Characteristic Relation Of Permeability In Soft Magnetic Materials

Soft magnetic materials with large AC permeability at high frequency have attracted significant attention,because they can improve the efficiency of power conversion for electrical and electronic equipment.However,even in the MHz frequency range,none of the available magnetic materials can fully release the potential of the wide bandgap semiconductors devices that can work at GHz.Therefore,To find soft magnetic materials with high susceptibility in the MHz range and above is still a challenge.In fact,the permeability of magnetic materials is not only determined by intrinsic dynamic mechanisms of magnetization but also affected by the distribution of extrinsic microwave magnetic field The former mainly includes natural resonance in the MHz and GHz,and the latter mainly comes from skin effect.In this thesis,a systematic study on the permeability mechanism and its characteristic relationship above MHz has been carried out on single domain particle,soft magnetic composites and skin effect modulation experimentally and theoretically.The main research results are shown as follows:1.Susceptibility model of bi-anisotropy soft magnetic composites（BSMCs）consisted of bi-anisotropy single-domain particle（BSDP）with a unified anisotropy which is a both polar and azimuthal angles related bi-anisotropy was established.We find that the product of initial susceptibility and resonance frequency in the BSDP is beyond Snoek’s law.We derive the susceptibility of BSMCs further based on that of BSDP and find it is related to the orientations of both magnetization in static state and easy magnetization plane.More importantly,the maximum susceptibility of BSMCs is the same as that of a BSDP when the magnetizations in static state of BSDPs are oriented in one direction or plane.Our model provides an approach to improve the susceptibility of BSMCs.2.Impact of skin effect on permeability of Permalloy films with different thicknesses was investigated.With eliminating multi domain structure by measuring field permeability spectrum,the picture of permeability determined by natural resonance and modulated by skin effect was demonstrated.It is found that both the value of permeability and line shape of the field permeability spectrum depend on thickness significantly due to skin effect.The real part of field permeability spectrum decreases from 974（0.04 μm）to 14（4 μm）at 3.2 GHz,and two loss peaks appear in the imaginary part in films thicker than 2 μm,which can’t be explained by previous work.We established a skin effect-related model of permeability with which the experimental permeability spectra can be well fitted.The calculated cutoff frequency is significantly reduced by skin effect from 3.5 GHz（0.02 μm）to 0.3GHz（4 μm）,which is dominated by the natural resonance frequency（t ≤ 1.46 μm）and the additional loss peak frequency caused by skin effect（t > 1.46 μm）successively.3.A new characteristic relationship of permeability was found and verified theoretically and experimentally.We theoretically obtained the characteristic relationship of real part and imaginary part,which are equivalent.The characteristic relationship of real part is universal and can fall back to Snoek’s law and Archer’s law.The actual characteristic relationship of materials is closely related to the skin effect.Its essence is the non-uniform distribution of AC magnetic field caused by eddy current in magnetic materials.