Effect Of Two-Dimensional Asymmetric Structure On The First Brillouin Zone Of Magnonic Crystals

Magnonic crystal is an artificial magnetic medium with the characteristics of periodic structure.Due to the non-uniform filling property of magnonic crystal as a periodic structure material,it is different from traditional materials.Moreover,spin waves have novel and efficient propagation properties in magnonic crystals,and have great potential applications in the field of microwaves in future.The wave length of the spin wave can last up to less than 0.1nm.Moreover,spin waves have novel and efficient propagation properties in magnonic crystals.Compared with the electromagnetic wave(photons)of the same frequency,the length of spin wave is shorter,and can be applied to the devices less than 10 nm.In addition,the frequency of spin wave covers a wide range and can carry on the regulation of artificial lens according to the demand.At the same time,the magnonic crystals allow spin information to be transmitted at room temperature,and no extra joules of heat are consumed in the process.In summary,energy saving,high efficiency,and many other advantages make the research on magnonic crystals to develop rapidly in related fields in recent years.These related applications of magnonic crystals are mainly to use the band gap generated in their energy band structure,so to make magnonic crystalsmeet more applications,the first research task is to obtain more band gap structures that can be cut manually.However,the difficulty of opening band gaps lies in the degeneracy generated by the high symmetry points in Brillouin region.The stronger the degeneracy is,the more unfavorable it is for the opening of band gap of magnonic crystals.Therefore,how to eliminate the degeneracy at these points of high symmetry is a key point to optimize the band gap of magnons.Reducing the symmetry of crystal structure is a very good method to eliminate the degeneracy state.Therefore,the degenerate states can be separated effectively by using the structure of asymmetric structure.In this thesis,the model of magnonic crystals with asymmetric structure is proposed.In the previous studies,it is generally believed that the irreducible Brillouin zone of crystal does not change,and the first Brillouin zone is reduced to the traditional small region,called the irreducible Brillouin zone corresponding to the symmetrical structure of magnonic crystals.However,due to the adjustable structure of the artificial crystal,the structure symmetry can be changed.It is found that the irreducible Brillouin zone can be changed when the lattice structure is asymmetric,that is to say,the irreducible brillouin zone is no longer the traditional small region and may be expanded.In addition,for the magnonic crystals with symmetric structure,since the extremum frequencies of each band is located on the high symmetry line of theboundary of irreducible Brillouin zone.So,the true width of band gap of the whole first brillouin zone can be obtained only by calculating along the highly symmetry line of the boundary of the reduced small region.But for the magnonic crystals with asymmetric structure,the position of the maximum or the minimum of the energy band is no longer located on the high symmetry line of the boundary of the irreducible region,but may appear in the region.Therefore,for these magnonic crystals with asymmetric structure,if the first brillouin zone is reduced to a traditional small region and the band structure is calculated only along its boundary,the accurate band structure information cannot be obtained.The complete band structure can only be obtained by expanding the research scope and even to calculate the band frequency in the whole first Brillouin zone.