Investigation Of Spin Dynamics And Magnon-magnon Coupling In Magnetic Heterostructures

The patterns of collective precession of electron spin are called as spin waves,whose quantizations are regarded as magnon.Magnons can transmit spin information without charge movement,thereby fundamentally avoiding heat dissipation in highly integrated micro/nano-magnonics devices.Therefore,magnons have important application prospects in the next generation of ultra-low power microwave magnonics devices.Meanwhile,magnons can carry out energy transfer through interaction with other quasi-particles,so as to realize information processing and logical calculation in magnonics devices based on low-energy magnons.However,magnon-quasiparticle couplings are indirect,and their strength is still weak,which cannot meet the requirements of actual devices.In addition,the size of the resonant cavity is too large to realize the integrated miniaturization of devices,which is greatly limited in application.Therefore,it has become the direction of researchers’ efforts to find a material system with larger coupling strength with magnons and conducive to the integration of micro-magnonics devices.The intensity of direct magnon-magnon exchange interaction is much greater than the indirect magnon-quasiparticle couplings.So the intensity of magnon-magnon coupling is expected to be greatly improved,and magnon-magnon coupling can be directly realized in a two-diamensional multilayer film system,which is beneficial to the integration and miniaturization of devices.Therefore,in this paper,YIG/Py(Yttrium iron garnet/Permalloy)heterogeneous structures are prepared by pulsed laser deposition system,the magnon-magnon coupling and spin dynamics of YIG/Py heterogeneous structures are explored by ferromagnetic resonance tests based on coplanar waveguides and spin-torque ferromagnetic resonance tests.The main contents of this thesis are as the following:(1)The growth conditions of single-crystal YIG films are studied,and the singlecrystal YIG films are characterized by various experimental methods.The crystal structure,ductility and surface roughness of YIG thin film were characterized via highresolution X-ray diffractometer,atomic force microscope and reflection high energy diffraction electron spectroscopy.The dispersion relationships and damping constants of YIG films prepared by different growth conditions are obtained by ferromagnetic resonance on the coplanar waveguide.The relevant effective magnetization of YIG single crystal films at different temperatures was obtained by coplanar waveguide ferromagnetic resonance and corresponding theoretical analysis.In order to study spin transport in YIG/Pt,the components of various torques and the existence of the spin pumping effect are analyzed.The experimental results of this part are in agreement with the theoretical values,which provides a reasonable basis for the subsequent experiments.(2)The magnon-magnon coupling in YIG/Py heterogeneous structure is observed,and various methods modulate magnon-magnon coupling strengths.YIG(100 nm)/Py(20 nm)samples are measured via ferromagnetic resonance on the coplanar waveguide,and analysis showed that magnon-magnon coupling between YIG(n = 2)mode magnons and Py uniform procession mode magnons are analyzed by Kittel’s formula.The anti-crossing gaps increase with increasing rf microwave power,which affects the dynamic dipolar interaction on the whole sample.The anti-crossing gaps decrease with increasing the angle of the external magnetic field,which makes the damping and magnon relaxation in the system larger.The magnon-magnon coupling maybe not be affected by an applied electric field,in which the positions of resonance peaks do not change significantly.According to the experimental results of this part,various methods are investigated to modulate the magnon-magnon coupling.(3)Spin mechanics in YIG/Py heterogeneous structures are discussed via spintorque ferromagnetic resonance in this part.Magnon-magnon coupling is observed by spin torque ferromagnetic resonance in YIG/Py heterostructure.The anti-crossing gaps will increase with reduced temperatures.The components of the symmetric and antisymmetric Lorentzian coefficients are analyzed by spin torque ferromagnetic resonance with the rotating magnetic field in the x-y plane.The experimental results show that the anti-crossing gap increases,and the spin pumping effect and the field-like torque in the z-direction will enhance the magnon-magnon coupling.