Synthesis, structural and microwave properties of nano ferrites

Abstract

The effect of copper, cadmium, cobalt and lithium substitution on the Mn-Zn ferrites has been investigated. Four series of spinel nano ferrites with nominal compositions Mn0.5CuxZn0.5-xFe2O4 (0.0 ≤ x ≤ 0.35), Mn0.5CdxZn0.5-xFe2O4 (0.0 ≤ x ≤ 0.35), Mn0.5Zn0.5-xCoxFe2O4 (0.0 ≤ x ≤ 0.50) and Li0.5Mnx/2Zn0.75-x/2Fe2O4 (0.0 ≤ x ≤ 1.2) were synthesized by chemical co-precipitation and solgel methods. Structural, electrical and microwave absorption properties of these spinel ferrites have been investigated. These ferrites belong to Fd3m space group and all the prepared samples exhibit FCC spinel structure. The variation in lattice constant (a) follows the Vegard’s law for all the samples. The average crystallite sizes calculated using Scherrer’s formula and found to be in the nanometer range (12 - 32 ± 2 nm). Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and atomic force microscopy (AFM) micrographs confirmed the spherical morphology of prepared samples that were also in nanometer range. Agglomeration in these materials was due to magnetic nature of these ferrites. FTIR measurements showed two major absorption bands due to octahedral (B-site) and tetrahedral (A-site) sites in the spinel ferrites. The temperature variation of DC electrical resistivity for all the samples exhibit semiconducting nature of the spinel nano ferrites. The doping greatly influenced the DC electrical resistivity of these ferrites. The resistivity up to 2.2 x 1011 Ω-cm obtained for Mn0.5Zn0.25Cd0.25Fe2O4 nano ferrites prepared through sol-gel method. The activation energies obtained from Arrhenius plots ruled out the band type conduction in these ferrites. The conduction due to hopping was dominant. Dielectric properties like dielectric constant and loss tangent were studied as a function of composition and frequency. The dielectric constant and loss tangent decreased with increase in frequency which is the normal dielectric dispersion behavior. Complex impedance spectroscopy was carried out to separate the resistance of grains and grain boundaries. Cole-Cole plots clearly indicate that resistance of grain boundaries was higher than grains due to increase in grain boundaries at nano scale level. Compositional variation in the magnetic properties like saturation magnetization, coercivity and remanence were studied from the M-H loops using vibrating sample magnetometer (VSM). The increase in saturation magnetization and remanence with composition is due to increased A-B interaction at A-B sub-lattices in these types of nano ferrites (Ms = 62 emu/g and Hc = 166 Oe for Co2+ doped). Magnetocrystalline anisotropy constant was also estimated using law of approach to saturation (LOA) for these polycrystalline nano ferrites. The microwave magneto-dielectric properties have been studied using RF/material analyzer in the range 1 MHz to 3 GHz. The high reflection loss (RL) (~ - 67 dB for Cd2+) having relatively wide band-gap (270 MHz) made Mn-Zn ferrites suitable to use in the lower microwave (MW) region. These ferrites can be used in high frequency single layered electromagnetic wave absorbing devices. The addition of dopants improved the magnetic and electrical properties of Mn-Zn nano ferrites. The synthesis methods and use of surfactant also greatly affected the structural, electrical and magnetic properties of the present system of nano ferrites.