Study of Structural, Magnetic and Dielectric Properties of Ferrite/Chromite Nanoparticles

Abstract

This thesis is schematically based on synthesis and characterization of cobalt chromite (CoCr2O4), maghemite (γ-Fe2O3) and nickel ferrite (NiFe2O4) nanoparticles, as well as selective coating and doping in host compounds in order to tune its structural, dielectric and magnetic properties. CoCr2O4 and NiFe2O4 nanoparticles were synthesized by sol-gel method, while γ-Fe2O3 nanoparticles were synthesised by microwave plasma technique. For chromite nanoparticles, the low temperature magnetic response of CoCr2O4 nanoparticles, magnetic and dielectric properties of Mg doped CoCr2O4 nanoparticles and magnetic properties of SiO2 coated CoCr2O4 nanoparticles have been studied in detail. X-ray diffraction revealed the cubic spinel structure of the nanoparticles. Zero field cooled and field cooled (ZFC/FC) curves revealed a paramagnetic (PM) to ferromagnetic (FiM) transition at TC = 97-100 K with conical spiral state at TS = 27 K and lock-in state at TL = 13 K. Negative magnetization is observed in the ZFC curve under 50 Oe applied field, which gets suppressed upon the application of higher field due to reorientation of the nanoparticles magnetization in the direction of applied field. The TC was shifted towards higher temperature with the application of higher field, while TS and TL remain unaffected which was attributed to strong B-B interactions which act as a frozen spins or canted spins at surface. M-H loops showed an abnormal decrease in MS which may be due to presence of stiffed/strong conical spin spiral and lock in states at low temperatures. Modified Kneller’s law showed a good fit for temperature dependent HC at higher temperature and deviated at low temperature (< 25 K) which was attributed to frozen disordered surface spins. Nanoparticles showed slow spin relaxation in both ZFC and FC protocols at 5 K, which signifies the presence of spin-glass like behavior at low temperatures. Mg doped CoCr2O4 nanoparticles showed non-monotonous trend in the average crystallite size and showed a peak behaviour with maxima at x = 0.6. The members CoCr2O4 (x = 0) and MgCr2O4 (x = 1) are FiM and antiferromagnetic (AFM), respectively. TC and TS showed decreasing trend with increasing x, followed by an additional AFM transition at TN = 15 K for x = 0.6. The system finally stabilized and changed to highly frustrated AFM structure at x = 1 due to formation of pure MgCr2O4. Dielectric parameters showed a non-monotonous behaviour with Mg concentration and were explained with the help of Maxwell-Wagner model and Koop’s theory. Dielectric properties were improved for xvii nanoparticles with x = 0.6 and is attributed to their larger average crystallite size. SiO2 coated CoCr2O4 nanoparticles showed decreasing trend of the average crystallite size and cell parameter with increasing SiO2 concentration. The decrease in average crystallite size is due to SiO2 coating which limits the growth of nanoparticles by generating more nucleation sites. All the magnetic transitions of CoCr2O4 nanoparticles shifted towards low temperatures which is due to decrease in average crystallite size. SiO2 concentration also decreased saturation magnetization (MS), which was enhanced surface disorder in smaller nanoparticles. In study of structural, magnetic and dielectric properties of ferrite nanoparticles, chromium oxide (Cr2O3) coated γ-Fe2O3 nanoparticles and NiCrxFe2-xO4 ferrite nanoparticles have been studied in detail. Simulated ZFC/FC curves exhibited large value of effective anisotropy of Cr2O3 coated γ-Fe2O3 nanoparticles as compared to bulk γ-Fe2O3 but less than bare γ-Fe2O3 nanoparticles which is may be due to weak interface anisotropy between ferrimagnetic γ-Fe2O3 core and antiferromagnetic Cr2O3 shell. Bloch’s law was fitted on T-dependent MS data and revealed the higher value of Bloch’s constant and lower value of Bloch’s exponent as compared to bulk γ-Fe2O3. Spin glass behaviour was investigated by using different physical laws for f-dependent ac susceptibility and they confirmed the presence of spin glass behaviour which is due to disordered frozen surface spins. XRD analysis of Cr doping at B site in NiFe2O4 nanoparticles confirmed the cubic spinel structure for all samples with x = 0, 0.2, 0.4, 0.8, 2.0 concentration. Saturation magnetization depicts decreasing trend with addition of Cr3+ concentration which is attributed to replacement of large magnetic moment of Fe3+ by smaller magnetic moment of Cr3+. HC reveals minimum value for NiFe2O4 nanoparticles and showed increasing trend with addition of Cr3+. This increase in HC may be attributed to change in magneto crystalline anisotropy. Dielectric constant showed increasing trend with the Cr+3 concentration due to less conductive nature of Cr as compared to Fe. In summary, a detail study of structural, dielectric and magnetic properties of chromite and ferrite nanoparticles have explored with tremendous results that will open a new insight in device applications such as automatic switching, magnetic memory and targeted nanotherapeutic.