Design of controllable magnon frequency comb in synthetic ferrimagnets

The magnon frequency comb provides opportunities for exploring magnon nonlinear effects and measuring the transmission magnon frequency in magnets, whose controllability becomes vital for modulating the operating frequency and improving measurement accuracy. Nevertheless, this controllable frequency comb remains unexplored. In this work, we investigate theoretically and numerically the skyrmion-induced magnon frequency comb effect generated by the interaction between the magnon excitation mode and the skyrmion breathing mode in synthetic ferrimagnets. It is revealed that both the skyrmion breathing mode and the magnon frequency gap are closely dependent on the net angular momentum δₛ, emphasizing the pivotal role of δₛ as an effective control parameter in governing the comb teeth. With the increase of δₛ, the skyrmion size decreases, which results in an enlargement of the breathing frequency and the distance between the comb teeth. Moreover, the dependences of the magnon frequency gap on δₛ and the interlayer coupling allow one to modulate the lowest coherent frequency of the comb via structural control. Consequently, the coherent modes generated by the comb may range from gigahertz to terahertz frequencies, serving as a bridge between microwave and terahertz waves. This work represents a substantial advance in the understanding of the magnon frequency comb effect in ferrimagnets.