Mechanisms Of Modulating The High Frequency Permeability Of Magnetic Nanostructures

With the rapid development of network and technology,some intelligent household appliances have gradually entered our daily life.To some extent.the application of these devices has brought convenience to our daily life,but also produces a large number of electromagnetic radiations,and then formed electromagnetic pollution.In order to overcome the electromagnetic interference(EMI)between various electronic devices,the use of electromagnetic wave absorption material has become an effective means of protection.Therefore,it is necessary that the material can absorb excess electromagnetic energy through magnetic resonance or eddy current effect.Generally,the imaginary part of the permeability is positive,and the larger the peak value of the imaginary part is,the better the absorbing property of the material is.In addition,as the working frequency of electronic devices gradually develops to high frequency,the working frequency of antielectromagnetic interference materials is required to increase accordingly,which requires that the peak position of the intrinsic high frequency permeability spectrum of materials can move to higher frequency,so that it can adapt to the different range of the electromagnetic waves and thus achieve the purpose of absorbing and attenuating electromagnetic energy.For magnetic absorption materials,the main methods to adjust permeability are changing the internal microstructure of materials by heat treatment,changing the internal orientations of magnetic moments by external field or changing the anisotropic properties by controlling the shape of magnetic materials.The first part of this thesis is based on this method,which indirectly adjusts the orientations of magnetic moments along nanowire arrays by changing the distance between nanowire arrays.Then the high frequency intrinsic permeability spectrum of nanowire arrays is obtained by applying a pulsed magnetic field.The magnetic information is characterized by FORC diagrams.It is found that the displacement of magnetic resonance peak is mainly related to the interaction along the materials.The second part of this thesis is mainly based on spin transfer torque(STT)effect.The interaction between spin-polarized electrons and local magnetic moments is used to change the precession behavior of magnetization,and then adjusting the magnetic permeability of materials.It is found that the changes of damping factor,adiabatic coefficient and current polarization do not affect the absorbing frequency of magnetic materials.It only affects the magnitude of wave-absorbing.In the third part of this thesis,Voronoi diagram is used to study the effect of the changes of nanostructures of Finemet nanocrystalline alloy on its permeability spectrum.It is found that the variation of nanostructure does not affect the position of the peak,but only affect the intensity of the peak.However,this effect is not obvious.