A dissertation submitted to the Department of Chemistry, Quaid-i-Azam University, Islamabad, in partial fulfillment of the requirement for the degree of

Abstract

In the present study, M-Cr (M = Co, Ni, Cu, Zn and Mn) substituted magnesium ferrite nanomaterials (Mg1-xMxCrxFe2-xO4 with x = 0.0-0.5) have been prepared by the polyethylene glycol assisted micro-emulsion method. Thermal and XRD analyses reveal that the complete spinel cubic phase formation occurred at 1123 K. The average crystallite sizes in differently doped series are in the range of 15-62 nm. The micrographs obtained from SEM analysis show that the synthesized materials are agglomeration of the individual particles. The energy dispersive X-Ray fluorescence (ED-XRF) spectrometric analysis reveals that the observed molar ratios of different components of the samples are in close agreement with their nominal compositions. Variation of Mössbauer parameters is explained on the basis of preferential site occupancy of the substituted cations. The center shift (CS) value for A-site is smaller than that of B-site due to difference in the Fe3+-O2- inter-nuclear separation, normally larger for B-site as compared to that for A-site. The value of quadrupole splitting (QS) is negligibly small which indicates that the overall symmetry of Fe3+ surroundings is not disturbed with the substitution of the dopant ions into a magnesium ferrite matrix. With the increase of dopant contents, the variations of hyperfine magnetic field (H) and site population area (A) are akin to the compositional variation of saturation magnetization, MS. Symmetric magnetic hysteresis loops are measured using a superconducting quantum interference device (SQUID) magnetometer up to an applied magnetic field of 50 kOe at 300, 200 and 100 K. SQUID analysis reveals narrow hysteresis loops with a coercivity (HC) and saturation magnetization (MS) varying for different compositions. The high field regimes of these loops are modeled using the Law of Approach (LoA) to saturation to extract information regarding magnetocrystalline anisotropy and saturation magnetization. In the present study, the saturation magnetization of magnesium ferrite increases by doping with Co-Cr, Ni-Cr, Zn-Cr and Mn-Cr, respectively, but decreases by doping with Cu-Cr contents. The coercivity (HC) of all the series studied here decreases with an increase in the substitution level. All the magnetic parameters i.e. MS, Mr, K1 and HC increase with decrease in the temperature from 300 K to 100 K. To determine the Curie temperature (TC), the temperature dependence of normalized moment is measured at an applied field of 5 kOe within the temperature range of 350-973 K, using a vibrating-sample magnetometer. The Curie temperature initially increases with Co-Cr and Mn-Cr contents, but start to decrease for higher level of substitution. In case of Ni-Cr doped series, TC value increases progressively with the increase in dopant contents, while it continued to decrease with the substitution of Cu-Cr and Zn-Cr contents. Temperature dependence of DC-electrical resistivity reflects the semi-conducting nature of the doped Mg-ferrites. The room temperature resistivity (ρRT) and activation energy increase up to a certain level of substitution with Ni-Cr, Zn-Cr and Mn-Cr contents but increase continuously for Co-Cr and Cu-Cr substitution contents. The dielectric constant (έ) and dielectric loss tangent (tan δ) decrease with increasing applied field frequency. The variations in the magnitude of drift mobility, dielectric constant and dielectric loss tangent are in close agreement with the trend of DC-electrical resistivity by increasing the dopant contents. With improvement in properties, the synthesized materials could be suitable for potential application in some magnetic and microwave devices.