Terahertz Permeability Of Permanent Magnets

With the advancement of telecommunication technologies toward higher frequency bands,THz technologies have just been bloomed.It is widely believed that traditional permanent magnets cannot find applications in the THz domain because their magnetic responses are extremely weak.Therefore,metamaterials are widely developed to obtain strong THz magnetic responses(i.e.,THz permeability).But the complicate processes and high cost of fabrication make metamaterials difficult for mass production.Therefore,it is urgent to develop some natural synthesized(non metamaterials)materials to realize strong THz permeability,which however will require such a material to have very large magnetocrystalline anisotropic field(H_k).In this thesis,we will select two categories of permanent magnets(Mn-Al alloy and Nd-Fe-B alloy)as the study objects,and employ the widely accepted micromagnetics simulation tool to study how to obtain significant THz permeability and the physical mechanisms to adjust THz permeability.In the first part,we have studied the THz permeability of Mn-Al permanent alloys.Mghan’s law proposes that due to the inhomogeneous microstructures,there are plenty of LLG type precession phenomena simultaneously happens when the magnetic material is under the excitation of external high-frequency field.Correspondingly,it will result in plenty of permeability spectra(called sub-spectra).The observed spectrum is the weighted average results of these sub-spectra.Our results show that:the THz permeability of hard magnet phase(τ-MnAl nanostrip)in MnAl alloys are different under different remanence states(M_r/M_s).If this material is in a spontaneously magnetized state,it will be unable to obtain strong THz permeability,despite of the fact that it has a very large H_kvalue.Nevertheless,based on the Mghan’s law,we have innovatively proposed an approach to obtain strong THz permeability by choosing the maximum magnetization field to control its remanence states and the distribution of volume fractions(weight ratio)of localized effective magnetic anisotropic fields in aτ-MnAl nanostrip.The proposed Mghan’s law can give reasonable explanations on the dependence of permeability spectra shapes on the various magnetized states.Especially,at a magnetized state when M_r/M_sis 1.0,only one significant THz permeability resonance type spectrum at 0.1697 THz can be obtained.In addition,THz permeability spectra can be slightly adjusted by changing the geometrical dimensions of single nanostrip and the nanostrips array(i.e.,textures of nano phase)made from the single hard phase(τ-MnAl).Finally,we have used the Voronoi diagram technique to simulate the microstructures of materials.The results show that obvious and strong THz permeability spectra still can be realized as long as the materials are in the proper remanence state even for the materials contain different distribution patterns of hard phase in their microstructures.Furthermore,the spectra shapes and loss peak frequencies of THz permeability will vary with the size distributions of hard magnet phase,and show a feature of“log-normal distribution”.We believed it is due to the log-normal distribution of shape factor of hard phase(τ-MnAl)in the microstructures.In the second part,we have studied the THz permeability of NdFeB permanent magnet alloys,which can give strong THz magnetic responses at an even higher THz frequency band.Our results show that a strong and significant THz permeability spectrum can be obtained at 0.2171 THz for single nanostructured strip made from hard phase NdFeB when its remanence state(M_r/M_s)is 1.0.Similarly,Voronoi diagrams were constructed to simulate the microstructures of polycrystallized NdFeB single hard phase,and have been used to study the impacts of average orientations,damping constants,remanence at different magnetizing directions,and average nanograin sizes on the THz permeability spectra.Among these factors,permeability spectrum is more sensitive to the average orientations of nanograins.All these results can be well understood by using the Mghan’s law.Finally,the studies in this thesis show that only if the permanent magnets with extremely large H_kvalues are in the proper remanence states,can we obtain significant THz permeability spectra.It is because we can adjust the distribution of volume fractions for localized effective anisotropic magnetic fields,and make the ones falling into the THz frequency bands to have the overwhelming volume fractions.Also,it is found that large magnetic losses can be acquired due to the LLG type natural resonance mechanism.This finding tells us that we can develop THz electromagnetic wave absorbing materials based on the Mghan’s law.