Ferrites for magnetic and piezomagnetic filter elements with temperature-independent permeability and elasticity

In recent experiments the compositions and methods of preparation of various ferrites were varied with the aim of achieving optimum magnetic, magnetomechanical and mechanical performance. Essential improvements of the temperature dependence of the permeability, the piezomagnetic coupling coefficient and the mechanical resonant frequencies have been obtained. These were achieved by small cobalt substitutions in nickel and lithium ferrite and in mixed nickel-zinc and lithium-zinc ferrites. The total variations of the mechanical resonant and anti-resonant frequencies of ferrite filter elements in the temperature range 20¿50°C are normally 0·10¿0·25% in existing commercial nickel and nickel-zinc ferrites, but have been reduced to 0·03% or less. The coupling coefficient at remanence of such a stable ferrite may be well above 0·20 with a suitably modified sintering treatment. Since the mechanical Q-factors are usually much better than 2000, such ferrite vibrators can be profitably applied to the construction of electrical and electromechanical band-pass filters. The cobalt substitutions, apart from leading to a very low temperature coefficient of the real part of the permeability, also decrease the imaginary part, so that these cobalt-substituted ferrites have a high figure of merit.