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Title: Effect of Doping of Various Metal Cations on Structural, Electrical and Magnetic Properties of Nano Cobalt Ferrites
Authors: Siddiquah, Mah Rukh
Keywords: Natural Sciences
Chemistry & allied sciences
Physical chemistry
Techniques, equipment & materials
Analytical chemistry
Inorganic chemistry
Organic Chemistry
Issue Date: 2008
Publisher: Quaid-i-Azam University Islamabad, Pakistan
Abstract: Cobalt ferrite, having an inverse spinel structure and the inherent properties of high coercivity, moderate saturation magnetization and high electrical resistivity, is a potential candidate for magnetic storage devices and high frequency applications. In the present study, cobalt ferrite has been doped with various dopants like Cr, rare earths (Sm, Ho, Er, Dy and Pr) and Zr co-doped with Mg, Mn and Ni, in order to improve the electrical and magnetic properties while maintaining a spinel structure and moderate saturation magnetization values a micro-emulsion method of preparation in which a cheap surfactant, namely polyethylene glycol, has been used. The formation of spinel phase occurs between 573 and 673K as indicated by the thermal analyses (TG/DTA), but a well crystalline and stable spinel phase is achieved at 1073K as evident from the powder X-ray diffraction studies of the synthesized samples. All the doped cobalt ferrite samples are in single spinel phase as confirmed by XRD and magnetic susceptibility measurements. The average crystallite sizes of the doped cobalt ferrite samples are in the range of 13 nm to 70 nm. The elemental composition of doped cobalt ferrites is confirmed by energy dispersive X-ray fluorescence analysis which shows an agreement between the theoretical and experimental compositions of the prepared samples. Electrical resistivity as measured at 293K the by two point probe method is found to have a value of 1.25 × 106 Ωm for un-doped cobalt ferrite which is enhanced by doping with Cr, Zr-Mg, Zr-Mn and Zr- Ni. For small contents of rare earth metal cations introduced into spinel lattice of cobalt ferrite the electrical resistivity (at 293K) increases to a larger extent due to insulating nature of rare earth oxides. The variation in electrical resistivity with composition and temperature has been discussed on the basis of hopping model of electron conduction in ferrites. The activation energy of hopping and drift mobility of the charge carrier is calculated from the resistivity data. The dielectric properties are measured by inductance capacitance resistance (LCR) meter in the frequency range of 100Hz – 1MHz and dielectric constant (έ), dielectric loss angle (tanδ) and dielectric loss factor ( ε ′′ ) are calculated from the capacitance data. These dielectric parameters are found to decrease with increasing frequency. This behaviour is typical of ferrites as explained by Koop’s model. The dielectric constant (έ) and the dielectric losses of the un-doped cobalt ferrite have been reduced by doping in the present work. Curie temperature has been determined from the low field AC-magnetic susceptibility measurements and was found to increase for specific contents of dopants as compared to the un-doped cobalt ferrite while for others a lower value of Curie temperature was observed. Saturation magnetization has been increased by doping with Cr up to x = 0.5, Zr-Mn content x = 0.1, Sm content x = 0.04 and Er content x = 0.08 while for the rest of compositions the saturation magnetization has been decreased as compared to the un-doped cobalt ferrite sample. Beside this, the coercivity of the materials prepared in the present study has been increased by doping rare earth metal cations, while it has been reduced by Cr and Zr co-doped with Mg, Mn and Ni, in cobalt ferrites. The reduction in magnetization has been discussed in terms of dilution of magnetization, crystallite size effects and the spin canting introduced by the dopants at octahedral sites.
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