| In the field of micro- and nano-structure. some artificial composite materials with physical properties not be found in natural crystals have been designed and fabricated by using the current technologies according to the realistic demands. For examples, photonic crystals are the artificial crystals with photon gaps, and phononic crystals are the new type of functional materials which are the periodic structures of elastic constants and mass density. In recent years, magnetic composites with the structure modulated periodically on the nanoscale are the subject of a very intensive research, and the interest in their properties is increasing. By analogy to photonic crystals and phononic crystals, they are referred to as magnonic crystals (MCs) which are another kind of artificial microstructures. The fully controllable behavior of spin waves propagating in MCs would be expected by researchers. Moreover, the wavelength of spin waves is much shorter than that of electromagnetic waves of the same frequency. This would provide more possibilities in miniaturizing magnetic devices.In the field of research, MCs can be divided into the infinite structure and the limited thin film structure based on the scale of system. Both bulk structure and thin film structure can be divided into one-dimensional, two-dimensional or three-dimensional system according to the dimension of the periodic structure. As for the thin film of MCs, the influence of static magnetic field is important to properties of spin waves in MCs. So. it is more complicated to theoretically investigate MCs, and rare reports of band structure in thin film can be found. In this paper, based on the previous researches of band structures in bulk MCs investigated by our group, a scheme of optimizing band gaps in MCs of thin film would be proposed again by rotating the nocircular scatterers. Band-gap structures are calculated for the thin film MCs including scatterers of square rods and rectangular rods, and the optimizing cases of band gaps are investigated after rotating two kinds of nocircular rods, respectively. At the same time, the influence of the edge length ratio of rectangular rods on the gap is analyzed. The numerical results show that band gaps of spin waves can be effectively generated, especially for the gaps of high frequency. Thereby, the adjustable spin-wave gaps can be achieved in the thin film MCs. The width of band gap becomes large as increasing the mismatch of the edge length in the thin film MCs, but which behaves as the opposite case in the bulk MCs. All of these can be used to design spin-wave devices of high frequency, and an effective approach has been provided in the field of spin-wave gap engineering.In the last, it is found that the traditional definition of irreducible Brillouin Zone (BZ) in the symmetry of crystals is not always correct in the artificial crystals. Consequently, some band gaps obtained from the ordinary band diagram are not complete band gaps. The irreducible BZ in artificial crystal is influenced by the shape of scatterers, which is not absolute identical to case of natural crystal. Because, atoms in natural crystals are always regarded as "spherical" ones, only the lattice is considered in the research of the symmetry. But for the case of artificial crystal, "atoms" are called scatterers, and the different shapes of scatterers which can be designed would be different symmetries even in the same lattice of "atoms". So, the irreducible BZ of artificial crystal has not the same presentation. That is to say, it is necessary to investigate a larger zone in the BZ to obtain the authentic band structure and band gaps in artificial crystals. |
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