Research On Inverse Spin Hall Effect And Magnetoacoustic Effect In Metallic Magnetic Thin Films

In metallic magnetic films,the interaction of time-varying electromagnetic field,strain field(acoustic field),and magnetic moment can generate rich physical effects,such as ferromagnetic resonance(FMR),spin rectification effect(SRE),spin pumping-inverse spin Hall effect(SP-ISHE),and magnetoacoustic effect(MAE),which are one of the frontier and hot directions of current research in the field of magnetism.Among them,the SP-ISHE can be used to realize the complete process of spin flow generation,transport,and detection and to study spin-related parameters of materials,such as the spin Hall angle,spin diffusion length,and spin mixing conductance.In addition,the SP-ISHE can convert microwave signals into DC signals,which can be used to develop new devices such as microwave magnetic field detectors and microwave rectifiers.The MAE can achieve acoustically driven ferromagnetic resonance(ADFMR)and study the physical phenomena of magneton and phonon interactions.Moreover,the MAE can also be used to develop new magnetoacoustic devices,such as magnetic surface acoustic wave(MSAW)sensors,acoustic non-reciprocal transmission devices,etc.This dissertation focuses on the pure ISHE signal acquisition in ferromagnetic metal(FM)/nonmagnetic metal(NM)bilayer structure and microwave rectifier design and preparation,SAW-based ADFMR effect,and MSAW sensor to carry out the research work.The main research works are as follows.1.Acquisition of pure inverse spin Hall signal.The inverse spin Hall effect(ISHE)is often accompanied by the SRE when the ISHE is generated in the FM/NM bilayer under the action of a time-varying electromagnetic field,making the detection of the ISHE signal vulnerable to strong interference from the SRE signal.Aiming at this problem,through electromagnetic simulation analysis,this dissertation found that for the Ni Fe film with a length l of 10 mm,when its aspect ratio was not lower than 200,the induced microwave current flowed mainly along the long edge,while the component along the short edge could be neglected.Through theoretical analysis of the SRE and ISHE,it was found that the SRE voltage along the long edge should be zero and the ISHE voltage should reach its maximum value when the induced current in the FM/NM bilayer was confined at its long edge direction,and the magnetic field direction was perpendicular to the long edge.Based on this finding,this dissertation proposed a method to directly obtain pure ISHE signals by controlling the film geometry.Using this method,a bilayer Ni Fe/Ta film with an aspect ratio of 200 was measured in the microwave frequency range of 2 GHz-3.8 GHz,and its FMR voltage curves showed a symmetrical Lorentzian line shape,which achieved the effective acquisition of pure ISHE signals in a wide frequency band.The study results provide a new fast and accurate means for quantitatively acquiring material spin-related parameters in FM/NM bilayer.2.Design and preparation of microwave rectifiers based on the ISHE.Microwave energy can be converted into DC energy using the ISHE,but the ISHE signal is weak,usually in the order ofμV or even n V.To address this problem,this dissertation proposed a cascaded array device structure based on multiple head-to-tail connected FM/NM microstrip films,which enhanced microwave energy conversion and harvesting ability by superimposing pure ISHE signals.Experimentally,Ni Fe/Ta microstrip film array rectifier devices were prepared by the MEMS process.It was found that when the microwave frequency and input power were 3.8 GHz and 200 m W,and the microstrip width and length were 14μm and 4 mm,respectively,the ISHE voltage and microwave absorbed power of the device increased linearly with the number of microstrips.When the number of microstrips was increased from 40 to 140,the ISHE voltage of the device increased from 28μV to 110μV,and the microwave absorbed power increased from 28μW to 106μW.The microwave-to-DC conversion efficiency was obtained as about 1 m V/m W by analyzing the ISHE signal strength at different input powers.The study results provide a new idea to enhance the wireless energy conversion and harvesting ability of ISHE-based microwave rectifiers.3.Research on the SAW-based ADFMR effect.For magnetic thin films under the action of strain fields,the form of ADFMR was derived from the Landau-Lifshitz-Gilbert equation,and the analytical solution of the magnetoelastic driving field was derived.Experimentally,firstly,the interdigital numbers,aperture,IDT spacing,and metallization rate of the SAW delay line were structurally optimized,and the optimized devices had significant 3rd,5th,and 7th harmonics in addition to fundamental wave.Secondly,Ta/Ni/Ta film was deposited on the SAW delay line,and the ADFMR phenomenon under the action of SAW at different frequencies was investigated.The results indicated that the device showed pronounced absorption peaks.The energy absorption coefficient amplitude showed asin~2(2θ_H)relationship with the applied magnetic field angle,consistent with the theoretical calculation.Moreover,the resonance field and SAW frequency were consistent with Kittle’s formula.These results confirmed that the absorption peaks originate from the ADFMR.Finally,the effects of film width and thickness on the ADFMR were investigated.It was found that the ADFMR absorption showed a positive non-linear relationship with the width and tended to increase and then decrease with the thickness,reflecting a different physical phenomenon from the microwave-driven FMR.The study results lay the foundation for further research on the acoustic spin pumping effect in the FM/NM bilayer.4.Research on the SAW-based magnetic field sensors.To improve the Q value,CMOS compatibility,and flexibility of magnetostrictive material selection for MSAW sensors,a new-type MEMS MSAW resonator based on Al N/Si/Fe Co Si B-Cr structure was proposed in this dissertation,which deposited piezoelectric and magnetostrictive films on both sides of a 30μm-thick Si substrate to ensure the high-quality growth of Al N piezoelectric film.Experimentally,the device samples were prepared through the MEMS process on a silicon-on-insulator(SOI)wafer.It was found that the device’s resonance frequency increased and then saturated with the applied magnetic field along the easy axis.The maximum frequency change and magnetic sensitivity were 20 k Hz and 2k Hz/Oe,respectively.Along the hard axis,the resonant frequency increased firstly,then decreased,and increased again and finally saturated with the magnetic field.The maximum frequency change and magnetic sensitivity were 76 k Hz and 11 k Hz/Oe,respectively.The trend of resonant frequency variation with the magnetic field was explained by modeling the magnetic domains.By analyzing the device’s Q value,it was found that it has a high Q value of about 3700 at the highest magnetic sensitivity position along the hard axis,which met the requirements of high sensitivity and passive wireless sensing.The study results provide a new approach for developing MSAW sensors with high CMOS compatibility and Q value.