Abstract
Spin-wave nonreciprocity arising from dipole-dipole interaction is insignificant for magnon wavelengths in the sub-100 nm range. Our micromagnetic simulations reveal that for the nanoscale magnonic crystals studied, such nonreciprocity can be greatly enhanced via synthetic antiferromagnetic coupling. The nonreciprocity is manifested as highly asymmetric magnon dispersion curves of the magnonic crystals. Furthermore, based on the study of the dependence of the nonreciprocity on an applied magnetic field, the antiparallel alignment of the magnetizations is shown to be responsible for the enhancement. Our findings would be useful for magnonic and spintronics applications.
Highlights
The MC considered is a 2μm-long, 30nm-wide ferromagnetic nanostripe decorated with a periodic array of nanocuboids, as shown in Fig. 1.15–17 The crystal has a lattice constant a = 20 nm and consists of two layers of equal thickness, between which the exchange coupling can be of either the AFM or ferromagnetic (FM) type
The enhanced nonreciprocity is a result of the large contrast in the respective magnetizations and the effective fields of the top and bottom layers of the MCs
It is evident that the distribution of the internal field of the AFM MC exhibits a larger contrast between the two layers than that of the FM crystal
Summary
An external magnetic field H0 is applied transverse, in the y-direction, to the long axis of the crystal. Boundaries anymore, because the dispersion curves of counter-propagating waves no longer intersect at the BZ boundary due to their different slopes.[5] Note that this property is a natural property of gyrotropic periodic materials.[23,24] Another interesting property of the band structure of the MC with AFM coupling is that the SWs acquire nonzero group velocities when k = 0.
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