Spin waves in Y<sub>3</sub>Fe<sub>5</sub>O<sub>12</sub>-CoFeB spin-wave directional coupler

Abstract

The spin-wave coupling device is used as a connection unit to solve the connection problem between spin-wave devices. However, the current size is too large in comparison with the nano-scale process, which is caused by the low efficiency of the spin wave within it. Therefore, we propose the spin-wave directional coupler based on Y<sub>3</sub>Fe<sub>5</sub>O<sub>12</sub>-CoFeB coupling which can improve the current dilemma to a certain extent. By filling the gap layer of two spin-wave waveguides (Y<sub>3</sub>Fe<sub>5</sub>O<sub>12</sub>) placed in parallel with CoFeB material, it is found that the dispersion relationship of the spin wave changes in the data calculation of the micromagnetic simulation software Mumax<sup>3</sup>. The existence of CoFeB makes the transmission efficiency of the spin wave between the two waveguides higher than in the case without any filling, the enhancement effect is about 4 times where coupling length is reduced from the original 2000 nm to 500 nm, which is conducive to the miniaturization and integration of the spin-wave directional coupler design. From the perspective of the entire device, further analysis indicates that owing to the high saturation magnetization of CoFeB (approximately 8 times that of Y<sub>3</sub>Fe<sub>5</sub>O<sub>12</sub>), the effective field in the Y<sub>3</sub>Fe<sub>5</sub>O<sub>12</sub>-CoFeB directional coupler is greatly enhanced, which leads the spin wave dispersion curve in the waveguide to change. At the same time, the energy of the entire system also increases several times, which is mainly caused by the increase of dipole energy and exchange energy. Then a greater contribution of dipole energy is obtained by changing the size of the device. After that, we study the relationship between the coupling length and the device size and the external magnetic field, then draw a general rule which can play a role in designing any directional couplers with similar structures. Finally, our view points are given from the different spin wave excitation frequencies, gap layer filling materials, internal roughness of the directional coupler, and spin wave lifetime by considering the problems that may occur in practical applications with the Y<sub>3</sub>Fe<sub>5</sub>O<sub>12</sub>-CoFeB directional coupler. In conclusion, our proposed Y<sub>3</sub>Fe<sub>5</sub>O<sub>12</sub>-CoFeB directional coupler structure can effectively enhance the coupling efficiency, and it can also provide a new idea for the application of the interaction between composite materials.