagnetic transition and these samples show antiferomagnetic transition at low temperatures the x 0 075 of LT sample and x 0 05 andx 0 075 of HT samples show a drop in the real part of AC susceptibility and a peak in the imaginary part of ACmagnetic susceptibility curves The peak is related to spin glass state in these samples This peak is frequencydependent and shifts towards higher temperature with increasing frequency The competition between FM doubleexchange interaction and AFM super exchange interaction produces spin frustration and is responsible for theoccurrence of spin glass According to the electrical resistivity measurements by the increasing of the substitutioncontent electrical resistivity increases and the in metal insulator transition temperature decreases so that x 0 075sample of both series have no insulator metal transition in whole of measured temperature interval Fitting ofelectrical resistivity plots with small polaron hoping SPH and variable range hoping VRH models demonstratedthat the nice agreement between the SPH model and the electrical resistivity of x 0 025 x 0 05 and x 0 075samples of both series Kay words manganite spin glass AC susceptibility sintering Temperature Ti doping superexchange double exchange

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ساخت و مطالعه خواص ساختاري، مغناطيسي و الكترونيكي منگنايت...

شيشه ي اسپيني , پذيرفتاري AC , دماي كلوخه سازي , آلايش Ti , برهمكنش ابر تبادلي , تبادل دوگانه