| Magnetic nanomaterials have excellent electromagnetic performance in the GHz band.They can not only meet the requirements of high-frequency devices,but also solve the electromagnetic interference problems associated with high-frequency devices.Based on the application,the requirements of high permeability and high cut-off frequency are proposed,and the two are restricted by the law of Snoek.In order to break through this restriction and regulate the electromagnetic properties of magnetic nanomaterials,we can start from the only non intrinsic parameter,shape anisotropy.When it comes to the control of the microwave electromagnetic properties of magnetic nanomaterials,we must make clear the relationship between the problem of the material system and the shape anisotropy.For traditional magnetic soft magnetic metals and alloys,the cut-off frequency determined by magnetocrystalline anisotropy is difficult to appear in the range of GHz,and eddy current and the impedance mismatch caused by high dielectric constant will impede its application.The key to solve these problems is to adjust the shape anisotropy of the material reasonably.In addition,the nanoscale of the material brings a new pattern to the anisotropy of the system,and what will it affect the high frequency electromagnetic performance?With these problems,this paper studies the following aspects.1.the magnetic permeability and dielectric constant of the flake nanocrystalline FeCuNbSiB alloy were studied,and the control was realized.(1).The shape anisotropy distribution of flaky powders is studied.The expression of permeability spectrum based on this distribution is put forward,and the problem of loss peak broadening of permeability spectrum is analyzed.(2).Based on the eddy current effect of the alloy composite system,we have selected different sizes of alloy powders,studied the connotation of effective permeability spectrum,calculated their intrinsic permeability spectra,and quantified the eddy current effect of the system by calculating the depth of the system.(3).The dielectric constant of alloy powders with different grain sizes is studied.The relaxation mechanism includes the interfacial polarization and the effect of conductivity of the alloy and the organic matter.Combined with its relaxation mechanism,dielectric spectroscopy was analyzed by conductance corrected Cole-Devidson model and Lorentz model.By analyzing the conductivity spectrum,the relationship between dielectric constant and polarization and conductivity was explained.(4).Based on the problem of high dielectric constant caused by high conductivity of alloy powder,coating with high resistance ferrite reduces the dielectric constant greatly,and achieves impedance matching,which enhances the electromagnetic wave absorption properties.2.A series of monodisperse anisotropic magnetic nanomaterials were prepared,and the effects of nanoscale on the microwave electromagnetic properties were studied.(1).The uniform porous Fe flakes with high saturation magnetization were prepared,and the high permeability and multi resonance phenomena in GHz range were also realized.(2).The Fe3O4 nanoribbons with an average size of 17.2 nm×64.3 nm×939.1 nm were prepared.When studying the microwave electromagnetic properties,it is found that 4 resonance peaks appear in the spectrum of in the measured frequency range,and the loss mechanism corresponding to the peak is the natural resonance and the exchange resonance.The dielectric constant is very low,and the loss of the system is mainly attribute to the magnetic loss.Moreover,the wide magnetic loss peak caused by multiple-resonance leads to excellent microwave wave absorption performance,and its absorption bandwidth(RL<-10 dB)is up to 4 GHz.The problems of eddy current effect,high dielectric constant and narrow absorption peak of micro-flakes are solved.(3).The monodisperse FeCo nano cubes with an average size of 239 nm×239 nm×239 nm were prepared.The microwave dielectric constant is 5-6 times lower than that of the micro-flakes,and the eddy current effect is greatly reduced.The microwave magnetic spectrum also shows the wide loss peaks,and the system has good absorption bandwidth.The magnetic moment distribution of the system and the loss mechanism of the magnetic permeability spectrum have been studied by micromagnetic method.Because of the nanoscale,the influence of surface state can not be ignored.The permeabilities are anisotropic in x,y,z three directions.In the measured range,the multi-resonance mainly comes from the natural resonance,the domain wall resonance and the exchange resonance caused by the non-uniform precession of magnetic moments in domain wall.3.Amorphous FeP nanowire arrays were prepared by postentiostatic electrodeposition.The effects of heat treatment on magnetic anisotropy,magnetization and exchange were systematically studied,and their relationships with coercivity and rectangular ratio were also studied.Finally,the microwave permeability of nanowire arrays with different aspect ratios were studied by micromagnetic simulation,especially for the resonance mechanism of the imaginary part of the permeability. |
PDF Full Text Request