Abstract:
Polycrystalline Ni0.50-xMnxMg0.5Fe2O4 ferrites with x=0.00, 0.10, 0.20, 0.30, and 0.40 have been synthesized by the combustion technique. Pellet and toroid-shaped samples were prepared from these powders and sintered at 1200, 1250, 1300 and 1350ºC in air for 5 hours. Structural and surface morphological measurements were performed by X- ray diffraction (XRD) technique and high resolution optical microscopy, respectively. The magnetic properties of these ferrites were characterized with high frequency (100kHz-100MHz) complex initial permeability and temperature dependent permeability measurements. DC magnetization of all samples was measured by the SQUID technique. The influence of microstructure, various cation substitution and sintering temperatures on the complex permeability of these ferrites are discussed. A possible correlation among sintering temperature, average grain size and density is also discussed.
The XRD patterns of all samples clearly indicate the formation of a single phase spinel structure. Lattice parameters are calculated using Nelson-Riley function. It is also found that the lattice constant, ao, increases linearly with increasing Mn content, obeying Vegard’s law. This is due to the fact that the ionic radius of Mn2+ (0.89Å) is greater than that of Ni2+ (0.77Å). Microstructural study shows that grain size increases with increasing Mn content in Ni0.50-xMnxMg0.5Fe2O4. However, the bulk density, ρB, decreases with Mn substitution. As the sintering temperature increases, the ρB increases, and the porosity decreases for all samples. The real part of initial permeability, μi/, and saturation magnetization, Ms, increase with increasing Mn content in Ni0.50-xMnxMg0.5Fe2O4 up to x = 0.30, and beyond x = 0.30, they decrease. The decrease of μi/ and Ms beyond x = 0.30 indicate the possibility of a non-collinear spin arragement. It is also observed that the μi/ increases with sintering temperatures for all samples because at high sintering temperature all samples have uniform grain size. The μi/ remains fairly constant in the frequency range up to some critical frequency which is called resonance frequency, fr. It is also observed that the higher μi/ is accompanied with the lower fr. The relative quality factor, Q-value, increases with increasing Mn content in Ni0.50-xMnxMg0.5Fe2O4 for a fixed sintering temperature. The highest Q-value at sintering temperature 1250°C for Ni0.10Mn0.40Mg0.5Fe2O4 is found to be 1820. The Néel temperature, TN, is measured from temperature dependent permeability. It is observed that TN decreases with increasing Mn content. It may be explained by the modification of the A-B exchange interaction strength due to the change of the Fe3+ distribution between A and B sites. The decrease of the TN is due to the weakening of the A-B exchange interaction. This can be attributed to the increase in distance between the moments of A and B sites, which is confirmed by the increase in the lattice parameter with increasing Mn content.
