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In order to study the effect of substitution of Fe 3+ by Al 3+ and Cr 3+ in Li 0.5 Fe 2.5 O 4 on its structural and magnetic properties, the spinel system Li 0.5 Al x Cr x Fe 2.5−2x O 4 (x = 0.0, 0.2, 0.4, 0.5, 0.6 and 0.8) has been characterized by X-ray diffraction, high field magnetization, low field ac susceptibility and 57 Fe Mössbauer spectroscopy. Contrary to the earlier reports, about 50% of Al 3+ is found to occupy the tetrahedral sites. The system exhibits canted spin structure and a central paramagnetic doublet was found superimposed on magnetic sextet in the Mössbauer spectra (x > 0.5).

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The dielectric constant spectra of Bismuth substituted nickel ferrite NiBi 0.20 Fe 1.80 O 4 has been reported. The variation of dielectric constant with temperature in the range 50 -500 C shows linear response at constant frequency. The variation of dielectric constant with frequency in the range 500 Hz -1 MHz (decade increase) at constant temperatures also shows almost flat response. This may be attributed to the substitution of Bi 3+ ions at octahedral sites. With higher substitutions of bismuth, this material may be useful in the GHz frequency range applications.