A Study Of The Magnon-magnon Strong Coupling Phenomenon

In past decade,the strong coupling between cavity photon and magnon has been extensively studied both theoretically and experimentally.However,the strong photon-magnon coupling systems are restrictive in device miniaturization,CMOS-integration and low power consumption.Because the sizes of microwave cavities and magnetic materials are usually required to be tens of millimeters and hundreds of microns,respectively,as well as the generation and transmission of light have energy loss.Since the propagation of magnon does not involve the movement of electron charges,it has the advantages of low energy consumption,short wavelength and easy modulation.The realization of strong magnon-magnon coupling has been attracting high attentions.For single layer magnetic nanostructures,the evolution of magnon modes has been studied widely,but there are rare reports on the magnon-magnon coupling in such system.For synthetic antiferromagnets,the reported magnon-magnon coupling usually has weak coupling strength due to the interlayer exchange coupling between bottom and top ferromagnetic layers is weak.In this work,we investigated the strong magnon-magnon coupling in both single and multilayers magnetic nanostructures by means of micromagnetic simulations and theoretical calculations.For the strong magnon-magnon coupling in a single magnetic nanodot,we investigated the coupling between the edge and the center magnon modes confined in an irregular hexagonal dot.The spatial interaction between those two modes results in the coupling between them.It is found that the coupling strength can be significantly affected by adjusting the geometrical shape,size and Gilbert damping.The obtained maximum coupling strength g reaches 0.494 GHz,and the corresponding cooperativity C=60.1.Since the coupling strength g is greater than the dissipation rates of the coupled magnon modes k _EM and k _CM when Gilbert damping?<7.65?10^-3,the interaction between the edge and the center magnon modes belongs to the strong coupling regime.For the strong magnon-magnon coupling in the synthetic antiferromagnets,we investigated the coupling between the in-phase acoustic and the out-of-phase optic magnon modes by micromagnetic simulations and theoretical calculations.Once the spatial symmetry of the system is broken,the magnon-magnon coupling will occur.It is found that the coupling strength g can be adjusted by tuning the tilting angle?of the bias field,the strength of the interlayer exchange coupling|J_IEC|,and the thicknesses of the ferromagnetic layers t.The g is proportional to the?and the magnitude of|J_IEC|,while,it is inversely proportional to the t.The obtained maximum coupling strength g reaches 9.94 GHz,and the corresponding coupling rate g/f_g=0.59,approaching the ultrastrong coupling regime.Moreover,the magnitudes of the magnetic anisotropies have significant effect on the coupling phenomenon,the observed magnon-magnon coupling will be destroyed when the values are large enough.