Investigation of magnetic properties of Ba4Ni2-xZnxFe36O60 hexaferrites

Abstract

Ba4Ni2-xZnxFe36O60 (0.0≤x≤1.2 with a step 0.2) were prepared by conventional solid state reaction technique. Stoichiometric amounts of the BaCO3, Fe2O3, NiO and ZnO were milled for12 h. After milling, compositions were subjected to sintering wherein it was controlled at 900°C and sintered powder was crushed into fine powder. The prepared fine powder was pressed into pellets and sintered at 1250, 1300 and 1350°C for 6 h. The Xray diffraction (XRD) patterns for different compositions were analyzed. This analysis and the comparison of obtained XRD patterns with previously reported XRD patterns of U-type hexaferrites confirm the formation of the proper phase. The lattice parameters ‘a’ and ‘c’, and c/a ratio for all samples are calculated. The value of lattice parameters are close to the usual values of a=5.88 Å and c=113 Å for U-type hexaferrite. There is an increase in lattice parameters are observed due to Zn substitution. The increase in lattice parameters with increasing Zn content can be explained on the basis of the ionic radii. The ionic radii of the cations used in Ba4Ni2-xZnxFe36O60 are 0.83Å (Ni2+), 0.88Å (Zn2+) and 0.69Å (Fe3+). Since the ionic radius of Ni2+ is less than that of the Zn2+, increase in lattice constant with the increase in Zn substitution is expected. The experimental density (dexp) of the samples is observed to be increasing with increasing Zn content. This increase in density with increasing Zn content can be explained on the basis of the atomic weight. Since, the atomic weight of Ni (47.88 amu) is less than that of Zn (65.39 amu) therefore, increase of the density is expected. Structural and surface morphology were studied by the high resolution optical microscope (Olympus DP-70) for different compositions. The micrographs show that grain size increases with Zn content. The ac magnetic properties of the hexaferrites are characterized within the frequency range 0.1- 120MHz. The real part (m ¢ ) of the complex permeabilitym * = m ¢ - im ¢ for different compositions indicates that m ¢ decreases with increase in frequency. The decreasing trend in permeability m ¢ with frequency is a general phenomenon and is ascribed to the limited speed of spin and domain wall movement (displacement/ rotation) in the samples. In general, multiple resonances are more prominently observed in m ¢ spectrum of U-type hexaferrites. But weak resonance peaks were found due to domain wall motion for all sample within measured range of frequency. It is observed that permeability increases for all samples as the sintering temperature increases from 1250 to 1300°C and above 1300°C the permeability decreases. The reason behind this is the samples being heated at higher temperature contain increasing number of pores within the grain which results in decrease in permeability. The observed increase of saturation magnetization (Ms) with Zn content is due to the fact that the diamagnetic Zn2+ cation takes the position on the spindown and tetrahedral sites of T- and S-blocks. In order to explain this lower value of saturation above x=1 can be explained by local spin canting with a magnetic component along the hexagonal c-axis. The variation of dielectric constant (e ) of Ba4Ni2-xZnxFe36O60 hexaferrites with frequency (1-100kHz) indicates that the dielectric constant decreases with increasing frequency and then reaches almost constant value due to the fact that beyond a certain frequency of external alternating field, the electron exchange between Fe+2 and Fe+3 cannot follow the alternating field. The addition of Zn2+ ions seems to disturb this equilibrium and could reduce the hopping conduction in the sample. On the other hand, Zn is also volatile in nature and some percentage of it escapes during sintering leaving some vacancies in the structure. The evaporation of Zn ions during sintering process may increase the number of Fe2+ ions at these sites, which in turn lead to an increase in hopping conduction.