Study On The Dynamic Magnetoelastic Coupling Effect And Its Device Applications

In the field of micromagnetics,a wealth of physical effects exist inherently within magnetic materials（e.g.,ferromagnetic resonance and spin wave resonance）,between magnetic materials（e.g.,dipolar coupling and exchange coupling）,and between magnetic and non-magnetic materials（e.g.,spin pumping and spin transfer torque）.These phenomena give rise to a variety of novel characteristics.By employing the magnetoelastic effect,various magnetic materials or magnetic structures can be integrated into a surface acoustic wave（SAW）platform,resulting in the formation of magnetic surface acoustic wave（MSAW）devices,which enable acoustic excitation,detection,and control of spin waves or spin parameters.In recent years,a number of MSAW devices have been designed,such as highly sensitive weak magnetic field sensors,integrated magnelectromechanical antennas,nonreciprocal SAW isolators and circulators.However,the true nature of magnetoelastic coupling at various frequencies still presents a major challenge in understanding and designing this kind of device.This dissertation focuses on the theory of acoustically excited dynamic magnetoelastic effects and its related device applications.The main content and findings are as follows:（1）High-performance magnetoelastic thin films were prepared,and their damping mechanisms were analyzed.MSAW devices pose stringent requirements on the performance of magnetoelastic thin films,such as high magnetic permeability,a low damping factor,a small coercive field,and a large magnetostriction.By optimizing the fabrication process of amorphous Fe Co Si B thin films,we have achieved a coercive field H_c of 1.4 Oe,an effective damping factorα_eff of 0.009,a low uniaxial anisotropy field H_kof 20 Oe,and high magnetic permeabilityμ>600.High-field ferromagnetic resonance test indicates that the intrinsic Gilbert damping factor of Fe Co Si B thin films can be as low as 0.0038 at room temperature.（2）The acoustically excited dynamic magnetoelastic effects were re-investigated,and a multi-physical field simulation model of MSAW device based on the dynamic elastic modulus effect has been established.This work combined the Landau-Lifshitz-Gilbert equation and the dynamic elasticity equations to derive frequency-dependent modulus effects.A’micromagnetics’module was added in the COMSOL finite element simulation software to couple with the piezoelectric module,which enables the precise simulation of the magnetic field response of MSAW devices.The field response of MSAW devices based on Rayleigh waves（R-SAW）and shear horizontal waves（SH-SAW）was assessed.It was found that the latter’s response was significantly stronger than the former’s,making it more suitable for developing a strong magnetoelastic coupling device.（3）A novel self-biased MSAW sensor was proposed.Resonator-type MSAW sensors were designed and fabricated on ST-cut quartz substrates.By adding the degree of freedom of theφ_EA angle between the easy axis of magnetostrictive film and the wavevector direction of SAWs,the frequency-magnetic field response of the device can be significantly altered.At a specificφ_EA,the symmetry of the magnetoelastic equivalent driving field concerning the magnetization can be broken,enabling self-biasing of the device.The measured resonant frequency sensitivity of the MSAW sensor is as high as630 k Hz/Oe under zero magnetic field.（4）The dynamic shear modulus effect was observed for the first time.A shear-wave delay line capable of exciting high-order harmonics was fabricated by utilizing split-finger interdigital transducer on a 42°YX Li Ta O₃ substrate and optimizing the metallization ratio.The phase-magnetic field response of the device at various harmonic frequencies was tested after depositing Fe Co Si B thin films of different thicknesses.TheΔG effect induced by the magnetic film under each harmonic excitation was extracted using a finite element model.Both theoretical and experimental results suggest that theΔG effect is related to the center frequency of the SAW and the effective damping factor of the magnetostrictive thin film.The closer the SAW frequency to the natural resonance frequency of the magnetostrictive film and the lower the effective damping factor,the more pronounced enhancement in theΔG effect can be obtained,reaching over 80%.（5）The phase noise of MSAW magnetic field sensors was analyzed,and the means to suppress phase noise were proposed accordingly.Magnetic field sensors based on SH-SAW delay lines were designed and fabricated.Measurement on the phase noise reveals that magnetic noise is the primary source of the sensor’s phase noise.A significant correlation was observed between the magnetic noise and the phase sensitivity,causing that the limit of detection（LOD）at low fields is largely independent of magnetic sensitivity.By suppressing edge domain walls and selecting the appropriate operating frequencies,LODs of 1.7 n T/Hz^0.5 at 1 Hz,563 p T/Hz^0.5 at 10 Hz,and 66 p T/Hz^0.5 at 100Hz were achieved.Further measurement along the easy-axis shown that the relationship between magnetic sensitivity and phase noise could be decoupled,which may be utilized to reduce magnetic noise and achieve lower LOD.（6）A nonreciprocal MSAW device based on anti-magnetostriction heterostructures and dynamic magnetoelastic coupling was proposed.A Ni/Ti/Fe Co Si B heterostructure with negative-positive magnetostriction was coated on a Li Ta O₃ substrate.Although the magnetic moments of the two magnetic layers are parallel,SH-SAW can excite optical mode spin waves at low frequencies via magnetoelastic coupling.At 2.333 GHz,the measured isolation ratio is as high as 41 d B（or 82 d B/mm）,and the nonreciprocal phase shift reaches 188°（or 376°/mm）.The experimental results can be well explained by the dynamic magneto-elastic model.This work paves a novel way to develop nonreciprocal solid-state acoustic devices.