Electrical and magnetic properties of NI-TI substituted perovskites.

Abstract

All perovskites samples studied in this work: (La,Bi,Sr)Ni0.25Ti0.25Fe0.5O3 were synthesized by a combination of high energy ball milling (HEBM) on a Retsch PM 400 instrument and heat treatment on a Sentro Tech (type: STT-1600C-3-24) high temperature tube furnace. These samples were selected in anticipation of a future study of gas sensing properties not undertaken in this work because of equipment constraints. The additional Ni0:5Ti0:5Fe2O4 spinel ferrite was synthesized by HEBM. The compounds were characterized by X-ray di raction (XRD), Fourier transform infrared spectroscopy (FTIR), high resolution transmission electron microscopy (HRTEM), M ossbauer spectroscopy, vibrating sample magnetometer (VSM) at temperatures between 10 K and 300 K, resistivity four probe method, Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BHJ) for surface area measurements. The X-ray di raction data for LaNi0.25Ti0.25Fe0.5O3 (LNTFO) showed the formation of single phase compound with orthorhombic structure which is similar to that of reported LaFeO3. The average particle size obtained from HRTEM was 91 5 nm. The cosubstitution of Ni2+ and Ti3+ at the Fe3+ site indicated enhanced magnetic properties. Magnetization measurements showed soft ferromangetic behavior with saturation magnetisation MS in the range 2.25 0.2 emu/g (at 300 K) to 2.61 0.2 emu/g (at 10 K). The coercivity increased from 0.36 0.02 kOe to 0.41 0.02 kOe with decreasing temperatures. M ossbauer data revealed the sample was magnetically ordered at room temperature. The isomer shift values indicated possible strong covalent bonds between metal and the oxygen ions and existence of only Fe3+ ions in the structure. Furthermore, the milled material showed semiconducting behavior with activation energy of 3.4 0.1 eV whilst the milled and annealed sample has lower activation energy of 0.9 0.06 eV. A BiNi0.25Ti0.25Fe0.5O3 (BNTFO) perovskite compound has been successfully synthesized by high energy ball milling and annealing. The crystallites size were obtained to be in the range 10 nm to 70 nm. The grains were observed to be semi-spherical with good surface coverage. We found improved saturation magnetization relative to the LNTFO sample with MS in the range 6.5 0.2 emu/g (at 300 K) to 7. 3 0.2 emu/g (10 K) and the coercivity in the range 0.49 0.02 kOe (300 K) to 0.62 0.02 kOe (10 K). M ossbauerspectrum reveals magnetic ordering at room temperature. Isomer shift values indicated only the presence of Fe3+ ions. Magnetic hyper ne values at 300 K were obtained to be 518 5 kOe and 510 4 kOe for the A and B sites respectively. A low activation energy of 0.66 0.02 eV was obtained for this sample. SrNi0.25Ti0.25Fe0.5O3 (SNTFO) belonging to an orthorhombic crystal system showed paramagnetic behavior at room temperature whilst at lower temperatures it was superparamagnetic. The magnetization MS was relatively low ranging from 14.14 0.23 emu/g to 0.0032 0.0001 emu/g and the coercive eld increased with decreasing temperature from 0.34 0.06 kOe to 1.14 0.06 kOe. The M ossbauer spectrum indicated the presence of both Fe4+ and Fe3+ iron ions which is consistent to that reported for SrFeO3 compounds. Particle size obtained from FETEM averaged 127 12 nm and the surface morphology was indicative of a rough absorber surface with semi-spherical grains of di erent sizes. An activation energy of 0.37 0.03 eV for the annealed SNTFO indicated good electronic conductivity at relatively higher temperature. An additional Ni0.5Ti0.5Fe2O4 compound was successfully synthesized by HEBM. The sample was characterized by quick phase formation. Prolonged milling destroyed the phase. From the structural analysis it was evident that starting precursors for a chemical reaction are of vital importance as they have great in uence on the reaction product. The mean particle size was obtained to be 45 9 nm. Particle size reduced with milling time whilst the strain increased. The coercivity and saturation magnetization appeared to follow the Stoner{Wohlfarth model in two distinct regions at high temperatures (300 K to 100 K) and low temperatures (50 K to 10 K) with approximately equal anisotropies in each temperature range. Saturation magnetization was obtained to be between 38.73 0.03 emu/g to 38.84 0.03 emu/g and the coercivity was between 820 32 Oe to 407 32 Oe. Room temperature M ossbauer spectrum revealed hyper ne elds of 446 1 kOe and 480 1 kOe for A and B sites respectively. Isomer shift values indicated co-existence of both Fe3+ and Fe2+.