Study on High-Frequency Permeability in Ferrite Ceramics and Ferrite Composite Materials

フェライト焼結体及びフェライト複合体の高周波透磁率に関する研究

中村 龍哉

The complex permeability spectra of Ni-Zn sintered ferrite and the ferrite composite materials were studied. The complex permeability spectra in the ferrite composite materials depend not on the ferrite particle size but on the volume loading of the ferrite particles. The real part of the complex permeability in low-frequency region decreases as the ferrite volume loading is reduced. The shoulder frequency of the real permeability shifts toward higher frequency with a decrease in the ferrite volume loading, hence the real permeability in high-frequency region does not always decrease with a decrease in the ferrite volume loading : it becomes larger in the ferrite composite materials than in the sintered ferrite. The imaginary part of the complex permeability in low-frequency region also decreases as the ferrite volume loading is reduced. The peak frequency of the imaginary part also shifts toward higher frequency with a decrease in the ferrite volume loading. These experimental facts cannot be expected using the semi-empirical logarithmic law.

In order to establish the physical picture on the complex permeability variation, we separate the complex permeability spectra into two contributions : the spin rotational and the domain wall motion components, using the numerical fitting called the inverse cole-cole technique. First, in the sintered ferrite, the domain wall component, the frequency dependence of which is resonance-like, contributes the complex permeability only below 100MHz region. The spin rotational permeability, the frequency dependence of which is relaxation-type, remains even at higher frequency such as 1GHz region. Next, from the experimental results on the ferrite composite materials, the spin permeability strongly depends on the ferrite loading : the static spin susceptibility decreases and the spin resonance frequency shifts toward higher frequency when the ferrite loading is reduced. The domain wall component depends not only on the ferrite loading and but also on the ferrite grain size : the static susceptibility of the domain wall motion decreases and the domain wall resonance frequency shifts toward higher frequency, as the ferrite loading is decreased. While, in low frequency region, the permeability of the ferrite composite materials is lower than that of the sintered ferrite, the permeability of the ferrite composite materials is greater than that of the sintered ferrite at high frequency such as 100MHz region.

These features can be explained by the magnetic circuit model calculation. In the model calculation, the parameters, such as static susceptibilities and resonance frequencies, are transformed using the gap parameter, which is a simple index for the averaged structure of the composite materials. In other word, we can estimate the complex permeability spectra for the ferrite composite materials using the magnetic circuit model combined with the spin rotation and the domain wall motion formulations. The magnetic circuit model provides changes in the magnetic connection with the ferrite loading. The introduction of the magnetic inert component causes not only the magnetic dilution but also a cut-off of the magnetic circuit in the materials. Thus, the permeability becomes lower than that expected from the semi-empirical logarithmic law. The feature is attributed to the demagnetizing field generated by the magnetic poles on the interface of the ferrite particle. Simultaneously, the addition of the demagnetizing field on the magnetocrystalline anisotropy field leads to a shift of the resonance frequency toward higher frequency.

Additionally, using this model calculation, we obtain the extended Snoek's law for the composite materials, and we can explain the temperature variation of the complex permeability in the ferrite composite materials. Furthermore, this magnetic circuit model can be applied to other phenomena such as the complex permeability variation in low-temperature sintered Ni-Zn-Cu ferrite ceramics and the electromagnetic properties and power losses in Mn-Zn ferrite sintered ceramics. This model calculation is thought to be a useful tool for the development in many high-frequency electromagnetic devices through the evaluation of the complex permeability in the materials.

物理学 [ 420 ]

英語

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広島大学