Spin Waves Exited By Acoustic Waves And Its Nonreciprocity Application

Surface acoustic wave（SAW）is regarded as a new means to excite and control spin waves in magnets.The efficiency of excitation is much higher than that of traditional electromagnetic waves,and it can also realize long-distance spin wave（SW）transmission.In addition,SAWs can also assist magnetization reversal by reducing coercivity,generate and drive the magnetic domain walls and skyrmions.Moreover,based on the SAW-SW coupling（phonon-magnon coupling）,a variety of new devices and applications have been demonstrated,such as high-sensitivity weak magnetic field sensors,integrated magnetoelectric antennas,solid-state acoustic isolators,nonreciprocal phase shifters,and so on.Therefore,it is of great value to study the physical mechanism of magneto-acoustic coupling,develop new magneto-acoustic coupling effects,and improve the efficiency of magneto-acoustic coupling.This dissertation focuses on the use of SAWs to excite SWs and its nonreciprocal phenomenon.The specific contents are as follows:1.Three important coupling mechanisms between SAWs and SWs are summarized,and the classic magnetoelastic coupling models are enriched and extended.Then two sets of processing methods are obtained to quantitatively solve various types of magneto-acoustic couplings.The susceptibility tensors of different magnetic structures and the equivalent driven magnetic fields of various magnetoacoustic couplings can be determined by the magnetization dynamics method,and the energy absorption caused by magneto-acoustic coupling can be quantitatively solved.The change of transmission loss and phase shift of SAWs generated by magneto-acoustic coupling can be solved by the coupled waves dispersion method.Moreover,both methods can be effectively used to solve the nonreciprocal transimission of SAWs.2.The design,preparation and testing process of magneto-acoustic devices are introduced.It includes the simulation of the characteristics of different types of SAWs on various substrates,the structure design of interdigital transducers,the process flow of device preparation and the test methods of both high-frequency magnetic parameters of films and magneto-acoustic devices.3.The vorticity characteristics of Rayleigh wave and shear horizontal wave,as well as the spin-polarization accumulation,alternating spin flow,spin-transfer torque equivalent driven field and spin-wave resonance induced power absorption in Ni Fe/Cu bilayers are investigated by simulation and numerical calculation.Then,experiments were designed accordingly and carried out to verify the calcuations.It is found for the first time that the spin-vorticity coupling of shear horizontal wave is four orders of magnitude stronger than that of Rayleigh wave for the same wavelength,and a theoretical explanation is given.In addition,the frequency dependence of spin wave resonance intensity excited by spin-vorticity coupling is also studied and analyzed.4.Based on the theoretical framework of magnetoelastic coupling,the characteristics and efficiency of SWs excited by three different types of SAWs are studied and compared.The results show that longitudinal leakage wave has the same four-fold symmetric power absorption characteristics as that of Rayleigh wave,but its exciation efficiency of spin wave resonance is higher.Due to the larger in-plane equivalent driven field,shear horizontal wave can excite SWs much more efficiently than Rayleigh and longitudinal leakage wave,and the angular dependence of its power absorption is complementary to the other two waves.In addition,the shear horizontal wave excites magnetostatic backward volume waves（M//k）or magnetostatic surface waves（M⊥k）with the highest efficiency.Rayleigh wave and longitudinal leaky wave are more efficient when the angle between M and k is 45°.5.The nonreciprocal transmission of shear horizontal SAWs on Li Ta O₃ substrate is realized by using FeCoSiB/Ni Fe Cu magnetoelastic bilayer structure.This structure uses two kinds of magnetic materials with a large saturation magnetization difference to form a bilayer structure.The large difference between their saturation magnetizations can break the spatial symmetry of the dipole field in the thickness direction.On the other hand,the introduction of a low saturation magnetization Ni Fe Cu layer can significantly reduce the spin wave resonance frequency,which is beneficial to realize nonreciprocal transmission of SAWs at a lower frequency.A very large isolation ratio of 30 d B（or 60 d B/mm）is achieved in our fabricated delay line at 2.33 GHz.